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Combi Boiler System Pressure ~ Problems with Central Heating System Pressure

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Combination boilers and system boilers both carry pressure gauges and we're often asked about the pressure shown on these gauges. Any sealed central heating system needs an expansion vessel, a pressure gauge and a pressure relief safety valve.


To explain a bit further:

If you heat water it expands. In an open vented central heating system (with a header tank) some of the water moves back up into the header tank. The expansion does not increase the overall volume of water by a huge amount (maybe 3 or 4 litres, depending on the size of the heating system) but it would be more than enough to burst the pipes if it had nowhere to go.


In a sealed system you have to allow for this expansion. This is done by adding an expansion vessel to the system pipework and it's often inside the boiler. The expansion vessel is just a chamber with a rubber diaphragm across the middle. One side of the expansion chamber is connected to the system pipework. The other side has a car tyre type valve so it can be pumped full of air.

In manufacture the air side is pumped full of air, typically to about 1 bar (about 15 psi). This forces the diaphragm right across to the opposite wall of the chamber. When the central heating system is filled with water and pressurised (to about 1 bar) the water pushes the diaphragm back to about the middle.

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combination boiler pressure problems ~ sealed central heating system pressure problems ~ system boiler pressure problems

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pressure rises in combi boiler ~ sealed system pressure too high ~  combi boiler pressure too high ~ combination boiler pressure dropping

When the system is heated up the water expands a little but this expansion is accommodated by pushing the diaphragm across partway and compressing the air pocket. The pressure gauge on the boiler or on the pipework will go up by maybe a ¼ bar or ½ bar.


If the air pocket has been largely or completely lost, the pressure will rise sharply as the system is heated, maybe by more than 1½ bar.


To stop the pipework bursting, the boiler or pipework is fitted with a 3 bar pressure relief safety valve (usually with a red cap). If the pressure goes above 3 bar this valve is forced open and water passes out of the system along a safety pipe and is dumped outside the building.


Though a failed or de-pressurised expansion vessel is the most likely culprit when water passes out through the safety pipe it is not the only possible cause. If the plate heat exchanger (secondary heat exchanger) fitted to most combi boilers is leaking internally water will probably be forced out through the safety valve and the pressure gauge will read consistently around 3 bar.

There are 3 ways the air pocket from an expansion vessel can be lost:


If the heating system contains a lot of water (lots of radiators in the central heating system) an extra expansion vessel will be needed from the start, or you will consistently get a big rise in pressure.

If, however, your system pressure has normally changed by only ¼ bar or ½ bar as it heats and cools but the pressure now swings much more widely, I would check the expansion vessel.


With the heating turned off I would release pressure from the Schraeder valve. If water comes out, the expansion vessel is knackered. If there's no obvious water, and the system pressure gauge still shows pressure on the water side, you may simply need to pump the expansion vessel up again.


First you must lose the pressure on the central heating water side by draining from a drain cock until the gauge shows zero. Don't use the 3 bar pressure relief valve to dump the pressure because, once operated, the seating may become contaminated and it may leak water through the safety pipe afterwards. If the system has been regularly dumping water through the safety valve you will probably have to replace the safety valve anyway.


With the water pressure down to zero you use a foot pump or large bicycle pump to pump up the air side of the expansion vessel to 1 bar (15 psi). The valve on the expansion vessel is the same as the valve found on car tyres or some bicycle tyres so you can use the same pump.

If the air pressure takes a long time to rise and you hear air bubbling into the water system, the diaphragm in the expansion vessel is probably ruptured. If, however, the vessel pumps up correctly you re-fit and tighten the Schraeder valve cap and then fill the water side again to increase the pressure. You want to be between 1 and 1½ bar when the system is cold. You then heat the system up and the pressure rise should not be excessive. A rise of between ¼ bar and ½ bar is typical. This pressure should drop back down as the system cools.


Note that the pump on some combination boilers and system boilers can cause a change in pressure when the pump kicks in but this pressure change is immediate and generally small.

If the expansion vessel is knackered you need to change it if you can get at it. If it's inside the central heating boiler it's not always simple to remove. If it's going to be very difficult to remove (and some boilers have to be removed from the wall) you can opt to leave it in place inside the boiler and fit an external expansion vessel instead. Make sure the cap on the Schraeder valve on the old expansion vessel is tight.


New expansion vessels come already pumped up to about 1 bar. If you need to fit an external expansion vessel you tee it into the central heating return pipe. Most plumbers' merchants carry expansion vessels for sealed systems and a typical size is 12 litres.


If your expansion vessel is knackered there is a temporary "fix" you can try.

Drain water from a drain cock and release a radiator bleed point to allow air in to the radiator. Allow quite a lot of air in so the radiator is only ½ full of water (you'll have to guess).

Close the radiator bleed and the drain cock and re-pressurize the system. Make sure that at least one of the radiator valves is open. The air in the radiator will now act as the expansion pocket. (This won't work if the radiator is piped from the top but most aren't.)

This should only be used as a temporary fix because the air will progressively dissolve into the system water, oxygenating it and making it much more likely to cause corrosion.


Finally, check that the system is not losing pressure through the safety valve. If the valve is letting by, without the pressure becoming excessive, you will need to change the valve. Don't be tempted just to keep topping up. In hard water areas (much of the UK) you will be bringing in lots of dissolved lime-scale with the water. This will precipitate out in the hottest place in the heating system, the boiler heat exchanger, and will make the boiler kettle loudly.