Sizing ASHP and hot water cylinder

[eandg]

Any ready reckoners/advice? 

 

Well insulated (u-values 0.11 floor, 0.13 walls and roof, 0.8 windows) and airtight c.200m2 two-storey house, UFH on the ground floor, two bathrooms and a shower room (showers, or at least in the ensuite anyway, likely to be power showers). Five occupants (two adults, three kids). 

 

There's also potential for ASHP to serve 42m2 summerhouse to be built later (with UFH in raft foundation, wash hand basin in w.c. and bar area). Is that doable and any pipework considerations that would need to be provided for at the outset?

 

Thanks

Edited February 5, 2021 by eandg

[AliG]

You would probably only need a 5-6kw ASHP for heating, but you might want a slightly lager one to heat up your hot water faster.

 

The stats are almost identical to my parent's house and we went for a 9kw for the hot water, especially with 5 occupants. You will probably also need a 300L hot water tank.

 

You could use a larger than necessary manifold with an extra output for the summerhouse that is capped off until needed. However, if the summerhouse is much worse insulated than the house the flow temperature probably won't be high enough to heat it fully or quickly. You would probably be better off having a fast acting heater in the summer house, it depends how much you plan to use it.

 

 

 

 

[Dan F]

We are 300m2, similair levels of insulation and have a PHPP calculated heating demand of 3kW.

 

We were between a 7kW and 10kW ASHP.   The reason we were looking at 10kW was i) UVC reheat time ii) UFH in a 40m2 garden room.   In the end we are going with a 7kW + Mixergy + WWHRS combination, the latter two components mitigating the need for such a quick reheat time from the ASHP.

 

If you have a buffer tank, you could have you buffer tank at 45C and run summerhouse UFH at 40C and house at 28C  (seperate manifolds)

 

How are you planning to hook-up your summerhouse UFH?  We used https://www.loco2heat.co.uk/products/preinsulated-pipe/rehau-preinsulated-pipe/rauthermex-duo

 

[eandg]

2 minutes ago, Dan F said:

We are 300m2, similair levels of insulation and have a PHPP calculated heating demand of 3kW.

 

We were between a 7kW and 10kW ASHP.   The reason we were looking at 10kW was i) UVC reheat time ii) UFH in a 40m2 garden room.   In the end we are going with a 7kW + Mixergy + WWHRS combination, the latter two components mitigating the need for such a quick reheat time from the ASHP.

 

If you have a buffer tank, you could have you buffer tank at 45C and run summerhouse UFH at 40C and house at 28C  (seperate manifolds)

 

How are you planning to hook-up your summerhouse UFH?  We used https://www.loco2heat.co.uk/products/preinsulated-pipe/rehau-preinsulated-pipe/rauthermex-duo

 

Thanks - and haven't thought that far! Was initially thinking infrared heating or a stove but now thinking I could slightly oversize the heat pump and go ufh in the slab for very little extra cost - plus we've solar pv which we'd want to maximise generation from. Will give that link a look. 

 

16 minutes ago, AliG said:

You would probably only need a 5-6kw ASHP for heating, but you might want a slightly lager one to heat up your hot water faster.

 

The stats are almost identical to my parent's house and we went for a 9kw for the hot water, especially with 5 occupants. You will probably also need a 300L hot water tank.

 

You could use a larger than necessary manifold with an extra output for the summerhouse that is capped off until needed. However, if the summerhouse is much worse insulated than the house the flow temperature probably won't be high enough to heat it fully or quickly. You would probably be better off having a fast acting heater in the summer house, it depends how much you plan to use it.

Thanks - will make sure and spec a 300l tank. Idea for the summerhouse is for it to be SIPs (0.16 u-value) walls and roof which could be supplemented with additional insulation. Hadn't thought about perhaps wanting a quicker response time given it's more infrequent use. One to ponder, thanks.

[Dan F]

3 minutes ago, eandg said:

Thanks - will make sure and spec a 300l tank. Idea for the summerhouse is for it to be SIPs (0.16 u-value) walls and roof which could be supplemented with additional insulation. Hadn't thought about perhaps wanting a quicker response time given it's more infrequent use. One to ponder, thanks.

 

We put a pre-insulated pipe in to DHW in summerhouse too.  But I didn't thnk about it too much and used the same 25mm pipe as for UFH.  This is going to mean it'll b a long wait for hot-water and so might not be a good idea.  Therefore we may end up refrofitting a quooker (and potentially an electric shower), but this will depend on how we use it.

Edited February 5, 2021 by Dan F

[eandg]

18 minutes ago, Dan F said:

 

We put a pre-insulated pipe in to DHW in summerhouse too.  But I didn't thnk about it too much and used the same 25mm pipe as for UFH.  This is going to mean it'll b a long wait for hot-water and so might not be a good idea.  Therefore we may end up refrofitting a quooker (and potentially an electric shower), but this will depend on how we use it.

 

What would you recommend instead? Due to their location our mains water and the hot water tank would both be closer to the summerhouse than the kitchen and bathrooms, so hoping (in ignorance) that that would not necessarily be an issue.

[Dan F]

14 minutes ago, eandg said:

 

What would you recommend instead? Due to their location our mains water and the hot water tank would both be closer to the summerhouse than the kitchen and bathrooms, so hoping (in ignorance) that that would not necessarily be an issue.

 

We should have used maximum 20mm RAUTHERMEX, or less potentially. This would have meant 6.2L in 30m of pipe instead of 10L, so less heat to loose and less time to wait for hot water.

 

Looks like you don't have this same issue, but still good idea to size hot-water pipes as small as you can (while accounting for pressure loss + velocity).

Edited February 5, 2021 by Dan F

[eandg]

40 minutes ago, Dan F said:

 

We should have used maximum 20mm RAUTHERMEX, or less potentially. This would have meant 6.2L in 30m of pipe instead of 10L, so less heat to loose and less time to wait for hot water.

 

Looks like you don't have this same issue, but still good idea to size hot-water pipes as small as you can (while accounting for pressure loss + velocity).

Makes sense and shall refer back to here when the time comes, thanks!

[PeterW]

9kW ASHP and 400 litre UVC with HP coil. 2 power showers will empty a 300 litre in around 14 minutes if it is at 55°C. 
 

Bin the idea of the summer house using UFH - you won’t have the PV spare capacity at the right time of year so just fit a 40A supply to it and an inline water heater and an air to air heat pump which will be change of £700

 

 

[J1mbo]

air to air will also achieve quicker warm up and can achieve COP of 6.

 

DHW is the killer to your config @eandg, reheat time with a small HP will be extended and you'll have no heating whilst it does it. Thermal mass of UFH downstairs will help though.

[SteamyTea]

3 hours ago, J1mbo said:

Thermal mass of UFH downstairs will help though.

Not once steady state is reached.

It could, possible, be a financial advantage.

[J1mbo]

Reduce the impact of having no heating for two hours, I mean.

Edited March 7, 2021 by J1mbo

[Nick Laslett]

On 05/02/2021 at 22:37, PeterW said:

9kW ASHP and 400 litre UVC with HP coil. 2 power showers will empty a 300 litre in around 14 minutes if it is at 55°C. 

 

 

Hey @PeterW, I’m re-reading a few posts due to the WWHR, which was new to me. You are such a great asset to the forum, because your posts are so clear. Just wanted to say thanks.