Why Does My Boiler Keep Losing Pressure?
A pressure gauge that keeps dropping means water is getting out somewhere and this is how to track it down.
The short answer
A boiler keeps losing pressure because water is escaping from the sealed heating system or a component has failed. The usual culprits are a leak in the pipework or a radiator valve, a faulty pressure relief valve, a flat expansion vessel or water let out when bleeding radiators. Check every visible joint and the relief pipe outside first, then call a Gas Safe engineer for faults inside the boiler.
A combi or system boiler runs on a sealed circuit of water, and the gauge on the front tells you how much pressure that water is under. When the needle keeps creeping down, the radiators struggle to heat up, the hot water can drop out and the boiler eventually locks out with a low pressure fault.
Topping the pressure back up gets the heating going again, and our guide to repressurising a combi boiler with the filling loop shows how. If you find yourself doing it every week or two, though, water is leaving the system somewhere and the cause needs finding.
What Pressure Should A Boiler Be At
Most boilers sit between 1 and 1.5 bar when the heating is cold, and the pressure rises a little as the water heats up. Your boiler manual gives the exact figure for your model, and many gauges have a green band marking the safe range.
Check the gauge with the heating off and the system cold so you're comparing like with like. Write the reading down, then check again the next day. A steady drop over a few days suggests a slow leak, while a sudden loss means something has let go or a valve has opened.
Why Is My Boiler Losing Pressure?
1. A Leak In The Pipework
A leak somewhere in the heating pipes is the most common reason for a boiler losing pressure. It can be anywhere along the run, from a weeping joint behind a radiator to a pinhole in a pipe under the floorboards or buried in a wall.
Start with what you can see. Check every radiator valve, the joints under the boiler and any pipework in airing cupboards and under sinks, wiping each one with a dry tissue to show up small drips. Damp patches on ceilings, stained floorboards and bubbling paint low on a wall are all clues to a hidden leak. With the heating running, a thermal imaging camera for tracing hot pipes shows where pipes run behind plaster and flooring and helps pin down a warm, wet spot.
Once you've found it, a loose compression joint can be nipped up, but a failed soldered joint or a corroded pipe needs cutting out and replacing.
2. A Leaking Radiator Valve
Radiator valves are a frequent source of slow leaks, especially where the spindle enters the valve body or where the valve meets the radiator tail. Look for a drip running down the pipe, a green or white crust on the brass, or a damp carpet below.
Tightening the gland nut a fraction stops some spindle leaks. If the valve is worn, swap it for a new one, which is a good chance to fit one of the thermostatic radiator valves we recommend or go a step further with smart radiator valves for room-by-room control.
3. Bleeding The Radiators
Bleeding radiators lets out trapped air, and as the air escapes the pressure in the system drops. If you or a plumber have recently bled the radiators, a lower reading is expected and topping up once will sort it.
If the pressure carries on falling after you've topped up, the bleeding wasn't the cause and something else is letting water out.
4. A Faulty Pressure Relief Valve
The pressure relief valve is the boiler's safety outlet, letting water out if the pressure climbs too high. When the valve wears or gets grit in its seat, it can let water out at normal pressure too.
Follow the copper pipe that runs from the boiler to the outside wall. If it drips, or there's a damp stain on the wall below it, the relief valve is passing water. Replacing it is a job for a Gas Safe engineer, as it's inside the boiler casing.
5. A Flat Expansion Vessel
The expansion vessel is a small tank inside or beside the boiler that absorbs the extra volume as the water heats up. It has an air charge on one side of a rubber diaphragm, and over time that charge leaks away.
A flat vessel makes the pressure shoot up when the heating is on, which forces the relief valve to dump water, and then the pressure falls too low once the system cools. If your gauge swings well into the red when hot and drops when cold, an engineer needs to recharge or replace the vessel.
6. A Leak Inside The Boiler
Seals, pumps and heat exchangers inside the boiler can corrode and leak, particularly on older units. Look underneath the boiler for drips or water marks on the casing, but don't take the cover off yourself. The casing on a gas boiler is sealed and should only be removed by a Gas Safe registered engineer, who can tell you whether a repair or a replacement makes more sense.
