Why Your Boiler Keeps Losing Pressure (And What To Actually Do About It)
There’s a particular frustration to checking your boiler and seeing the pressure gauge sitting somewhere it shouldn’t be. Not dramatic enough to feel like an emergency, just low enough to make you wonder how long it’s been like that, and whether this is the start of something expensive.
The short answer: pressure loss is never random. Water doesn’t disappear. It either leaks out somewhere, or the boiler dumps it through a safety valve. That’s it. Once you understand that, the troubleshooting becomes a lot more logical.
Start With the Gauge But Don’t Trust It Blindly
Most combi boilers sit around 1.0 to 1.5 bar when cold. That’s your baseline. If you’re looking at 0.5 bar or less, the system is losing pressure in a meaningful way.
Before doing anything else, take a photo of the gauge and check it again in a few hours. A slow, steady drop points to a genuine leak. A gauge that jumps erratically may just be faulty, and chasing a pressure problem that’s actually a broken gauge is a good way to waste an afternoon.
It’s worth knowing this before you start pulling radiators apart.
Look for Leaks First The Obvious Ones
Get down low and actually look. Damp patches under radiators, drips from valves, staining on the wall near pipework, these are easy to miss with a quick glance from standing height. A pinhole leak spraying fine mist behind a radiator can stay invisible until you get a torch in there at the right angle.
Check under the boiler too. Internal drips can land on components and evaporate before they ever reach the floor, but you’ll often see white mineral staining or faint corrosion marks that give the game away.
If you find moisture anywhere, that’s your answer. Don’t move on until you’ve ruled it out.
The Filling Loop Gets Overlooked More Than It Should
The filling loop that braided hose (or built-in valve system) used to top up the pressure is a surprisingly common culprit.
If someone topped up the boiler recently and left a valve slightly open, the system can overfill. When it heats up, pressure climbs too high, the pressure relief valve kicks in and dumps water outside, then pressure drops again. From the outside it looks like the boiler is losing pressure for no reason. Really, it’s just doing exactly what it’s designed to do.
Check that both filling loop valves are completely closed.
Check the Discharge Pipe Outside
Most boilers have a copper pipe that exits through an external wall, usually pointing down near ground level. This is the pressure relief discharge pipe, and it’s worth checking.
If it’s dripping, or shows signs of previous dripping (staining, moss growth below it), then the boiler has been releasing pressure when it heats up. That means something is causing pressure to climb too high, most likely a failing expansion vessel or an overfilled system.
Water coming out of that pipe isn’t the problem. It’s a symptom. The problem is upstream.
Expansion Vessel Failure: The Pattern That Repeats
If your boiler pressure climbs high when the heating runs, approaching 3 bar then drops low again once it cools down, that’s a classic expansion vessel issue.
The expansion vessel absorbs the pressure increase as water heats and expands. When it fails, there’s nowhere for that extra pressure to go. The relief valve opens, water gets dumped, and the system ends up losing pressure by the end of the cycle. Then someone tops it up, and the whole thing repeats.
This one tends to sneak up gradually. Plenty of people top up every couple of days and assume that’s just how boilers work. It isn’t. And beyond the inconvenience, constantly introducing fresh water into the system accelerates internal corrosion, which creates a different set of problems down the line. Consider upgrading your expansion tank to help with this issue.
Hidden Leaks Are the Worst Case, But They Happen
If you’ve checked everything visible and still can’t account for where the pressure is going, it might be under the floor or behind a wall.
Signs worth paying attention to: a section of flooring that feels warmer than it should, a persistent damp smell in one room, or pressure that just keeps dropping slowly over days with no obvious source. Sometimes the only visible clue is a slightly warped floorboard above a weeping joint.
It’s annoying. But a slow, hidden leak left long enough causes far more damage than the repair itself.
After Bleeding Radiators
One quick note: if you’ve recently bled radiators, that’s almost certainly why the pressure dropped. Bleeding removes air, which gets replaced by water, which lowers the pressure reading. Top it back up to the correct range and watch it over a few days. If it holds, you’re done.
If it keeps dropping after that, the bleeding wasn’t the cause, it just coincided with the timing.
FAQ
How often is it normal to top up a boiler?
Once every few months after bleeding radiators fine. Once a week, not fine. Frequent topping up is always a sign something is wrong.
Can a faulty pressure gauge cause a false reading?
Yes, more often than people realise. If the reading seems inconsistent or the gauge needle behaves erratically, the gauge itself may need replacing before you diagnose anything else.
Is it safe to run the boiler while it’s losing pressure?
Most boilers will lock out automatically below a certain pressure (usually around 0.5–0.8 bar), which is a built-in protection. Running it briefly isn’t catastrophic, but it shouldn’t be left in that state — especially if the cause is unknown.
What’s the difference between the pressure relief valve and the filling loop?
The filling loop is how you add pressure manually. The pressure relief valve is a safety device that releases pressure automatically when it gets too high. They do opposite jobs, and confusion between the two is common.
Why does my pressure only drop when the heating is on?
That points toward the expansion vessel. When everything is working correctly, pressure rises slightly during heating but stays within normal range. If it spikes hard then crashes, the vessel has likely lost its charge.
If you’ve been topping up regularly and hoping it stabilises, it probably won’t. Track the pressure over a few days, note when it drops and by how much, and give that information to our tech . A clear pattern is faster to diagnose than a vague complaint.
