Akvaterm Thermal Store (Bits n bobs Q's)

[AlBargey]

I'm in the planning stages before finally installing the 750L Akvaterm Thermal store we have which will be heated by a diesel boiler, and 2 x 6kW immersions from excess solar or as a base load on the generator. One day when we get a mains connection I'll get an ASHP plumbed in.

A few questions though..

I've calculated the whole system to hold around 888 litres, ideally I want it to be unvented (F+E tank placement is a problem) I've tried finding sites for the maths for sizing the expansion vessel but none are very clear and generally systems have a higher head to the top rad (Ours would only be around 1-1.5m) and system pressure would be 1.5Bar max, the rule of thumb seems to be 10%, so lets say 100l, is that ok, based on 1.5bar safety valves?

The Akvaterm tank is rated 1.5bar, so can we use standard 1.5bar safety valves, or should we get fancy ones set at I.E. 1.4Bar?

P.S. This is for a boat so building regs don't apply, however I intend to conform to them, and will get a heating engineer for commissioning and smoke test on the diesel boiler.

[Andy]

Here is a calculator for the volume increase based on a temp delta. At least you can check it makes sense. https://gegcalculators.com/water-therma ... alculator/

[AlBargey]

Here is a calculator for the volume increase based on a temp delta. At least you can check it makes sense. https://gegcalculators.com/water-therma ... alculator/

Thanks but that didn't work for me.

Engineering toolbox has an expansion from 888L @10° C to 80°C expanding to 932L which is around 4.5% so not sure why general recommendation is a 10% expansion vessel, 45L seems too small though for a 750L store, but that's still not the calc expansion vessel suppliers use for sizing which must have some extra room for error / internal volume discrepancies.

There's a calculator here but they want to round everything up to more standard numbers https://mikrofill-evc.wilxite.com/

[spread-tee]

Water will expand by about 4% between 4degC and 100degC, so assuming you will have a standard T&PRV on it that will be the worst case scenario you EV will have to cope with. As you said earlier it is about 45L expansion, you dont want the pressure rise to be to much so you need an EV sized to accept at least double that volume to keep the pressure rise in check.

Remember Boyles Law P1*V1=P2V2

So if your EV starts with an initial charge of 1bar and you stuff half its volume into it on expansion your final pressure will be 2 bar.

To limit your final pressure to 1.5 bar will need a pretty big EV, a 100L one would do it if you can start with an initial charge pressure of around a half bar.

Too big is good and dont skimp on the safety measures as pressurised stored hot water is DANGEROUS*

*not meaning to patronise anyone but I have seen some real horror stories in my time.

Desp

PS I am halfway down a bottle of red so this may not be spot on

[Andy]

Water will expand by about 4% between 4degC and 100degC, so assuming you will have a standard T&PRV on it that will be the worst case scenario you EV will have to cope with. As you said earlier it is about 45L expansion, you dont want the pressure rise to be to much so you need an EV sized to accept at least double that volume to keep the pressure rise in check.

Remember Boyles Law P1*V1=P2V2

So if your EV starts with an initial charge of 1bar and you stuff half its volume into it on expansion your final pressure will be 2 bar.

To limit your final pressure to 1.5 bar will need a pretty big EV, a 100L one would do it if you can start with an initial charge pressure of around a half bar.

Too big is good and dont skimp on the safety measures as pressurised stored hot water is DANGEROUS*

*not meaning to patronise anyone but I have seen some real horror stories in my time.

Desp

PS I am halfway down a bottle of red so this may not be spot on

At equilibrium before taking up the slack the expansion tank would be about half full. Hence the 10% which is almost double what you are trying to absorb.

[ecogeorge]

Have installed several unvented systems , .......... always gone with 5% min expansion vessel with no problems. Fit vessel on coldest part of cylinder.
Off course t+p valve at 3 bar / 95c .....
George

[spread-tee]

Water will expand by about 4% between 4degC and 100degC, so assuming you will have a standard T&PRV on it that will be the worst case scenario you EV will have to cope with. As you said earlier it is about 45L expansion, you dont want the pressure rise to be to much so you need an EV sized to accept at least double that volume to keep the pressure rise in check.

Remember Boyles Law P1*V1=P2V2

So if your EV starts with an initial charge of 1bar and you stuff half its volume into it on expansion your final pressure will be 2 bar.

To limit your final pressure to 1.5 bar will need a pretty big EV, a 100L one would do it if you can start with an initial charge pressure of around a half bar.

Too big is good and dont skimp on the safety measures as pressurised stored hot water is DANGEROUS*

*not meaning to patronise anyone but I have seen some real horror stories in my time.

Desp

PS I am halfway down a bottle of red so this may not be spot on

At equilibrium before taking up the slack the expansion tank would be about half full. Hence the 10% which is almost double what you are trying to absorb.

If you have a 100l vessel half full cold, then stick another 45l into it you may have problems. Ideally you want no water in the vessel cold and then half full hot, that would double the initial charge pressure.

[AlBargey]

Water will expand by about 4% between 4degC and 100degC, so assuming you will have a standard T&PRV on it that will be the worst case scenario you EV will have to cope with. As you said earlier it is about 45L expansion, you dont want the pressure rise to be to much so you need an EV sized to accept at least double that volume to keep the pressure rise in check.

Remember Boyles Law P1*V1=P2V2

So if your EV starts with an initial charge of 1bar and you stuff half its volume into it on expansion your final pressure will be 2 bar.

To limit your final pressure to 1.5 bar will need a pretty big EV, a 100L one would do it if you can start with an initial charge pressure of around a half bar.

Too big is good and dont skimp on the safety measures as pressurised stored hot water is DANGEROUS*

*not meaning to patronise anyone but I have seen some real horror stories in my time.

Desp

PS I am halfway down a bottle of red so this may not be spot on

Thanks Desp, that's useful. Warnings heeded!

Yes I was assuming I'd have to pre charge at only 0.5 bar, and as no ones mentioned it I assume a 1.5 bar T&PRV if I can find it or as separates should be ok on the Akvaterm rated at only 1.5 bar?

Thanks George also, I like the idea of only a 5% EV but not much difference if I just went for a 100l ish, and I think the lower pre charge allows more expansion than standard 3 bar PRV, so could be more necessary on this system.