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Buffer tank on a 2 x 14kw Ecodan system

 
Air Source Heat Pumps (ASHPs)
Last Post by fosterl2 1 hour ago
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 fosterl2
(@fosterl2)
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Joined: 9 months ago
Posts: 17
Topic starter 10/11/2026 5:51 pm   [#3220]

We have a setup where there are 2 x 14kW Ecodan units (PUZ-HWM140VHA) recently fitted in a 6 bed bungalow that has an indoor pool. There are a couple of radiator circuits, and three UFH circuits, and a circuit for the pool heating. At the moment, the system is consuming over 60kWh of electric a day, and it has not really got that cold down here in Suffolk yet...

What seems questionable is the buffer tank set up: There is a 45lt Vaillant tank (VP RW 45/2 B), which if needed at all, seems quite small, or piped up wrong.

image

The image above shows the set up - on the LHS of the tank, the pipes, starting at the top, are 1) ASHP1 flow, 2) ASHP1 return, 3) ASHP2 flow & 4) ASHP2 return. The top 3 pipes are 120mm apart. On the RHS, top pipe is flow to the emitters, bottom one is return. Just out of shot on the left, there are the circulation pumps that are on the return sides, so effectively "sucking" the return to each ASHP (correctly), so the draw for the system is the pumps on each of the circuits on the RHS.

It does not make sense to me that for ASHP1, no sooner is the hot water sucked into the buffer tank, the return pipe immediately below is sucking it out again, and if ASHP2 joins in, its  flow in is immediately below the return of ASHP1, which again is going to be sucked out by ASHP1 before getting across the tank to the emitter side. The flow temp from both ASHPs is set to 40c (to achieve a pool temp of 29c), and the flow pipe on the emitter side of the tank measures 37c, so a loss of 3c before anything starts circulating.

Looking at the manufacturers info on the tank, it shows it is used for a combined gas boiler / single ASHP set up, with the top two pipes being for a gas boiler flow and return, and the pipe 3 is ASHP flow, and pipe 4 at the bottom is for ASHP return (which makes sense).

What I am looking for is advice on whether that buffer tank is completely wrong for my 2 x ASHP set up, and if so, what would be correct? No buffer tank at all? A separate buffer tank for each ASHP (if so, is 45lt appropriate?) or perhaps a 100lt buffer, with the flow in the top two pipes, and the returns at the bottom, or volumisers rather than a buffer set up, or possibly a completely different solution!

Thanks in anticipation... 😀 



   
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