Two ASHPs. How best to use them?

[saveasteading]

Based on info on Buildhub I estimated a 15kW ashp, but a local specialist says it is not enough.

 

Also the floor arrangement requires a secondary feed and manifold to reach the distances.

 

The building will be in the form of a main building and an annex (for us old ones and b&b).

 

Plans below shows the layout, with central courtyard. All the GF to be ufh. The upper areas (front and left wings in roof) to be radiator (headroom is tight).

Outside is 21 x 19, inside 18 x 16 approx.

 

Insulation will be very good, to new-build targets, although some trading off for practicality (esp headroom at existing doors), so the floor will be 125 to 150 PIR.

 

Our thoughts are as follows.

1. 2 pumps needed so do they both feed the one plant room or 1 for main build and 1 for annex?

2. If the former, do they work in tandem or split to space heating and water heating?

3. Split the load as 2 identical pumps sharing the load, or large and small?

4. Best to have 2 water tanks anyway, for volume and to avoid arguments between the 2 spaces?

 

270m2 GIA ground floor, 110m2 upper floor.

 

The plant room is shown,  to the left next to the stairs.

There is  a roof void at bottom right that could house more plant.

 

BTW the current favourite installer seems good, but it is early days and I like to understand and question.

Any comments welcome, not just as my 1-4.

 

 

Edited December 27, 2021 by saveasteading

[SteamyTea]

1 hour ago, saveasteading said:

If the former, do they work in tandem or split to space heating and water heating

My intuitive feeling is that separate HPs for Space and Water heating.  They are different things, at different times and at different temperatures.

My thinking is that you can get a small, but better, modulating one for space heating and a non modulating one for water heating.

The reasons being is that weather effects both the HPs performance and the house heating load, but water heating is generally quite fixed in the amount of energy needed.  So you can find HPs that better suit the two separate tasks, rather than compromise.

And you could divert, via a buffer tank the DHW HP to space heating during exceptionally cold spells.

But if it is worth it financially, I am not so sure.

[PeterW]

What’s the use of the building in totality ..? How many bathrooms / showers etc ..?

[saveasteading]

Just now, PeterW said:

How many bathrooms / showers etc .

6 en-suites, 2 wc, 2 kitchen , 2 utility.

 

[PeterW]

All in use at the same time ..???

[saveasteading]

9 minutes ago, PeterW said:

All in use at the same time ..???

It is possible that at the end of a long, muddy, winter's  walk, they are all used within a short time frame.

ProDave has mentioned adding an inline heater for times of extreme use which is perhaps worth a thought.

But I think we need generous hot tanks, and efficient shower heads.

[PeterW]

Generous tanks and shower controls won’t get away from the laws of physics…! Even with an 8l/min flow to a shower, you’ll need heading toward 50 litres a minute on your mains supply. That is heading toward hydrant levels of flow - what has your water provider said about connections ..? Assuming here you’re not having a 22mm water meter ..?! 
 

The other issue will be recovery rate and storage - ASHP doesn’t give you lots of rapid heat so you need to oversize. As this has variable usage patterns then I would domino the tanks using two or three smaller tanks - gives you flexibility and you’re not heating 6-900 litres of water when you don’t need to. 
 

I would go with three 300 or 400 litre Telford Heat Pump UVCs in a domino set up, but get them upgraded to 32mm cold flow and hot outlets. I would also get the first with twin immersions - this is your “daily” tank so you need to be able to boost it. A hot return will be needed but I think you may have to return it to lower than the top of first tank so a custom tank connector would need to be specified. 
 

Control wise, I would put a 12kW ASHP into the set and use diverters to alter the return flow - you’ll need to manually switch these to give you the tank volume so would also suggest getting industrial valves that don’t have a hold open current. Not expensive but means you don’t end up munching synchro motors. 
 

This needs a fair amount of space - how much have you got planned for the plant room ..??

[saveasteading]

3 hours ago, PeterW said:

what has your water provider said about connections ..

Private water supply gurgling slowly to a storage tank. From there a pump, either to a header tank or hard into the system.

 

3 hours ago, PeterW said:

how much have you got planned for the plant room

4m x 1.5m

[PeterW]

Ok so is the storage tank very large and away from the main property ..? You’re going to need something like a multi stage CH4-50 pump to get the pressure and flow, I’d even consider a tandem set up so you have redundancy. Header tanks are pointless as you won’t get the pressure. 
 

https://www.anchorpumps.com/stuart-turner-ch-4-50-5-bar-centrifugal-multi-stage-booster-pump-240v

[saveasteading]

2 hours ago, PeterW said:

storage tank very large and away from the main property

Building a new one, about 20m away and at about ridge level. Can be any size as we are still designing it.

 

From once having a rainwater harvester I am aware of how easy that was: the pump in it kicked in whenever a tap was turned on, and it was sheer pressure and no tanks.. On the other hand, the pump failed in 2 years, so the tandem idea is good.

[SteamyTea]

1 hour ago, saveasteading said:

pump failed in 2 years

Shame the one @PeterWsuggested only has a 2 year warranty.

My twin impeller show pump has lasted over 13 years now. Was a 100 quid from Screwfix.

[saveasteading]

I came across Willis heaters elsewhere in BH. Are they an option to either boost the hot water after the tank, or add to each shower for use in high demand?

 

I am thinking worst case is all the Christmas visitors came back cold and wet from a walk (or kayaking)  and all head for the showers.