pressure problem

[george765]

I have a worcester R28 HE boiler that was installed in 2006. The problem I have is that the C/H system pressure gauge on the front of the boiler does not remain constant.
When the c/h is on the pressure comes up near to the top of the green section, but when the c/h is off the needle drops almost to the bottom of the gauge.
I have checked for leaks and there doesn't appear to be any.
Could someone please help by telling me what the problem could be.
Thanks in advance, george

 

[hazfiend]

I'm not an expert - don't rely on my opinion...

Depends on the model I suppose but mine has a gauge which goes from 0 through 1, 2, 3 and 4, BAR I think. It's supposed to range from 1 when not doing anything to 2 when fully hot and pressurised, which is what it does at the moment.

On mine, I have a filling tap below the boiler and I open this to raise the pressure to 1 if it has dropped below this, which hasn't actually been necessary so far, although I overfilled it trying to trouble shoot a different symptom. After that it was higher than 1 and went up to about 3 or so before, I suppose, venting the excess through a relief valve and returning to normal.

I'm assuming your system has leaked some fluid and therefore the pressure has dropped to the current low point and when heated up it is rising to "current + 1 bar" like it would be expected to if there wasn't anything else wrong. Where your filling tap might be I have no idea.

 

[MANDATE]

Water expands when heated so where does the extra volume go to.
On a vented system it is just pushed back to the tank.On a sealed system a expansion vessel is provided. The vessel incorporates a diaphragm so you have water on one side and air on the other side.
Think of the diaphragm as a balloon, so to ensure the vessel is empty of water the balloon is inflated to approx0.8bar.
Mains water is put into the system and also set to 0.8bar (or just over).
Now when the water gets hot, the extra is pushed against the diaphragm and compresses the air, this the air pressure rise and of course so does the water pressure.
The size of vessel is dictated by the total volume of water in the system and is designed so the extra volume creates a rise in pressure of approx 1bar.
So with heating on the pressure would reach 2bar and anything below 2bar
would be your green area. With heating off pressure would drop back to the 0.8bar ( or what it was set to )
If your boiler operates as above it's normal.
Problems arise from a failure to check and maintain the air pressure in the vessel. Air leaks out slowly (like the car tyre). The vessel gradually takes in water and reduces the volume of air. When the reduced volume of air is compressed it results in a much higher pressure, this in turn is a danger so it opens the pressure relief valve. This discharges water out through a discharge pipe and results in the pressure dropping to almost zero basically.
To operate the boiler the pressure has to be re-set by re-filling.
Of course the problem will just repeat itself until the vessel has been corrected.
PRVs also once opened are liable to be the source of another problem because they tend to leak a little and replacement is usually required.
Things to watch! With heating on what pressure does water go to?
The PRV opens at 3bar.
With heating off does pressure drop over night
secure plastic bag to discharge pipe to see what it collects and prove if PRV is passing water.

 

[george765]

What a great answer ! I now understand much better how the system works and have some clues to the problem. Thanks very much indeed.