ASHP buffer tank options

4 Jul 2021
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United Kingdom
I'm about to order a 12kw Air Source Heat Pump for my new home. For various reasons, I already have an electric boiler and a 2000 litre Akvatherm heat bank. On approaching a Daikin installer, they said I need a mains pressure buffer tank, otherwise Daikin won't support the warranty.

I understand the general need for a buffer tank, and I think I'm right in saying the Daikin buffer tank uses an indirect glycol loop to heat the buffer tank water.

Is this universal for ASHP suppliers, or are other options available?

The purpose of the large heat bank it to use off-peak power to heat the heat bank water, then use heat exchangers to provide dhw and ufh (already working fine).

It wouldn't be the end of the world to install (say) a 50 litre buffer tank, but it seems like needless complexity. The water in the heat bank is already treated and softened.

Andy W.
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I'm not familiar with the Akvatherm heat bank, but if its contents are heated directly by the ASHP (other than by an indirect coil) then Daikin may have been spooked by the quantity of water in the heating primaries (and the amount of corrosion inhibitor or other chemicals required). Then again they may have objected to the amount of expansion capacity needed for 2000 litres of heating fluid.

Manufacturers and their reps sometimes don't seem to like unusual applications, or applications they haven't considered; or maybe they just want to sell you more kit. Maybe you can resolve this by dialogue with their technical dept., or by slipping the name of their main competitor into the conversation.:sneaky:
Thanks for the reply. Good tagline! - Are there any UK-made ASHPs?

I understand your point, but in this case there was a quite specific series of events devoid of actual technical explanations:
  • They sent the office apprentice to do the initial site survey - nothing wrong with that, she know what to take pictures of and what questions to ask.
  • They noticed the Akvatherm, and said "oh, that's fine - we can use that at mains pressure to be the buffer tank" - without noticing (until I pointed it out) that it was not rated for mains pressure (1 bar maximum).
  • So they said "...then we'll need to install a Daikin buffer tank ahead of the heat bank - 'cos reasons (warranty)"
I'm not entirely surprised - I can imagine Dakin have been bitten by things like the output lines to the Daikin running dry and damaging something inside. The first thing a major manufacturer does when someone claims under warranty is to call in legal and exclude that case from the warranty conditions. I would have though a dirty great tank of water taller than the heat pump and topped off by a mains-driven header tank would be just as safe. (You're right about the cost of corrosion inhibitor - £150; the header/expansion tank is 200litres). Buffer tanks can leak, mains pressure systems can run dry. I'm more worried about the unnecessary complexity of the control and monitoring for the extra pump and sensors. I have the control system sorted, all I need is a "demand heat" contact.

I just want to know what I'm missing - it's possible Daikin are just being anal about covering themselves, hence the question - is it all ASHPs, or just Daikin?
the volume of glychol and cost for that size accumulator would be enormous. it would require around 20lt for every 100lt. so it would cost in the region of £1500.

No one in there right mind would do this. And then there is the requirement for safety and storage in the event of a drain down.
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There's a distinct difference between a buffer tank and plate separation.

assuming the store is directly heated ie its not heated via a coil, You will need plate separation so that you have a glycol loop at one pressure for the HP and separate fill water&pressure in the store.

A buffer serves 2 main purposes in HP units, one is to allow correct flow rate through the heat pump as it must be carefully maintained, the other is to give the system some volume so it can defrost the heat pump from the buffer without stealing heat from the heating system.

With such a large tank, you should be able to use a plate heat exchanger and run it without a buffer, as essentially the store is one giant buffer. It's actually a very good way to run it assuming the calculations for the store size suit the property (ie your not heating a store that size every day but only using a small portion of it) as you would have virtually no cycling (the hp compressor doesn't start and stop it could just run for a long period and do it's job in one go)
> With such a large tank, you should be able to use a plate heat exchanger and run it without a buffer, as essentially the store is one giant buffer
That's a good thought! Needs another pump from the secondary side of the heat exchanger, but that could be wired with a mains relay or secondary contact from the heat pump.

> you would have virtually no cycling (the hp compressor doesn't start and stop it could just run for a long period and do it's job in one go)
Yes, that's the whole design principle of the heating system - use off peak power to run the heat pump and fill the heat bank when conditions allow; use the heat in the heat bank as required. It seems to be able to retain heat and remain stratified for several days.

My only concern the simplistic, anal approach of Daikin: "no Daikin buffer tank, no warranty". I'll either see if I can get the installer (ecoBubble) to engage with Daikin, or switch to another supplier/manufacturer.

Thanks for your thoughts. That's really helped.

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