Shaun Dunn f your boiler keeps losing pressure, water’s usually escaping from the sealed heating system somewhere. That could mean a small leak, a problem with the expansion vessel, a pressure relief valve that isn’t sealing properly, or a one-off drop after you’ve bled your radiators.
You don’t need to know your way around the inside of a boiler to start narrowing it down. There are a few safe checks you can make yourself, and the way the pressure moves tells you quite a lot on its own.
As a general guide, most boilers should sit somewhere around 1 to 1.5 bar when the heating’s cold, though your own manual should always come first. If you’re topping the pressure up every few weeks, or it won’t hold for more than a day or two, you may need a repair.
Key Takeaways
- The most common causes are a leak, a failed expansion vessel, a pressure relief valve that’s letting water escape, or a one-off drop after bleeding your radiators.
- A healthy system rises by around 0.3 to 0.5 bar as it heats up. If yours shoots above 2 bar when hot and drops below 1 bar again when cold, the expansion vessel is a likely suspect.
- An occasional top-up two or three times a year can be within normal seepage around seals and valves. Monthly or more often means water is leaving the system somewhere.
- You can safely check visible pipework, radiator valves and the pressure relief pipe outside, and you can repressurise and bleed the system yourself. Anything behind the boiler casing is Gas Safe work.
Why Your Boiler Keeps Losing Pressure
Most pressure-loss problems fall into a handful of familiar patterns, and you can check for several of them without taking anything apart or buying specialist equipment.
Leaks in the Heating System
A leak doesn’t have to mean water pouring across the floor. A radiator valve that’s slowly weeping, or a pipe joint hidden under a floorboard, can lose enough water to bring the pressure down without leaving an obvious puddle.
Start with the parts you can see:
- Check both valves on each radiator.
- Look where radiator pipes disappear into the floor.
- Check visible pipework underneath and around the boiler.
- Watch for damp patches, staining or dark marks around carpet edges.
- Look for green or white crusting around brass fittings and joints.
If you can’t see a leak, that doesn’t mean there isn’t one. Pipework runs beneath floorboards, behind kitchen units and through screed floors. If you’re in an Edinburgh flat or tenement, it’s common for a hidden leak to show up as a mark on your neighbour’s ceiling before you notice anything in your own property.
Tracing one can involve pressure testing sections of the circuit, and sometimes thermal imaging, before anything gets opened up. Check your home insurance before you pay out of pocket, as many policies include trace and access cover.
Expansion Vessel Problems
The expansion vessel helps your boiler deal with the fact that water expands as it heats up.
You don’t need to find or touch it yourself. What matters is what your pressure gauge is doing.
A healthy system rises by roughly 0.3 to 0.5 bar between cold and hot. If yours starts below 1 bar when cold, shoots past 2 bar once the heating’s running, then falls right back down again by morning, the expansion vessel is one of the first things we’d suspect.
If the vessel loses its air charge, it can’t absorb that extra pressure properly. Some can be recharged, while a failed internal diaphragm means the vessel needs replacing.
Pressure Relief Valve Problems
Your pressure relief valve protects the heating system if the pressure gets too high. On many domestic sealed systems it opens at around 3 bar and sends excess water outside through a discharge pipe.
Go outside and look for the small copper discharge pipe coming through the wall near the boiler, usually pointing downwards. Water dripping from it, or a green stain or limescale trail underneath, means the valve may be weeping.
Sometimes the valve has opened because another fault pushed the pressure too high, and then debris or scale stops it sealing properly again.
Air and Recently Bled Radiators
If you’ve just bled several radiators, a one-off pressure drop is expected. Bleeding releases trapped air from the sealed system, so the pressure falls afterwards, and doing four or five radiators can bring the gauge down by around 0.3 bar depending on the size of the system.
Top the pressure back up and see whether it holds. If it keeps dropping over the following days or weeks, the bleeding wasn’t the real cause.
Other Internal Boiler Leaks
Not every leak shows up around a radiator or a visible pipe.
A failed automatic air vent, worn seals or a corroded heat exchanger can all let water escape inside the boiler itself. These faults sit behind the casing, so they’re not something to investigate yourself.
A leaking heat exchanger can be a significant repair. On a boiler over ten years old, it’s worth weighing the repair cost against replacing the boiler rather than putting more money into an ageing appliance.
What Does the Way Your Pressure Drops Tell You?
The pattern can give you a useful clue before anyone starts investigating.
| What you notice | What it can point to |
|---|---|
| Pressure drops slowly over several months | A small amount of seepage can be normal. Keep an eye on it and check your manual for the correct cold pressure. |
| You’re topping up monthly or more often | Water is leaving the system somewhere. Check for visible leaks and the outside discharge pipe. |
| Pressure rises above 2 bar when hot, then falls below 1 bar when cold | The expansion vessel is a likely suspect. |
| The outside discharge pipe is dripping | The pressure relief valve may be leaking, or another fault may be pushing the pressure too high. |
| Pressure dropped after bleeding radiators | A one-off fall is normal. Repressurise and see whether it holds. |
| Problems start when the heating comes back on in autumn | An existing leak or fault may be showing itself once the system is heating and cooling every day. |
| You have an older system with a tank in the loft and no pressure gauge | You may have an open-vented system rather than the sealed type covered here. |
In Edinburgh flats and tenements, hidden pipework can make leaks harder to spot. If the pressure’s falling but everything around your boiler and radiators looks dry, don’t assume the water’s disappeared. It may just be escaping somewhere you can’t see.
