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You topped the boiler up last week. Now the needle’s back below 1 bar, the heating has cut out and the radiators are cold again. So why does my boiler keep losing pressure? Short answer: water is getting out of a system that’s meant to be sealed, or a part that controls the pressure has failed.
Below you’ll find what normal pressure looks like, what usually causes the drop, the checks you can do yourself and how to top up properly. We’ll also tell you when it’s time to get a Gas Safe registered engineer in. One rule before you start: never take the boiler casing off yourself.
A boiler that keeps losing pressure almost always has a water leak, a faulty pressure relief valve or a failed expansion vessel. Top it up to 1?1.5 bar once. If it drops again within days or weeks, look for leaks and a dripping outside pipe, then book a Gas Safe registered engineer.
Most combi and system boilers in UK homes run on a sealed heating circuit. The gauge on the front (or the digital display on newer boilers) shows the pressure in that circuit, in bar.
Most manufacturers want around 1?1.5 bar when the heating is off and cold. It’ll creep up a bit once the system is hot, because water expands as it warms. Brands differ slightly, so check your boiler manual.
| Gauge reading (heating off) | What it usually means |
|---|---|
| Below 1 bar | Too low. The boiler may lock out and show a fault code. |
| 1?1.5 bar | Normal for most boilers. |
| Around 2 bar when cold | Higher than needed. Check the filling loop is fully closed. |
| Near 3 bar | Too high. The pressure relief valve may start to release water. |
If the pressure climbs high when the heating’s on and then sinks when it cools, that’s a clue in itself. More on that under the expansion vessel below.
A sealed system should hold its pressure for months. When it doesn’t, either water is getting out or the system can’t cope with normal expansion.
This is the most common reason. A slow weep from one radiator valve is enough to drop the pressure over a few days. Check the radiator valves, pipe joints, towel rails and the pipes under the boiler.
Plenty of London houses and converted flats have heating pipes running under floorboards or buried in walls. Leaks there are hard to spot. Look for damp patches, stained ceilings, lifting flooring or a musty smell.
Every sealed boiler has a pressure relief valve (PRV), a safety part that dumps water if pressure gets too high. On most domestic boilers it is set to open at around 3 bar. Its discharge pipe usually runs through an outside wall and ends outside.
It’s easy to miss, as it often pokes out low down or at the back of the house. If the end is wet or dripping, the valve may be faulty, or something is pushing the pressure too high. Worcester Bosch’s own advice lists this as one of the first things to check.
The expansion vessel is a tank, usually inside the boiler casing, with an air cushion on one side. It soaks up the extra volume when water heats up. Over time that air charge leaks away.
Once that happens, the pressure shoots up every time the heating comes on. The PRV lets water out, and when the system cools the pressure is low again. Topping up just starts the cycle over. Recharging or replacing the vessel is a central heating repair job for an engineer.
Bleeding radiators lets air out, so the pressure falls. A one-off drop afterwards is normal. Top up once and it should stay put.
Pressure can drop after a radiator has been changed, a new valve fitted or the system drained. Air works its way out over the next few days. One or two top-ups during that time are not unusual.
A slow drift over many months is normal for a sealed system. Which? says most boilers need topping up once or twice a year.
This is a very common situation, and it’s frustrating. Work through these checks before you call. What you find helps an engineer get to the fault faster, which usually means a shorter visit.
Found nothing? Then the water is probably escaping under a floor or through the PRV. That’s when we’d test the expansion vessel and start tracing hidden leaks.
Topping up is fine for most people to do, as long as you use the filling loop and leave the casing alone. The filling loop is usually under the boiler. It might be a braided hose between two valves, a built-in lever or a blue key. The valves are often stiff if they haven’t been touched in a while, so turn them slowly.
Worcester Bosch, Vaillant, Baxi and Ideal all do it slightly differently. Some models use a key and others a built-in lever. If you’re not sure what you’re looking at, your manual or the manufacturer’s website will show the layout for your model.
Never remove the boiler casing, adjust the PRV or try to recharge the expansion vessel yourself. Anything behind the casing must be done by a Gas Safe registered engineer. If you smell gas, turn off the supply at the meter, open windows and call the National Gas Emergency number on 0800 111 999.
Topping up every week? That’s a fault, not a habit to get used to. Call London Emergency Plumbing on 020 8064 1046 or see our boiler repair service in London.
A reading of 0 bar means the system has lost almost all its pressure. The boiler will lock out to protect itself. Don’t panic, but do find out why before you keep refilling it.
Call a Gas Safe registered engineer if any of these apply:
Renting? Tell your landlord or letting agent in writing. In most tenancies the landlord is responsible for repairing the boiler and heating, so you shouldn’t be stuck topping it up all winter.
Testing the expansion vessel and internal valves is a job for a Gas Safe registered engineer.
Our Gas Safe registered engineers cover every London borough, day and night. Call 020 8064 1046, book through our boiler repair page or check our pricing first.
Low pressure on its own isn’t dangerous. The boiler will usually lock out and stop heating rather than run unsafely. The real risk is whatever’s causing it. A hidden leak can rot floorboards and stain ceilings, and a failing PRV can let hot water out. Get the cause found rather than topping up forever.
Some rise is normal, because water expands as it heats. A big jump followed by a drop usually means the expansion vessel has lost its air charge. The PRV then releases water each time the system heats up, so the pressure falls when it cools. An engineer can test and recharge or replace the vessel.
No. A filling loop left open, even slightly, lets mains water creep into the heating system. Pressure then rises until the PRV opens, which can wear the valve out. Close both valves fully after every top-up. If your loop is the removable hose type, disconnect it as the manufacturer advises.
On a combi boiler, usually yes. Most combis lock out completely when the pressure drops too low, so you lose hot water as well as heating. Topping up to 1?1.5 bar and resetting the boiler normally gets both working again, as long as there is no other fault.
Fix the cause rather than the symptom. Have leaks repaired, get the expansion vessel checked, and have the boiler serviced once a year. A service is a good time for the engineer to check the PRV and vessel charge before they fail. Check the gauge every few weeks through the winter.
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