What does it mean when your boiler keeps losing pressure?
A combi or system boiler runs on a sealed circuit. Once it has been filled and bled, the same water goes round the radiators for years. Nothing in normal operation uses it up. So if the needle on the pressure gauge keeps sinking and you are opening the filling loop every week or two to bring it back, water is leaving the circuit somewhere. The system is not "using" pressure. It is losing water.
There are only three places that water can go. It can be pushed out through the pressure relief valve and down the discharge pipe. It can escape from the boiler itself. Or it can escape from the pipework, radiators and valves around the house, including the parts you cannot see under floors and inside walls. A proper diagnosis works through those three in order, because the first two are quick to rule in or out and the third is where most of the time goes.
What pressure should my boiler be at?
Most domestic sealed systems are set at around 1 to 1.5 bar when cold. Your boiler manual gives the exact figure for your model, and many gauges have a green band or a marker arm showing it. When the heating is running, the pressure normally rises by a few tenths of a bar, because water expands as it warms. It should settle back to roughly the cold figure once the system cools.
The useful habit is to read the gauge at the same point every day, ideally first thing in the morning before the heating comes on. Readings taken at random, some hot and some cold, can make a stable system look as if it is dropping and a slowly leaking one look fine. Write the figure down with the date. A week of cold readings tells an engineer more than any description over the phone.
How fast is your boiler pressure dropping, and what does the rate tell you?
The speed of the drop is the first real clue. It will not identify the leak on its own, but it narrows the list of suspects.
| Pattern you see | What it usually points to | First thing to check |
|---|---|---|
| Falls to near zero within hours of topping up | A significant leak, often visible somewhere, or a relief valve stuck open | Radiators, the floor under the boiler and the outside discharge pipe |
| Falls by around half a bar over several days | A small, steady leak, often hidden in a floor or wall, or a weeping valve where the water evaporates | Radiator valves and exposed joints, then a survey |
| Climbs high when hot, sits low when cold, and repeats | An expansion vessel that has lost its charge, with the relief valve discharging at the top of each swing | Watch the gauge through a full heat-up and check the discharge pipe |
| Only drops when one zone or the hot water is running | A leak on pipework that only carries flow when that zone is open, or a joint that opens up as it warms | Note which zones were calling for heat during the drop |
| Dropped once after bleeding radiators, then held | Not a leak. Bleeding removes air and that volume needed replacing | Top up once and keep an eye on it |
If yours matches the last row, you may not have a problem at all. Every other pattern deserves investigation.
Is the pressure relief valve letting water out?
The pressure relief valve (PRV) is a safety device on the boiler. On most domestic boilers it opens at 3 bar and vents water through a copper discharge pipe that usually runs through the wall to the outside and turns down towards the ground or a drain. Follow that pipe outside and look at the end. A drip hanging off it, a wet patch below, or a green or white stain on the wall all mean the valve has been passing water.
A PRV that has opened once or twice can fail to reseat properly, often because a fragment of debris from the system has lodged under the seal. It then lets a little water through all the time, and the pressure falls with no visible leak indoors. We go into this in more detail in our guide to a combi boiler pressure relief pipe dripping. For diagnosis the rule is simple: if the discharge pipe is wet, start there before looking for a leak in the house.
Could the expansion vessel be causing the pressure loss?
The expansion vessel is a steel tank, inside the boiler casing or mounted nearby, split in two by a rubber diaphragm. One side holds system water and the other holds air or nitrogen at a set pre-charge pressure. As the water warms and expands, it pushes into the vessel and compresses the air, so the system pressure only rises a little.
When the vessel loses its charge, or the diaphragm splits, the expanding water has nowhere to go. The pressure shoots up every time the heating runs, reaches the PRV setting, and water is dumped outside. When the system cools, the pressure ends up lower than where it started, because that water has gone. You see low pressure in the morning, top up, and the cycle repeats. It behaves exactly like a leak, but the fix is a vessel recharge or replacement, usually with a new PRV, rather than a hunt under the floorboards.
Checking and recharging an expansion vessel inside a boiler is a job for a Gas Safe registered engineer. What you can safely do is watch the gauge from cold to fully hot. A climb of well over a bar, especially towards 3 bar, is a strong sign the vessel needs looking at.
Boiler losing pressure but no leak visible: where is the water going?
This is the question behind most of the heating calls we take. The discharge pipe is dry, the vessel has been checked, and there is no puddle anywhere. The water is still going somewhere, and the usual answers are these:
- Pipes in or under solid floors. Many London flats and post-war houses have heating pipes running through a concrete screed. A small leak soaks into the screed and the ground beneath and may never reach the surface.
- Pipes in suspended timber floors. Water drips into the void under the floorboards. In a well ventilated void, water from a hot pipe can evaporate almost as fast as it arrives.
- Radiator valves and tails. A weep at a valve spindle or a radiator tail nut can dry off the hot metal before it ever forms a drip. Look for white or green crusting around the valve body.
- Pipes boxed in or chased into walls. Leaks here tend to show later as a damp patch, bubbling plaster or a mark on the ceiling below.
- The boiler itself. A weeping heat exchanger or pump seal can drip onto hot components inside the casing and dry off. That needs a Gas Safe engineer to open up and inspect.
Half a bar a week on a typical house system is a small amount of water, which is exactly why it disappears so easily. Small does not mean harmless. Every top-up brings in fresh, oxygen-rich mains water and dilutes the corrosion inhibitor, which speeds up corrosion inside radiators and pipes and can create new pinholes over time.
