Oil combi pressure rising quickly.

[bengasman]

To get back to the subject, as for system size, I said in the first post the system was 4 rads.

The pressure at start was 1 bar air, 1bar water. The system gauge was approximately right.

I take it from the trivial replies that no one can offer an explanation based on experience.

Indeed oilman, it seems very odd.
12 litre ev seems more than adequate, and as it is new and charged we should be able to rule out it is the culprit.

You probably checked, but have to eliminate all options; did the pressure go back down when the system cools off?
Could it be that there is something that contains a lot of water that is connected to the system which causes the unexplained pressure rise?

 

[Norcon]

I 'll take up your challenge Bengasman if I may.
If you had an 18/L expansion vessel on a system which is sized correctly ( but you have no idea in the first place if its sized correctly or not) and you release some air and the pressure guage drops significantly.
By your analogy you then deduce that the expansion vessel size is ok ?

Now we go to a second system which is the same capacity in volume but the expansion vessel fitted is only 2/L.
(Which we know is grossly undersized)
You release the air and voila , the guage drops significantly also.

In both of the above cases you cannot see the physical size of the expansion vessels. You only have access to the shrader valve and the pressure guage.
Do you now deduce that the expansion vessel is sized ok on the second system also?

 

[bengasman]

Yes, you may, please do; I am not here to prove a point but to improve knowledge, mine as well as others'.

I don't recall ever having seen a 2 liter ev, nor have I heard of one so I presume for the moment this is a hypothetical situation.
If I found a 12 rad system with a 2 litre ev, I would expect to find a heatline or another typical cowboy choice with it, indicating the whole system was suspect and check it from start to finish.

More to the point; I would not expect a 2 liter ev to be capable of compensating a 12 rad system.
If the precharge was as prescribed by the manufacturer, the system free of air, and releasing the air from the ev would bring the system pressure down from say 2, to 0.5 bar, I reckon it could still be balanced so that the cold system pressure was high enough to engage the low pressure switch and keep the hot pressure below the safe limit.
Purely based on experience, I would still recommend the client to get a larger ev to make the system less tricky.

 

[Terrywookfit]

The only possible explanation, is that the EV is not working. This is not Rocket science!

 

[bengasman]

That would have been my reaction if the questions was asked by a householder. But how do you explain that this would mean the old ev was broken, AND the new ev is broken AND oilman has checked the precharge?

 

[numpty1]

ev could be ok,oilman has checked the connecting pipework,i would be looking for a blockage/restriction on the connecting pipework be it at the ev or where it connects to the hydroblock.another place to look would be around the pump outlet some sh1te blocking the connection.

 

[bengasman]

But how can the system pressure go down when you depressurise the ev?
I have seen a fair few strange problems, but can't figure this one out. And oilman is not exactly a beginner himself.

 

[kevplumb]

But how can the system pressure go down when you depressurise the ev?

you are removing the air pressure same as bleeding a rad

took that in the wrong context

if the pressure drops then there can't be a blockage

 

[bengasman]

Exactly my train of thought.
Interesting fault this one.

 

[numpty1]

does it have that stupid grundfoss twin head pump set up that are fitted to some oil combis?if so check the pump internals the plastic innards have a habit of giving wierd symptons.

 

[oilman]

The pressure drops as the water cools.

The pump is a single, not one of the twin pumps.

 

[bengasman]

The pressure drops as the water cools.


.

That would indicate it is purely a temperature/expansion issue.
I will just enjoy being baffled for the moment.

 

[Norcon]

Could it just be due to the design of the expansion vessel?

 

[wilhelm]

not overfired is it?



but you should be able to hear that..

 

[oilman]

Pressure rises quickly when it"s filling too. I wonder if the rubber is welded to the EV and there is only a small effective space.