Central Heating pressure issues

DavidS14

DavidS14

DavidS14

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Joined
2 Apr 2004
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UK
Worcester 24i Junior Boiler with high central heating water pressure problems.

When the heating is on all of the radiators get hot with no cold spots or noises.

Cold pressure set at 1Bar but goes to >3 Bar when high pressure burner is on and drops back to under 2 Bar when the system drops back to the low pressure burner. Pressure gauge on the boiler seems to vary more with what the burners are doing rather than the system temperature.

Checked expansion vessel on boiler and seems OK, drained the air out and no water followed. Dropped system pressure by venting a radiator and pumped up the expansion to 11psi as per manual, refilled system to 1 Bar and vented radiators just in case.

Put the heating on and problem persists, it's acting as if there is no expansion vessel but there is one :confused:

Anything else I can try or any ideas before I get someone in ?
 
Make sure that when you are pressurising the expansion vessel that the boiler is fully drained and that the drain off **** is left open while you charge the vessel, as the excess water has to be pumped out. It would also be a good idea to make sure the diverter valve is not causing any hydraulic lock while you do this, tho that would be quite rare.

If you can charge it up to 1Bar then it maybe a good idea just to watch the pressure for a while to be happy its holding steady.

Other than that you might be looking a a clogged up expansion vessel pipe.

Or it could just be that the system volume (amount of water that makes up the entire system) is beyond what the expansion vessel can cope with. Then you need to fit an additional external vessel to the return pipework near the boiler.

Mick
 
Mick,

Thanks very much for the info.

It's a small boiler but there are only six radiators in the house and the system used to run ok pressure wise.

I've watched the system heat up from cold again, goes up to 3Bar when Hi burner on, drops back to 1.5Bar when Lo burner is on and maintains this as long as the burner is on.

As soon as the burner goes out the pressure gauge drops back to the 1Bar until the burner comes back on again and it rises to 1.5Bar again. Why would the pressure gauge show 1Bar (cold system pressure) on a hot system ?

I'm confused :confused:
 
Does the PRV blow off out of the overflow pipe? I would say it sounds like a restriction in the expansion vessel hose/pipe. Would probably be a good idea to check it out before it blocks completely and causes you problems.

Mick
 
Thanks again :)

Nope PRV has never vented afaik and I did check it a few times and the pipe exit is dry.

I tried and experiment, turned half of the rads off and put the heating on.

System started at 1 Bar, Lo pressure burner on up until about 1.25Bar the Hi came on. Now as the system it was heating was smaller the pressure in the system only went up to 2 Bar before the return from the rads was hot enough that the Hi pressure burner stopped and the Lo carried on. Pressure stabilised at 1.5Bar. I then opened the closed rads and the system warmed up with no issue staying at 1.5Bar.

Now as the system is fully hot, all the rads are on and hot would that not mean I should be at a decent system pressure even when the burner is off ?

What I cant understand is why as soon as the burner turns off even though the system is hot the pressure on the gauge drops from the 1.5Bar it sits at with the Lo burner on back down to pretty much the cold system pressure within a couple of minutes.

I'd have though that the hot system pressure would be regulated by the expansion vessel to say 1.5Bar but would not vary much with either the burner on or off and only fall very slowly as the whole system cools and would only drop back to 1 Bar when fully cold.

If the boiler was in a cupboard then I'd be blissfully unaware of any issues as other than the pressure gauge readings everything works, CH comes on, gets hot, TRV's all work and then turns off when programmed to and DHW is all OK as well.

Grrrrrrr :confused:
 
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