Ashp design

[junglejim]

I’m trying to get my head around heating design. We are building close to passive house timber frame. I have ufh downstairs and want provision for radiators upstairs. I’d like the ashp to part heat dhw if possible so presumably unvented indirect cylinder. A plumbing company locally have shared suggested design but is generic and what I’ve read is that low loss headers are not best with passive house. Any advice welcome as Would like to meet with them to talk through.

 Thanks

Proposed ASHP Layout - Charmaine.pdf

[SteamyTea]

That diagram shows very little in reality.

 

Have you had a heat loss calculation done?  This is what sets the size of the heat pump (or any other thermal source) before DHW considerations, the length and spacing of the UFH pipework, the number, type and size of any radiators.

There are also other factors to take into account such as ACH, MVHR volumes and efficiencies, window sizes and orientations, PV.

 

The main things to understand is the difference between power [kW] and energy [kWh], temperature differences [ΔT], mass flow rates [], heat and specific heat capacities [C and c], temperature [K or °C] is not power or energy and that generally, in a modern house or any sort, the space heating loads are quite small, often in the region of 2 to 3 kW at a Δ20K.

Some rudimentary understanding of weather is also helpful i.e. we very rarely get extremely low, or high, temperatures for very long periods of time.  If you want that, move to Canada.

 

The main thing to watch out for is grossly oversized system, which your diagram seems to show.  Generally buffer tanks/ volumisers/low loss headers are not needed on a well designed system, but they do all have their place in some designs.

It may seem like a mine field, but once numbers are put into the design, it all starts to make sense.  Without the numbers, you get badly designed systems that are inefficient.

 

[DownSouth]

@junglejim Do your heat calculations say you need a pump that size? If it’s close to passive I’m assuming it must be a massive house you are building to need that sort of size. 
 

[JohnMo]

1 hour ago, junglejim said:

building close to passive house timber frame

And they are showing a 15kW heat pump. So unless you house is several 1000m² it's a bit large. 

 

Are you going for the grant or doing outside the grant?

 

I have found very little difference in running costs doing DHW via immersion only. Which is outside the grant allowed method of heating. But direct immersion heating reduces install cost and cylinder cost.

 

Heating design you have two options, but both options require a room by room heat loss calc, this allows sizing of room UFH array and radiator sizing. 

 

Option 1 is to run everything designed for the same flow temp. Then ideally run as a single zone.

 

Option 2 run at different flow temperature. Then you may need zones, your smallest zone defines system volume. Which you need to add water volume to meet a 20L x min output kW if heat source. But a mixer driven by ASHP could have it's own WC curve so be operated as a single zone.

 

No option above needs a LLH or buffer.

 

Another option is to do fan coils in rooms upstairs and this gives you bedroom cooling (radiators don't), the UFH can do heat or cool anyway.

[SteamyTea]

Here is a pdf of a useful book.

 

 

Heat_Pumps_for_the_Home_-_John_Cantor.pdf

[SimonD]

2 hours ago, junglejim said:

A plumbing company locally have shared suggested design but is generic

 

That's not  design, it's just a basic schematic. For a design you need heat loss, cylinder and emitter sizing calcs at minimum outdoor design temperatures as an absolute minimum. You also really need the same things at your average outdoor temps to ensure your system can modulate sufficiently.

 

48 minutes ago, JohnMo said:

Option 1 is to run everything designed for the same flow temp. Then ideally run as a single zone.

 

That really isn't an option, even if loads of plumbers and some heating engineers think it is, especially with a house to passive standards.

 

7 minutes ago, SteamyTea said:

Here is a pdf of a useful book.

 

 

Heat_Pumps_for_the_Home_-_John_Cantor.pdf 15.75 MB · 0 downloads

Or try this one https://renewableheatinghub.co.uk/the-ultimate-guide-to-heat-pumps-a-comprehensive-resource-for-homeowners/ 

 

Although not available for free!

[JohnMo]

1 hour ago, SimonD said:

 

That really isn't an option, even if loads of plumbers and some heating engineers think it is, especially with a house to passive standards

Explain? It close too which means what?

