Boiler choice FAQ

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editors note, recovered from archive of Ed's original page - note that some information in this document may now me out of date as of time of posting (March 2014)

Frequently Asked Questions about Choosing a Boiler for uk.d-i-y (news:uk.d-i-y)

by Ed Sirett

Version 071217

The condensing gas boiler is a unique appliance in that it is the only appliance which is connected to all four mains services.


If it's your own home and you are going to live there, buy the best boiler you can afford. The extra in the cost of the boiler is small in comparison with the fitting labour cost (even your own). Sadly and cynically its a different story if you are choosing a boiler to repair the heating in a place that's to be sold. In this case you need to spend the minimum it takes to convince a surveyor the heating is working.

I do not recommend any particular make, rather this is a guide to the features of various boilers and how they might affect your choice.

Putting this FAQ in its place.

The question of "My old boiler has broken down. I need a new one, what should I get!" comes up in some form almost every day on uk.d-i-y. This FAQ is written to assist Usenet news group news:uk.d-i-y. This FAQ is not an endorsement or a criticism of any particular method of domestic heating or hot water (HW) production. The intention is that regular posters can make reference to this FAQ to save them having to write out the same basic stuff repeatedly.

People coming to this FAQ maybe reading this to gain help in a planned upgrade to the heating or they may be in a crisis of no heating in the middle of winter. People in the latter situation may have less options available to them for changes but I hope a bit of extra info should be useful.

One of the aspects about choosing a boiler is that it is closely bound to questions about what heating system might be best for you. In particular, questions about different types of heating systems can produce very long threads. These threads frequently veer off topic and can sometimes stray into flame wars and even personal criticism. Let's be quite clear: I am not advocating any particular method of domestic heating or hot water production. Similarly I'm not advocating for or against any specific manufacturer except that some produce models which I think have better features than others. Obviously when a boiler is up for renewal then questions about the general state of the heating and plumbing naturally get raised. It may be that improvements to efficiency, performance or control of the entire system can be addressed with less additional disruption at that time. Some of these changes are mandatory, others are matters of necessity or just convenience. Methods of HW production can also come under review, some people can have firmly held opinions. This subject is covered at a basic level in the main FAQ of uk.d-i-y under the heating & plumbing pages ( here )

At many points in this FAQ I am assuming that you are getting at least some professional help. The issues surrounding heating system replacement by yourself are dealt with in the section "What are the issues with doing it myself?". Many people just want to know what the options are. Others may want to know if there are aspects of the work that they can do, whilst leaving other parts to a professional.

Help! Help! The boiler has died! What Next?

There are a number of likely reasons why you may wish to replace the boiler. If the circumstances are inconvenient then it might just be possible to avoid the big problem until a warmer season. Obviously not in every case. Let's review some of the common circumstances.

Often there is going to be a lot of work done to the house (e.g. A loft conversion or an extension added). It might seem that the additional cost, hassle and disruption of a boiler upgrade can be 'hidden' as a marginal extra in the other works or the replacement is forced on you because you want the space where the boiler currently sits.

Others have been made aware that the boiler is getting older. Often this is done by a larger heating service company. Perhaps these are the words: "We aren't able to offer you any service cover for this boiler any longer, due to lack of spare parts.". The company then takes advantage of the situation by offering a quotation to replace the boiler and do other works at somewhat over the going rate. The supply of the critical spare parts may be far better than is suggested. Nevertheless you do well to heed the general warning that the boiler is getting older.

Sometimes the boiler has been 'condemned' or more formally Labelled as "Immediately Dangerous" or "At Risk" of becoming so. (For more info see the Gas fitting FAQ Section 2c). You should definitely seek a professional opinion as to the prospects of repair if it was not given to you earlier. As your boiler may be repairable until a less desperate time of the year arrives.

Example: The boiler's thermostat fails and the boiler boils, the noise alarms you, so you turn it off and call for help. The fitter arrives and finds that the boiler does indeed have failed thermostat and since s/he can't repair it there and then s/he draws up a 'notice' and 'labels' the boiler, which is their legal duty. It will only take £20 of parts and ½ hour to fix this problem, a good fitter should make you aware of the options.

Possibly it's broken down but it's one of those awkward faults that no fitter seems to be able to diagnose and repair correctly. A number of people have tried some of the major components of the boiler (at great expense). The phrase good money after bad keeps springing into your mind. You should be aware that fitters do vary somewhat in their ability to diagnose boiler faults. An experienced fitter may often say "I've narrowed it down to one of two or three parts", since I can't tell which we should try the cheapest first. Intermittent faults are especially difficult for the fitter to diagnose and sometimes apparently irrelevant observations from you may help. Less experienced fitters may be tempted to say "The boiler's finished, we've got to replace it." Fitters from the manufacturers may have been better trained on diagnosing problems with your particular appliance. They may well be aware of an upgrade that is available to rectify a relatively less relaible component. However manufacturers service 'teams' often get very stretched especially in winter and they may attempt to control this by responding only to warranty call outs.

