Expansion volumes for heating systems

[Ron Lowe]

Hi, All.

I'm getting pressure loss on my sealed system, and I suspect it's going out the expansion pressure relief valve. I'll tie a bag on to it to verify.

I've noticed the pressure rises a lot when the system is hot. The expansion vessel is inside the boiler, and I'll get round to checking / pumping up shortly.

However, the vessel inside the boiler is very small, and the system is fairly large. All systems I've seen of comparable size have external expansion vessels too.

So:

1) What's the normal max hot pressure you'd want to allow under normal operation?

2) Is there a rule-of-thumb way to estimate the required expansion volume based on system volume? And a rule-of-thumb way to estimate the system volume in the first place?

I may go ahead and juat add external expansion anyway.

3) Is it conventional to add the expansion volume on the flow, or return?

[Lobster]

Don't have specific answers, bit if your system works OK with the size of boiler you have, that suggests to me it's not undersized, and that therefore the internal expansion vessel should be adequate?

Also, your post implies (maybe I'm wrong?) that the depressurising issue is new - if so, suggesting something's gone wrong rather than there being a fundamental installation problem?

Certainly when my own system developed problems like you report (involving leakage from the PRV), it was down to an expansion vessel having become depressurised.

David

[gremlin_95]

1) Not much above 2 bar. 2) A 10 litre expansion vessel is usually sufficient for 100 litres of system volume. 3) I have usually seen it on the return.

Has your system only recently started doing this? Sometimes you can get away by pumping the expansion vessel up slightly more than usual.

[Newshound]

Had this sort of problem with last combi on a larger than average house, cured by adding external vessel. When I replaced the boiler a few years later it turned out that the internal vessel had failed but that might have happened later. Remember to include an isolation valve between the new vessel and the system so that you can isolate it to check the vessel pressure, to save depressurising the whole system.

[Andrew Gabriel]

I would aim for 2 bar max when system running at max temperature. You could probably go up to 2.5 bar, but that stresses everything just that little bit more.

You might also be able to reduce the cold pressure, which would give you more headroom. The cold pressure required is 1 bar for every 30'/10m height of the system above the pressure guage, plus a bit to allow for the pump's dynamic pressure difference.

1 bar is the norm, but many systems could get away with less, although it give you less margin if the system is losing water.

I recall finding tables listing this, when I was installing my CH, but I don't have any to hand now.

You should have one near the pump inlet. If you think the internal one is dead or dying, then position the new somewhere in the pipework which is not far from the pump inlet. Otherwise, it doesn't matter.

[Ron Lowe]

Thanks for the replies.

No, it's not really a new behaviour. The system was converted from open vented to sealed back in 2008. It's always gone up to high-ish pressures. But it's got worse over the last year. I think the built-in expansion is in need to topping up, but I think it's marginal at best.

I noted the pressure was just a tad under 3bar this evening, from a cold

0.75 bar.

So I'm going to whack on an external bottle. I see screwfix do a range of vessels: 8, 12, 18, 25l, and the necessary mounting kits.

I just need to decide on a size.

[gremlin_95]

on the EFD stimulator link, there is plenty of information to help you size an expansion vessel.

[Heliotrope Smith]

Ron Lowe wrote in news:jaomli$utc$ [e-mail]:

The symptoms you describe indicate vessel failier or under pressure. Pump up to around .75 bar when system empty.

If failed then estimate system volume in litres and multiply by 0.075 to get required size in litres for external vessel.

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