7. A Faulty Pressure Gauge
Now and then the gauge itself is the problem. If the needle doesn't move when you add water through the filling loop, or the reading doesn't match the boiler's digital display, the gauge is stuck or faulty. Stop adding water at that point, because overfilling a system you can't read accurately pushes the pressure too high.
Problems You Can Fix And Jobs For An Engineer
Plenty of pressure problems are well within reach for a confident DIYer. Tightening a weeping valve, replacing a radiator valve and topping the system up through the filling loop are all reasonable jobs. Changing a valve or replacing a leaking radiator means draining down first, and our guide to draining a central heating system walks through it.
Anything inside the boiler casing is different. Pressure relief valves, expansion vessels and internal leaks all need a Gas Safe registered engineer, both for your safety and to keep the boiler warranty valid.
Why A Pressure Drop Shouldn't Be Ignored
Topping up every few days hides the problem rather than fixing it. A leak that's left running gets worse, soaks into floors and walls and can lead to damp, rot and stained ceilings. Each top-up also brings fresh, oxygen-rich water into the system, which speeds up corrosion and sludge build-up inside radiators and the boiler.
Once the leak is sorted, a magnetic central heating filter catches the sludge that's already circulating and helps protect the boiler from here on.
Other Questions We Get Asked
Is it dangerous if my boiler keeps losing pressure?
Low boiler pressure isn't dangerous in itself, because the boiler shuts down safely when pressure drops too far. The risk comes from what's causing it. A hidden leak can damage floors, ceilings and walls, and repeated top-ups bring in fresh water that speeds up corrosion. Find the cause rather than topping up indefinitely.
How regularly should a boiler need topping up?
A healthy sealed heating system rarely needs topping up, as no water should be leaving it. Needing to repressurise every week or every few days means water is escaping, either from a leak or through the pressure relief valve. Note how quickly the pressure drops, as an engineer will ask.
Why does my boiler pressure go up when the heating is on and then drop?
Boiler pressure that rises sharply when the heating is on and then drops when it cools points to a flat expansion vessel. Without its air cushion, the vessel can't absorb the expanding water, so the pressure relief valve opens and dumps water. A Gas Safe engineer can recharge the vessel or replace it.
Can I still use my boiler if the pressure is low?
Not for long, because a boiler shuts down and shows a low pressure fault code once the pressure falls too far. Topping up to the level in your manual through the filling loop brings it back on. If the pressure drops again quickly, keep an eye out for leaks and arrange for an engineer to check the boiler.
What's the first thing to check when my boiler pressure keeps dropping?
Start with the leaks you can see. Check every radiator valve, the joints under the boiler and any pipework in airing cupboards and under sinks, wiping each one with a dry tissue to show up small drips. Then follow the copper relief pipe to the outside wall, because a drip or damp stain there means the pressure relief valve is passing water.
How can I tell whether it's the gauge or a real leak?
A faulty gauge shows itself when the needle doesn't move as you add water through the filling loop, or when the reading doesn't match the boiler's digital display. If that happens, stop adding water, because overfilling a system you can't read accurately pushes the pressure too high. A real leak shows as a steady drop over several days.
Can I replace a leaking radiator valve myself?
Yes, replacing a radiator valve is a reasonable job for a confident DIYer. Try nipping up the gland nut a fraction first, as that stops some spindle leaks, and if the valve still weeps, drain the system down before swapping it. Anything inside the boiler casing, such as the relief valve or expansion vessel, needs a Gas Safe registered engineer.
Does topping up the pressure every few days do any harm?
Yes, because it hides the leak rather than fixing it. A leak left running gets worse and can soak into floors and walls, leading to damp, rot and stained ceilings. Each top-up also brings fresh, oxygen-rich water into the system, which speeds up corrosion and sludge build-up inside radiators and the boiler.
Richard has spent years renovating properties and turning them into rentals and Airbnbs. Like a lot of people he started out as a beginner, and these days there isn't much DIY he won't take on.