Generally faults involving the main heat exchanger and other major components on older appliances will truly put the boiler beyond economic repair. As a rough rule of thumb using 2005 figures I estimate any repair of less than £100 is worth doing. For a boiler less than 20 years old you may want to allow a bit more on a sliding scale so that with a younger boiler you are prepared to spend much more for its repair. Obviously if you are able and intending to make the repairs yourself then cost of the repair will be the parts and whatever you want for your time and trouble, this latter, is, in all honesty, more than nothing. I am saying this to illustrate the point that repair v. replace is not quite as simple as some try to make out it is. A low cost repair may get you through the winter to a more convenient season to replace the boiler. Having said all this, modern boilers consume less gas and can easily justify replacement against old and inefficient systems on gas saving grounds alone.

The fear of having a cold house in winter can push some people to make unwise or hasty decisions. Hopefully reading this may put you in a better position to deal with the crisis. Whilst no heating and HW in winter is very unpleasant it is not a matter of life and death (except possibly for the very elderly or infirm). You can often just heat one room with an electric heater or gas fire. HW problems can be got around by a visit to the sports or health club , a friends house or boiling kettles. If you have a stored HW system you can turn the electric immersion heater on (if the original installer added one!).

Why can't I have 'old faithful' back again?

As soon as you have found out that your boiler really is dead (i.e. beyond plausible economic repair) your first idea will be very likely. "I just want a straight replacement.". This is understandable your old "Hades Utopia 80 CF" has been in the kitchen for 35 years. A couple of times it's let you down but it's always been easy to fix. Over the years it's actually had 5 new thermocouples, 3 new pumps, a new wall thermostat in the hall, a new time switch and perhaps a new gas valve but overall it's been your friend. Now it's leaking brown water and you reckon it doesn't owe anyone anything, but what you really want is for it to be rejuvenated.

As soon as any fitter comes round the first thing they say is: "Sorry, you're going to have to have a new type of boiler, it's regulations. I couldn't get hold of one of these anyway." You are disappointed - 'old faithful' was a real friend. The cat used to curl up on top of it and watch the world go by until it got too hot. Everyone used to come in from the cold and rain and lean over it to warm up, it was like having an Aga. You had this gadget which clipped round the flue and you could hang tea-towels on it to dry.

At this point you realize that the repair is not going to be like a glorified swapping of the washing machine. You then start to hear all the different and apparently conflicting advice (some of which is due to hearing only parts of the whole story). You might post a question to a few newsgroups on the net. You get some helpful and some confusing replies leaving you even more confused.

For better or worse, we (the UK) are now signed up to trying to reduce CO2 emissions along with most of the rest of the world. The practical outworking of this is that new boilers since 1st April 2005 have had to be the latest condensing technology. Such boilers together with other improvements to the control systems can save about 30% off you gas bill when compared to the worst case older systems. These boilers extract more heat from the fuel by condensing the water vapour produced during combustion. These improvements are mandatory. [Footnote: There is an exemption procedure but unless you have a back-boiler in a terraced house or flat you can forget it.].

Many people take the view or have heard a myth that condensing boilers are unreliable and expensive to fit and repair. Like all good myths there is actually a small core of truth which has become greatly exaggerated. Some manufacturers did produce some truly dire models in the 1990s when they brought out their first designs. In reality the condensing boilers are very little extra work to fit. They just need a drain for the condensate. This drain can go any into any waste water service: surface, foul or combined. It can also go into a small soak-away outside filled with limestone gravel. It should not go through metal pipes. It is a debatable grey area whether or not it can go into a metal drain after being joined with other waste water. The condensate is slightly acidic, the same as fruit juice, it might just rot a metal pipe given time, but not overnight.

The real difference is not so much between condensing and non-condensing boilers and but between modern boilers with lots of integrated electronics and older low-tech boilers. In fairness much of this difference (fans, air flow switches and electronic ignition & flame detection) are also the things that help modern boilers save a lot of gas.

I suspect that part of the like that some people have for older boilers stems from a sense of confidence. A low-tech boiler was simple to understand and so diagnose and repair. I would point out that just because a boiler has electronics does not mean that it has to be unreliable, The Potterton Netaheat was considered to be a very reliable boiler, and has the electronic features listed earlier.

Modern boilers can go in a large range of locations and are usually wall hung. This is not a disadvantage since nearly always there is more wall space than floor space. The flues are small (100mm) and can go up through roofs (both flat and pitched) or out through walls. If necessary some models can use high quality (MUPVC) 56mm plastic pipe with outstandingly long runs, some models only use this plastic pipe. If necessary boilers can be hung on the wall just above the floor but care is needed to check that all the relevant access clearances are complied with.

What other types of boiler are there?

You now realise that replacing the boiler is going to be a pile more upheaval than you originally thought. So then you ask yourself "What other options do I have?". As far as the heating is concerned the main issues will be:

Always got too hot.

