water pressure needle on combi boiler
- Thread starter milly
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B
breezer
think you will find its not a problem, its just the water expanding / contacting as its being heated, although perhaps the expanion vessel may need a bit more air
Breezer !
I can't see how giving the expansion vessel more air will help. If you increase the pressure on the air side it will increase the pressure on the water side, because the pressure of the water and air is equal is it not ?
I can't see how giving the expansion vessel more air will help. If you increase the pressure on the air side it will increase the pressure on the water side, because the pressure of the water and air is equal is it not ?
B
breezer
yep
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If the pressure is getting up to 3 (presumably 3 bar), then the expansion vessel probably has little air left in it. It needs repressurising or the vessel needs replacing. Search the forum for expansion vessels, there are lots of posts on checking the pressure.
The air pressure should be checked at every service on sealed systems, combi or otherwise.
(Some expansion vessels are connected wothout a flexi, and are supported by the air valve sitting on the ground, so it's impossible to check the system without draining the system or freezing the connecting pipe and removing the vessel. Some clever installers around. )
The air pressure should be checked at every service on sealed systems, combi or otherwise.
(Some expansion vessels are connected wothout a flexi, and are supported by the air valve sitting on the ground
, so it's impossible to check the system without draining the system or freezing the connecting pipe and removing the vessel. Some clever installers around. )
B
breezer
MANDATE see what oilman posted
The expansion vess on the 350 is easy to check aas the air valve points down at the controll box, if you remove the front panel then the iner panel then remove the dust cap and take the pressure with a tyre gauge if water comes out (any amount ) then its had it. Its one of the easy ones to replace and costs in the £50 mark so shouldnot be to big a bill. Your pressure should be within the 1 to 2 bar range.
Breezer!
Yes I've seen the posts by oilman and cog. As they offer solutions I take it they feel a rise of 2 bar is a problem.not that I understand the reasoning.
My thoughts are that if you have the same volume change on a larger air space the pressure will be less, so I would have thought the expansion vessel is too small to match the volume of the system.
Yes I've seen the posts by oilman and cog. As they offer solutions I take it they feel a rise of 2 bar is a problem.not that I understand the reasoning.
My thoughts are that if you have the same volume change on a larger air space the pressure will be less, so I would have thought the expansion vessel is too small to match the volume of the system.
The lower pressure rise there is in a system, the less stress the system is under, and particularly the less flexing the boiler is subjected to, so the longer it lasts.
You may well be right that the vessel is too small, they often are. But even if they are the right size, they still need the air space regenerating from time to time.
"My thoughts are that if you have the same volume change on a larger air space the pressure will be less, so I would have thought the expansion vessel is too small to match the volume of the system.
"
Well yes, but wot happens is that the air space gets smaller so that at say 1 bar(cold) there's hardly any volume of air in the e.v. . Say there's 1.2 litres. The water expands regardless of the pressure, say 1 litre. That means that 1.2 litres has to squash to 0.2 litres so its pressure will go up to 6 bar!
When youprecharge an EV to 0.7 or 1 bar, there should be NO pressure on the water side, so the EV is FULL of air. Say it's 10 litres. Then the 1 litre of water expansion will only raise the pressure about 0.1 bar.
So the problem isn't that the EV precharge pressure is too low (0.001 bar would be ok as far is expansion is concerned), as long as it's FULL of air.
So what happens as time passes IS that the EV effectively shrinks in size.
Sometimes you only have to drain a cupful of water to take the gauge pressure down to nothing - a sure sign that the EV is full of water.
Does that help?
"
Well yes, but wot happens is that the air space gets smaller so that at say 1 bar(cold) there's hardly any volume of air in the e.v. . Say there's 1.2 litres. The water expands regardless of the pressure, say 1 litre. That means that 1.2 litres has to squash to 0.2 litres so its pressure will go up to 6 bar!
When youprecharge an EV to 0.7 or 1 bar, there should be NO pressure on the water side, so the EV is FULL of air. Say it's 10 litres. Then the 1 litre of water expansion will only raise the pressure about 0.1 bar.
So the problem isn't that the EV precharge pressure is too low (0.001 bar would be ok as far is expansion is concerned), as long as it's FULL of air.
So what happens as time passes IS that the EV effectively shrinks in size.
Sometimes you only have to drain a cupful of water to take the gauge pressure down to nothing - a sure sign that the EV is full of water.
Does that help?
COG -I've got a suspect EV on a 35cdi - are they the same? It's in a cupboard though so I suppose it'd have to come off the wall?
And don't you find that there's often a little water coming out of the air valve?
And don't you find that there's often a little water coming out of the air valve?
OIlman & Chris R,
Thanks for response, I can now understand it a lot clearer.
Thanks for response, I can now understand it a lot clearer.
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