Sizing an expansion vessel

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

Reply to
BruceB
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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

Reply to
RobertL

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?

Reply to
BruceB

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?

Reply to
John Rumm

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!

Reply to
BruceB

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.

Reply to
Ed Sirett

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