Gas fitting FAQ

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Wiki editors note: Main text by Ed Sirett, Leak testing and purging sections by "The Boilerdoctor", Editing and much constructive feedback by Phil Addison - Originally published on Ed's website


WG = "Water Gauge" - a U Tube manometer used to indicate gas pressure


This section of the FAQ contains the very basics about domestic gas fitting for natural gas. Just as, reading a book on surgical procedures does not give you a licence to perform surgery, so reading this FAQ does not give you a licence to do gas fitting beyond your level of expertise.

Legal Requirements & Safety

Is it legal to do DIY work on gas?

In short, yes it is, but only if you are competent to do so.

So how is competence judged? You are to be your own judge of whether you are competent or not, but should some catastrophe result from your work, then it would seem clear that you were not competent and should not have tackled the work. That could leave you vulnerable to legal action (assuming you were still alive).

This question arises almost weekly on the news group uk.d-i-y and can generate heated and inconclusive discussions, sometimes propagating wrong information.

Like some other aspects of DIY there are laws governing gas fitting. These are the Gas Safety (Installation and Use) Regulations 1998 published by the Heath & Safety Executive (HSE), ISBN 0-7176-1635-5, and are online at:

Regulation 3 (1) effectively says that anyone who works on gas must be competent. Regulation 3 (3) effectively says that anyone who is rewarded for gas work (in money or otherwise) must be registered with Gas Safe (formerly CORGI). The guidance notes published along with the laws state that level of competence must match the work being done.

I don't think there has been a legal ruling yet on precisely what competence means in regard to DIY gas fitting, but for professionals it means they have the knowledge, experience and exam passes in the relevant aspects of gas fitting. There are dozens of exam categories covering domestic, commercial, liquefied petroleum gas (LPG), and more exotic gas fitting skills.

Opinions vary amongst DIYers as to the range of gas work they may undertake. The HSE and Gas Safe are unsurprisingly keen to stress that gas work should be done by competent people. It is my belief that the law is really aimed at eradicating the unregistered professionals whose motivation is their pocket and for whom public safety and customer satisfaction are at best secondary. Incompetent DIY gas fitting is inevitably unstoppable (for instance, I once saw a photo of a home made boiler). The HSE state that the majority of fatal gas incidents occur to the old, the poor, students, and those in rented housing. The regulations specifically try to address this situation.

Whilst gas fitting is potentially the most dangerous of all DIY activities, the public perception of the dangers of gas are often over-estimated, perhaps because when occasionally a gas explosion occurs it is reported as a national news item. To put it bluntly, you are at least 50 times more likely to be killed on the road than by a faulty gas installation.

Most DIYers would agree that at least some work on a gas appliance is permitted, and hardly any would say you should not even change a thermocouple. At the other extreme, replacing a multi-function gas control in a boiler would be beyond most DIYers. There are shops and mail order companies that sell spare parts for gas appliances, in fact obtaining the correct spare part for a gas appliance seems easier than for most other type of appliance. However some shops may refuse to sell "gas carrying parts" to someone who is not Gas Safe registered, but will sell thermocouples, pressure switches, fans, etc.

It should be noted that some work which is not thought of as being gas-related may in fact be: eg in some boilers accessing the terminals to connect an external programmable thermostat requires opening the front panel which may be classed as a gas carrying part; in other boilers it may not be.

Work on flues, chimneys and compartment ventilation for gas appliances is also controlled by the Regulations. Particularly for older open-flued appliances, any work affecting the flue or ventilation may make an otherwise safe appliance potentially or immediately unsafe.

You may have seen notices in some of the big DIY chains saying they "don't give advice about gas fitting, and are thus complying with the law". I don't think there is anything in the regulations that forbids them, but perhaps their legal department fear that giving advice might be thought of as aiding and abetting incompetent, and therefore illegal gas fitting.

A word of warning:

Some jobs that look straightforward, for example, fitting an inset gas hob consists essentially of "cut hole, drop in hob, secure, and connect up to gas supply". However, this hides the fact that there is often a stack of standards of be complied with which the installation instructions may or may not tell you about. For instance, in the case of a gas hob the room in question must have a door, window or other vent that can be opened directly to outside air and the room must have a volume of at least 10 cubic metres.

Professionals should do many checks on their work, some of which may seem unnecessary at first. Before doing your own gas fitting, you should know and understand the relevant regulations and standards relative to every aspect of the task in hand, and your level of experience must be appropriate. For instance, if you are not confident that you can make a good soldered joint for a water pipe then do not even think about doing gas pipework.

