Combi Boiler Pressure

[pemba]

Hi there the highest radiator in my central heating system was cold so i knew it needed bleeding. The system is powered by a Valiant Combi boiler and the pressure was low (0.5), so i topped it up as recommendedon several posts, to about 1.4, whilst bleeding the radiator. Now when the boiler runs the pressure increases to above 2.2 (not too sure how high it would go as i'm switching it off when it gets to 2.2). After reading several other posts i'm now not so sure that i've used the correct "taps" for topping up the pressure.

I intend to let the system cool down and then release the pressure back to where it was and run the system again. Any advice on what could have gone wrong? is the increase in pressure normal? am i worrying unduly?

 

[PrenticeBoyofDerry]

You can get a pressure rise of up to 2.0 bar, if the pressure reaches above it's expected value the PRV should activate.
How large is the installation.

 

[pemba]

Many thanks for the respons.
It is a reasonably large semi-detatched house with 4 bedrooms and 2 reception rooms, so not massive.
I've dropped the pressure to 0.8 and put the heating back on and it went up to 2.6 before I lost my bottle and switched the heating off again!

 

[ChewChew]

It could be an issue with the expansion tank. Its interesting to note you didn’t have this problem before you bled the cold rad. It might be the rad full of air was acting like an expansion tank which prevented the pressure build up.
Things to check – blocked hose to expansion tank, tank needs repressurising or replacing.

 

[pemba]

Thanks ChewChew that is an interesting suggestion.
I hate it when things apparently happen for no associated reason but the radiator acting as you suggest could make sense. Problem i've got now is that trying to check anything like that is probably way beyond my ability. I've got the system running now and it is sitting about 2 Bar above the normal when it is running fully hot (central heating). This is obviously outside of the Green area but also below the Red area. As i mentioned before it is a relatively large system heating 15 radiators - do you think it acceptable that it has a pressure differential of 2 Bar as previously suggested by PrenticeBoyofDerry?

 

[martyndc88]

sounds like the expansion vessle is flat/faulty. you should get it checked out

 

[MANDATE]

The increase in pressure is related to the size of the vessel and the " air charged ".
These dictate the volume of air that is available to be compressed.
A large system holding more water would need a larger vessel with larger volume.
Initially before any water is pressurised the 'air' side needs to be pre charged to about 1bar.
This forces the diaphragm to the water side and the volume of air available for compression is at it's Max.
If the vessel was not air charged the diaphragm would move to the opposite end of the vessel with no volume of air to compress.
Most problems relate to the partial loss of air pressure and it's not too difficult to correct it.
There is one important condition and that is ensuring there is no water in the vessel where air should be.
To ensure this the water side need's to be open to atmosphere.
Note a blocked pipe from vessel would have similar effect
Adding another vessel would increase the total capacity.
Normally most systems are set about 1bar air pressure followed by 1bar water pressure.
The 1bar water pressure should rise by 1 or 1.5 bar but should not reach 3.
If it does reach 3 then the pressure relief valve opens, the excess water is discharged and the valve closes and the pressure drops.
Also note that prvs once opened don't always seal correctly, leaving a small leak and another problem to deal with
?

 

[mcmoby69]

expansion vessel charge is usually checked when serviced. or 'should' be. when was it last serviced?

 

[pemba]

Guys loads of brill information, really appreciate your help, and i'll get on the case tomorrow and let you know the outcome.

 

[pemba]

Guys as you suggested the expansion vessel was at fault. Simple job (for a professional!) to relieve the water pressure, top up air pressure and repressurise the water side. System now sitting at 0.8 rising to 1 when operating.

Many thanks for all the information.