Hi new to site wondered if anyone can help? I have a microgenus combi boiler that loses pressure at a very rapid rate (less than 2 weeks) I've checked all the pipe work I can including outside overflow from boiler and cant see any leaks, I have two radiators upstairs that seem to fill with air before any of the others, they are I believe at the end of the run from the boiler, everything else works fine, i top up pressure and all is ok again but it seems scary how much water sounds like is going in (where is it disappearing to?
Central heating pressure drop
- Thread starter DLT
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Hi, when I refill the system it takes about 10 seconds to get the needle to the top end of the blue section which is about 1.5 bar, depending on how empty it is, the pressure increases by just less the 1 bar when the heating fires up
I think you misunderstood twgas question.
Does pressure rise quickly to 3bar after you've topped it up?
The volume of water expands when heated, that extra water has to go somewhere. The expansion vessel is there for that purpose. The vessel has a membrane (diaphragm) and when vessel is pre charged it holds no water (or very little). The extra water pushes against the membrane and compresses the air in the vessel.
If the vessel has lost some or all of its air charge, there will be no air to compress, or there will be a quick rise in pressure when you put the heating on.
Of course the pressure won't rise beyond 3bar because the Pressure Relief Valve opens at 3 bar and spills out the excess water.
You may like to try securing a plastic bag on the PRV outlet pipe to see if the PRV is opening.
In fact I understand once a PRV has opened it may not give a good seal when closed.
So PRV maybe letting water pass. Expansion vessel may have lost some or all of its air charge.
Or the membrane in the PRV could be ruptured, allowing water to pass from the water side to the air side
Does pressure rise quickly to 3bar after you've topped it up?
The volume of water expands when heated, that extra water has to go somewhere. The expansion vessel is there for that purpose. The vessel has a membrane (diaphragm) and when vessel is pre charged it holds no water (or very little). The extra water pushes against the membrane and compresses the air in the vessel.
If the vessel has lost some or all of its air charge, there will be no air to compress, or there will be a quick rise in pressure when you put the heating on.
Of course the pressure won't rise beyond 3bar because the Pressure Relief Valve opens at 3 bar and spills out the excess water.
You may like to try securing a plastic bag on the PRV outlet pipe to see if the PRV is opening.
In fact I understand once a PRV has opened it may not give a good seal when closed.
So PRV maybe letting water pass. Expansion vessel may have lost some or all of its air charge.
Or the membrane in the PRV could be ruptured, allowing water to pass from the water side to the air side
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As I open both the taps to fill the system to get some pressure the pressure increases steadily as the water is entering the system, then when I have a suitable reading and I turn the taps off the pressure remains the same
Yes the pressure after you have filled it should remain as set. That is until you switch the heating on. It will then rise about 1 bar as the water expands and when heating switches off it should return to your set figure.
As explained the extra water now goes into the expansion vessel and compresses the air which increases the pressure.
As explained the extra water now goes into the expansion vessel and compresses the air which increases the pressure.
So, as it looks like my system is working as it should, why does it lose pressure so quickly without any visible signs of a leak?
Could be the PRV is not making a good enough seal and letting some water pass out.
It would pay to secure a plastic bag on the outlet pipe and see what it collects.
It would pay to secure a plastic bag on the outlet pipe and see what it collects.
I have a Ravenheat Combi boiler and have been losing pressure - currently drops to bout 0.5. Strangely at this level I can still get hot water and when turn central heating on it goes up to 3 bar and gradually comes down when heating is goes off. I can repressure it but drops down again over some hours. Ravenheat say that if you don't see any water dripping from boiler then there must be a leak elsewhere. Is it true that only a leak can cause water pressure to drop?
Yes! only a leak can cause pressure to drop. But don't forget initially you have a expansion vessel that is full of air. The water is separated from the air by a diaphragm and when the water expands the excess volume enters the expansion vessel. This compresses the air and increases the pressure.
It's important that the vessel is adequate size for the volume of water in the system. What happens is the vessel loses a little of its pressure, (perhaps through the shader valve) and so does the water. So you top it up, but you're no longer starting with a vessel full of air, it now contains some water and less air capacity.
After this process has been carried out a number of times, the capacity of air becomes so small that the pressure rises well above that set on the pressure relief valve, so the PRV opens and discharges some water out of the system. The pressure drops to almost nothing, so it gets topped up again and the whole process is repeated.
The pressure is expected to rise about 1 bar so if it rises say 2 bar, that tells you the air capacity of the vessel must be reduced.
This means the vessel needs recharging and the excess water has got to be removed from the vessel.
Unfortunatley PRVs once opened don't always re-seat properly due to dirt on the valve seat and you can get water escaping at a slow rate.
Any leakage at valves or joints, or even bleeding a radiator will cause the pressure to drop.
It's important that the vessel is adequate size for the volume of water in the system. What happens is the vessel loses a little of its pressure, (perhaps through the shader valve) and so does the water. So you top it up, but you're no longer starting with a vessel full of air, it now contains some water and less air capacity.
After this process has been carried out a number of times, the capacity of air becomes so small that the pressure rises well above that set on the pressure relief valve, so the PRV opens and discharges some water out of the system. The pressure drops to almost nothing, so it gets topped up again and the whole process is repeated.
The pressure is expected to rise about 1 bar so if it rises say 2 bar, that tells you the air capacity of the vessel must be reduced.
This means the vessel needs recharging and the excess water has got to be removed from the vessel.
Unfortunatley PRVs once opened don't always re-seat properly due to dirt on the valve seat and you can get water escaping at a slow rate.
Any leakage at valves or joints, or even bleeding a radiator will cause the pressure to drop.
Thanks for this. I am a novice (could use the word thick) to all this so does your answer mean that the issue you raise about expansion vessels and PRV's can be the reason for the pressure drop and can be fixed i.e. if that is the problem is that classed as a 'leak' and therefore does not mean I have a leak in joints etc.
There are many who have similar problems and very few find leaks at valves or pipe work. Mostly they are due to the expansion vessel not being checked and corrected at regular intervals.
You've already stated the pressure goes to 3 bar from 0.5 bar. That indicates there is insuffient air capacity in the vessel.
It needs the excess water removing and pumping up to about 0.9 bar.
Pumping it up to 0.9 bar will not cure it if the excess water remains.
You've already stated the pressure goes to 3 bar from 0.5 bar. That indicates there is insuffient air capacity in the vessel.
It needs the excess water removing and pumping up to about 0.9 bar.
Pumping it up to 0.9 bar will not cure it if the excess water remains.
Just had an engineer visit and it was the expansion vessel causing the problem which is exactly what you alluded to. Thanks.
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