Why does my boiler keep losing pressure?
Your boiler keeps losing pressure because water is escaping from the sealed circuit of boiler, pipes and radiators. On a combi or system boiler that circuit is closed, so once it has been filled, the pressure should stay roughly steady for months. When it keeps dropping, the water has to be going somewhere.
The usual routes out are:
- A small leak at a radiator valve, a bleed screw, a towel rail or a pipe joint. A slow weep can evaporate before it leaves a puddle.
- The pressure relief valve, which dumps water through a copper pipe to the outside when the pressure gets too high, or when the valve itself is worn.
- A failed expansion vessel, which makes the pressure swing up when the heating runs, so the relief valve opens and water is lost.
- A leak inside the boiler, for example at an internal seal or the automatic air vent. You will not see this without the case off, which is a job for an engineer.
- A hidden leak under the floor, in a wall or at an underfloor heating manifold.
Bleeding radiators also lowers the pressure, because air leaves and water moves in to fill the space. That is expected and is not a fault.
How much pressure loss is normal?
A small loss over months is normal, but a loss you can see from one week to the next is not. Worcester Bosch says the pressure in a heating system will usually need topping up once or twice a year, and that if you are having to repressurise much more often, you should contact your installer.
Two things can look like pressure loss when they are not:
- The drop from hot to cold. The pressure rises when the heating runs and falls back as the water cools. A system that reads 1.7 bar hot and 1.3 bar cold is working normally. Always compare cold readings.
- The drop after bleeding radiators. Each radiator you bleed lets out some air, and the gauge falls a little. Top it up once and it should then hold.
For what the gauge should read in the first place, see our guide to normal boiler pressure.
Pressure dropping on the boiler every day
If the pressure on your boiler drops every day, you have an active leak or a relief valve that is opening, and you should find it rather than keep topping up. Each top-up brings in fresh mains water, which carries oxygen and speeds up corrosion inside radiators and the boiler's heat exchanger.
Start a simple log. Note the cold reading at the same time each morning for a week, and whether the heating ran the day before. A fall of 0.1 to 0.2 bar a day points to a slow weep. A sudden drop after each heating cycle points to the relief valve and expansion vessel.
Step by step: from visible leaks to hidden ones
- Look under the boiler. Without opening the case, check the pipes and valves underneath for drips, damp patches, white crusty deposits or green staining on copper. Put a dry tissue on the floor or shelf below it overnight. If you find water here, our guide to a boiler leaking water covers the next steps.
- Check the relief valve pipe outside. Find the copper pipe that comes through the outside wall near the boiler, often ending pointing down or turned back towards the wall. A wet end, a damp patch or a limescale mark on the wall or ground below it means the relief valve has been discharging. Our boiler pressure relief valve guide explains why.
- Check every radiator. Wipe a dry tissue round each valve spindle, the nut where the valve joins the radiator, and the bleed screw. Look for brown staining on the carpet or a crust on the valve body. Do not forget towel rails in bathrooms.
- Check exposed pipework. Look along any pipes you can see, in cupboards, under sinks and in the airing cupboard. Check round the magnetic filter under the boiler, if you have one, and the filling loop connections.
- Watch the gauge through a heating cycle. Note the cold reading, run the heating for an hour and note the hot reading. A rise of more than about 1 bar, followed by a lower cold reading the next day, points to the expansion vessel.
- Look for signs of a hidden leak. Stains on ceilings below pipe runs, lifting floorboards or laminate, a warm patch on a solid floor or a musty smell can all mark a buried leak.
Check that the filling loop is fully closed before you start. A partly open valve can hide a leak by letting mains water trickle in, and it can also push the pressure up until the relief valve opens.
Boiler losing pressure but no leak
When a boiler is losing pressure but there is no leak you can see, the water is usually leaving through the relief valve pipe outside, or through a leak you cannot see, inside the boiler or under the floor. Because the relief valve pipe discharges outdoors, the most common hidden cause often goes unnoticed for months.
The classic pattern is a failed expansion vessel. The vessel has lost its air charge, so when the water heats and expands, there is no cushion to absorb it. The pressure shoots up, the relief valve opens near 3 bar and lets water out, and when the system cools the pressure is lower than before. This can happen every day while the heating is in use and stop in summer.
Other hidden routes include a weeping automatic air vent or internal joint inside the boiler case, and leaks in pipework buried in walls, screed or under floorboards. An engineer can pressure test the system in sections to narrow it down.
Combi boiler losing pressure
A combi boiler losing pressure has the same causes as a system boiler, because both run on a sealed circuit with an expansion vessel and relief valve inside the case. The only real difference is that on a combi everything is packed into the boiler, so internal checks always mean an engineer taking the case off.
Regular (heat only) boilers on an open vented system are fed from a small tank in the loft, so they do not usually lose pressure in this way. A leak on those systems shows as the loft tank refilling more often than normal.
Can a faulty expansion vessel cause pressure loss?
Yes, and it is one of the most common causes when you cannot find a leak. The vessel itself does not leak water; it makes the pressure swing so high that the relief valve opens. Our guide to the expansion vessel explains how an engineer checks and recharges one.
Symptoms and likely causes
| What you notice | Likely cause | Who fixes it |
|---|---|---|
| Slow, steady drop, damp or stained radiator valve | Weeping valve spindle or joint | Plumber or heating engineer |
| Pressure jumps well over 2 bar when hot, then drops | Expansion vessel lost its charge | Gas Safe registered engineer |
| Wet end or stain below the copper pipe outside | Relief valve discharging | Gas Safe registered engineer |
| Water or drips under the boiler | Leak at an external or internal joint | Gas Safe registered engineer |
| No visible water, ceiling stain or warm floor patch | Hidden pipe leak | Heating engineer with leak detection |
| Drop only after bleeding radiators | Normal air removal | Top up once yourself |
What to do while you wait for an engineer
You can keep the heating running by topping the pressure back up to the normal cold range using the filling loop. Our guide to boiler pressure that is too low shows how to do this safely, and close the loop fully afterwards.
Do not open the boiler case to look for a leak. The internal parts sit next to gas and electrical connections and must only be worked on by a Gas Safe registered engineer. If water is dripping onto electrics or a ceiling light, switch the power off at the consumer unit and call an electrician.
When you book, tell the engineer your cold and hot readings, how often you top up, whether the relief valve pipe is wet and what you found at the radiators. With that log, they can often go straight to the expansion vessel or relief valve instead of spending time searching.