This would be due to a system that had no wall thermostat or a thermostat placed in an untypically cold location.

Never got really warm.

I have known one house where this was due to the user simply not being prepared to switch the heating on often enough and long enough. It can also be because there are not enough radiators or big enough ones. Radiator encased in boxes give out only about half the heat as when they are uncased. It can also be due to an underpowered boiler (or one that functioned that way).

One or more areas that were always too hot.

The radiator in that area is not controlled by a Thermostatic Radiator Valve (TRV), or the TRV is faulty or set too high.

One or more areas that were always too cold.

There is no radiator or not a big enough radiator in the area. Note that often the area where an old boiler is located is heated by the boiler. This may need radiator as modern boilers give off next to no heat. Problems can also occur if the radiators are not balanced so some get hot and others don't.

Most professionals have a good idea what the requirements will be from just looking and walking around. They would want to allow for possible problems and future extra load (extensions and loft conversions). There is also the fact the an under-heated house will be a major problem to the user and the installer whereas a slightly over sized boiler will be accepted without a thought. Modern boilers can adjust their output power over a range automatically, therefore over-sizing is less of a sin that it used to be, nevertheless getting this right will save some gas. You can use this calculator web page to work out the required boiler power. This calculator tends (in my experience) to produce slightly smaller boilers that I would feel comfortable installing, however it should identify the cases where the boiler is grossly over (or under) sized. Combi boilers are sized for HW flow, the minimum is 24kW but you should install a bigger one if you can.

As far as HW production is concerned there are many options they all have strengths and weaknesses. Briefly they are:

Vented HW cylinder.

This is still the norm for most houses. With a modern cylinder and correct controls there should be no problem with running a bath every 15-20 minutes. Cylinders are now required to have a good level of insulation (not the old jacket insulation). New replacement cylinders will be able to heat the whole cylinder from cold in around 20-25minute, and be able to replenish a bath-full sooner. Flow is usually good, pressure for showers will depend on location of cold water storage cistern. Flow & pressure can be boosted by a pump.

Unvented HW cylinder:

Offers HW at mains pressure together with excellent flow. These are expensive and can only be installed by qualified people. To justify the expense you really need to have an excellent water main at least 25mm diameter plastic pipe. Results will be adequate but probably not worth the cost if used with an ordinary lead or 20mm plastic incomers.

Thermal store:

Offers the advantages of an Unvented cylinder, costs are similar but may be diy installed. Can be used to integrate other sources of heat (solid fuel, solar) or facilitate underfloor heating. Will still need and good mains supply, say 20 litres/min or more.

Normal combi boiler:

Heats water only when it is needed. Instant availability (after the HW has drawn through and the boiler settled down – about 10-40 seconds). Never runs out. Simplified installation. Limited flow rate, but mains supply still needs to be average or better.

Small storage combi:

As above but holds a store of HW to give a prompter response and improved flow rates for a while until the stored HW has run out.

Large storage combi:

Washing machine sized floor standing combi boiler. Contains pretty much all of the benefits of a HW cylinder whilst still having a simplified installation. Saves the space that a HW cylinder would take.

There are multiple decisions to be made based on trades-off between size, cost, flow rate, warm up times etc.

What are the issues if doing the work myself?

Since the 1st April 2005 there are rules associated with building regulations. Since the 1st Jan 2005 there might be aspects of the electrical work that come under Part P of the building regulations. Gas fitting itself is highly regulated and is discussed in the Gas fitting FAQ.

Most boilers are installed in a kitchen and a good proportion of the rest are installed in bathrooms. Bathrooms are special locations as far as Part-P of building regulations are concerned (kitchens were until the most recent edition of Part P). In a special location, work that is more than direct repair or replacement now has to be done or certified by either someone or a company that is registered for "Part-P" work. Alternatively you can approach you local building control office and do the work yourself. Different local authorities are found to be charging differing amount for inspection work although there seem to be some evidence that they ought to charge a flat fee which is around £100+VAT. With all these new regulations the level enforcement is very limited.

Likewise anyone who installs a heating appliance in a dwelling either has to be able to self certify the installation complies with the building regulations or they have to submit a building notice to the local authority. To self certify there are a number of trade bodies which cover various aspects of the work. Anyone who fits gas appliances for reward has to be Gas Safe registered. Gas Safe members are permitted to self-certify all aspects of the work including minor electrical work (up to one new circuit) provided they have passed the right assessments. If you are not able to self-certify, then the law is that you must submit a building notice application. The same is true for replacing the HW cylinder even a normal vented one.

With new ideas arriving in 2021, a heat-pump boiler is occasionally possible in small dwellings. Monobloc types are self-contained, like a refrigerator, so can be DIY-installed. They sit outside the building, so not in a special location. See Heat Pumps. They are permitted development too

Why can't I replace just the boiler?

