Sealed system FAQ

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Editors note, originally published on Ed Sirett's web site - preserved here

The sealed heating system section of the FAQ for uk.d-i-y Version 41024 by Ed Sirett


Central heating systems currently being installed in the UK are increasingly of the sealed primary circuit type. This means the water heated by the gas (or oil) burner in the boiler and which circulates around the radiators is held in a closed circuit under pressure. Such an arrangement has a number of advantages over ‘open’, ‘vented’ or ‘conventional’ heating systems. Sealed systems are low maintenance but they are not maintenance free. Combination boilers invariably have sealed circuits.

The advantages of sealed systems are at least the following:

  • They are simpler to install since there is no header tank in the roof space and any of its possible problems such as over-flowing or freezing.
  • They are easier to fill since air is expelled under pressure when bleeding the radiators.
  • There can be no problems that stem from a number of installation faults occurring on open systems that end up drawing air into the system causing air locks, corrosion and sludge.
  • The increased pressure raises the boiling point of water in the boiler. In an older boiler hot-spots cause localised boiling that often make kettle-like noises or even loud clanks or bangs.
  • In the unlikely event of major damage to the heating system the resulting flood is limited. In contrast with a conventional system where the automatic top-up on the header tank will contribute an indefinite amount.
  • They save space in flats where there is usually no good location for the header tank anyway.
  • The system can be flushed out under pressure using the mains water supply.

In fairness the following are slight draw backs:

  • It is a bit harder to introduce chemicals (such as inhibitors and cleansers).
  • The system pressure needs checking from time to time.


Most, sometimes all, of the following are found within the boiler casing although it is possible for a sealed system to be built using individual component parts. Some manufacturers offer all the parts as an additional packaged option that is installed onto their ‘normal’ boiler (usually directly above or below it).

A filling point. This is where you add water to the system to initially fill it and to top up losses from time to time. It is very often is a braided metal sheathed flexible hose about 50cm long. Installers often put this directly below the boiler although some favour trying to place it in an awkward or obscure place. (If you cannot find it try looking under the sink, in the airing cupboard, or behind the back of a kitchen unit.) This hose is known as the filling ‘loop’ it is a removable connection. This is to comply with the water bylaw that states something like 'no closed circuit shall be permanently connected to the main supply’.

The loop also contains two non-return valves and is never actually removed, at least in my experience. The valve to let water into a system is usually a plastic knob or a straight screw-head slot. It operates by turning just 90 degrees between open and shut, turning it more than that will most likely damage it. The screw head slot will be aligned across the hose when the valve is shut.

Some older installations may have a fill point made with rigid pipe work and a couple of stop valves. To use: open one a turn or two and control the flow of water with the other - turn both off afterwards.

A couple of boilers that I know about have built in filling points. A) The Bosch-Worcester Cdi range - this requires a large hand sized plastic key which is inserts into the underside of the boiler and turns a quarter turn then another small knob is opened nearby. If you lose the key (a bad idea) you can get replacements from many plumbers’ merchants for a few pounds. B-W have introduced a new range which uses a smaller key but the comments apply.

B) The Vaillant Turbomax/Ecomax range: a valve with a straight slot underneath the boiler takes the small key provided which operates through a quarter turn. If you lose the key a screwdriver will work fine (chubby version if access is limited). Some models have two such valves and both need to be opened to fill.

A pressure relief valve and discharge pipe. In the unlikely event that all the boiler controls, cut-outs, and trips have failed and the boiler is still roaring away then this is the last line of defence. As the water heats up the pressure will rise to around 3 bar at which point the hot water travels along the discharge pipe. The water will be disposed of safely outside. The end of the pipe must not point towards someone who might be standing nearby, so you will often see the pipe have a downwards or even hooked end. The relief valve will have a test knob on it that lets out water into the discharge pipe. It is a bad idea to operate this as the slightest piece of debris on the valve seat will cause the valve to dribble.

An expansion vessel When water is heated it expands, the expansion is about 2.5% from 25C to 75C. If this expansion were not possible because the water is in a closed container then the pressure rise would be an astonishing 50 bar (750 psi), this would be bad news for the pipes. In order to limit this pressure rise an expansion vessel is included in the system usually tucked away at the back of the boiler. The expansion vessel is a metal container divided in two by a rubber diaphragm. One side is connected to the pipe work of the heating system and therefore contains water. The other, the dry side, contains air under pressure and you will find a car-tyre type valve for checking pressures and adding air. When the heating system is empty or at the low end of the normal range of working pressure the diaphragm will be pushed against the water inlet. As the water expands so the diaphragm moves compressing the air on its other side and giving rise to a moderated increase in pressure that you can see on the pressure gauge.

