Boiler losing pressure? Causes, fixes and when to call

What does it mean when a boiler loses pressure?

When a boiler loses pressure, water has escaped from the sealed central heating system, so the gauge drops below the normal 1 to 1.5 bar it should read when cold. The two most common causes are a small leak and a recently bled radiator. You can safely top it up once; if it keeps falling, a Gas Safe registered engineer needs to trace where the water is going.

This guide explains why pressure drops, what you can safely do yourself, and the point at which it stops being a homeowner job. Smart Plan is a home service plan, not insurance.

A low reading on its own is not a gas emergency. It is a water issue, not a gas one. The National Gas Emergency Service line, 0800 111 999, is only for when you can smell gas or a carbon monoxide alarm is sounding.

Why does my boiler keep losing pressure?

A one-off drop is normal maintenance. Pressure that falls again within a day or two, or that has you topping up every few weeks, almost always means water is leaving the system somewhere. There are four usual culprits, from the harmless to the engineer-only.

A small leak on a radiator valve, a pipe joint or a system component is the cause that keeps coming back, letting pressure creep down over days or weeks. Recently bled radiators are a harmless one-off, because releasing air also releases a little water, so the gauge drops and a single top-up sorts it. A weeping pressure relief valve can push water out through the small overflow pipe on an outside wall. And a failed expansion vessel can no longer absorb the water expanding as it heats, so the pressure swings up when hot and drops back when cold.

Can a boiler lose pressure with no visible leak?

Often, yes. A tired or flat expansion vessel is the most common hidden cause when nothing looks wet, because the fault sits inside a sealed component rather than out on the pipework. A slow weep from the pressure relief valve outside, or a leak hidden under floors or inside walls, can do the same. One more possibility is the gauge itself: a faulty or stuck pressure gauge can read low or even zero right after you've topped up, while the heating still works normally. None of these is something to chase yourself, but all are straightforward for a Gas Safe registered engineer to find, and the gauge is quick for them to test and replace if it is the culprit.

Why does the pressure drop overnight or when the heating is off?

Pressure reads a little lower when the system is cold, because water contracts as it cools, so a small change between cold and hot is normal. A larger drop that leaves you below 1 bar by morning points to a genuine leak or a failing expansion vessel rather than normal contraction. If you are repressurising most mornings, treat that as a fault to trace, not a routine to keep repeating.

Boiler losing pressure: causes at a glance

How do I safely repressurise my boiler?

Do this with the boiler switched off and the system cold. Top up only once.

• Switch the boiler off and let the system go cold. Find the pressure gauge or pressure display so you can watch the reading.
• Locate your filling device underneath the boiler: a silver braided filling loop with a valve at each end, a keyed filling port, or a blue keyless filling lever.
• Open the valves slowly, insert and turn the key, or pull the blue lever. You'll hear water flowing in and see the gauge start to climb.
• Stop the moment the gauge reaches about 1 to 1.5 bar when cold. Go slowly so you don't shoot past 1.5 bar.
• Close the valves firmly, or return the key to the open padlock position and remove it, or release the blue lever. Detach an external filling loop once you're done, so it can't back-contaminate the mains.
• Turn the boiler back on and reset it once if it needs it. Never keep resetting or topping up if the pressure falls again, because that only masks a leak and can accelerate internal corrosion.

What does an engineer check when pressure keeps dropping?

If the pressure will not hold, the job moves from topping up to finding out why the water is leaving. A Gas Safe registered engineer typically checks three things in turn: the expansion vessel, which may need recharging or replacing; the pressure relief valve, which can weep and let water out through the external overflow; and the system itself for a hidden leak on pipework, joints or a radiator. A healthy system usually needs topping up only once or twice a year, so anything more frequent is worth investigating.

How much does it cost to fix?

Repair costs vary with the fault behind the pressure loss, from a simple valve to a hidden leak. Rather than quote a figure here, see our sourced UK boiler repair cost statistics for typical ranges with named sources and dates. Book a one-off repair and we'll trace why the pressure won't hold. A one-off repair carries a £95 call-out fee, and one-off repair visits run Monday to Friday, 09:00 to 17:00.

Does boiler cover include losing pressure?

A plan covers future breakdowns, so it won't fix a fault your boiler's showing today, but it can cover the next one. Smart Plan is a service plan, not insurance, and it is modular, so you pick only the modules you want and don't pay for what you don't use. Boiler and central heating cover includes parts and labour up to £500 per 12-month period if your boiler is under 7 years old, or up to £200 if it is 7 years or older, so on an older boiler a larger repair can cost more than the cap. Call-outs under a plan run Monday to Friday, 08:00 to 18:00. A £95 call-out fee applies in defined cases, including the first 30 days of a new agreement, and a 12-month agreement period begins once you use a service, with a 14-day cooling-off period that ends once a service is carried out.

Prefer it covered before it breaks?

If your pressure keeps creeping down, a boiler module covers the next fault before it leaves you cold: choose only the cover you want and see your price in the online builder. Call-outs under a plan run Monday to Friday, 08:00 to 18:00, a £95 call-out fee applies in defined cases including the first 30 days, and a 12-month agreement period begins once you use a service. A service plan, not insurance. Questions? Call 0333 772 6247.

Boiler losing pressure FAQs

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