Improving the efficiency of the boiler itself is only one aspect of complying with Part L of the current building regulations. The other part involves making sure that the controls are up to scratch. The contribution to improvement in efficiency is about equal between the boiler and all the other stuff. The following is a summary of what it will take.

  • Thermostatic radiator valves on all but one radiator – usually the radiator nearest the thermostat.
  • A wall mounted thermostat usually in the hallway. (You may already have this).
  • A time control for the heating. (You almost certainly already have this).
  • A boiler interlock which simply means that when there is nothing to do then the boiler stays off, this is achieved by the way the controls are wired up to the boiler.
  • For large houses the heating must be divided into independent zones (e.g. No need to heat the bedrooms during the day. )

For non-combi boiler systems where there is a HW cylinder you also will need:

  • Independent time control for the heating of HW.
  • Temperature control of the HW (usually done by a cylinder thermostat).

If the HW cylinder needs to be replaced then it too will have to comply with new regulations. The new cylinder itself will comply (it will be adequately insulated and have a 'fast recovery' indirect coil so that the water is reheated quickly). The surrounding pipework will have to be insulated as well.

So when an installer says all this other stuff needs doing you can be assured that this is not a work creation scheme, but the correct comments of someone who wishes not to cut corners.

Why are (good) boilers so expensive?

I expect that it costs about the same to produce a boiler as it does to produce a washing machine. They arguable use a similar amount of similar materials and employ about the same level of technology. So why are boilers roughly twice the cost? I don't know for sure but I suspect that the following are some possible factors.


If you ran your washing machine for roughly 25% of the entire time it would soon wear out, even the up market models. Even the low end boilers should be able to do normal duty for at least a few years with a reasonable chance of being problem free.


Some parts of the boiler are gas carrying parts and understandably these must be manufactured to a high standard. Gas valves which leak, or fail into the open state would be unacceptably hazardous.

Type Approval:

All boilers that are legal to fit will have been 'type approved'. The design and construction are scrutinised and tested to ensure that the boiler remains safe enough to use under the worst case operating conditions. For instance, will it work rightly if the gas supply pressure drops to 13mbar and the supply voltage to 207V? I am informed by several sources that the process of type approval is very expensive.

What's with all these new boiler features?

Certainly there are features to be avoided and those to be sought after.

Features which I consider good:

  • Stainless steel heat exchanger.
  • Pre-mix burners.
  • Integrated pump.
  • Integrated sealed primary circuit components.
  • Integral bypass circuit.
  • Ability to choose lots of different flue types at reasonable cost.
  • Flexibility for location of flue.
  • Ability to install in a cupboard with small clearances and no extra ventilation.
  • Smart or remote diagnostics.

Features I consider questionable:

  • Aluminium heat exchangers.
  • Two-part heat exchangers.
  • Atmospheric burners.
  • Upward firing burners.

The features explained:

Heat Exchanger material. With Stainless-steel these don't corrode either from the inside or the outside. I have been to at least one installation with an aluminium heat exchanger which was corroding from the water side despite the correct inhibitor being used. I really don't know if Silicon coated Aluminium is a good material or not, some well respected makes use it.

Burner firing direction and heat exchanger layout. In a condensing boiler if the flames go upwards the condensation tends to want to fall onto the burner, this is not desirable. A number of boilers have this design usually because the design is very clearly a rework of an existing non-condensing boiler. These reworked boilers often incorporate the original design of burner and heat exchanger unaltered and then further cooling of the flue gases is done in a second heat exchanger. All the condensation happens in the second heat exchanger (we hope). The one piece type designs are smaller for a given power and tend to make for a smaller boiler.

Burner type. Older non-condensing domestic boilers had what were known as 'atmospheric' burners. That is they mixed the gas and air together by virtue of injecting the gas into the air and then ducting the gas/air mix to the burner. The better condensing boilers use a forced pre-mix burner where the gas air mixture is forced into the burner. The advantages are two fold firstly the power of the burner is adjustable over a wider range by adjusting the fan speed. This enables the boiler to better match its power to part load settings (the most common need). An older boiler must cycle on and off which is a source of some inefficiency. Secondly, with the forced pre-mixing, combustion is more tightly controlled so that pollutants like nitrogen oxides are kept to very low levels.

Integrated pump: The pump which circulates the water is in the boiler casing this means that boiler directly controls the pump and can often adjust it's output to suit the operating circumstances saving wear, electricity and gas. The down side is that when it fails the pump will very likely be customized unit for the boiler and not a cheap generic pump. Combi boilers will always have this feature. Boilers without this feature are known as plain heating boilers or sometimes 'Compact'. Integrated sealed system components: Sealed primary circuits are an advantage, see the Sealed CH FAQ . This saves having these components elsewhere but it makes the boiler box bigger. Combi boilers invariably have this feature.

Integrated bypass: Modern boilers need to keep the water circulating for a short while after the gas has cut off, this prevents localized boiling in the heat exchanger. This is achieved by having some pipework (called the bypass circuit) through which water can circulate when there is no other path. If it's in the boiler then it's one job less for the installer. Combis will usually have these.

