Why Your Boiler Keeps Losing Pressure (And the Fixes That Finally Worked for Me)

A boiler losing pressure is one of those things that creeps up quietly. One day everything is fine, and the next you’re looking at the gauge wondering why it’s slipped down again. At first you top it back up and think nothing of it. Then it drops again a few days later. After that, you start getting into the habit of checking it more often than you check your emails.

If your boiler keeps falling below 1 bar, there’s always a reason behind it. It doesn’t happen randomly, and once you understand the different causes, it becomes much easier to narrow down what’s going on. When it happened to me, it took a while to work out the pattern, but once I did, the fix was surprisingly straightforward.

Here’s how boiler pressure actually behaves, what makes it drop, and how I eventually stopped mine from losing pressure all the time.

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Understanding boiler pressure in the simplest way

The pressure in your heating system is just the force that pushes hot water through the pipes and radiators. If the pressure is too low, the boiler can’t circulate water properly, so radiators take ages to heat or don’t heat at all. If the pressure is much too high, the boiler automatically releases water to protect itself, which then causes the opposite problem — low pressure again.

Most boilers are happiest between 1.2 and 1.5 bar when the system is off, and usually rise a little when the heating is running.

If the gauge drops below 1 bar and does it repeatedly, something in the system is allowing water to escape, or something inside the boiler isn’t controlling pressure correctly.

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Where I started: the simple things

When mine began losing pressure, the first thing I did was look for obvious signs of leaks. I checked around radiators, valves, under sinks, and anywhere the boiler pipes were visible. Most leaks aren’t dramatic, and many never leave puddles. A small damp patch, a bit of white limescale around a joint, or a slight oily feeling on a valve can all indicate slow water loss.

In my house, I didn’t see anything clearly leaking at first. Everything looked dry. That’s why pressure loss can be confusing — most of the time the leak is tiny and evaporates before you ever see it.

The next thing I did was think about whether the problem started after bleeding radiators. Bleeding them releases trapped air, but it also releases pressure. If you bleed several radiators in a short space of time, the pressure will definitely drop. Sometimes it’s nothing more than that. But if you top the boiler up and it drops again without touching radiators, then something else is going on.

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Radiator valves and slow leaks

Eventually, I noticed a small crusty deposit on one radiator valve upstairs. It wasn’t dripping, and it didn’t look dramatic, but it was there. These tiny leaks don’t run down the wall or cause damp. They just let out a small amount of water over time, and that’s enough to drag the pressure down day after day.

If your boiler drops pressure every few days rather than every few hours, a small leak like this is often the cause. I had the valve tightened and resealed, and that alone made a noticeable difference — the pressure held for longer, but it still wasn’t perfect.

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The outside pipe told me more than the radiators did

If your boiler has ever discharged water, it will do so through a copper pipe that leads outside. It’s usually right by the boiler wall. This pipe is connected to the pressure relief valve, which is designed to release water if the system becomes over-pressurised.

If this valve wears out, it may drip even when the pressure isn’t too high.

I checked mine and noticed that after the heating turned off, there was a slight bit of moisture around the pipe. Nothing dramatic, but enough to make me realise the valve might not be sealing fully.

This type of issue can cause pressure to drop steadily, especially overnight when the system cools down.

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The expansion vessel issue

The final piece of the puzzle was the expansion vessel inside the boiler. This part absorbs the extra pressure when water heats up and expands. If the vessel loses its air charge, the system has nowhere to store expansion. Pressure spikes, the relief valve lets water out, and then when the system cools, the pressure ends up too low.

This is exactly what mine was doing. Pressure would climb too high when the heating was on, then sink too low when the heating turned off.

Once the expansion vessel was recharged properly, the pressure stopped swinging so dramatically.

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Putting it together: why mine actually dropped pressure

In the end, the problem wasn’t one big issue — it was a combination of small ones:

• A radiator valve upstairs letting out a tiny amount of water
• An expansion vessel that wasn’t holding charge properly
• A pressure relief valve that occasionally released water

None of these issues were serious on their own, but together they were enough to make the system lose pressure almost daily.

Once the valve was sealed, the vessel re-pressurised, and the relief valve checked, the boiler held pressure exactly where it should be. No constant topping up. No waking up to cold radiators. No worrying about the gauge every time I walked past.

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If you’re trying to work out your own pressure drop, here’s the pattern I learned

Pressure loss behaves differently depending on what’s causing it:

• Drops slowly over several days: usually a small leak at a valve, radiator joint, or under flooring
• Drops after every heating cycle: often linked to the expansion vessel or pressure relief valve
• Drops immediately after bleeding radiators: the pressure loss is simply from bleeding
• Drops at random times of the day: could be the filling loop letting water out or a faulty valve
• Drops and heating becomes uneven: air entering the system or a circulation issue

Just watching how the gauge behaves over a day or two can give you far more information than you’d expect.

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What topping up too often actually does

A mistake I made early on was topping up the pressure every day. I assumed it was harmless. What I didn’t realise is that every time you add fresh water, you introduce oxygen into the system. Oxygen speeds up corrosion inside radiators and pipework. More corrosion means more air and eventually more leaks.

So while topping up seems like the quick fix, it actually makes the system deteriorate faster if you’re doing it constantly.

Once the real issue is sorted, you shouldn’t need to top up more than once every few months.

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When it’s worth calling an engineer

Not every pressure drop needs a professional, but there are clear signs that it’s time to get someone out:

• Pressure rises too high when heating is on, then drops too low when it switches off
• You find damp patches under floorboards or carpets
• The relief pipe outside is regularly dripping
• You hear unusual noises from the boiler
• Pressure drops within hours instead of days
• You’ve topped up the boiler more than twice in a week

Most fixes are straightforward and fairly inexpensive. The important thing is catching the pattern early so the system doesn’t develop a larger issue in the long term.