Boiler Losing Pressure:Common Causes and How to Fix It
| DIRECT ANSWER BLOCK | A boiler losing pressure has lost water, through a leak in the system, a radiator bled recently, or a failed part inside the boiler. How fast pressure drops tells you which. A small drop over several months is normal settling. A drop every few days points to a leak worth tracing. |
Most homeowners keep topping up and hope the problem settles. Below is how to read the rate of loss, find a leak yourself, and judge when a repair makes more sense than another top up.
How fast is your boiler losing pressure?
Rate of loss is the most useful clue you have, and the one nobody asks about.
| How often you top up | What the rate means | What to do |
| Once a year or less | Normal. Sealed systems lose a trace of water over time. | Nothing. Your engineer tops up at the annual service. |
| Every few months | Minor. Often follows radiator bleeding or recent plumbing work. | Top up and note the date. Watch whether the gap shortens. |
| Every few weeks | A slow leak or an expansion vessel losing its charge. | Check joints and valves for damp. Book a diagnostic if nothing shows. |
| Every week | A real leak. Losing 0.1 bar a week adds up to half a bar in six weeks, enough to reach lockout within a season. | Book a repair. Topping up is masking the fault. |
| Every day or two | A significant leak, or a failed internal part. | Switch off and call out. Daily water loss causes damage somewhere. |
Can a boiler lose pressure without a leak?
Yes, in two situations. A failed expansion vessel cannot absorb expanding water, so pressure swings hard between hot and cold and the gauge reads low once the system cools. And a pressure relief valve passing water lets pressure out through the discharge pipe outside, which looks like a leak with nothing wet indoors. Both are engineer repairs and neither shows a damp patch in the house. If water is appearing inside, read boiler leaking water instead.
Everything else involves water leaving the system somewhere, even if the escape point stays hidden under a floor.
How to find a hidden leak yourself
1. Check every radiator valve, top and bottom, for damp or a small white crust where water has dried.
2. Run a dry tissue around pipe joints. Condensation wets the tissue evenly, a leak leaves a wet point.
3. Look at skirting boards and floorboards near pipework for discolouration, swelling or lifted paint.
4. Lift any loose floorboard over a known pipe run and feel underneath.
5. Check the ceiling below first floor pipework for a faint brown ring.
6. Look at the discharge pipe outside, usually copper and near the boiler. Water there means a pressure fault, not a leak.
If nothing shows after these six checks, the leak sits under a solid floor or inside the boiler. Both need tracing equipment rather than guesswork, and our boiler breakdown and repair team carries leak detection kit for exactly this.
The common causes, most likely first
A radiator bled recently. Bleeding releases water as well as air. One bled radiator can drop pressure by 0.3 bar. Top up and move on.
Weeping radiator valves. Valve glands dry out and seep slowly. Often the easiest fix on the list, sometimes just a tightened gland nut.
Corroded pipework. Pinholes form in older copper, particularly where pipes pass through screed or sit against masonry. Slow, hidden, and the most common cause of a leak nobody can find.
A failed expansion vessel. Loses its gas charge, so pressure cannot hold between cold and hot. Recharge or replacement, usually under an hour.
A passing pressure relief valve. Lets water out continuously through the outside pipe. Cheap part, straightforward swap.
Auto air vent failure. A sticking vent inside the boiler releases water along with air.
Heat exchanger failure. The expensive one. Water escapes inside the casing under pressure, and the repair sometimes costs more than an older boiler justifies.
Why topping up repeatedly makes things worse
Each top up draws fresh mains water into a sealed system, and fresh water carries dissolved oxygen. Oxygen plus steel radiators produces iron oxide, the black sludge that settles in radiator bottoms and blocks flow. A system topped up weekly for a year has taken in a lot of oxygen, so a leak left alone eventually becomes a leak plus cold radiators plus a struggling pump. Fixing the leak early avoids the second problem entirely.
Is it safe to leave a boiler on low pressure?
Safe, but not sensible. Modern boilers lock out below roughly 0.5 bar rather than run in a damaging state, so you lose heating and hot water rather than risk the appliance. Running at the bottom of the range makes the pump work harder and leaves no margin before the next lockout. Restore pressure to the normal range, then deal with the cause.
How to stop pressure loss returning
Once the leak is fixed, ask for corrosion inhibitor to be added to the system. Inhibitor slows the reaction between water and metal and should be topped up whenever the system is drained. A magnetic filter on the return pipe catches circulating debris before it settles. Both get checked as part of an annual boiler service, along with the gauge, the vessel charge and the relief valve.
Frequently asked questions
Is it normal for a boiler to lose 0.1 bar a week?
No. A steady weekly loss adds up to half a bar in six weeks and points to a slow leak. Losing a trace over a whole year is normal.
Why does my boiler need topping up every day?
Daily topping up means a significant leak or a failed internal part. Switch the boiler off and book a repair rather than keep refilling.
Can a boiler lose pressure without a leak?
Yes. A failed expansion vessel or a passing pressure relief valve both drop pressure with nothing wet inside the house.
How much does it cost to fix a boiler losing pressure?
A tightened valve gland costs a short visit. A vessel or relief valve needs a part plus labour. A hidden leak under a solid floor costs more because of the tracing work. We quote once we know the cause.
Will a heating leak show up on my water meter?
Usually not. A sealed heating system holds a small volume and leaks slowly, so the loss rarely registers against household use.
Does low boiler pressure stop the radiators heating?
Below about 0.5 bar the boiler locks out and nothing heats. Between 0.5 and 1 bar, radiators often heat unevenly as the pump struggles.