Wide variety of flue types, locations and positions: There are a considerable number of places where the boiler may/may not be sited and in particular the flue terminal may not be sited. A model whose rules are less restrictive is to be favoured. Note that due to the visible 'steam', or plume, from a condensing boiler's flue, the flue terminal should not point towards a property boundary. IIRC 2.5m is the minimum distance to avoid the problem of creating a nuisance, vertical flues can be used to avoid this. Boilers which can be installed in cupboards even kitchen wall cupboards without additional ventilation are obviously less trouble to the installer.

Diagnostics: The better boilers will often keep information about how it is performing, some even allow a laptop PC to be plugged in to gain access to this information. Such features can be invaluable when trying to track down intermittent problems. Some models offer the ability to have remotely mounted status and warning indicators fitted. At the top end of the market the boiler will probably monitor HW cylinder and outside temperatures. This attempts to better control the boiler to match the circumstances and save a little more gas without loss of function.

At the end of this FAQ is a glossary of terms and features that apply to heating and boilers.

Marketing: Truth, lies and bribes!

As with all consumer products there are perceived rankings of the 'quality' or 'reputation' of the manufacturer. The matter is further compounded by the fact that there are three distinct groups who the marketing people wish to address, and who may have very different perspectives on the quality of a manufacturer, the manufacturer may give very different weights to their opinions.

The different groups are:

Specifiers - These are people like architects who might simply specify a particular make and model for dwelling. If a team of architects working for a large multiple unit house builder choose a particular make and model this is very significant. Another person might be someone like a maintenance manager for a large social housing estate. Getting one of those people to specify a particular model to replace all the ageing boilers on an estate of 500 hundred flats would be the dream of every boiler sales rep. I hear rumours of holidays etc. being offered as incentives.

Installers - We don't mind what the name on the boiler is (unless the end user has an opinion). What we want is roughly in decreasing order:

  • Loads of flexibility on the installation rules so we can put the flue and boiler wherever the customer wants it.
  • Ease of Installation
  • High availability - next day at the merchants (or better) and high availability of spares.
  • High reliability (for at least a few years), long guarantee period.
  • Very easy to repair and service.
  • Low cost.
  • Light weight.

End-users – want the same make as any boiler they have been happy with and a different make to any boiler they have been unhappy with.

Clearly the opinion of people in the first group can have a large influence. In my opinion a certain manufacturer is still around because they used to make very reliable boilers and still have a big reputation. Left to the installers who have to fix their more recent products they would have gone out of business.

The second group, the installers, is less influential but still worth trying to please. I know of one case where a manufacturer struck a deal with a gas training & assessment centre. The deal was that the boiler maker would pay a small sum of money to the centre for every student who went through their doors. The centre in turn agreed to give a 5 minute demonstration to all students about the boiler; showing how easy it was to service and repair. I was impressed with some aspects (good layout, 5 year guarantee) and unimpressed with the electrics positioned ready to catch all the drips from work on the water carrying parts (they are by no means the only offender in this area).

I have heard of incentive deals offered to installers if they install so many from one manufacturer in a given time. I have yet to come across one myself though. The most I have ever gotten was an Easter bunny fridge magnet.

The third group obviously have relatively little influence. One manufacturer did some TV advertising the other year (autumn 2003 IIRC). I'm not sure how much impact it had, it pleased me as they were the same make as I often install. [Footnote: It had shots of geese flying away for winter and a young girl keeping warm because the house had their new (high efficiency) boiler. It was a "Euro-ad": pictures only with a voice over.]

Finally, there is one German manufacturer who is major and well respected in continental Europe that we don't have: "Junkers". You'd have thought they could have changed their name to market themselves in the UK, but I guess the name would keep slipping through to the UK on spares packets and the back of manuals.

Why do some spares cost much more than others?

I found it most surprising that the cheaper the boiler the more expensive its spares. This was very counter intuitive. I'd have supposed that an expensive boiler would need expensive spares but hopefully needed less often. To illustrate this: A bottom of the range combi boiler has a 24hr time switch which costs £80+VAT, this is a flimsy unit, the On-Off-Timed slider switch breaks easily. A top-end boiler has a well built timer which is larger (so easy to set) and costs £30+VAT but the boiler costs more than double the low-end make initially.

My best guess about this is that the more likely a spare part is to go wrong the more demand there will be for it, more demand means higher prices. The production cost of a spare part is next to nothing compared with what it costs you to purchase it. What you are really paying for is the whole industry set up to have spares available for you when you need them. To further illustrate the point any boiler which has a 'stock' fault will generally have a very expensive replacement part for the fault. There is a common boiler model which has a lot of trouble with its PCB. The replacement PCB is over £200 whereas most PCBs are around £100 give or take.

What's the bottom line?