A pressure gauge This is a gadget to tell you the pressure of the water in the system pipe work. However some manufacturers like to put them in awkward locations (like underneath the boiler at an odd angle) others decorate them with coloured sectors, ‘limit’ lines and moveable pointers. Some installers favour placing the filling loop just out of sight of the gauge to add an extra challenge. The gauges are invariably marked in bar, the numbers typically mean 0 - system empty, 1- typical pressure when system is cold, 2- typical maximum pressure when hot, 3 - excessive pressure.


What should the pressure gauge be set to?

If you have the user and/or installation instructions for your boiler that will give you the answer. Otherwise set it to about 1.25 bar when cold, but if you have a 3 storey house and the boiler is on the ground floor then maybe a little more, perhaps 1.4 bar. These are the upper limits when cold. The system should be topped up when the pressure falls to about 0.8 bar when cold (or whatever the instructions say).

Why does the pressure seem to shoot up and down?

As the system heats up some rise in pressure is to be expected up to around 2.0 bar. If you have a very large house it is possible but unlikely that the expansion vessel in the boiler is insufficient. However the most likely cause is that the expansion vessel has failed in some way (see below). Usually what happens is that you add water because you see the pressure is low then you turn the heating on. The pressure rises quickly and excessively, the relief valve opens and lets some water out but later when the boiler cools down the pressure is too low. Alternatively there may be a largish leak - if it were inside you would know about it - so most likely it is on the relief valve (see below).

The pressure drops over a period is this a problem?

In a system with good quality radiator valves and no microscopic leaks it is possible for the system pressure to hold up adequately between annual services. Most people seem to find that they have to top up a few times a year, and some people more often. If you find that you are needing to top up more than once a week it might be worth trying to find the problem or see if the relief valve is leaking (see below). If topping up annoys you (or it’s awkward) then you might be able to get the system to hold pressure better by adding an internal leak sealer such as Fernox LS-I. This seals very small leaks but it is worth checking that the boiler manufacturer allows its use especially if the boiler is still under guarantee.

The pressure just goes on rising until it is very high, why?

Sometimes a filling point control valve can become defective. This will let a trickle of water into the heating circuit so its pressure continues to rise. This continues until either the heating system and mains pressure are similar or the relief valve opens and begins dripping. Disconnecting the hose section of the filling loop should confirm this problem. Replacement of the control valve section should be sufficient to fix the problem. The protential problems if the problem is left unfixed are two fold corrosion and pressure relief valve leaks. Corrosion will lead to sludge, and eventual early failure of the radiators and boiler.

What happens if I leave the system entirely alone and never top up?

To begin with nothing unusual at all but there will come a time when there is no pressure in the highest radiators. This might encourage air into the system that is a bad thing and one of the biggest advantages of a sealed system is lost. Furthermore in many installations the boiler is the highest point and will invariably contain an automatic air bleed unit, these devices let air out but not water, they also let air in. They work well if they only operate occasionally and are under pressure but if they draw in air they can become faulty and then fail to hold in water. Finally if the pressure falls below a certain point the boiler may have a low pressure cut out. This will stop the heating but is much better than having a pump fail because its bearings are dry or heat exchanger damage due to a ‘dry fire’ incident. Many boilers detect the pressure change across the pump the absence of which they might report as ‘pump failure’.

What should I do with the pointer on the pressure gauge?

Pressure gauges often have coloured sectors, lines or digits to indicate limiting or acceptable values. The meaning is clear if you know that 0.5-1.5 bar is the acceptable range of pressure when cold, 1.0-2.0 bar would be the range when hot, anything over 2.5 bar is wrong and anything under 0.5 bar means empty. There is sometimes an adjustable pointer on the gauge this can indicate whatever you wish, for example.

  • The pressure below which you should top the system up.
  • The pressure that you should top up to.
  • The highest pressure you have ever seen in normal operation.
  • Alternatively you can just ignore it.

How do I refill the whole system when working alone?

When you have an assistant you simply get them to fill the system to required pressure and when the pressure drops as you bleed radiators then they add some more. Alone, I simply pressurise the system to about 2 bar then bleed about half the radiators, add some more to bring the pressure back to about 2 bar and bleed some more radiators. Should the air become slacker at coming out then you add some more water. If you end up with too much pressure then you might have to let a little out via a drain point, but with experience you can usually judge things so that will not be necessary.