A compromise that many DIYers arrive at is to find a registered fitter and work together so that the fitter advises of the finer points of the regulations and approves the plan for the work, and then checks everything over at the end. Some have found that it can be difficult to find someone prepared to work this way and, no doubt, there are some registered fitters who will not do this, but some certainly will.

I would suggest that for larger jobs, such as fitting a boiler, you pay for a consultation at the outset and discuss the work in detail. Do the work, and then get the professional in at the end to check things over and commission the appliance. For smaller jobs, e.g. boiler repairs, it can save a lot of time and expense if you can narrow down the fault even if you are not up to making the repair yourself. For instance, if you can say to the fitter "I'm not sure if the problem is the gas valve, but there is gas on the pilot flame, and 230V on the gas valve, but there is no click and no main flame, it is a Baxi Bermuda with serial number nnnnnn and GCN nn nnn nn", then the fitter can arrive with the correct spare part and this will save you both time and money.

This is never so true as with intermittent faults, which can be difficult to track down quickly, and may not even show when the fitter turns up.

Note that LPG is significantly more hazardous than natural gas (NG), because being heavier than air it collects in low lying places just waiting to be ignited. LPG appliances are forbidden below ground level for this reason. Of course, people might move a LPG cabinet heater into a cellar but I would reckon on the instructions forbidding it.

b) How can I check fitters' qualifications?

All registered professionals should carry a photo identity card from Gas Safe. The slogan is "If they don't show you the card, show them the door!"

You can check someone's registration by phoning Gas Safe or on their web site, which can also be used to find registered gas installers in your area.

On the back of the identity card is a list of gas fitting skills that cover both domestic and commercial work, and columns for NG and LPG. There are ticks in the relevant places for the skills that the cardholder has.

From time to time one hears about the work of some of the less thorough gas fitters that are around. Hopefully as time goes on they will be weeded out by the more stringent entry requirements that started in 1999, or because their work has been inspected, found wanting, and having failed to put it right (at their own expense) were struck off.

If you think that gas work has been done incorrectly, firstly tactfully ask the fitter about it. If that does not bring a good outcome, contact Gas Safe who will inspect the work and decide if it was done safely and correctly, but they will not resolve money disputes.

What about warning labels and notices?

A registered fitter may fix LABELS to appliances or meters (National Grid (formerly Transco) use triangular labels), and draw up a NOTICE (form GA02) on which the details and reasons are stated. If anything untoward happens when you operate an appliance that has been labelled, it is you and not the fitter who will be held liable.

The fitter fixes a label or issues a notice according to the following circumstances:

(i) Sub-standard
The mildest type of problem is known as "Not to current standards" or "Sub-standard". This is something that when the original installation was done was acceptable, but nowadays would not be done. There is no substantial risk to anyone of anything, but it falls short of modern safety standards and best practice.
Example: A pre-1970s cooker in good order but the oven has no flame failure device. This means that, if unlit, it could fill the oven with unburnt gas at the full rate.
If the fitter finds such a condition, he should draw the owner's attention to the risk, but he is not allowed to disconnect it, or put a notice on it.
(ii) At Risk (AR)
At Risk is when there is a risk of an appliances becoming Immediately Dangerous at a later date, i.e. no one is about to be hurt and nothing is about to be damaged but there could come a time when the situation changes.
Example: A back boiler and gas fire is fitted in a room with no additional purpose-provided ventilation. However, the operation of both fire and boiler are fine as confirmed by closing all doors and windows and using smoke matches to check for spillage of flue gases into the room. Perhaps the room has single glazed windows and is quite draughty at present, but what might happen if someone changed them to double glazed units, or even just draught proofed them?
Ideally, when a fault is found it is best to put it right there and then, but for many reasons this may not be possible. In this situation, the fitter makes out an "At Risk" notice. This is done on the same form as the "Immediately Dangerous" category described below, which makes the situation look a lot more serious that it actually is. The fitter will then request to disconnect the appliance, but has absolutely no power to insist on it being disconnected. He may also label the appliance with either stickers or tie-on notices.
You should consider what has been identified as at risk, and evaluate the risk carefully, the matter rests with you, on your own head be it should anything nasty happen.
(iii) Immediately Dangerous (ID)
Immediately Dangerous is the most serious sort of labelling when a registered fitter has reason to believe that continued use of an appliance or installation would endanger life or property. You should consider this a very serious matter indeed.
Here are some of the things that would make for an ID label:
  • Gas leakage above permissible limits or user smelling gas.
  • Pressure regulator missing from meter inlet or it is malfunctioning.
  • Fumes reported, or a CO detector goes off, or soot marks showing spillage of combustion products for whatever reason (flue, blockages, no vents, debris, lack of servicing…).
  • Scorching of any combustible material by an appliance.
  • Uncapped/unplugged pipework connected to a live gas supply even if the supply is currently turned off - incidents have happened after someone rips out an appliance when they leave and someone else turns the gas on when they arrive.
  • Flues terminating into an internal space like a conservatory that was built after the boiler was installed.
  • Flue missing where required.
  • Appliance faulty due to neglect.
  • Unsuitable appliance for the gas being used.
  • Any appliance installed in a place that is forbidden for its type and size.
  • Safety devices having been defeated, modified, or bypassed and/or failing to danger.

