I have a flat in Scotland, UK. We live on the 2nd floor. We have been here for about 18 months and we have a (new-ish) Combi boiler holding a static pressure measurement of around 1.6 bar.

Boiler water pressure indicator increases by about 0.2 bar when central heating is on.

First Notice:

We usually have no need to check the pressure and we found to our surprise a few days ago that the hot water (washing) was not heating, checking the boiler it had a stated pressure of only 0.3 bar, and so the boiler was not firing.

The water flowed out the taps are at a suitable speed.

I hooked up the repressuring inlet tube and raised the boiler pressure to 1.9bar. This is the first time we needed to do this in 18 months.

Ongoing Concern:

From that day, I have been keeping an eye on the stated pressure of the boiler -- and the pressure is decreasing by about 0.2bar a day, right now it is 1.1bar . There has been a little central heating usage in the last few days (1-2 hours only) but I'm curious as to why the pressure seems to be dropping so persistently and if this cause would bring it down to under 0.5bar as before.


  • We live in Scotland. The water does not contain limescale.
  • I have extensively checked all visible pipes and no indication of any leak was found.
  • Tap pressure hot/cold are roughly equal, cold tap pressure is not an issue.
  • We have a new (18 months ago) Combi gas fired boiler fueling hot water and central heating.
  • My only point of pressure measurement is the boiler display which is dial rather than digital.
  • Radiator heating circuit is set to be turned on manually so we never have the heating on without doing so ourselves.
  • Boiler is "Logic Combi ESP1 30". Boiler was installed new at property purchase.

Other Q&A on here:

I have read Low water pressure which sounds similar but the question deals with a lot of (technical?) stuff I don't recognise (their pressure guage for instance). I also viewed and a few similar posts but which seem to deal with fast/sudden pressure drops rather than gradual persistent pressure drops over time.


It feels like something's changed because previously this has never been a notable issue and certainly pressure has never fallen below 1bar in the last 18 months.

What else could/should I be looking at/for in order to try and establish the cause of this?

How can I fix this if needsbe?

  • 2
    I'm no expert on boilers, but what did you use to raise the boiler pressure back to 1.9 bar? If it was air, I'd suspect that you have an air leak, not a water leak. Also, you may want to edit in the brand/model # of the boiler, as that may provide additional insight for someone (who isn't me).
    – FreeMan
    Commented Dec 6, 2022 at 14:08
  • @FreeMan there is a system where the internal cold pipe is manually diverted into the boiler to force repressuring -- this is per the guide given on the boiler manual. I have done similar things on previous boilers and this in itself was fine.
    – Martin
    Commented Dec 6, 2022 at 14:14
  • 2
    You are adding pressure to the water that flows though the radiators. There is obviously a very small leak somewhere associated with a radiator or the pipes to the radiator. There could also be an internal leak in the boiler. It is nothing to do with the mains water pressure.
    – HandyHowie
    Commented Dec 6, 2022 at 14:18
  • 2
    The radiator/pipes could still be leaking even without the heating being on. It is obviously leaking somewhere, since the pressure is dropping.
    – HandyHowie
    Commented Dec 6, 2022 at 14:24
  • 1
    @ArtemPtushkin the issue may have been the expansion tank as after another round of some careful exploration I'm confident the pipes are not breached.
    – Martin
    Commented Jan 22, 2023 at 13:03

2 Answers 2


If the "sealed loop" portion of a hydronic boiler system is losing pressure, there's a leak. There's typically a small variance between cold and hot, (so it goes up a bit as it heats and drops a bit as it cools) but any drop in pressure at the same temperature is an indication of a leak.

If the pressure loss is slow, the leak is small/slow.

The leak can be located anywhere on the sealed loop. Hydronic radiators go "on" and "off" by whether a pump is running, and perhaps also a single valve on the radiator itself blocking or allowing flow, but they are connected the loop at all times unless there are valves completely closing off both supply and return paths, which would be unusual. So a leak can be in that portion without running the heat.

I would hope that at merely 18 months of age since newly installed your heating system supplier/installer would sort this out without additional charge, if its a defect in their work or the boiler itself.

  • 1
    On the one hand, thanks for this clarification -- I didnt realise that the water loop remained connected when turned off (I assumed a valve or similar). On the other hand. "Oh shit I've got a leak, somewhere". 😆😒
    – Martin
    Commented Dec 6, 2022 at 14:44
  • 1
    @Martin See additional edit. I do hope it applies. Ah. perhaps not - You've been there 18 months but the boiler predates that as "new-ish." Ugh.
    – Ecnerwal
    Commented Dec 6, 2022 at 14:49
  • Thanks. To be clear; if the pressure level on the boiler is decreasing but there's not a corresponding drop on cold pressure (even when it was 0.3bar the cold tap was working ok), I can assume that the leak is within/behind the boiler ring(s) rather than in any cold pipes? Cheers
    – Martin
    Commented Dec 6, 2022 at 14:56
  • And yes, the boiler was installed brand new just prior to our purchase of the property. However we do have the paperwork from them and the boiler will probably be under guarentee for 5years or so, I'm sure. 🤞
    – Martin
    Commented Dec 6, 2022 at 14:58
  • 1
    Right - you obviously have adequate supply pressure to the flat, so I would expect both cold water and hot water taps to run fine (since hot water for taps is not in the "sealed" portion of the boiler, even though it's heated by the boiler - though we have different boiler naming conventions to aid "separated by a common language." The portion of the boiler to which you have to add pressure due to the leak will be the closed loop, with the radiators (since the other part would just drip and be refilled by the mains with no loss in pressure.) Glad to hear it should be under guarantee after all.
    – Ecnerwal
    Commented Dec 6, 2022 at 15:02

Presumably your boiler has a bladder-type expansion tank. If so, the bladder may have failed. The gas pressure in the bladder would force bubbles into the hydronic system, where they would be accumulated and eliminated by an automatic air vent. This may be the cause of your slowly falling pressure.

If this is the case, your expansion tank will eventually lose all of its air and it will be completely full of water. The next time the heating system warms up after that, there will be a pressure spike because the warmed, expanded water will have no place to go, and (hopefully) your safety pressure relief valve will spill some water on the floor to reduce system pressure.

If the Schrader valve on your expansion tank points downward, release a tiny bit of air from it by pressing the center pin. If water comes shooting out instead of air, the tank is failed and it must be replaced.

  • Would I be able to qualify this by waiting for the pressure to fall to certain base level (eg 0.8bar)? The pressure originally fell to under 0.3 bar (minimum operating pressure) which feels like the bladdar wouldn't be big enough to reduce the pressure by this amount (1+ bar).
    – Martin
    Commented Dec 6, 2022 at 16:09
  • I have opened the boiler and found the valve you mention and this ejects high pressure air and no water. Thanks.
    – Martin
    Commented Dec 6, 2022 at 16:32
  • 3
    @Martin I was expecting an external expansion tank! See page 54 of the manual you referenced. It appears that the Schrader valve is on the side of the tank, so if the bladder is less than half flooded, you won't get water spraying out. It's worth checking anyway, and it looks like it's readily accessible by removing one external panel. Note that if the bladder in the expansion tank is completely flooded and all of your piping is copper (no PEX), the boiler pressure can go from 0 when cold to safety release when hot, i.e., 2+ bar.
    – MTA
    Commented Dec 6, 2022 at 16:34

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