For various consumer goods, opinions vary about the best way to achieve the lowest total cost over the long term. For washing machines various arguments can be put forward for buying the cheapest or the mid-range or the dearest products. The same might hold true for cars but with bigger numbers. When it comes to boilers these arguments no longer apply. Whilst the cost of exchanging a washing machine is about £50 the cost of exchanging a boiler can easily be £750 done professionally. With d-i-y this situation does change but your time may well be worth more than nothing and there may be building control costs. You may need to hire or buy some tools you don't already have. Even so this pushes the whole decision for boilers strongly towards the top end of the market. There is a big difference to the reliability and just as importantly the repair-ability of the top end boilers relative to the low end units, this is ever more so as the boiler ages. The superior initial build quality allows for successful repairs on aged boilers which will happen less often using cheaper parts.

There is one exception. All the above assumes that you are staying put (for more than a year or two) with your new boiler. If you are going to sell then everything changes. The effect on house prices of a boiler is as follows: Working boiler: no effect. Broken boiler: deduct at least £3000. The conclusion is that you should spend the minimum it takes to get the boiler working. Working in this context means: working well enough that a surveyor does not point it out as a possible problem on a survey. It may not actually have to work all that well. With all due respect to surveyors, if it looks 'cared for' and lights when turned on, that'll probably pass as OK.

Landlords should also fit the best boiler they can afford. They also do well to make sure that every tenant is properly informed in the use of the boiler. Doing this latter step will often save a small fortune in plumbers' charges.

There is a 'grey' market for low end boiler installations (a segment of the market I wish to have little to do with). These installations don't comply with all the latest regs. The up side is short term cost saving, the down side is lack of paperwork. The manufacturers might not honour the guarantee, but the chances are that you are not going to need help between 1month and 24 months after installation. The local authority won't find out but if they did they could come down on you very hard. There may be irregularities in various bit of paperwork when you sell, but it remains to be seen just how much of a problem that will really be.

The fitter is recommending X. Is it any good?

An honest fitter will be happy to let you know what the boiler costs. A small discount may be available to him/her but you can easily check the price online (don't forget to allow for the flue and VAT as needed). Try looking at and searching for specific boiler models. There are many similar sites. The manufactuers site tells you everything they want you to know and never tells you the price.

Here are some of the good reasons why your fitter may be recommending a certain model:

  • He knows the model and has fitted several before. He does not need to spend time reading the instructions because he knows them already. He knows if there are any 'gotchas' like bits that need to be fitted before something else gets fixed. He knows where all the wires and parts go. All this saves time.
  • He knows he has had little trouble with the model in terms of call backs and warranty claims.
  • The local merchant has the boiler either in stock or can get it easily.
  • He is familiar with the range of accessories (especially for the flue) and other options.

Here are some bad reasons:.

  • There is a regulatory change coming up and there was an amazing deal available to anyone who bought five units. The fitter now needs to install them promptly before they become worthless.
  • The fitter was offered a cash back deal or free holiday if he agreed to fit ten units in 4 months. He's just fitted the ninth.
  • He got hold of the boiler on ebay and it's now cluttering up his garage/store room.

It all comes down to trust in your installer.

I will not spoil my best attempt at a neutral point of view by recommending any one make. I simply say that there is a correlation between purchase price and likely quality and reliability. To find out which are the better units go to an online boiler supplier and compare the offerings from the different makers. Look for the features from the features list section.

Are there any bargain boilers?

Not really.

Sometimes you can find a weird pricing structure put out by the manufacturers. If you only need a 12kW boiler (quite plausible for a smallish terraced house or flat) you might find that the 12kW boiler is almost identical in every respect to the 15 and 20kW versions but is cheaper. Because so many combis are sold (throughout Europe) they can sometimes be cheaper than the non-combi version even though the latter has less components! If you need a 24kW non-combi boiler it might still be worth using a combi for the kitchen sink tap and/or a shower and using the heating side to do both heating and heat the HW store/cylinder.


There seem to be 3 sorts of boiler on offer.

Normal brand new boilers (boxed with the flue and instructions). The reserve prices are such that even if you get lucky on the bidding you won't be saving a lot relative to an online store or local merchant.

Used boilers that have been removed whilst they were still working (e.g. An extension was being built). The vendor thinks there is someone somewhere who can get some more life out of it. The flue and/or the instructions are likely missing. The flue might be obtainable the instructions certainly. The vendor usually wants you to collect due to the great difficulty of shipping the unit. Unless you are installing it yourself just to get a property sold you might as well forget it. Most professionals would be reluctant to reinstall a second hand boiler unless fairly new.

New boilers at apparently huge discounts. They are often advertised as "boiler only", no flue or manual. How did it come about that the flue and manual went missing? Have you ever thought where the boilers that are stolen from building sites go? Many boilers have an installation frame it may well be missing. There is no guarantee that the boiler has not been dropped.

Future choices

If or when fossil fuels are phased out, what will be the major considerations for a replacement? Switching to electric heating or other fuels present quite significant changes.

