Most modern boilers will automatically shut down if the pressure in the system gets too low. But when the system is enclosed, how can the pressure possibly drop? There are several ways, from leaks to component malfunctions, and we’ll go through the most commons ones below.

It’s usually pretty easy to get your pressure back in its operating range (1–1.5 bar in most systems) by opening the inlet valves and letting some more water in. It’s important to note, however, that if you’ve lost pressure it’s because something isn’t right, and topping up the water will only be a temporary measure. Have a look through the items below one by one. If the problem keeps coming back, speak to a professional central heating engineer, who should be able to diagnose the issue – it could be time for a new boiler or a repair.


The most likely issue is that there’s a leak somewhere in the 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 are puddles, something clearly needs fixing, so call in a plumber to sort it out – it’s probably a 5-minute job.

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 pressure over time.

Not enough inhibitor in system

The water that circulates around the system should have a chemical mixed in with it called inhibitor. This slows down the chemical reaction between the water and the metal of the radiators (usually steel). As well as preserving the hardware itself, inhibitor has a secondary use – it prevents gas build-up. Because the reaction between water and steel releases gases, this gas will form bubbles and eventually start replacing water in the system. That will reduce 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 system with pure mains water, it’s likely that the concentration of 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.

Air vents

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 some slight drop in pressure over time and will need to top up the water. But if the air vent is blocked or jammed, air will be allowed to enter the system and get worse. So, both a working air vent and a stuck one can lead to reduction in pressure, but again, the real problem is elsewhere. But if the vent is blocked or damaged, it needs fixing or replacing.

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 it 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.

Pump damaged

If the pump is damaged, it will not be as efficient, and could struggle to maintain pressure. This should come up during your annual inspection, but if pressure drops suddenly it could be down to the pump. Call in an expert.

Expansion vessel not working

Like all materials, water expands as it heats up, and when it cools it contracts. 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.

That’s why every pressurised central heating system has an expansion vessel. It 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 the rubber bursts or tears, water will fill the air gap, and the pressure in the system will drop. If you have a sudden drop in pressure and there’s no obvious sign of a leak, this could be a candidate for investigation. It would not be advisable to re-fill the central heating system, as the increase in pressure without the safety of an expansion vessel could cause a failure.

Too much pressure in the system

Ironically, low pressure can be caused by having the pressure 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 dump water to protect the system. Once it cools down again, you might find that the pressure has dropped considerably. Always fill the system to the recommended pressure level.

Pressure relief valve damaged

In a central heating system, the pressure relief valve is one of only a few links with the outside world. It 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.

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

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