I'm currently trying to deal with a sealed central heating system in which the pressure is dropping by 0.2–0.3 bar per day (this is obviously far too fast, because the intended pressure for this system is 1 bar). Although I intend to call in a plumber to fix the issue, it seems important to find out the location of the problem first, so that the plumber can access the appropriate place to fix it (e.g. if the problem is under the floor, there's a need to move floorboards to provide access).

One common cause of this appears to be a malfunction in the pressure release valve. However, on one occasion after a loss of pressure, the circulating pump in the system was running continuously and (based on the sound it was making) failing to pump water; this was almost certainly because the water level had fallen below the location of the pump. The pressure release valve is higher than the pump is; does that mean that the pressure release valve can't be responsible (on the basis that the pump should be underwater even if the pressure release valve were opened entirely), or is it possible that the water in the system could fail to reach the pump despite the only issue being with the pressure release valve?

The other possible issue is a leak, but I haven't been able to locate the leak, if indeed there is one. Is there any technique that can be used to determine where the leak is? One thing I tried was to leave the system off for several days and then attempt to bleed an upstairs radiator – on the basis that a leak below the radiator should make it impossible to bleed due to the lack of pressure – but air still seems to be coming out. Would that test automatically rule out a leak below the radiator, or might it still be bleedable even with a leak below it?

One piece of information would be helpful would be the approximate rate (in terms of volume/time) at which I can expect the water to be leaking out; this would aid both in identifying a leak (because it would predict what the leak would look like), and in determining how long to wait before testing to see if the radiators were still bleedable.

  • Where does the outlet of the pressure relief valve go? Is it piped straight into the drain? It shouldn't be, so that you can immediately see if there's any water coming out.
    – TooTea
    Mar 24, 2019 at 13:56
  • @TooTea: the valve I'm aware of is the one in the expansion vessel, whose outlet goes out of the wall of the house (but not into the main drains, it's a separate outlet). That should be set to over 2 bar, though (thus, if it's operating, it's faulty). I haven't seen any water go that way but it's quite hard to observe small amounts of water in that location. It's possible that there are also other pressure release valves that I'm unaware of.
    – ais523
    Mar 24, 2019 at 16:16
  • Followup: it was in fact a leak (an incredibly slow one, the lower the pressure the slower it became), we found the leak eventually (via guessing where it might be and placing paper underneath it to see if the paper became stained). That said, other people may well have the same problem but a different solution, so additional answers are still useful in case they help someone else.
    – ais523
    Sep 5, 2019 at 20:03

2 Answers 2


Check the expansion bladder has correct precharge pressure: with boiler barometer at 0bar air pressure inside the bladder should be 1 bar.
Also check the automatic de-aerator are working properly (maybe thosae are clogged by limestone or just failed): if those fail to discharge air outside air could build up inside the system and act, de facto, as if there is no water.


I assume that you have a hot water heating system. If that is the case, the water should never leak out from anywhere. As the water in the system changes temperature the system pressure will change; warmer water causes the water to expand and cooler water causes the water to contract. If you see water discharge from any device, pipe or fitting, or anywhere, that is a problem that needs addressed. If the pressure relief valve is leaking or releasing water then that item and the cause of the discharge need addressed. Anytime that you can purge air from the system the pressure will fall. My recommendation would be to call a reputable service company to have them address your concerns and problems.

  • I'm aware that water shouldn't be leaking from the system (it's a sealed hot water system, yes); the issue is trying to determine a) whether or not it is, b) if so, where, c) if not, why it's losing pressure (e.g. is the pressure release valve responsible?). (Note that the system is losing pressure even while turned off, so this is nothing to do with temperature.) This answer is basically a repeat of the question, rather than an actual answer to the question.
    – ais523
    Mar 24, 2019 at 11:31
  • If the relief valve is leaking water you should see the water on the floor. If the system is loosing pressure then you have a leak or it is flowing back into the supply line due to poor maintenance or a bad device. If everything is working correctly then you have to find the leak.
    – d.george
    Mar 24, 2019 at 11:50

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