The fitter should ask permission before disconnecting the appliance, fill in the notice, and label the appliance or installation. If permission is not given to disconnect, and there is reason to believe a danger to people or property will persist then National Grid may be called, who have powers of forcible entry.

There is of course some room for subjective judgement in these matters but it is the fitter's opinion that counts. I have noticed a tendency for some heating repair companies (both very large and otherwise) to condemn appliances more often than is justified on safety grounds (i.e. the appliance is just plain broken) and all that is needed is a repair, but they are hoping to win a replacement job.

In summary, treat ID notices issued with great respect, it's the law and however inconvenient, it's there to protect you.

Working on Gas Pipework

What size pipes should be used for gas?

The pipework must be sized adequately to give sufficient gas pressure at the appliance, regardless of whether any or all other appliances are on or not.

The pressure of the gas in the service main buried in the road and arriving at your gas meter will usually be somewhere between 22 mbar and 30 mbar or more (if I recall correctly the maximum is 75 mbar). It might be less than 22 mbar at certain times of the day but if so you can expect National Grid to be digging the road up within the medium term to upgrade the main. There is a pressure regulator, known as the main governor, fitted above the inlet (left hand) side of the gas meter.

Larger consumption or more complex domestic gas appliances usually have their own pressure regulator in the appliance, whilst smaller or simpler appliances rely on the main governor to supply gas at the correct pressure which is 20 mbar +/- 1 mbar. Even though an appliance has its own regulator, it is still important that the gas reaches it at the correct pressure. For instance, a boiler's main burner may only need 10 mbar, but a permanent pilot will work best if it is operated at 20 mbar. In practice, most appliances are tolerant of being supplied at slightly the wrong pressure, this is so they will work safely even when the gas supply is very poor (down to 15 mbar). However this feature is for safety and not an excuse to have inadequate pipe work.

If the pressure is too low then the appliance will not work as powerfully as it should, and secondly the burner will not mix the gas with enough air and so some highly toxic carbon monoxide may be produced. This is especially serious with grills because they are deliberately designed to produce floating slightly under-aired flames, and the products of combustion go straight into the room. If the gas pressure is too high then too much heat may be produced, and the flames may lift off from the burner, burn noisily, or even blow out.

If you connect a U-gauge (a manometer) to the test point on the outlet of the meter you will probably find that the pressure reads anything from 20 to 30 mbar when no gas appliances are in use. This is known as the standing pressure (as compared to the working pressure) and it simply tells you that there is gas present at the meter. When the gas is being consumed, the working pressure at the meter outlet should be in the 20 to 22 mbar range. If the pressure is much outside of this, varies or jumps erratically, or varies widely with the rate of gas being consumed, then the emergency service provider (National Grid 0800 111 999) should be called to rectify the fault. You should not attempt to adjust the governor yourself as it is sealed. It is not unknown for governors to be damaged by some fitters who like to "tweak" them up a bit (and then some).

The pipework up to and including the meter is National Grid property and must not be tampered with. Between the meter and the appliances is your installation pipework through which a maximum drop in pressure of 1 mbar is allowed no matter how many appliances are operating and at whatever settings. The actual pressure drop will depend on the gas flow rate, the bore of the pipe, the length of the pipe and the number of bends, tees, elbows, and couplers in the pipework. It is beyond the scope of this FAQ to go into the details of pipe sizing calculations. Most domestic installations of a typical size and layout are usually satisfied with 22 mm (3/4") pipe from the meter to the boiler or water heater, and with 15 mm (1/2") branches as needed for hobs, gas fires, etc. This is true even though a 33 kW water heater may only have a 15 mm gas inlet connection, as it will still need a 22 mm pipe from the meter to keep the pressure drop within 1 mbar. The rule of thumb "22 mm to within 1m of the boiler" is widely held as a quick, easy and fairly reliable guide for domestic gas pipe sizing, but if your house is larger than average then it might not be adequate, or conversely if the meter is right next to the boiler then 15 mm might be sufficient. In general, an appliance should never be supplied through a pipe smaller than the appliance inlet connection.

