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.

12 hours ago, JohnMo said:

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

As we've said before John, you can have your cake and eat it - Cool Energy are still doing their free heat pump offer which includes the MCS certification -> https://coolenergyshop.com/pages/free-heat-pump

So they get the grant, you do the install and they do the MCS certification of the install.

I have a heat loss of 3.5kW so similar to this thread and the Cool Energy don't supply anything suitable currently, their small ASHP is too small, the next one up too big. But other scheme providers are available to go that route. Was happy to pay myself.

What @JohnMo says.

I followed his comments on here and ditched the buffer tank and pumps and mixers and actuators to run open loops on WC. Saves a ton of money, is more efficient, and moves the only pump noise to outside. Local installers were useless and extortionate. They refused to consider an install without buffers and room thermostats and actuators. Our plumber did the pipework, spark did the electrics. Couldn't have been simpler. I spent more time insulating pipes that the plumber took to fit them.

We don't get any grants in Northern Ireland so that was never a consideration.

We have total heat demand of about 2.5KW. Our 5KW panasonic monobloc is hardly ticking over in this weather. I can't speak highly enough of our panasonic heat pump. Best toy I've bought in years. And cheap too.

I agree other comments about fitting minimum 300 litre tank and not pre-plumbed. We have a load of solar PV and a hot water cyclinder costs less than a battery.

Edited Thursday at 10:033 days by Mr Blobby

many suggest undersizing heat pump and if needed for those few days supplement heat. id go for 4kw no question. bin the buffer which lowers efficiency (if not needed) adds cost and complecations. you shouldn't need it as you will likely have plenty of water volume.

On 21/10/2026 at 20:14, sansserif said:

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)?

Before specifying your kit, you need to go back to first principles.

1. On you heat pump size, don't just look at peak output at design outdoor temps. You need to also look at the unit's minimum modulation to see if it will run at your average outdoor temperatures with your heat loss at those temperatures. With a peak demand of 3.7kW you could be well below a lot of heat pump's minimum modulation, so check with the manufacturer, and if they can't give you the figure, go and find someone who does.
2. as has already been suggested, the value of a pre-plumbed cylinder is questionable, unless your doing this commercially and you save yourself a day or 2 in labour - sometimes worth the extra cost in headache time too.

3. calculate the open system volume to determine whether you need to add more system volume using a volumiser

4. Carefully consider your control strategy. If you're going to go for lots of zones, then a buffer is something that you're likely to need to help the system run efficiently (I know I'll get flamed but the actually decision between a buffer/no buffer is more nuanced than most people understand and they're not necessarily the no-no people seem to think they are). If you decide that you don't want lots of zones, or you have a couple of mixed zones on different curves, then you may not need it.

5. As already mentioned, very carefully consider if you really want fan coils and ufh to be duplicated. It's not always the plumbing that's the issue but the control and whether your chosen system has adequate control to do this (even though the additional plumbing for fan coils can be significant if you want full cooling capacity from them because you may need condensate drainage as well as individual power outlets etc). But also consider whether UFH cooling would suffice and then as @ProDave said using ufh downstairs and fan coils upstairs where you're most likely to want the cooling too.

6. Carefully consider your DHW sizing. The MCS size calculator is quite crude and for a family or 4 you'd get to something like 250l anyway, but the new CIBSE method is much more detailed based on taking some time to calculate draw off compared to likely consumption, lower storage temperatures etc. For example in my instance, MCS calc gives 220l requirement so a 250l cylinder. Using the CIBSE method recommends a 400l cylinder. As a basic calc you can use 45l per person plus 40l as a start.