For a wider look at heating leaks and the ways they are repaired, see our central heating leak detection guide.
How is a hidden central heating leak traced?
A methodical survey follows much the same order every time. The aim is to shrink the search area before any detection equipment is pointed at the floor.
- Confirm the loss. The engineer records the cold pressure, repressurises if needed and watches the gauge. On some jobs a separate test gauge is fitted so the drop can be read more precisely.
- Rule out the relief valve and expansion vessel. The discharge pipe, the visible boiler connections and the vessel behaviour are checked first, because a fault there makes the rest of the survey pointless.
- Walk the visible system. Every radiator, valve and exposed joint, plus the pipework in cupboards and under kitchen units, is inspected and checked with a moisture meter.
- Isolate sections. Where the layout allows, parts of the system are valved off and each section is watched for a drop. If the loss stops when the downstairs circuit is closed, the leak is downstairs.
- Thermal imaging with the heating running. Hot water in buried pipes shows through screed and plaster as warm lines. Escaping hot water shows as a warm spread that does not follow the line of the pipe.
- Acoustic confirmation. A ground microphone picks up the sound of pressurised water escaping, which helps pinpoint the spot where the thermal image is broad or unclear.
- Tracer gas where needed. If the pipe is deep or the other methods are inconclusive, the affected circuit can be drained and filled with a hydrogen and nitrogen mix, which is then detected where it comes up through the floor.
- Mark and report. The leak position is marked and recorded in a written report with images, which is what an insurer will usually want to see if you have trace and access cover.
Our central heating leak detection service follows this sequence, so nothing gets lifted until the engineer is confident of the spot.
Should I keep topping up the boiler pressure in the meantime?
If the drop is slow and there is no visible water damage, topping up to the correct cold pressure keeps the heating and hot water working and is reasonable for a short while. Do it with the system cold, open the filling loop slowly, stop at the recommended figure and close both valves fully afterwards. A filling loop left slightly open will feed the leak continuously, hide the symptom and can push the pressure up to the PRV setting.
Topping up is not a long-term plan. Apart from the corrosion problem, a hidden leak keeps wetting the floor, wall or ceiling around it, and the repair grows the longer it runs. If a survey is booked, keep the system pressurised and working until the visit. Thermal imaging and acoustic equipment both rely on hot water moving under pressure.
If you are tempted by a chemical leak sealer, check your boiler manual first. Sealers can clog narrow boiler waterways and filters. Our guide to central heating leak sealer covers which leaks one can close and how to tell it has failed.
When should you call a leak detection specialist?
Call a Gas Safe engineer first if the discharge pipe is wet, if the pressure swings sharply between hot and cold, or if you can see water inside or under the boiler. Those are boiler faults and a heating engineer is the right person.
Call a leak detection specialist when the boiler side has been checked and the pressure still drops, when you are topping up more than once or twice a month, when a damp patch, warm spot or ceiling stain appears, or when your insurer wants the source found and documented. A standard survey including thermal imaging starts from £150, and a combined thermal, acoustic and tracer gas survey is £250 to £500. Full figures are on our pricing page. Every survey is no find, no fee, and you can reach us on 020 3912 0200.
How do you repressurise a boiler, and when should you stop?
Your boiler manual is the guide to follow, because filling loops differ between models. On a typical combi, the steps are broadly these:
- Switch the heating off and let the system cool, so you are reading the cold pressure.
- Find the filling loop under the boiler. It is usually a braided hose with a valve at each end, or a built-in key or lever.
- For an external loop, check both ends are connected firmly. Open one valve, then open the second slowly. You will hear water flowing in.
- Watch the gauge and close the valves when it reaches the cold pressure given in the manual, often around 1 to 1.5 bar.
- Close both valves fully, then disconnect an external filling loop. Water regulations require a temporary filling connection to be removed after use, because it stops heating water, which contains inhibitor, being drawn back into the drinking water supply.
- Switch the boiler back on and reset it if the manual says to.
If you overshoot, stop and ask an engineer rather than draining water from the boiler yourself. Never remove the boiler casing. Topping up once after bleeding radiators is normal. If you find yourself repeating these steps every few weeks, the system is losing water, and each refill is a sign to find the leak rather than a fix.
Frequently asked questions
Why does my boiler keep losing pressure when there is no leak?
There is almost always a leak, it just is not visible. The usual causes are a relief valve passing water to the outside, a failed expansion vessel making the relief valve discharge, or a small leak under a floor or behind a wall where the water soaks away or evaporates.
How often is it normal to top up boiler pressure?
A healthy sealed system should hold its pressure for months. A top-up after bleeding radiators is normal. Needing one every week or two is not, and means water is leaving the system.
Is a boiler losing pressure dangerous?
Modern boilers lock out when pressure is too low, so the usual result is no heating or hot water. The bigger risks are water damage from a hidden leak and corrosion from repeated top-ups. If you see water inside the boiler casing, switch the boiler off and call a Gas Safe engineer.
Why does my boiler pressure rise when the heating is on and then drop?
A small rise when hot is normal. A large rise followed by a fall below the starting figure usually means the expansion vessel has lost its charge and the relief valve is releasing water at the top of each cycle.
Will a leak detection survey fix the leak?
The survey finds and documents the leak. With the position marked, many repairs can be made through one small opening, and the report supports a trace and access claim if your policy includes that cover.