Pipework that is adequate for natural gas will always be adequate for LPG since the latter has a higher calorific value (i.e. is more "concentrated") and hence needs a lower flow rate than NG. LPG appliances are supplied at 28 mbar for butane or 37 mbar for propane LPG. LPG appliances often do not contain their own pressure regulators.

Installing new gas pipework

All of the following applies to above ground pipework only, and for DIY work this would normally be copper. This document should not be considered a primer for gas pipework fitting.

It is assumed that the reader is already proficient at installing copper water pipes, and that the gas pipework is not currently connected to the gas supply. Refer also to the chapters on testing the pipework for soundness, and purging of air.

  • Acceptable materials are copper pipe and steel pipe. Plastic pipe is forbidden.
  • Acceptable joints are soldered, threaded, and compression (see exceptions below). "Threaded" refers to a manually cut thread on an iron pipe screwed into a fitting.
  • Threaded joints are to be sealed with "Boss White" or similar, to BS 6956 or EN 751-2-1997.
  • Hemp is not to be used (except between a back-nut and long screw coupler where it must be used). This is a screwed equivalent of a slip coupling, but with a lock nut to stop it unscrewing (a back-nut being what engineers call a lock-nut, i.e. one nut tightened against another to jam the threads).
  • Threaded joints may also be sealed with thick (which may be yellow tinted) PTFE tape to BS 6974 (not the ordinary very thin tape used on water fittings), overlapping the tape half its width each turn.
  • Compression joints may be used where they are accessible. Under the floor, you should use only soldered or threaded joints.
  • There is no special type of flux for gas, but it is to be used sparingly and cleaned off after the joints have been made.
  • A visual check that the solder has flowed all the way around must be made on every soldered joint.
  • Pipes that go through walls must be sleeved to prevent damage in the event of settlement, to prevent any leaks entering the cavity, and to avoid corrosion. There must not be any joints within the wall (i.e. within the sleeving). The pipes must go directly through the wall.
  • Pipes must not be run inside wall cavities as any leakage there is very dangerous.
  • The pipe must be sealed to the sleeves except on the end of the sleeve that is outdoors. This is to allow any leak inside the sleeve to be vented outside.
  • The gap (generally quite small) between the sleeve and the wall it is passing through should be sealed.

One way to put a 22 mm gas pipe through a wall, is to core drill 30 mm diameter and bash though a piece of 28 mm plastic water pipe (e.g. Hep2o) which is caulked to the wall, and then put the copper pipe through the Hep2o. The copper is then caulked to the Hep2o on the indoors side only.

  • Suitable materials for sealing are heat resisting silicone, intumescent mastic. Mortar, cement, plaster, or filler should not be used as they cause corrosion to copper and steel.
  • Pipes may be run inside partition walls provided they are corrosion protected with anti-corrosion tape.
  • Pipes are to be clipped at least every 1.5 m (15 mm pipe) or 2 m (22 mm pipe).
  • Gas pipes should be no less than 150 mm from electric meters and consumer units.
  • Gas pipes should be no less than 25 mm from electric cables.

There is a stack of rules governing pipes buried in floors, walls, ducts, passing between buildings, and between floors of flats.

There are additional points to watch for when working on existing pipework:

  • If the job is to be left unattended at any time then the meter must be capped, or all the pipework capped or plugged as needed.
  • When breaking into existing pipework a temporary electrical earth bond should bridge the pipe to prevent any electric shock hazard or sparks igniting the gas left in the pipe. Car jump leads are suitable for this.
  • When soldering gas pipework, the pipework should be disconnected from the meter and the meter capped. This is to prevent flashbacks to the meter, ingress of dirt or accidental large gas escapes.

Note that a different jointing compound is used on LPG pipework.

Connecting to the gas supply

(i) Connecting the new pipework.

