Over pressurising CH and DHW hot-cold-hot

18 Sep 2010
Reaction score
United Kingdom

Its a Worcester 35CDI MK2 combi, in our new house, installed about 5 years ago according to the paper work that was with it

The hot water was cutting in and out and with help from here I took off the secondary plate heat exchanger and de-scaled. This seemed to fix the problem.
We haven't moved in yet so when I left the house I turned it off to be safe. Now we're nearly in I've programmed it and left it to run twice a day to dry the house out.

The pressure rose quite high when it was hot so I released pressure from the PRV and it leaked, I replaced the PRV.
I have re-pressurised the expansion tank as it had no pressure at all and then released the air and no water came out. Re-pressurised it again and left for a couple of days and it seems to be holding pressure.

I have reduced the pressure in the CH to 0.5 bar and left the boiler off thinking if the HE is holed then the mains water pressure will leak through and raise the CH pressure, it didn't.

Now the Diverter valve seems to buzz when there is DHW demand and the hot water cycles with boiler lighting and shutting down as it did before.

if the boiler is left alone for a week or so the pressure when hot stays high enough to vent water but doesn't drop enough when cold to cause the boiler to not fire up from cold in the mornings. This suggests to me water is leaking across to be filling the CH. Is that a possibility anywhere other than the secondary HE?

I don't want to replace the HE and it make no difference as money is very tight and I'd have to borrow to buy it.

Any help greatly appreciated

Sponsored Links
I initially set it to 0.8 bar but have upped and lowered it since and it hasn't made much of a difference
If the expansion vessel had no pressure, that would mean it was full with water with little or no 'air'.
Pumping up the vessel to 0.8bar is required, but it is essential the water taking up the space in the expansion vessel is discharged. otherwise there will be insufficient volume of air to compress.
In normal operation the water pressure should rise about 1bar, this being the result of the air being compressed in EV.
If the volume of air is allowed to halve, then the pressure will double so instead of 1.5bar rising to 2.5 bar it would go to 5bar. but the PRV opens at 3bar.
In most cases its the loss of air pressure that allows more and more water to go into vessel so the rise in water pressure keeps increasing until the day the PRV opens. Once that has happened you have two problems instead of one.
PRVs once opened rarely seal and need replacing with a new one.
The initial questions should be water pressure when heating off and pressure when on.
Sponsored Links
If the pipe connecting the exp vessel to the water section of the boiler is choked then this would give you the same problem as no air/too much air in the vessel.
I've had the pipe to the expansion vessel off and it was clear. To clear the vessel I turned the isolators off where the pipes enter the boiler and drained it with the PRV as I pumped the vessel up.

Thinking about it I suppose if the vessel is on the other side of the pump to the PRV then the might not be able to escape like that. Does anyone think it is worthwhile taking the water pipe back off the vessel and pump the vessel up to clear the water out. I don't want to drain the whole system down as it has treatment in it, second lot at £25 a go
Stopped off at the house this morning to see what the pressure is cold and it was still up at a little over 3 bar. I think this confirms that water must be leaking across to the CH system? The filling key is out and has been all the time. Does this just leave the secondary HE or can the diverter valve cross link if the diaphragm is split? (I think I'm just being optimistic because a diaphragm is £8 over an HE for £130...)

Do I need to worry about the buzzing? I seems as the motor(solenoid?) pulls the pin out of the diverter valve when the DHW is on it doesn't "park" or is getting stuck so it buzzes like a sticky relay

Thanks for the responses so far
Agree if it's rising to 3bar with heating off, its got to be coming from the filling loop or the heat exchanger where cold water at mains pressure is heated.

A split diaphragm could not have any affect if the heating is off.No increase in temperature therefore no increase in volume. no increase in pressure

The diverter just diverts the CH water to a smaller circuit which includes the heat exchanger as against the radiator circuit

Anyway to isolate either the 'filling loop' or water going to the HEX and re test, it would indicate which was at fault.
Thanks for the help so far,

This weekend I turned the cold in back on and had the heating on but didn't run the hot tap at all and the pressure in the CH rose. This, to me as a non boiler engineer, suggests it might not be the HE as the water didn't pass through it

Is this right? or is the only place where the DHW could pressurise the CH loop in the HE?
Hi Adam! water may not have been passing through the heat exchanger, but it is in there at mains pressure. just like your kitchen cold tap.

It is just on standby, waiting till the hot tap is opened. The opening of the hot tap releases and reduces the pressure. It's this reduction in pressure that allows a diaphragm to move the diverter and ignite the boiler.

If pressure increases without CH being on, then the mains supply must be the cause. In some cases it might be due to the filling loop not having been removed and letting by, or it could be the heat exchanger.

Do you know if there is any valve on boiler to stop the Cold inlet that leads to Hot out
it didn't rise as such with the heating off but it just didn't drop as it got cold so I suppose its the same thing.

is there any way other than to take it apart and look to see if its the HE passing the water or the diverter or do you think its pretty much certain that its the HE?

I can isolate the cold feed into the boiler, that's how it is now so the heating can be on without the pressure rising, we just had a load of wooden flooring delivered so I want to keep it at normal temps so it doesn't warp

Thanks for the help

Which cold feed into the boiler did you isolate?
Is that (a) the cold water feed that comes out of the hot tap? or (b) is it the cold feed to fill and control the pressure.
With boiler off and one of the above isolated, the pressure will either rise or not, showing what effect the isolation had.

The above would be better done with heating off.
Pressure is going to rise with heating on, but I understand what you mean.
When heating switches off it does not drop back to original set pressure
There is only one feed into the boiler it then must be split up to the DHW and the filling loop. I'll have a look to see if there is any way in the boiler to isolate it separably


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links