Sizing an expansion vessel

Feb 26, 2009 5 Replies
B

I would be grateful for someone to confirm or otherwise my thinking on the required size for an expansion vessel.



I am refurbishing a large old mill. Heating and hotwater will be via heat pump (22kW). I need an expansion vessel for the sealed heating circuit Capacities in litres are: CH Manifold 1 73 CH Manifold 2 82 CH Manifold 3 98 CH Manifold 4 105 Other Pipes 15 Jacket for unvented tank 450



I have drawn on tables from rwc.co.uk and a formula from

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and my thinking is that a 24 litre vessel should do it. Any other offers with reasons? Regards Bruce


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R

Do you really have roughly 1000 litres of water? IIRC the expansion is about 4% in volume if you go from cold to boiling. that suggests

40 litres of expansion so you'd need a vessl much larger than that.

Robert

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B

Do you really have roughly 1000 litres of water? IIRC the expansion is about 4% in volume if you go from cold to boiling. that suggests

40 litres of expansion so you'd need a vessl much larger than that.

It seems a lot, but the figures are correct. However the operating temperatures are much lower than boiling, say 45 for the water circulating in the building and 60 for the hw tank. I think that ambient to those temperatures is all the expansion vessel needs to allow for, as any higer temperatures are a failure mode and the relief valves can do their job?

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J

What exactly does this mean? Is that actually the water content on the primary side of the heating circuit, or the DHW content of the tank?

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B

volume of primary circuit 450 volume of DHW 300

I am coming to the conclusion I have undersized it and might need a second expansion vessel!

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E

I would strongly recommend against sealed primaries over 200 litres. Install as a small pressurized primary and use a big water-water heat exchanger.

If you really have 450 litres of sealed primary then about 45l of expansion vessels should do it.

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