Having installed the new pipework there comes a point when it needs to be connected to the existing gas supply. The procedure for this is basically to isolate the supply at the main valve. If soldering is to be undertaken then the meter outlet should be disconnected and capped. Often a blank plug has been left by the meter or you can obtain a thin disc which rests in the outlet of the meter effective sealing its outlet. The new pipework is connected to the old and the meter reconnected. The pipework is then tested for leaks as described below. Lastly the air or gas/air mixture is purged out of the pipes.

(ii) Check for soundness against leakage

Original text by "The Boilerdoctor" posted to uk.d-i-y on 15/12/1998, revised 29/1/2002.

This test only applies to a U6 domestic meter; other sizes of meter require different figures for allowable pressure drop. The gas under test is Natural Gas. LPG requires a different treatment; longer test periods, higher test pressures, and no drop at all is permitted. A standard manometer capable of reading 0 to 12" WG or 0 to 30 mbar, and a length (approximately 18") of 1/4" rubber hose is required, and also a supply of leak detect solution.

During the test, DO NOT SMOKE or allow any sources of ignition near.

1. Check that all outlets and appliance taps are off and pilot lights extinguished. If the installation includes a cooker with a drop down lid which automatically extinguishes the rings when closed make sure that this lid is raised but the taps are off - this is to ensure that as far as possible the cooker internal connections are included in the test.
2. Turn off the gas service valve at the live side of the meter, remove the test point screw on the discharge branch of the meter and connect the manometer to the test point using the flexible hose.
Open the gas valve SLOWLY until a reading of approximately 8" WG or 20 mbar is obtained in the system, then close the valve. Allow to stand for two minutes to stabilise. The pressure should remain close to the initial pressure, though thermal effects may cause a slight rise or fall. If the pressure has risen at all, bleed a little off to ensure that the system pressure is not exceeding 8" WG or 20 mbar. If the pressure is above this level, a condition known as Governor Lock-Up may have occurred causing a misleading reading.

Governor lock up occurs when the pressure downstream of the governor is at or above the set pressure, and hence the governor internal valve is closed. This traps the high pressure gas upstream and if the supply valve is then closed a quantity of high pressure gas remains trapped between the supply valve and the governor. This is a reservoir of high pressure gas and a small drop downstream of the governor will result in the governor valve passing some of this gas. A small loss will thus be masked by this replenishment of the governed side by the high pressure side. Testing at just below the set pressure means the governor valve will be open and thus the test is valid all the way back to the supply valve.

3. If this happens, repeat the procedure ensuring the gas valve is opened very slowly.
4. Take an initial reading of the manometer. Wait two minutes. Take a final reading of the manometer. Subtract the final reading from the initial reading and record any loss of pressure.
5. If the system pressure has risen during the test, the most likely cause is "let by" of the gas valve, in which case the test is invalid and you must seek help from National Grid to repair the fault on the Meter Control Valve (MCV). If no detectable pressure rise has occurred, bleed off the pressure until the gauge reads about half of the test pressure. Observe the pressure again to ensure that the pressure does not rise from this reduced level. This is to check that the gas valve is not letting a small amount of gas by, which just might match a loss rate at full test pressure.
6. If all appliances are still connected to the system, a permissible pressure loss of 1.6" WG or 4 mbar is allowed provided there is no smell of gas! If the appliances are isolated from the system under test or, in other words, the test is on pipework only, then no drop is permitted. If the above conditions are not satisfied, the leak must be traced and rectified, and the system re-tested in its entirety. This is because of the possibility of more than one leak.
7. If the test is satisfactory, remove the gauge, replace the test point screw, and restore the gas supply. Then test the test point for leakage with leak detection solution.

This is not to encourage amateur gas work but to illustrate how a professional, who will not leave home without test equipment, should do it. Some operatives carry out the let-by test before raising the pressure for the pressure drop test, but as this involves moving the gas valve again it does not assure that the pressure test is without let-by, particularly if the valve is old and possibly worn. If you employ anyone to do gas work, expect to see this test carried out, and if not ask why! I have assumed that you have already satisfied yourself that he/she holds a current Gas Safe identity card showing that they are approved to carry out the class of work you have engaged him/her to do.

As well as the soundness test, other important areas of concern are Workmanship, Flueing, Ventilation, Methods of construction and installation, Positioning of appliances, Stability, Hearths and fire surrounds, Pipe-sizing, and Equipotential earth bonding. A study of BS5440 parts 1 & 2 is often useful if you have access to a copy at your library (many libraries make access available online to library card holders).

(iii) Purge the air from the pipework

Original text by "The Boilerdoctor" posted to uk.d-i-y on 26/2/2001.