How to Fix Boiler Pressure Loss Yourself
There are a few things you can safely do before calling us. Just remember that topping the pressure back up isn’t the same as fixing whatever caused it to fall.
Check for Visible Leaks
Have a look around the radiators, exposed pipework and underneath the boiler.
If you find water around a radiator valve or joint, don’t keep tightening it and hoping more force will solve the problem. A leaking compression fitting usually needs remaking properly, and over-tightening tends to make it worse.
If there’s active water coming from a radiator, close the valves at both ends to isolate it until it’s repaired.
Repressurise the Boiler
If the pressure’s low, check your boiler manual first.
Many boilers use a filling loop, usually a braided silver hose with a valve at each end connecting the mains water supply to the sealed heating circuit.
- Turn the boiler off and let it cool for at least 30 minutes.
- Open the filling-loop valves slowly and watch the pressure gauge as water enters the system.
- Close both valves fully as soon as the gauge reaches the correct cold pressure, usually around 1 to 1.5 bar.
- Don’t keep adding water once you’ve reached the target. It’s easy to overshoot.
Some boilers use a keyed internal filling device instead, and some models restrict repressurising to an engineer. If yours doesn’t match what you’re looking at, check the manual rather than guessing.
If you have a detachable filling loop, disconnect it once you’ve finished. Leaving it connected risks heating-system water flowing back into the mains supply.
Bleed Trapped Air
If a radiator’s cold at the top, or you can hear gurgling when the heating comes on, it may have trapped air inside.
- Switch the heating off and let the radiators cool completely.
- Use a radiator key on the bleed valve at the top corner and turn it about a quarter turn anticlockwise.
- Let the air hiss out and close the valve as soon as water appears.
- Work through the downstairs radiators first, then upstairs.
- Check your boiler pressure afterwards and repressurise if needed.
Check the Pressure Relief Pipe Outside
If there’s no obvious water indoors, have another look at the copper discharge pipe outside near the boiler. Dripping water or staining underneath it points to the pressure relief valve losing water. Checking it from outside is safe, but don’t try to repair or replace the valve yourself.
When to Stop and Call Us
If the pressure drops once, you top it up and it stays there, there may be nothing more to do.
If it keeps falling, repeated top-ups only mask the fault, and each one brings fresh, oxygenated mains water into the system, which can speed up corrosion over time. A regular boiler service is usually where a fault like this gets caught before it becomes a bigger job.
Give us a call if:
- The boiler won’t hold pressure for more than a day or two.
- You’re topping it up monthly or more often.
- The pressure shoots up when the heating comes on and drops again when it cools.
- Water’s dripping from the pressure relief pipe outside.
- You’ve found a leak at a radiator, valve or pipe joint.
- The pressure keeps falling but you can’t see where the water’s going.
- The fault appears to be inside the boiler.
If there’s an active leak you can’t stop, switch the boiler off. If it’s coming from a radiator, close both radiator valves. If you can’t isolate it, turn off the mains stopcock and switch the boiler off at the isolating switch while you wait for help.
Don’t open the boiler casing yourself. The expansion vessel, pressure relief valve, heat exchanger and other internal components are all Gas Safe work, and you can check that anyone working on your boiler is registered and qualified at gassaferegister.co.uk.
It’s also worth checking whether your boiler warranty or service plan already covers the repair before you book.
If your boiler keeps losing pressure in Edinburgh, we can trace what’s actually causing it rather than asking you to keep topping it up and hoping for the best. We’ll check the system, explain what we’ve found in plain English and give you a fixed price before we start the repair.
Book a Gas Safe engineer with SD Plumbing & Heating
Frequently Asked Questions
What Pressure Should My Boiler Be At?
Most sealed heating systems should read around 1 to 1.5 bar when the boiler’s cold and the heating’s off.
The exact figure varies between boilers, so always check your manual. Once the heating comes on, a healthy system usually rises by roughly 0.3 to 0.5 bar before settling back down as it cools.
How Often Should I Need to Repressurise My Boiler?
An occasional top-up two or three times a year can be within normal seepage through seals, valves and joints.
If you’re repressurising once a month or more, it’s worth getting the system checked.
Is Low Boiler Pressure Dangerous?
Low boiler pressure on its own isn’t normally dangerous.
Most modern condensing boilers have a low-pressure safety switch that shuts the boiler down below a set level, commonly around 0.5 bar. The bigger concern is what’s causing the loss, particularly a hidden leak soaking floors or plaster where you can’t see it.
Does Cold Weather Make Boiler Pressure Drop?
Cold weather doesn’t normally make a healthy sealed system lose pressure on its own.
What winter does is make an existing fault more obvious. Once you’re using the heating every day, repeated heating and cooling cycles can expose a small leak, an expansion vessel problem or another issue that went unnoticed over summer
