It’s relatively common for boilers to lose pressure, as they’re enclosed systems, which means there’s only one way for water to go if there’s a leak, and that’s outwards. But what about gaining pressure? How is this possible? Let’s look at some potential causes, and what you should do about it.
The filling loop
The first place you should check is your filling loop. It’s normally a valve or two on the underside of the boiler, connected by pipes or tubes to the water mains. Sometimes the tube is completely detachable; other times it’s permanent pipework. Its job is to fill up the central heating system when it’s new, if you have a leak or if you have just bled your radiators, in which case pressure will have dropped.
It’s quite easy to start filling the system and then forget about it, in which case the pressure will keep rising until it trips. If you’ve done this, you’ll need to drain the system – check your manual to see how you do this.
It’s also possible that you’ve charged the system but not quite closed the valves enough. Since mains pressure is greater than pressure in the boiler, it’ll keep slowly filling. Make sure any filling valves are closed. Finally, the valve could be damaged. There’s no way for a homeowner to check this, so it’s time to call the professionals.
Faulty pressure gauge
Pressure sensors do sometimes jam and fail, so it might be that the pressure is actually fine, but the gauge is giving the wrong reading. You can possibly diagnose this by relieving the pressure a little and seeing if it makes any difference.
If there’s no change in the reading, your pressure gauge will need replacing. It’s safest to turn off the boiler in the meantime, although there should be secondary and tertiary safety measures to stop the pressure building to dangerous levels.
Build-up of grime in the system
If there’s anything in the pipework or heat exchanger that slows down the flow of water, pressure will be high behind it (where there’s a build-up of pressurised water), but low after it (where the normal flow is reduced to a trickle).
It will definitely need looking at, though. It could mean a replacement boiler, but often a power flush will do the trick.
Expansion vessel issues
The expansion vessel is the part of the boiler that regulates pressure as it goes through its hot (high pressure) and cold (low pressure) phases.
If this is damaged, it’s possible that pressure will rise considerably when the boiler is switched on, then gradually drop to normal levels when it’s off. This is definitely not a task for the amateur as it’s a vital component, so get an engineer to inspect it.
If in doubt, call us out
There aren’t many causes of high pressure in a boiler that can be dealt with by a regular homeowner, except perhaps when you’ve accidentally overfilled it.
If you’re in the Dublin area and are worried that your boiler is above the normal pressure range (1–1.5 bar), give us a call and we’ll send someone round to have a look at it.
Published: 15 March 2022