This is based on the procedure laid down in the Viper "Gas Point" training booklet that is used for the ACS assessments.

For clarity, a single pipe system is considered, but where branch pipes are present, these will also require purging. Large systems will require an allowance for the increased pipe volume.

Pre-conditions: Whenever a gas supply is made available to an installation the person making it available must ensure that all air and gas, other than the gas to be supplied, is purged from the installation. New installations, and existing installations that are being commissioned, must be purged of all air and gas/air mixtures.

The following pre-conditions must be satisfied:

  • A satisfactory soundness test must precede the purge.
  • No gas must be allowed to build up in a confined space.
  • No operation of any electrical switches, e.g. light switches, electrical appliances etc.
  • No smoking or other possible ignition source is allowed in the proximity of the purge area.

To enable the correct purge volume to be calculated the capacity of the meter is required, and this is shown on the meter badge plate. This can commonly be found on the panel that houses the registration dials, or alternatively, stamped on the front of the meter. A typical example is given where a U6 meter has a volume of 0.071 ft³ per revolution (of the inner measuring device not the test dial). U6 meters commonly measure in cubic feet. NB 1 cu.ft. = 0.0283 cu.m. (see Gas units for more details on conversion factors)

The purge volume is taken as five times the badge capacity. This is based on four times the capacity for clearing the meter and it has been found that with very few exceptions the fifth volume is adequate to ensure clearance of the pipework in a domestic premises.

Purge Procedure

1. Tell everyone present that purging of the gas supply is being undertaken and draw attention to the no smoking/ignition sources requirement.
2. Calculate the purge volume (5 x badge capacity). In the case of the example this will be 5 x 0.071 = 0.355 ft³. This figure is the minimum amount of gas that should be passed through the meter to purge the meter and installation.
3. Ventilate the area by opening doors and windows.
4. Ensure the emergency control at the meter (i.e. the main gas valve), and any appliances, including pilot lights are turned OFF.
5. Open the gas point furthest from the meter.
6. Turn ON the gas and allow the correct amount of gas to pass through the meter as indicated by the index reading on the meter. On reaching the correct quantity, turn OFF the gas.
7. On completion of the purge, seal and test the purge point for gas soundness with leak detection fluid. (See section on checking for soundness). The official test fluid is basically soapy water but the detergent used is manufactured to a closely controlled spec and is non-corrosive to all the commonly used materials for gas systems, including the very thin stainless steel "Anaconda" tube which National Grid use to connect to the governor. Ordinary soaps and Fairy liquid type detergents contain chlorides (I think) which have been known to corrode through Anacondas in a few months.
8. Relight all appliances, if any, ensuring they burn correctly. Allow any gas to clear the property via the ventilation before doing this. If when relighting an appliance, an obvious defect is found, the appropriate action must be taken for an unsafe situation.

Purging a U6 electronic meter

The usual standard gas meter is a displacement meter, but the E6 is a non invasive electronic unit which measures gas flow by ultrasonic means. The "innards" of a E6 consist basically of a U-shaped piece of tubing with transmitter at one end and receiver at the other, so very little volume of contained gas. As a result there is less volume to purge also a higher (8 mbar) permitted pressure drop for this type of meter. It would appear that the E6 is being withdrawn in favour of the U6 again as it has not been reliable enough for the suppliers.

Because of the low volume of gas actually contained in an E6 meter, it is only necessary to pass a volume of 0.010 cubic meters of gas through the meter to purge it fully. Due allowance must again be made for the volume of the pipework system.

Questions about specific appliances

How do I install a hob or cooker?

Here is a checklist of some of the things to watch out for when fitting a gas hob or free-standing cooker. This list does not include the checks for commissioning.

  • Follow the instructions supplied by the manufacturer.
  • The appliance must be suitable for the type and pressure of the gas supply.
  • The room must have an openable window or door, direct to outside air
  • The room must either be larger than 5m³ and have 50 cm² of ventilation to the outside, or be bigger than 10 m³.
  • The gas supply must included a cut-off for servicing (this is an item even some registered fitters sometimes omit)
  • Avoid siting hobs and cookers next to a window or door where a strong draught might blow out the flame.
  • In the absence of manufacturer's data to the contrary:

For hobs, there is an exclusion zone that extends over the hob and a further 50 mm horizontally in every direction along the work surface, and upwards for 760 mm. In this zone, there may be no combustible materials. This usually means there is almost no margin for positioning the hob to avoid the wall cupboards each side. Note that as of autumn 2004 if you fit a 700mm wide hob (usually 5 burner units) then you must have at least 700mm gap or "bridge" in any wall cupboards.

