A boiler that keeps losing pressure is almost always losing water from the heating system. The usual causes are a small leak in the pipes or radiators, a faulty pressure relief valve, or an expansion vessel that has stopped working. Topping the pressure back up is only a short-term fix. If you find yourself doing it regularly, the system needs checking by a Gas Safe registered engineer.
Below, we explain what your pressure should be, what causes it to drop, the checks you can safely do yourself, and when to give us a call.
What should my boiler pressure be?
Most combi and system boilers should read between 1 and 1.5 bar when the heating is off. When the heating is running, the water expands and the gauge usually rises a little, often to around 1.5 bar or slightly more. That is normal.
If the gauge drops below 1 bar, the pressure is too low. Many boilers will show a fault code or stop working altogether until it is topped up, which is often how people first notice the problem.
Your boiler manual gives the exact figures for your model, and many gauges have a green zone showing the safe range.
This only applies to sealed systems, which covers most combi and system boilers. Older regular boilers fed by a water tank in the loft (open vent systems) work differently and don’t usually lose pressure in the same way. Not sure which you have? Our open vent boilers page explains the difference.
Why does my boiler keep losing pressure?
Your heating system is a closed loop of water. Pressure only drops when water escapes from that loop. These are the five most common reasons we find.
1. A leak in the pipework or radiators
This is the most common cause. Leaks often start at radiator valves, pipe joints, or connections under the floor where you can’t see them. A slow leak may only lose a few drops a day, but that is enough to bring the pressure down over a week or two. Look for drips, rust or white crusty marks on valves, stained floorboards, or damp patches on ceilings.
2. Water escaping through the pressure relief valve
Every sealed system has a safety valve that releases water if the pressure gets too high. It connects to a small copper pipe that usually runs through an outside wall near the boiler. If that pipe is dripping or the wall below it is wet, water is leaving the system this way. The valve may be worn, or it may be doing its job because the pressure is rising too high, which points to cause 3.
3. A failed expansion vessel
Water expands as it heats up. The expansion vessel, a tank inside or next to the boiler, gives that extra water somewhere to go. Over time it can lose its air charge and stop working. The pressure then shoots up when the heating comes on, the relief valve releases water, and when the system cools the pressure drops lower than before. A gauge that climbs high when the heating is on and falls low when it is off is a classic sign.
4. You have recently bled your radiators
Bleeding radiators lets out air, and some pressure goes with it. A small drop afterwards is normal and you simply top it back up. If the pressure keeps falling after that, one of the causes above is more likely.
5. A leak inside the boiler
Seals and components inside the boiler can wear and leak. You may see water under the boiler or staining on the casing. This needs a Gas Safe registered engineer, as the inside of a gas boiler is not something to open yourself.
Safe checks you can do yourself
Before you call anyone, these checks can help pin down the problem, and they make it quicker for an engineer to find the cause.
- Note the readings. Check the gauge when the heating is off and again when it has been running for a while, over a few days. Write down how fast it drops and whether it jumps when hot.
- Look along visible pipes and radiator valves for drips, rust, or white crusty deposits.
- Check under the boiler for water or staining.
- Find the small copper pipe outside near the boiler and see whether it is dripping or the wall below is wet.
- Check ceilings and floors below rooms with radiators for damp patches.
- Top up the pressure using the filling loop or filling key, following your boiler manual. Bring it back to the level the manual gives, close the valves fully, and don’t overfill.
What not to do
- Don’t open the boiler casing. By law, only a Gas Safe registered engineer should work on a gas boiler.
- Don’t keep topping up and ignoring it. Every top-up adds fresh water to the system, and that can speed up corrosion and sludge in your radiators over time.
- Don’t overfill. Too much pressure makes the relief valve release water, which can start the cycle again.
- Don’t block or cap the outside pipe. It is there for safety.
When should I call an engineer?
Give us a call if any of these apply:
- You need to top up the pressure regularly
- The pressure drops within hours or a day or two of topping up
- The outside copper pipe is dripping
- You can see water around the boiler or damp patches anywhere in the house
- The boiler keeps showing a fault code or switching off
If water is actively leaking, or you have no heating or hot water, call us straight away on 0121 709 0000.
How we fix a boiler that keeps losing pressure
Our engineers start by finding where the water is going. That might mean checking the relief valve and expansion vessel, testing the pressure when the system is hot and cold, or tracing a hidden leak with our leak detection service. Once we know the cause, we repair or replace the faulty part, refill the system to the correct pressure, and test it before we leave.
We see this problem a lot across Birmingham and Solihull, especially in older properties where radiator valves and pipework have been in place for many years. If we find sludge in the system while we are there, we will tell you honestly whether a power flush or magnetic filter would help, with no pressure to go ahead.
MS HEATING AND PLUMBING has been looking after boilers and heating systems for over 28 years. Our engineers are Gas Safe registered, and we are a Which? Trusted Trader, so you know the work is done safely and properly.
Get your boiler pressure sorted
The quickest way to reach us is WhatsApp. Send a photo of your boiler pressure gauge and the outside copper pipe, and tell us how often you are topping up. For a boiler fault, call us on 0121 709 0000 so we can talk it through and book you in. Find out more about our boiler repair service.
