Any Boiler experts here?

LizardKing · 29 Nov 2010 at 17:29

You probably need to bleed your radiators if you can hear them fill up

If you need to release then you can bleed your radiators till it drops to a reasonable level (thats if you cant find the PRV)
Also condenser boilers do loose pressure over time but if your having to fill it up often then you have a leak.

Kol · 29 Nov 2010 at 17:50

Don't worry about the pressure being too high. The majority of systems 'should' have a relief valve but 2.5bar isn't too bad, I've seen systems that high before.

St0rmer66 · 29 Nov 2010 at 17:54

LizardKing said:
You probably need to bleed your radiators if you can hear them fill up
If you need to release then you can bleed your radiators till it drops to a reasonable level (thats if you cant find the PRV)

Not necessarily, could just be a few air bubbles which makes it SOUND like it's filling up. The PRV is internal on this boiler, there's no way to get to it from the outside (and they shouldn't be used just to vent pressure where possible anyway as then they start passing water all the time).

LizardKing said:
Also condenser boilers do loose pressure over time but if your having to fill it up often then you have a leak.

No. They don't. Not unless there is another problem.

OP, the fault you have is "heat exchanger overheat" (there's a sensor on it) caused by (you guessed it) lack of water in the system. If your pressure is going way up (3bar is when the valve lets water outside) and then dropping down to 0 when cold, it will just be a flat expansion vessel. Needs pumping up with a foot pump that's all. Easy job but should technically be done by a Gas Safe engineer as it involves "working" (in a legal sense) on the boiler. P.S. I work for BG too

.

The expansion vessel takes up the expansion of the water as it gets hot, it the vessel is flat (happens every 3yrs on average) then the water has nowhere to expand and the pressure goes way up... leaks out of the pressure relief valve (will just drip while it's on) and then when it cools back down there isn't enough water in the system, hence the gauge showing 0.

Kol · 29 Nov 2010 at 19:23

St0rmer66 said:
snip

Solid advice! Pretty much what my mate explained to me this afternoon (with an abundance of holiday logged, I've been 'labouring' for my mates plumbing business), always learning when motorsport is in the 'off' season ...

Fourstar · 29 Nov 2010 at 20:05

St0rmer66 said:
it will just be a flat expansion vessel. Needs pumping up with a foot pump that's all. Easy job but should technically be done by a Gas Safe engineer

I have one question if you don't mind.

Is there an easy conversion factor to work out what the precharge pressure should be when the system is pressurised?

If the expansion vessel is precharged to say 30psi then you fill the system to 1.5bar/20psi then pressure the air pressure in the vessel will have gone up.

I've been wanting to check mine as it's been there for 6 years but short of draining the system I have no idea what modifier to apply to the pressure on the vessel's label.

Or, as has just occurred to me, should the air pressure in the vessel be at recommended precharge value while the system is pressurised and cold?

RobertN · 29 Nov 2010 at 20:36

http://www.ultimatehandyman.co.uk/TOPPING_UP_A_COMBI_BOILER.htm

St0rmer66 · 30 Nov 2010 at 14:57

Fourstar said:
I have one question if you don't mind.

Is there an easy conversion factor to work out what the precharge pressure should be when the system is pressurised?

If the expansion vessel is precharged to say 30psi then you fill the system to 1.5bar/20psi then pressure the air pressure in the vessel will have gone up.

I've been wanting to check mine as it's been there for 6 years but short of draining the system I have no idea what modifier to apply to the pressure on the vessel's label.

Or, as has just occurred to me, should the air pressure in the vessel be at recommended precharge value while the system is pressurised and cold?

MOST expansion vessels internal to a boiler just have a set pre-charge of 15psi/1bar when there is NO pressure in the system. If you really want to do it while the system is full (not really recommended) it's just a case of adding the two together I think, so 1bar of water pressure + 1 bar of precharge = 2bar that you would pump it up to on your pumps gauge. I always just take the pressure out though, you'll only need to remove LESS THAN a bucket of water to drop the pressure. Keep in mind that as long as you're only opening one outlet (a drain point for example) then on a sealed system it will form a vacuum since no air can get in, so as soon as you get to 0bar (which is atmospheric pressure) the water will stop coming out.

There's usually a table/graph of values in the manufacturers instructions for the specific boiler if adding an external expansion vessel to give you an exact size for how many litres of water and the pre-charge needed etc.. but generally speaking you can't have an expansion vessel that's too big in that situation and the stock 8/10litre ones that come with the boiler will do an average house with no problems. I imagine there is a conversion factor but to be honest, you just don't need to be that exact... you can always just oversize it a bit.

Fourstar · 30 Nov 2010 at 16:27

Thanks for the reply

It had completely slipped my mind that a vacuum would form when releasing the pressure. There was I thinking 2 storeys of water pressure bearing down.