For cookers the zone is 20 mm side clearance on the lower part of the cooker (oven sides) and then 150 mm each side for the top part of the cooker (above the level of the burners and pan supports). The upper zone extends vertically to either 610 mm above the top of the cooker if it has an eye-level grill, or 760 mm if the highest point is the hob burners. Note that often the pan supports on older models of cookers are below the standard 900 mm level of the work surfaces to the side.

Connecting cookers and hobs

A flexible hose connection should be used for a cooker. The bayonet connector should have the open end pointing downwards to prevent ingress of dirt when disconnected and help the hose hang in a neat unstrained U-shape.

Generally fixed appliances are supplied with fixed pipework, but unless prohibited by the manufacturers instructions, may also be connected via a flexible hose (note there is some confusion in this area since subsequent issues of BS 6172 went from initially being silent on the issue, to prohibiting the use of flexible hoses, and then altering the language subtly between revisions such that they were again permitted).

The connection to a cooker is invariably 0.5" BSP female iron. Hob connections tend to be 0.5" BSP male iron, 8 mm compression (like many gas fires), or 15 mm stub pipe

Cookers must have a stability device fitted to prevent the cooker toppling.

New cookers usually come with a simple L-bracket, one arm of which goes into an opening at the back of the cooker. Older ones must be restrained by a hook and chain that you will have to supply yourself. The chain can be unhooked to let you move the cooker out to clean behind it.

What are the rules for siting a boiler flue?

The main standards for this are laid down in BS 5440 Part 1. You might be able to find a copy of this in a reference library.

Since this question is mostly asked in connection with "Where can I site my replacement boiler?", I will give the data that is relevant to a room-sealed forced-draught (i.e. with a fan) flue terminal for an appliance of 14 kW to 32 kW rated input. That covers over 90% of domestic boilers that are currently being installed.

If you already have the manufacturer's instructions, they may give details. If there is disagreement with the figures below, the manufacturer's data should be used.

If the flue is less than 2 m above a place where someone can stand, i.e. a path, drive, balcony etc., then a terminal guard must be fitted over the flue terminal.

These are the minimum distances from the flue to the feature described:

600 mm below an opening, open-able window, vent (but see * below)
75 mm from gutters, soil pipe, down pipes and stacks
200 mm below the eaves
200 mm below the underside of a balcony or a carport roof
300 mm from a corner of a building (internal or external)
300 mm above the ground, a balcony floor, or a flat roof
600 mm from any surface, structure, fence or boundary facing the terminal
1200 mm from another terminal facing directly opposite
1200 mm from any opening or door in a car port area
1500 mm from another terminal above it or below it
300 mm from another terminal on the same wall
  • Note that building regulations now require a minimum 600 mm separation to any gas flue terminal in any direction from any opening into the building.


General Many DIYers choose to leave this matter to a professional as much for peace of mind about what has been done as any other reason. Commissioning is a procedure for checking that everything has been installed correctly, that everything is working safely and in the right way.

It also gives an opportunity to check over all the other gas appliances in an installation at the same time.

Many manufacturers will explicitly list out the required checks and adjustments in the installation manual for the appliance. These should be followed precisely, of course. In the absence of specific instructions at least the following checks should be made:

  • Flues
  • Supply of air for combustion and appliance cooling
  • Gas soundness
  • Gas pipe purging
  • Flame picture.
  • Supply inlet working pressure
  • Burner pressure(s).
  • Rate of gas consumption.
  • Operation of normal controls.
  • Operation of safety controls.
  • Handing over to the user.

Flues and airways In cases where the installer and the commissioner are the same fitter then these checks will probably have been done at the time of installation. Flues needs to be visually checked for blockages and incorrect installation. Chimneys need to be checked with smoke pellets to ensure that they draw adequately (after warming if necessary) and that they do not leak into the building, other chimneys or a loft space. Likewise airways and vents are checked that they meet the regulations and are free of blockages. Test for flues are performed under worst case conditions i.e. shut all doors and windows and operate any extractor fans (on full) in the room or an adjoining one.

Gas soundness and purging See earlier sections under gas pipework.

Flame picture A simple check that can instantly tell you everything about the combustion process is to look at the flames. You may remember school chemistry lessons about Bunsen burners, how you could change the flame from yellow through blue to turquoise by adjusting the air inlet collar. All domestic gas burners (except large sand-box type display gas fires) work in just the same way. The gas is squirted out of a small nozzle (called the injector or jet) and mixes with air in a tube. The mix burns as it emerges from holes at the end of the tube. When all is well the gas burns in neat sharply defined light blue or turquoise coloured cones which are surrounded by a sky blue or slightly lilac outer flame.