How do I add chemicals to the system?

If you have one of those ladder type towel rails you can simply add inhibitor, de-sludger, leak sealer or whatever with a funnel into the top, after removing the bleed plug or blank plug. There are various methods for adding stuff via radiators either by injecting it through the bleed hole (with a tool for this purpose) or using funnels or hoses via the hole made by removing the blank plug. If you only have roll-top radiators (I have yet to encounter this situation) then perhaps the best approach is to add a T fitting somewhere convenient but unobtrusive. Add a short length of pipe with a service valve that you can then open to put in the chemicals via a short length of hose. Some people have had sucess with garden plant srayers and using the filling loop hose.

I suspect the expansion vessel is faulty, how do I check?

There are two ways the expansion vessel can fail to function properly, either the rubber diaphragm has punctured or the air on the dry side of the diaphragm has leaked away. Either way the effect is the same, namely the pressure rise as the system heats up is too large and the relief valve opens. You can check the vessel by removing the dust cap of the car-tyre type valve and pressing the pin. If water comes out the diaphragm has split, otherwise you can test the air pressure with a tyre pressure gauge. The pressure should be whatever the manufacturers say it ought to be or failing that about the same as the lowest acceptable pressure for operating the system typically 0.5-1.0 bar. If you decide that the pressure is low and needs some extra you can use a pump to add air. The most important thing is to do this while the system is de-pressurised and open (so that the displaced water has somewhere to go). Be aware too that the diaphragm may still have split even though no water came out, thus as you add air the vessel will not pressurise but the air will go into the heating system.

Expansion vessels are generally very reliable but if you need to replace the unit you will often find that it is tucked away at the back of the boiler. It is inaccessible without major work on the boiler including dismantling gas carrying parts. A simple way around this is to add a complete new vessel to the system pipe work at some suitable location, perhaps the airing cupboard. You add a T fitting to the pipe work and connect the vessel to it. A good plumbers’ merchant will be able to advise you on the correct size for the vessel or the boiler installation instructions may help. About 10% of system volume is needed. You can crudely but conservatively (over large) estimate would be to allow 0.5 litre of expansionper kW of boiler output. In practice the radiators rarely equal the power of moder boilers and you coudl get away with much less.

A new vessel will have been pre-filled on the dry side to about 3 bar or more so you will need to let some air out to get to the correct pressure for your system, typically the same as the lowest aceptable pressure say 0.75 bar. For the pedantic please note that I have been calling the gas in the vessel on the dry side air. However the manufacturers may well initially fill the vessel with nitrogen or even argon presumably to extend the life of the diaphragm.

If you find yourself with a system which has a failed vessel which you can't do anything about immediately then here is a work around. Choose a radiator (or two in a 6 or more radiator installation) which you don't need to heat. Partially drain the system so as to completely empty the chosen radiator(s). Refill but don't bleed the air from the chosen radiators. This should give enough cushion to operate the system for a while.

The relief valve might be leaking how do I check?

Sometimes slow pressure drops from the system might be caused by a leaking relief valve. You can confirm this by tying a polythene bag around the end of the relief pipe. If the valve is leaking it might be possible to dislodge dirt on the valve by operating the test knob (often red plastic) very briefly. If this fails after a couple of tries then the valve will have to replaced. It is a bad idea to simply keep topping up a system as this dilutes the corrosion inhibitor and keeps introducing dissolved air (containing corrosive oxygen) into the circuit.

Can I convert my existing system to a sealed system?

Yes, if the boiler manufacturer permits it. At the very least the boiler needs to have components that are all capable of withstanding 3 bar and at least one high temperature safety cut-out that requires manual resetting. Old radiators and their valves might start leaking but the benefits are worth it.

Why doesn’t my landlord give me a the filling key for the boiler? This can happen with the Bosch/Worcester Cdi boilers and some housing association landlords. Perhaps the landlord has made a decision that more harm than good might come of giving the filling keys to the tenants who will, like everyone else, lose them. Indeed that may be why your Landlord has not got one to give you.

What happens if I put too much water in the system?

If it is only a bit too much I would leave it. If you are much above 1.5 bar when the system is cold then you can try to reduce the pressure by bleeding every radiator. If you still have too much pressure the you will have to let some out via a drain point.

What happens if I leave the filling point open?

Sometimes some people just leave the filling point open all the while and this is a very bad thing. Water might come out of the relief valve, corrosion inhibitor might be diluted and the relief valve will sooner or later fail, however these symptoms may not appear if the mains pressure is low.

See Also