UFH buffer store size - is it dependant on the volume of water in the UFH pipes

[Bramco]

Hi,

 

We'll be following the advice to fit a buffer store between the ASHP and the UFH in our new build and it occurred to me that the size of the store required might be dependant on the volume of water in the UFH pipes.  We have 12 loops, each about 100m in length. Would 50l be enough, too big, or just right?

 

Simon

 

PS apologies if this has been asked before

[joe90]

I was advised to have 80 litres, with 5 x100m loops ? (with immersion in case of failure or servicing of the ASHP).

Edited October 18, 2021 by joe90

[SteamyTea]

What is the smallest volume of water that the ASPH can deal with?

Then make it larger.

[Bramco]

  On 18/10/2021 at 12:52, SteamyTea said:

What is the smallest volume of water that the ASPH can deal with?

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Not sure I understand that - the ASHP simply circulates heated water through the coil in the buffer tank - so surely it can deal with a v small tank which it would heat quickly, or a v large tank which it would heat slowly?

 

I reckon there will be roughly 140l of water in the UFH pipes.

 

Simon

[SteamyTea]

  On 18/10/2021 at 12:58, Bramco said:

 

Not sure I understand that - the ASHP simply circulates heated water through the coil in the buffer tank - so surely it can deal with a v small tank which it would heat quickly, or a v large tank which it would heat slowly?

 

I reckon there will be roughly 140l of water in the UFH pipes.

 

Simon

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It is to do with the reheating time versus the heat loss time.

If the time difference is too short i.e. you are only heating one small loop in the system, say it is spring and the kitchen door is open because you are gardening. Then the losses will be relatively high, but the input from the ASHP will be much higher.  This will cause the ASHP to switch on and off quite quickly.

That is not good for the CoP and not good mechanically for all the components that have to switch on and off.

[Bramco]

  On 18/10/2021 at 13:07, SteamyTea said:

f the time difference is too short i.e. you are only heating one small loop in the system, say it is spring and the kitchen door is open because you are gardening. Then the losses will be relatively high, but the input from the ASHP will be much higher.  This will cause the ASHP to switch on and off quite quickly.

That is not good for the CoP and not good mechanically for all the components that have to switch on and off.

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Hi, now I understand the comment!   ?

 

However, I should have added I guess that the UFH is built into an insulated slab and the house is almost PassivHaus levels of insulation, so we'd be following the advice from other members with similar construction to heat the slab once or twice a day and not use thermostats for every room.  So there shouldn't be an issue of the ASHP turning on and off.

 

Simon

[SteamyTea]

  On 18/10/2021 at 14:44, Bramco said:

 

Hi, now I understand the comment!   ?

 

However, I should have added I guess that the UFH is built into an insulated slab and the house is almost PassivHaus levels of insulation, so we'd be following the advice from other members with similar construction to heat the slab once or twice a day and not use thermostats for every room.  So there shouldn't be an issue of the ASHP turning on and off.

 

Simon

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I would still fit one. You are assuming that there will always be enough water volume, at the right delta T, to make the ASHP function as intended.

Higher than needed return temperature, or too low a flow rate can also throw an error that stops the pump.

Edited October 18, 2021 by SteamyTea

[Bramco]

  On 18/10/2021 at 15:00, SteamyTea said:

I would still fit one.

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We will - the question was more about sizing. How do you work out what size is the right size?

 

Simon

[SteamyTea]

  On 18/10/2021 at 15:20, Bramco said:

How do you work out what size is the right size

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It is based on the reheat time.

Any heating system will take a while to start producing thermal energy at the desired temperature. Once at that temperature, and assuming the set up is correct, maximum CoP will be delivered for the conditions.

So basically it is ((stored energy + added energy) - consumed energy) / time.

So you need to work out those to get s volume.

No need to go to 3 decimal places, nearest kWh will do.