Why is my boiler losing pressure?

Typically, if the pressure in modern boilers gets too low, the system will shut down. But if the system is enclosed, how can the boiler pressure possibly drop? Well, there are several ways, from leaks to component malfunctions, and we're exploring the most common causes of low pressure below.

We’ll also be explaining what boiler pressure is and what you should do if your boiler is losing pressure. It’s important to know what to do in this circumstance and when there is nothing that can be done, except upgrading your boiler.

What Is Boiler Pressure?

Inside your boiler system, there needs to be a certain level of pressure to ensure water flows freely through the pipes and out of the taps in your home when needed. As this pressure is what moves the water through your heating system, without it your boiler simply wouldn’t work and you’ll be left with no hot water or heating.

What Is The Impact Of Low Boiler Pressure?

If your boiler pressure drops, the system won’t work properly, as the pressure won’t be at the required level to pump water through the boiler system. Low boiler pressure will cause your boiler to work harder when trying to heat your home, meaning more energy is used, which could result in your energy bills increasing. This won’t cause any damage to your boiler, though, nor will it reduce its lifespan. 

What Should You Do If Your Boiler Is Losing Pressure?

If you notice your boiler is losing pressure, there are steps you can take to repressurise the system. It’s usually pretty easy to get your boiler pressure back within its operating range (1–1.5 bar in most systems) by opening the inlet valves and letting some more water in. 

Here’s how to repressurise your boiler system:

1. Turn off your boiler

2. Check each end of the filling loop is intact and in working order

3. Turn the valve so that it’s open, allowing water to flow into the system

4. Monitor the pressure gauge and wait for it to measure at 1.5

5. Once this reading is achieved, turn off both valves.

6. Turn your boiler back on

It’s important to note, however, that if you’ve lost pressure it’s usually because something isn’t quite right with your system, and topping up the water may only be a temporary measure. Have a look through the items below one by one. If the problem keeps coming back, speak to a Gas Safe-registered engineer, who should be able to diagnose the issue – if there is a more serious underlying issue causing your low boiler pressure, it could be time for a new boiler or a repair.

Is Your Boiler Leaking?

If there are puddles or excessive amounts of water around your boiler, you could have a leak inside your boiler system. It’s unlikely that the pipework has burst, so the most common places to look are at the joints. Check around the radiators where the pipework enters and leaves. If there is standing water, you should call a plumber to come and take a look.

Puddles aren’t always in evidence where there’s a leak, however. Slow leaks can often evaporate before they form puddles, especially in the summer. Look for clean spots on hard floors that are surrounded by dust, or black marks where persistent dampness has taken hold. Check ceilings for stains, especially around the edges where they meet the walls. Central heating leaks are rarely torrential, but they will certainly lower the boiler’s pressure over time.

Read our guide to find out what to do if your boiler is leaking.

Is There Enough Inhibitor In The Boiler System?

The water that circulates around the system should have a chemical mixed in with it called inhibitor, which slows down the chemical reaction between the water and the metal of the radiators (usually steel). It also prevents gas build-up, as the reaction between water and steel releases gases, which forms bubbles and will eventually start replacing water in the boiler system, reducing the pressure. You’ll be able to raise the pressure by bleeding the radiators and putting more water in, but it’s a temporary measure.

If you’ve regularly been topping up the boiler system with pure mains water, it’s likely that the concentration of the inhibitor has become too low. You should get a central heating specialist to drain the system and refill it with the correct amount of inhibitor.

Are The Air Vents Blocked?

If the air vent is blocked or jammed, air will be able to enter your boiler system but will be unable to leave. Many modern systems have air vents built in to relieve air build-up before it becomes noticeable. If the air vent is working well, you might notice a slight drop in pressure over time and will need to top up the water. So, both a working air vent and a stuck one can lead to a reduction in pressure but the real problem is whether the vent is blocked or damaged. You'll need a professional engineer to diagnose the issue as the vent may need fixing or replacing if you suspect a more serious underlying problem. 

Is The Boiler’s Pump Speed Too High?

In every pressurised central heating system there has to be a pump to get the water moving around it. If the pump’s speed is too high, however, it can cause air bubbles to enter the flow of water, and that will eventually lead to the problems covered above. A boiler engineer should be able to set the pump to the correct speed.

Is The Boiler Pump Damaged?

If the boiler pump is damaged, it will not be as efficient and could struggle to maintain pressure. If the pump is extremely hot to touch, water is leaking or the water flowing through the pump is cold, it is likely that your boiler pump is damaged. This should come up during your annual inspection, but if pressure drops suddenly it could be down to the pump. If you have concerns about your boiler pump being damaged, you should call in an expert.

Is The Expansion Vessel Working?

Like all materials, water expands as it heats up, and when it cools it contracts. To account for this, every pressurised central heating system has an expansion vessel, however, if the rubber bursts or tears, water will fill the air gap, and the pressure in the system will drop. 

Obviously in a pressurised system this would be a problem. The volume inside the system doesn’t change, so like over-inflating a tyre, you could cause a burst.

An expansion vessel is basically a small tank with a rubber diaphragm going across it (think of a syringe except the plunger is attached to the inside of the tube). On one side of the rubber is the pressurised water from the central heating system; on the other is compressed air. When the temperature rises, the volume of water grows and pushes the diaphragm towards the air. When it cools, the air pressure in the expansion vessel presses back against the water, keeping the system pressure constant. If there is a fault with your expansion valve, this can affect your boiler pressure directly, due to its integral role in the inner workings of a boiler.

If you have a sudden drop in pressure and there’s no obvious sign of a leak, the expansion vessel could be a candidate for investigation. It would not be advisable to refill the central heating system, as the increase in pressure without the safety of an expansion vessel could cause a failure.

Is The Boiler Pressure Too High? 

Ironically, low boiler pressure can be caused by having the pressure set too high. Your system should have a pressure relief valve to keep it safe – the water outlet is usually located outside the home, behind where the boiler is located. If your pressure is a little too high when the system is cool, turning it on will make it even higher, and it will release water to protect the system. Once it cools down again, you might find that the pressure then drops considerably. Always fill the system to the recommended pressure level to avoid issues with your boiler pressure.

Is The Pressure Relief Valve Damaged?

The pressure relief valve is essentially a tap that switches itself on when pressure gets too high, and like all taps, it can wear out and start leaking. If this happens, the pressure will drop just as it will with any other leak. Check the valve for drips of water, because in normal operation it should hardly ever be wet. If you notice water around your pressure relief valve, call a Gas Safe-registered engineer to investigate the issue for you.

That covers the most common causes of boiler pressure drops. Most of them are professional jobs that should not be attempted by homeowners. Pressure drops always have an underlying cause. If it is happening regularly, you should really get the system checked out.

For more information and help with issues your boiler may have, head over to our tips & advice page. Also, keep an eye on our socials, such as Facebook and Twitter, for regular updates and support.

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