Massive variation in heating pressure from cold to hot !

[paulbhalton]

Hi, I wonder if anyone can give me advice on the problem I am having of varying central heating pressure ?

I have a Vaillant ecoTEC plus 937 boiler. Even since installation (3 years ago) there has been a small pressure loss from the system requiring a "top-up" every two to three months. This I could live with (bearing in mind I couldn't see a leak anywhere visible)

My current problem is that the last time I re-pressurised to 1.3 bar I noticed that after 30 to 40 mins the pressure had risen to 3.0 bar and error code S 42 was showing (Pressure > 2.9 bar). I let 0.5 ltr out of system, pressure back to 2.2 bar.

Next morning no heating F24 displayed and 0.4 bar showing, presurised to 0.8 bar, boiler started but at full temperature (62 C) showed 2.6 bar.

Why is the pressure changing so much ?

I have checked the expansion vessel (pressed the valve, no water came out and there was pressure in it).

I had the plumber who installed the system look at it, he is now saying that he thinks the system needs a secondary expansion tank ! why ? It has been fine for 3 years and we have made no additions to the system since it was installed !

Any advise , things to try greatly appreciated.

I was told to half empty one of the rads to allow the other half to behave like a secondary expansion vessel, haven't tried it yet, thought I would ask here first.

 

[cozzmic]

Expansion vessel needs pumped up. check out the Stickies at the top of the page.

 

[doitall]

As above the vessel pre-charge needs topping up.

 

[Agile]

As site rules say, read the FAQ first before posting.

All these common problems are well covered!

 

[MANDATE]

Well it all starts with the total volume of water in the system and having a expansion vessel capable of taking in the extra water as a result of the water expanding when heated.
Because the air side pressure of the vessel has gone down a little, the diaphragm which should be fully biased toward the water side is most likely no longer in that position before the heating is switched on.
This means the total volume of system water has gone up a little and the volume of air which gets compressed has gone down. Compressing a smaller volume of air results in a much higher pressure, the PRV prevents it going past 3 bar, but in discharging water the pressure drops and topping up is needed.
Just pumping up the expansion vessel to 0.8 bar will not cure the problem unless you can also ensure there is no water in the vessel and the max amount of air is available for compression.
There are two other possible problems related to vessels.
The diaphragm could be 'holed' allowing water to pass to the air side.
The pipe joining the vessel to the system could be blocked.
Also be aware that after correcting the vessel problem you may still finish up with a small leak from the PRV, because they don't always seal fully after having being opened. This may result in a small drop in pressure overnight without the heat on. So replacing PRV should be considered.

 

[paulbhalton]

Well it all starts with the total volume of water in the system and having a expansion vessel capable of taking in the extra water as a result of the water expanding when heated.
Because the air side pressure of the vessel has gone down a little, the diaphragm which should be fully biased toward the water side is most likely no longer in that position before the heating is switched on.
This means the total volume of system water has gone up a little and the volume of air which gets compressed has gone down. Compressing a smaller volume of air results in a much higher pressure, the PRV prevents it going past 3 bar, but in discharging water the pressure drops and topping up is needed.
Just pumping up the expansion vessel to 0.8 bar will not cure the problem unless you can also ensure there is no water in the vessel and the max amount of air is available for compression.
There are two other possible problems related to vessels.
The diaphragm could be 'holed' allowing water to pass to the air side.
The pipe joining the vessel to the system could be blocked.
Also be aware that after correcting the vessel problem you may still finish up with a small leak from the PRV, because they don't always seal fully after having being opened. This may result in a small drop in pressure overnight without the heat on. So replacing PRV should be considered.

After reading all your replies and having already read the FAQ on this subject which stated 1 bar pressure in the expansion tank, I could find no real reason why it has suddenly gone wrong as the plumber said there was good pressure.
I obtained a copy of the installation istructions, in there in section 7.5 it described the checking of the combi boiler expansion tank, it was then I realised that the plumber who had checked it had looked at the shift load storage tank which is indeeed small and is pressurised to 4 bar !!.

I have removed the covers myself and checked the correct pressure vessel, it was too low on pressure.

Also as you said above I now have a leak due to the plumber running the system at too high a pressure and the PRV activating.

Thanks & regards

 

[ALEC1]

to check the EV you have to empty the boiler circuit to get that to atmospheric pressure... I doubt you did that..

 

[Agile]

About 50% of plumbers dont correctly repressurise them with the system at atmospheric, including most British Gas people in my experience!

Tony