There are four exceptions to this (that I can think of).

  • Very small pilot flames (smaller than many boiler pilots) may just have a simple pin hole jet. The gas rate is so small they get all the air they need as they burn.
  • Grills are deliberately designed to mix slightly less than the required amount of air into the gas; this causes the flames to be an even blue colour. The flames also tend to merge together (known as impingement) and extend over a larger area.
  • Modern gas fires that imitate coal-fires are also slightly under aired, the correct flame for these is blue when cold and a very convincing orange when hot.
  • Pressurised burners found in some boilers usually the most sophisticated condensing models. Discussion of these burners is beyond the scope of this document. The commissioning of these burners is best done (realistically can only be done) with a combustion analyser.

Gas inlet pressure and burner pressure. A check should be made that the pressure of the gas supplied to an appliance meets the manufacturers requirements or otherwise 19-21 mbar. Appliances like boilers often have two test points, one for the inlet pressure and one for the regulated burner pressure. Less sophisticated appliances where the burner pressure is simply the supply pressure will have just one test point. A gas hob may have no test point (you attach the manometer to one of the burner jets) or it may be hidden away underneath or inside. The appliance should have its burner pressure data marked on it somewhere; you may find that a range of pressures are specified. If a range is given then you should set the burner pressure to a specific value and mark it on the data plate. What setting you choose would be according to what rating you need from the appliance.

If the inlet pressure is outside the range 19-21 mbar then you should check the pressure at the meter outlet to see whether problem lies with the meter and main governor (National Grid's problem) or with the installation pipework (your problem).
If the burner pressure needs to be altered then the manufacturer's instructions will tell you how.

Rate of Gas Consumption or Heat Input Very simply how much gas the appliance consumes. This story might illustrate the importance of this subject: I once went to service a water heater and found that the burner pressure was much too low and the flames looked wrong, I adjusted it to the manufacturer's setting and found that the flames flared up nearly out of the top of the appliance. On checking the gas rate I found the appliance was consuming 40% too much gas! It turned out that at some time during its lifetime the wrong sized gas jet had been fitted. The right one (supplied free by the import agents) imperceptible different gave a good neat flame, no pyrotechnics and burnt gas at 98% of the specified rate.

To measure the gas rate make sure all other appliances are not in use and their pilot lights are off. Measure the time taken to consume a whole number of cubic feet or litres (0.001 cu. m), but measure for at least 60 seconds so as to minimise the effect of small errors.

For cubic feet the formula is kW of heat input = 1092 x (volume in cu. ft.) / time in seconds for measurement.

For litres the formula is kW of heat input = 38.6 x (volume in litres) / time in seconds for measurement.

If the appliance only specifies the heat input as BTU/hr note that 3412 BTU/hr = 1kW

If the appliance specifies the heat input using the word "Net" or "NCV" then that means the heat input is based on the lower calorific value (where the energy available by condensing the water vapour formed in combustion is not considered) the values of 1092 and 38.6 above should then be 983 and 34.7.

The appliance should burn gas at a rate between 90% and 105% of the specified rate.

If it is outside these limits advice from the manufacturers should be sought.

Operation of normal controls

Check that all user controls operate as you'd expect and/or the manufacturers state. Check that any normal (as opposed to safety) thermostats respond correctly.

Operation of safety controls

The manufacturers may specify a technique for testing a particular safety control. For instance a small flue-less water heater might have a device that detects various faults by a temperature sensor in the outer casing. The makers may say to block the vent at the top with a heat proof plate and check that the sensor cuts the pilot and the gas in under 90 seconds.

Pilot lights with thermocouples : Extinguish these by blowing them out or interrupting the gas supply and check that the gas valve closes within 60s (longer times are permitted for fires and cookers) but 60s should be enough anyway.

Lidded hobs and cookers: Many (but not all) cookers and hobs with drop down lids are interlocked to the hob rings, check this feature out.

If the manufacturer does not specify a way of testing a particular safety device and there is no way that you can test it safely (e.g. a high temperature cut-out thermostat) then you may assume it is satisfactory.

Handing over to the user

If you are not going to be the user, then explain to the user(s) what they need to know to operate the appliance safely and effectively. Keep all the manuals and instruction books.