Breakdowns and leaks
Boiler Losing Pressure? Causes, Checks and When to Call
A boiler that keeps losing pressure has a short list of usual causes, from a small pinhole leak to a faulty pressure relief valve, and the first checks are ones you can make yourself. Here is what the gauge should read, why the pressure drops, the checks to make in order, how to repressurise a boiler and when to call a heating engineer.
Normal boiler pressure readings
Read the pressure gauge on your boiler twice, once with the heating off and the radiators cold and once with the radiators hot, because a normal reading is higher when the heating is running. These are the readings in Checkatrade’s boiler pressure advice.
| Gauge reading | What it means |
|---|---|
| Just above 1 bar, with the heating off and the radiators cold | Normal for a boiler sitting idle |
| Around 2 bar, with the radiators hot | Normal for a boiler that is running |
| 1 to 2.5 bar | The normal working range |
| Below 1 bar | Generally not enough pressure to circulate hot water |
A reading that drops once, straight after you have bled the radiators, has an obvious cause. A reading that keeps falling after you top it up points to a fault in the heating system or the boiler.
Why your boiler is losing pressure
Each cause below leaves its own sign, so start from what you can find.
| What you find | Likely cause |
|---|---|
| Damp under a radiator, on pipework or around the boiler, and a gradual drop | A leak, such as a small pinhole leak |
| A drop straight after you bled several radiators in one session | Bleeding the radiators |
| A drip from the safety discharge pipe outside, near a combi boiler | A faulty pressure relief valve |
| An error code on a gas boiler’s display | A faulty part or safety device |
Combi boiler losing pressure
On a combi boiler, look outside as well as in. A drip from the safety discharge pipe near the boiler points to a faulty pressure relief valve, which debris or scale can cause, and fixing the valve on a gas combi is a job for a Gas Safe registered engineer.
Boiler losing pressure but no leak
When the radiators, the pipework you can see and the boiler are all dry, and you have not just bled the radiators, the likely cause is a faulty part such as the pressure gauge or the pressure relief valve. Finding which one has failed is a job for a heating engineer.
What to check when the pressure drops
A small pinhole leak shows as damp and a slow fall in pressure. If a leak has turned into water pouring or spraying from a pipe, the boiler pressure can wait until you have turned the water off at the stopcock.
Otherwise, work through these checks in order, starting at the gauge. Write down what you find, because it tells a heating engineer where to start if you end up calling one.
Checks to make when your boiler loses pressure
- The gauge reading written down, with the heating off and the radiators cold
- The floor under every radiator checked for damp patches
- The pipework you can see, and the boiler itself, checked for damp
- The safety discharge pipe outside, near the boiler, checked for drips
- The boiler display checked for an error code, and any code written down
- The pressure topped back up, following the manual for your boiler model
How to repressurise a boiler
Most boilers can be repressurised by the owner. Follow the instructions for your exact model, in its manual, rather than steps written for a different boiler.
When you have finished, read the gauge with the radiators cold. A normal cold reading is just above 1 bar, and if the pressure falls again after a top-up, a heating engineer needs to find the cause.
Do not keep topping up a boiler that keeps losing pressure
A top-up puts the reading back without fixing anything. The leak, the faulty relief valve or the faulty part is still there, the pressure drops below 1 bar again, and below 1 bar there is generally not enough pressure to circulate hot water.
When to call a heating engineer
Call a heating engineer when the pressure keeps dropping after a top-up, or when your checks point to a faulty part. On a gas boiler, HSE’s gas safety guidance for home owners says anyone you employ to work on it must be a Gas Safe registered engineer, competent in that area of gas work. Each engineer’s competencies are marked on the back of their Gas Safe Register ID card, so turn the card over before the work starts.
Among the Nottingham plumbing and heating firms ranked on their registrations and guarantees, Warm For Life in Beeston takes 24/7 emergency boiler repairs, Nottingham Gas & Plumbing in Forest Fields is Gas Safe registered and has emergency plumbers on call 24/7, and M & S Plumbing & Heating in Gedling takes 24/7 emergency call-outs.
When you call, pass on what your checks found. The boiler type, the gauge reading, where any damp is, whether the discharge pipe drips and any error code tell the engineer what they are coming out to.
Describing lost pressure to a heating engineer
Do this
Gas combi. The pressure drops below 1 bar within 2 days of each top-up. No damp under the radiators, on the pipes or around the boiler, but the discharge pipe outside is dripping. No error code on the display.
Not this
My boiler keeps losing pressure. How soon can you come out?
What it costs to fix a boiler losing pressure
Checkatrade’s example prices for emergency jobs are £182 for loss of boiler pressure, where the job is repressurising the boiler with a filling loop, and £275 for a boiler breakdown fixed with a simple part repair such as a new electrode.
When a part has failed, Checkatrade’s boiler repair figures put a boiler repair at £120 to £750, with an average of £300, and an emergency repair at £225 to £675, with an average of £450, all excluding VAT. Which part has failed, and whether the job is an emergency, decides where the repair bill lands.
A gas engineer charges £50 to £80 an hour plus parts, an average of £65, excluding VAT, so a fault that takes one visit to find and a second to fix is charged for both.
A pressure fault found on one visit and fixed on a second
| First visit to find the fault, 2 hours at £50 to £80 an hour | £100 to £160 |
|---|---|
| Second visit to fit the new part, 1 hour at £50 to £80 an hour | £50 to £80 |
Total £150 to £240
Prices exclude VAT, and the part itself is extra. Before the first visit, ask whether the engineer charges by the hour or a fixed price for the repair.
An older boiler that keeps losing pressure may be more cost-effective to replace than to keep repairing, so get the repair priced before you agree to it.
Common questions
What should my boiler pressure be?
Your boiler pressure should read just above 1 bar with the heating off and the radiators cold, and around 2 bar with the radiators hot. The normal working range is 1 to 2.5 bar, and below 1 bar there is generally not enough pressure to circulate hot water.
Why is my boiler losing pressure but not leaking?
With no leak anywhere, the likely cause is a faulty part such as the pressure gauge or the pressure relief valve. On a combi, a drip from the safety discharge pipe outside, near the boiler, points to the relief valve. Finding and fixing the faulty part is a job for a heating engineer, and on a gas boiler for a Gas Safe registered engineer.
Can I repressurise my boiler myself?
Yes, most boilers can be repressurised by the owner, following the instructions in the manual for that model. If the pressure keeps dropping after you top it up, a heating engineer needs to find the cause, and on a gas boiler that engineer must be Gas Safe registered.
Why does my combi boiler keep losing pressure?
The likely causes are a leak, bleeding several radiators in one session, or a faulty pressure relief valve, which debris or scale can cause. A drip from the safety discharge pipe outside, near the boiler, points to the relief valve, and an error code on a gas combi points to a faulty part or safety device.
Does bleeding radiators lower boiler pressure?
Yes, bleeding several radiators in one session can make a boiler lose pressure. Read the gauge when you have finished, and if it shows below 1 bar, repressurise the boiler following the instructions for your model.
How much does it cost to fix a boiler that is losing pressure?
A boiler repair costs £120 to £750 excluding VAT on Checkatrade's figures, and its example price for an emergency job to repressurise a boiler is £182. An emergency boiler repair costs £225 to £675, and a gas engineer charges £50 to £80 an hour plus parts, both excluding VAT.
Where the figures come from
- Checkatrade, what to do if your boiler loses pressure read 2026-09-16
- Checkatrade, plumber call-out fee cost guide read 2026-09-16
- Checkatrade, boiler repair costs guide read 2026-09-16
- HSE, gas safety for home owners read 2026-09-16
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