I'm designing the heating and cooling system for my new build.

I've done the thermal modelling, and my peak heating demand is 3.7kW, peak cooling is around 5.5kW.

~190sqm of underfloor, no radiators.

I'm thinking roughly this kit:

• Monoblock ASHP, 6-7kW.

• >250L DHW cylinder. Pre plumbed.

• >80L buffer tank

• Heating emitted by UFH

• Cooling emitted by fan coils, with insulated supply loop. Modulated by fan speed.

• Valves to switch between heating and cooling and prevent cold water ever entering UFH loops

I plan to run most of the pipework in advance of the equipment being installed. My electrician to do all the high voltage runs ahead of time too. I will supply and fit the fan coils further down the line.

I think the remaining work for the heating side will be: (1) physically installing the ASHP and hot water tank, (2) plumbing the bits together in the plant room, then (3) testing, programming and commissioning the system.

I've seen kits (Stiebel Eltron has one) where the DHW, buffer tank and ASHP controller are all integrated into a single unit, which seems like it would make install even easier? Are there other kits like this?

I chatted with a few MCS installers who quoted figures pushing towards £20K for supply and fit, which seems wild when so much of the work will already be done. I can spec high end kit for about £10K all-in -- and these quotes were not high end! Am I underestimating the install? What's the most cost effective way to do all this, and still get the BUS grant (if it still exists by then)?

I would not double up on UFH and fan coils. In the basis heat rises and cold falls, I would fit UFH downstairs and fan coils upstairs.

Assuming you only intend to heat the HW in the tank to a low temperature (for best COP) I chose 300L. No point in pre plumbed, there really is no advantage and it constricts your ability to plumb it the way you want, not someone else's idea of how to do it.

Don't put the HW tank in the plant room, find (or create) a cupboard for it somewhere central to points of HW use to get the shortest runs and thus quickest time to HW out of the tap.

Size for heating not cooling, you will regret the higher running cost of running a oversized heat pump. UFH in screed is a huge buffer of heat and cold. If you peak cooling is 5.5kW that heat demand can be spread over many hours.

Example I removed a 6kW and installed a 4kW in January this year. I was getting CoPs between 7 and 8 this summer doing cooling. This time last year doing heating my CoP 3.72 for the month, due to very light heat loads and plenty of standby hours, same house, same plumbing a CoP of 5.15 this year with the 4kW heat pump.

With 192m² of UFH your volume of water is huge and would prevent short cycles. Unless you want to zone every room. Your fan coils are constant water flow and variable fan speed based on room temp.

Cooling via UFH is no issue as long as you don't force 7 degs water through it. Run between 12 and 16 degs in the UFH loops you have cooling.

Pre plumber cylinders are a waste of money for a heat pump as they have lots of stuff you don't need.

36 minutes ago, sansserif said:

I can spec high end kit for about £10K all-in

You can spec the kit you need for under half that if you eliminate all the things you don't need.

38 minutes ago, sansserif said:

What's the most cost effective way to do all this,

I never bothered with a grant, don't believe they should exist, they don't add value. I did it DIY.

My view

Direct cylinder heat via immersion. You make it smaller and heat hotter. Charge with solar an ToU tariff.

Heat pump to direct to UFH to heat and cool. No buffer, house ideally a single zone, no buffer. If you are desperate for room temperature control gets an Adia Thermal. If you want fan coils get ones that run from the same flow temp that you should flow through your UFH in cooling mode

4 hours ago, sansserif said:

I've seen kits (Stiebel Eltron has one) where the DHW, buffer tank and ASHP controller are all integrated into a single unit, which seems like it would make install even easier? Are there other kits like this?

Makes install super easy but also makes long term maintenance hard and expensive by locking you in to the manufacturers proprietary parts (and likely approved service network). While it might be a bit more labour to install, going with off-the-shelf multiple source parts for as much of the system as possible will likely save you headaches down the line if you are someone who wants to get into the detail/wants flexibility in who you choose to do servicing. If you are completely non-technical and want to pay for the approved service/oem parts then the integrated stuff can make sense.