  • Electric heaters and heat recovery systems fit indoors, same as gas boilers, but heat pumps need to be outdoors*
  • A 5kW air-to-water heat pump is physically bigger than a 25kW gas boiler and runs at a much lower temperature, needing more radiator area.
  • A 5kW air-to-air heat pump is just as big and can only warm two or three small rooms.
  • A pellet boiler is much bigger than an oil or gas boiler

Heat pumps giving more than the 5kW example above increase in size and weight much more than the equivalent gas or oil boiler

*outdoors, for a heat pump, can also mean a shed, a greenhouse, a balcony or an attic with a dormer exhaust opening. See Heat Pumps

How can I check that the boiler will fit before I buy?

Many manufacturers have the information available online. The manufacturers will tell you the info you need if you ask, this is clearly not as satisfactory as publishing the info online. Some manufacturers have web sites where the some of the info is only available to registered installers. When requested by a web page for a CORGI registration number you could, of course, put in 999999 or 123456 and see if it opens up. There is next to no chance that it will link to CORGI's database or that they want to really find out who you are. Frankly, the most is they just might want to junk mail the installer and might use the reg number to find out where to send the brochure, but that'll be the end of it.

Glossary of abbreviations, terms and features of boilers and heating.

Advance / Override. A feature on many time controls where the the user turns the heating on or off for now but the timer will resume its normal programme in due course.

Air Pressure Switch/ Fan proving switch / APS / FPS A device usually a differential air pressure switch used to detect that there is an air flow through the boiler. Pre-mix burner fans have a tachometer sensor which is checked.

Anti short cycling Controls in a boiler which prevent short cycling usually by inserting delays before relighting after a thermostat recloses.

Auto-air vent A vent that can let air out but not water. Often lets both or neither out.

Balanced Flue / BF Flue gases removed by convection, air inlet is at the terminal of flue (big rectangular metal object) . Less susceptible to problems from the weather conditions than a conventional flue.

Boost A feature on the time controls where the heating is manually put on for a while regardless of the normal timer settings.

By-pass circuit Pipework that allows the primary water to circulate when no other path is available.

Carbon Dioxide / CO2 A gas produced by burning fuels containing carbon (oil/gas/coal/wood). Greenhouse gas that causes global warming.

Carbon Monoxide / CO A very toxic gas produced by incomplete combustion of fuels containing carbon. A 0.4% concentration is fatal in minutes.

Combi boiler A boiler which instantly heats the HW as and whenever needed. Some models have a store of HW so producing a spectrum of models with features more towards a 'conventional' system with separately stored HW.

Combustion Analyser A device for measuring the contents of flue gases. More or less essential to adjust a pre-mix burner.

Condensate drain Where the product of condensation goes.

Condensate trap

A trap similar in purpose to the trap under a sink. It seals the combustion chamber from the condensate drain.

Cylinder (Thermo)Stat A thermostat mounted on the hot water cylinder (about ¼ the way up from the bottom) to control its temperature.

Diaphragm valve / DHW flow detector. A device in a combi boiler that detects that the HW is running and switches the boiler on. Often but not always implemented as a differential pressure valve sensing the movement of water.

Diverter Valve A valve in the primary circuit which can choose to heat the radiators or the HW. Usually electrically powered by the control system. This component is inside combi boilers and can be operated by all sorts of means.

Domestic Hot Water / DHW Hot water that comes out of taps.

Electronic Ignition The boiler lights the gas each time it is neded by means of a high voltage spark.

Expansion Vessel One of the components for a sealed primary circuit. Also needed on some unvented HW cylinders. Sealed CH FAQ

Weather compensation A boiler or heating control which takes note of the outside temperature and make adjustments accordingly.

Fan Flue / FF The air and flue gases are moved through the boiler by a fan.

Filling Loop One of the components for a sealed primary circuit. Sealed CH FAQ

Flame detection / FSD / Flame Proving / Flame Sensing The process of detecting that the gas is alight. Flames conduct electricity a bit. Older boilers used a permanent pilot light and a thermocouple.

Flame picture The look of the gas flame – blue good - yellow (like a candle) bad. A simple method of crude combustion analysis.

Flue Any pipe or duct used to move the products of combustion. A chimney is one type of flue.

Flue-Air Duct A combined air intake and flue usually coaxial with the flue in the middle. Very common.

Fully pumped A system where none of the primary water is moved by 'gravity', but by a circulation pump.

Gas valve A device to let gas through when an electric current is applied often containing other gas safety related components.

Glow Plug A newer type of ignition, said to be more reliable.

Gravity / Gravity Hot Water An older system where the primary water is moved (slowly) by convection due to the difference in density of hot and warm water. Implies poor HW recovery.

Heat exchanger The part of the boiler where the hot gases are cooled and the primary water is heated. One of the largest and most expensive part of any boiler.

Holiday Mode. A feature on the controls where the heating will be set to a very low setting (or off) until you return.

HW Recovery Time The time taken to restore the HW store to its original temperature.

