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What Low Gas Boiler Pressure Means and What You Can Do About It

John is an experienced freelance content writer with an eclectic employment history.

This article will break down what low gas boiler pressure means and what you can do about it.

This article will break down what low gas boiler pressure means and what you can do about it.

Gas boilers are often the most cost-effective method of heating a home—at least, it is for homes with a gas line. Boilers convert gas into hot water by passing the water through a heat exchanger that is heated by a gas flame. This transfers the heat of the flame into the water, which can then carry on to wherever it is going, typically a central heating radiator or a hot tap.

When we are dealing with the central heating aspect specifically, a loss of water pressure is a big problem, because it changes the thermal properties of the water—and not in a way that benefits you, the bill payer who is sitting in your home feeling cold!

Why Is a Loss of Water Pressure Bad for Your Boiler?

Imagine a bowl of boiling hot water. The water will produce steam that, while extremely hot, will very quickly dissipate and cool down. In fact, under normal room temperature conditions, that steam would go from scolding hot to room temperature in mere seconds. But what about the water in the bowl? The bowl of water will retain its heat for much longer, and it could be minutes before it has cooled down enough to safely dip your hand in.

So, how is this relevant to boiler pressure?

On a molecular level, hot water is just water molecules moving around. The more movement, the hotter the water, but also the more molecules the more potential for movement. Water retains its heat longer than steam because the heat energy has to pass through far more water molecules before it can reach the cooler air around it than steam does. The same principle applies in a heat exchanger.

When the water pressure is right, the water molecules are densely packed together, and they can retain plenty of heat as they make their way around the system. If the pressure is lower, the heat retention capacity is reduced. And if there is air in the system, that heat retention drops dramatically. The less ability the water in your system has to retain heat, the cooler it will be by the time it reaches your radiators.

If the water pressure in your central heating is too low, you will get far less heat for your money.

If the water pressure in your central heating is too low, you will get far less heat for your money.

What Causes the Loss of Water Pressure?

There are a few things that can cause your gas boiler system to lose water pressure. If you’ve been experiencing problems after having some major work done, such as a new radiator or boiler being fitted, it could just be that there wasn’t enough water fed back into the system afterwards.

If there hasn’t been any significant work and the problem seems to have developed out of nowhere, your most likely culprit is a leak in the system. Water leaking out of the system reduces the water pressure in there, and if air is able to find a way in to replace that lost water, it will further reduce your boiler system’s ability to retain heat.

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How to Fix Low Water Pressure in Your Gas Boiler

The quick answer to this question is; “put more water in the system”, however, it is crucial that the cause of the low pressure is established first. If it is caused by a leak somewhere, for example, feeding more water into the system will only be a temporary fix—if it fixes it at all—but will mostly just result in more water leaking out of your system. You will need to drain the system before attempting to fix it, or you will almost certainly end up with a very wet mess to clean up. And, of course, you certainly need to make sure the boiler is not running. If the water is drained it won’t affect you (though it could be bad for the boiler) but if you have forgotten to drain the water and the boiler is running, you could end up with serious burns to go with the water damage caused when you crack the seal.

Fixing low pressure in your boiler is as simple as putting more water in the system... as long as you've cured the problem that cause the low pressure in the first place!

Fixing low pressure in your boiler is as simple as putting more water in the system... as long as you've cured the problem that cause the low pressure in the first place!

Regarding getting the water into your system once any necessary repairs have been completed, there should be an inlet feed pipe with a valve or tap on it going into your boiler. The valve is not part of the boiler itself but something the plumber or engineer will have installed. As such it could be located anywhere along the inlet pipe. As a general rule, it will have been installed close to the boiler itself, but if it isn’t, follow the inlet pipe back until you find it.

What Pressure Should My Boiler Read?

For the vast majority of boilers and central heating systems, the optimal pressure range should be between 1.5 and 2 bars (21–29 psi). However, always check the details on your specific boiler before refilling or topping up the system.

If your boiler is a relatively recent model, it will probably have safeguards in place that will stop it from trying to operate if the pressure is outside the acceptable range, but the risk you would face if the pressure is too low would be reduced performance, resulting in less effective heating and potentially higher gas bills. This is because the amount of heat produced by the boiler is the same as it always is, it is the transfer to the water in the heat exchanger that is lacking, so you are using the same amount of gas but getting less heat as a result.

As far as high pressure goes, the main risks there are damaging the pump in your boiler and, of course, water forcing its way out of a weak point in your system, such as a radiator pipe joint.

One of the easiest ways to relieve some of the pressure in your central heating system is to bleed your radiators. If there is air in the system, this will get rid of that air and reduce the overall pressure.

Don’t Risk It: Hire a Professional

Unless you are competent at dealing with plumbing, it really is best to get a professional on the case. The risks from inadequately repaired central heating pipes are quite severe. And, of course, anything that involves working with gas should absolutely be left to those who are certified to work with gas.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 John Bullock

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