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Why Do Boilers Lose Pressure?

April 5, 2021

Why is my boiler losing pressure? Is a question we get asked a lot at Heatsy, friends, family, and sometimes fellow heating engineers can all be stumped when it comes to figuring out exactly why your boiler is losing pressure.

Unfortunately, pressure loss on domestic gas boilers and heating systems is something many of us are familiar with, and some of us have even become quite used to it.

Diagnosing the exact reason why your boiler is losing pressure can be quite tricky, and time consuming, but it shouldn’t be something you just have to deal with.

Later, we will go through the exact process that we’ve developed to diagnose nuisance pressure loss calls, but first a little bit about why your boiler is pressurised in the first place.

In the old days heating systems were designed as gravity fed systems, gravity fed simply means you have a large storage cistern in the loft full of water and a feed pipe connected to your heating system, learn more about them here.

On a gravity fed system, if you had a small leak under the floor, or bleed the radiators, gravity would take over and force the water in the cistern through the feed pipe to refill your heating system.

While they may well sound like a good idea there are some distinct disadvantages to a gravity fed system. Firstly, they’re much more prone to air ingress, air or gasses in heating systems is about the worst thing you can have for the health of your boiler, you can learn more here but in short, any type of gas - including Oxygen and air will cause the inside of your system to basically rust and block up with what you probably know as sludge. Another reason is efficiency because they’re cleaner pressurised systems tend to be a bit more efficient.

Lastly, space, there’s no need to have huge storage tanks in the loft if you have a fully pressurised heating and hot water system so you could get that fourth bedroom you’ve always wanted.

There are lots of other reasons why a pressurised system is better than a gravity fed heating system, but for us, these are the three main benefits.

Now, to the question at hand, why is my boiler losing pressure? Well, there are two places your boiler can lose pressure, either from the boiler itself or the heating and hot water system.

If one of the water carrying parts with in the boiler i.e., diverter valve, pump, aav etc begin to leak you’ll usually notice water dripping from the boiler soon after you noticed the pressure kept going down and it should be a simple job for your boiler repair engineer to find out exactly where the boiler is losing pressure from - although it might not be as easy to fix it.

Another reason the boiler will leak is if the expansion vessel is faulty.

As pressurised systems do not have vent pipes there is nowhere for the heated water to go as it expands, the expansion vessel is half full of Nitrogen and half of water so as the water heats up the vessel can accommodate the extra volume.

If the vessel is faulty, when the heating is on the extra volume of the heated water will have nowhere to go and cause the pressure in the system to rise and the pressure relief valve (PRV) will release the water to reduce the pressure.

On heating systems it’s very common for the thermostatic radiator valves (TRV) to leak, as they are being turned on and off frequently the pins or seals will eventually perish and begin to leak.

Another common culprit on a heating system is the automatic air vent.

Radiators, pipework, isolation valves and pumps are all unfortunately prone to leaking, but the number one reason why they leak is poor water quality.

The water quality in your heating system is so important, it affects everything from the efficiency of your boiler to the life of your heating system.

As the gasses in the water begin to corrode the inside of pipes and radiators the system becomes acidic, this in turn then speeds up the corrosion process and amplifies it to the point where, if left unchecked it will cause joints on the pipeswork to weep, or pin holes to appear in radiators or seals in valves to perish and even if you do not notice a leak at first, you’ll experience it as your boiler continuously losing pressure, then you’ll add more fresh, untreated water to the system and that will feed the process further.

If you are experiencing pressure loss in your heating system and you haven’t seen any water dripping from your boiler you could try looking by the radiators.

Are there green or brown marks on the pipes, wet carpets or maybe you can see the corrosion coming through the radiator.

You can also look at the radiator valves, try unscrewing the TRV head to see if the pin inside is leaking a bit. If the pressure has been dropping for some time you might notice damp patches on ceilings or walls, or bowed floor, these are all signs that a leak maybe present.

Our process to find leaks is one that we’ve used for a while, and we’ve had some good success.

The first thing we do is ask you a few questions about how often you need to increase the pressure in the boiler and if you notice the pressure seems to drop at certain times.

Then we’ll carry out a thorough inspection of the boiler, looking for any signs of water damage, we’ll also check the PRV to see if there is any staining on the wall or water around the rim of the pipe.

If all is good here, we will investigate the heating system, we’ll check each radiator and valve, we’ll also check the hot water cylinder and any other components like the zone valve or heating pump.

If the leak isn’t immediately visible, we’ll need to move on to part two of our diagnostics procedure.

To miniseries the disruption of a full investigation we’ll start by adding some leak sealer.

Leak sealer is a chemical that solidifies when exposed to the atmosphere. When it is in the central heating system it stays as a harmless liquid, but when it finds the leak and begins to drip out it sets and hopefully seals the leak.

We usually advise to leave the sealer in for up to two weeks to give it a chance of finding and sealing the system, if after this time the pressure is still dropping, we’ll move on to the next part of our diagnostics process.

To determine if the leak is from the boiler or the system, we need to separate them, by closing the isolation valves below the boiler we can separate the heating system from the boiler. We usually need to leave the heating off overnight to give the pressure time to drop, if the pressure still drops with the valves closed, we know the leak is on the boiler. If the pressure has held overnight, when we reinstate the system by opening the isolation valves if the pressure drops suddenly, we know the leak is on the heating system.

The reason we need to do this is because sometimes the main heat exchanger can split inside and the water will not leak from the boiler, instead it will drip down the condensation pipe and we are unable to see it.

If the boiler has lost pressure overnight and we know the leak is on the boiler we’ll talk you through your options, a new heat exchanger can be very expensive and depending on the age of the boiler and its general condition it might be more cost effective to replace the whole boiler.

If after all this your boiler is still losing pressure the last thing, we do is use an infra-red camera to look for hot patches beneath the floors and in the walls.

We run the heating on high for an hour then begin to walk around the entire house looking for anything that stands out.

Unfortunately diagnosing the exact reason why your boiler is losing pressure can be quite a long and tricky task, thankfully it’s usually found by step one and two and something that can be repaired relatively quickly.

We hope you found this article interesting and useful, if you didn’t please let us know why and what we could do to improve it.

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Please remember, we’re heating experts so if you need your boiler repaired, a new heating installation or if you’re struggling with anything we’ve discussed here feel free to get in touch with us now so we can see how our solutions might help you.

Thank you for reading.

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