HWC / Hot Water Cylinder A cylinder storing HW for use later. Heated by a coil connected to the primary water circuit, or an immersion heater.

Immersion Heater Electrical heater to heat the contents of a Hot Water Cylinder. Useful as a backup.

Kettling / Knocking / Thumping Anything from wheezing sounds through to the sound of tumble-drying rocks. Causes by localised boiling in the main heat exchanger, due to build up of debris or some condition causing poor primary flow.

Lock-shield valve / Balancing valves The non-user operated valve on a radiator. Or more generally any valve that is not intended for casual use or tweaking. Often arranged so that a tool is needed to move it.

Lockout A condition caused by over-heating, lack of gas or some other problem where the boiler won't operate until it is manually reset. Usually done by a relighting the permanent pilot or pushing a reset button.

Low water pressure switch A switch to prevent operation of the boiler with inadequate primary water pressure, so preventing damage.

Mid-position Valve. Like a diverter valve but can stay in the mid-position so as to heat the house and the HW cylinder simultaneously.

Modulation A feature where the boiler can alters it power output usually to suit conditions.

Modulator Part of the gas valve that adjusts the flow of gas in response to an electrical signal. Common on modern boilers.

Open Flue / Conventional Flue /CF / Natural draught. Flue gases removed by convection due to their temperature and a supply of fresh air.

Party Mode A feature on the time controls where the heating is manually turned off for a while regardless of the normal timer settings.

PCB / Circuit card / Controller board / Electronics Module Any electronics: invariably a printed circuit board which can vary from a single relay to a dedicated microprocessor controller. Sometimes blamed for boiler troubles lying elsewhere.

Plume The cloud of steam (actually a mist of tiny water droplets) often seen coming out of a condensing boiler flue in cold weather. Can be considered a nuisance by the neighbours from hell and so begin an air-space rights feud.

Power-flushing A process where debris and corrosion sludge are removed from a primary circuit using a powerful pump and chemicals. Often offered at great cost by larger service companies to treat the symptoms whose causes may still need to be found.

Pre-mix Burner A type of burner used in condensing boilers.

Pressure Relief Valve One of the components for a sealed primary circuit. Also needed on unvented HW cylinders. Sealed CH FAQ

Primary Water/Circuit The water which circulates in your boiler and radiators and may also heat the HW indirectly.

Programmable (Thermo)Stat An Electronic Device which contains both the features of a thermostat and timer whose usefulness greatly exceeds the two devices installed separately.

Proportional Controls State of the art controls which tell the boiler not just on or off but how strongly it should heat.

Pump / Circulator A device that pushes the primary water around.

Pump Overrun A feature where the boiler keeps the pump running after firing to prevent localised boiling until the residual heat has been carried away. Controlled by a thermostat or timer.

Room Sealed / RS The combustion chamber is sealed from the room. Air for the burner comes in directly from outside. Greatly increases the safety of the boiler against carbon monoxide (CO) poisoning hazards. Balanced and fanned flues are usually thus.

Scale Reducer An attempt to control lime scale problems by either dosing the water with phosphates (which works) or electromagnetically bewitching the input mains water (efficacy uncertain).

Sealed Primary System A type of heating system where the primary water is sealed under pressure. See Sealed CH FAQ

Secondary Heat Exchanger / DHW Exchanger / Sardine-can. The part in a combi boiler where the DHW is heated by the primary water. Prone to filling with lime scale in hard water areas on the DHW side. Acts as a filter for all debris on the primary side.

Set-back A sophisticated feature on some heating controls where the boiler is turned on earlier or later according to the current inside and outside temperatures.

Short cycling A process where the boiler is firing on and off in short cycles. This is a source of inefficiency.

Smart bypass valve An adjustable valve that only let water flow through when there is sufficient pressure. Used to prevent the bypass circuit from short circuiting the heating in normal operation. Mandatory requirement on new or replacement heating systems.

Speed Selector Pumps usually have 2 or 3 speed settings leave well alone unless you know what you are doing.

Stratification Water tends to be hotter at the top of a HW cylinder and less so at the bottom.

Syphonic condensate trap A condensate trap that lets the condensate out in bursts. This reduces the risk of freezing in an external drain pipe.

Terminal The end of a flue. A chimney pot is a type of terminal.

Thermistor / NTC An electronic component used to measure temperatures.

Thermocouple / T/Couple A bi-metal junction that sits in a permanent pilot flame, makes a tiny amount of electricity to keep a gas valve open.

TRV / Thermostatic Radiator Valve. A valve on a radiator which can be used to control the temperature in the area by controlling the flow through the radiator and thus the radiator's temperature.

Volt-free Control terminals on a modern boiler that are not at mains voltage. The term is used rather loosely, may well be at 230V.

Wiring Centre A box where all the control wiring is brought together.

Zone Valve / Zones / Zoning A simple on/off valve in the primary circuit operated by the controls to selectively heat the HW, a part of, or all of, the house. The process of doing this.

See Also