[SimonD]

1 hour ago, JohnMo said:

Explain? It close too which means what?

 n = 1.3 v n = 1.0 - 1.1 - you can't balance the system across the output range, only at a single temp. And the lower your Mean Water to Air Temp difference is the more difficult it is to do. Most who do this will find their customers compromise on indoor air temps at some ranges. You really do need to run 2 different curves and therefore have one mixed circuit.

[junglejim]

Thanks for all the help. Clearly have a lot of homework to do! I was hoping the plumbing company would help with this. Here’s the heat calcs they’ve run but I’m not sure they have factored in the efficiency of the build and other aspects such as MVHR. Im not sure where to start in terms of doing calcs myself so any help really appreciated. 
 

ps. Location is Guernsey so no extremes in temperature.

Edited yesterday at 07:09 by junglejim

[SimonD]

1 hour ago, junglejim said:

Thanks for all the help. Clearly have a lot of homework to do! I was hoping the plumbing company would help with this. Here’s the heat calcs they’ve run but I’m not sure they have factored in the efficiency of the build and other aspects such as MVHR. Im not sure where to start in terms of doing calcs myself so any help really appreciated. 
 

ps. Location is Guernsey so no extremes in temperature.

By the looks of that, you really do need to find yourself someone else to do the work. You don't add DHW cylinder demand to a system with a heat pump as it's priority hot water - the heat pump switches from doing heating to doing hot water, so even if your house needed 8kW for heating, which I seriously doubt, this would be fine for your hot water. How big are your kitchen/dining, living & upstairs bed? They must be massive.

 

You can go and use my tool at https://openheatloss.com to create your own heat loss and system design. This is supported by a number of videos at https://youtube.com/@OpenHeatLoss taking you through how to use the tool and what to input - the video library is developing day-to-day

[JohnMo]

@Simon, beat me to it with the cylinder comments.

 

For context on heat demand, NE Scotland total are 234m², with bed and living space at 192m² @-9 3.5kW. Not Passivhaus.

 

So unless you are huge your heat loss calcs are way off.

[junglejim]

Thanks. Building is 15x10m with upstarts in roof space so total floor area approx 300m2

 so not massive.

 That’s really interesting to know what you’re using in Scotland.

[junglejim]

2 hours ago, JohnMo said:

@Simon, beat me to it with the cylinder comments.

 

For context on heat demand, NE Scotland total are 234m², with bed and living space at 192m² @-9 3.5kW. Not Passivhaus.

 

So unless you are huge your heat loss calcs are way off.

 

2 hours ago, SimonD said:

By the looks of that, you really do need to find yourself someone else to do the work. You don't add DHW cylinder demand to a system with a heat pump as it's priority hot water - the heat pump switches from doing heating to doing hot water, so even if your house needed 8kW for heating, which I seriously doubt, this would be fine for your hot water. How big are your kitchen/dining, living & upstairs bed? They must be massive.

 

You can go and use my tool at https://openheatloss.com to create your own heat loss and system design. This is supported by a number of videos at https://youtube.com/@OpenHeatLoss taking you through how to use the tool and what to input - the video library is developing da

2 hours ago, SimonD said:

By the looks of that, you really do need to find yourself someone else to do the work. You don't add DHW cylinder demand to a system with a heat pump as it's priority hot water - the heat pump switches from doing heating to doing hot water, so even if your house needed 8kW for heating, which I seriously doubt, this would be fine for your hot water. How big are your kitchen/dining, living & upstairs bed? They must be massive.

 

You can go and use my tool at https://openheatloss.com to create your own heat loss and system design. This is supported by a number of videos at https://youtube.com/@OpenHeatLoss taking you through how to use the tool and what to input - the video library is developing day-to-day

Thanks that’s really interesting. I’ll take a look.

[JohnMo]

On 25/05/2026 at 09:36, junglejim said:

have ufh downstairs and want provision for radiators upstairs.

The alternative to this if you're not going grant route, is to design the UFH to provide almost the whole house heat load, then have electric panel heaters in bedrooms, which only get switched on once in a while on very cold days if you feel you need to.  Plenty on here have zero heat in bedrooms. Open bedroom door for an hour you bedrooms will be the same temperature as everywhere else.

 

And make sure your heat pump does cooling, quite a few don't.  Then you get the option to cool the floor.

 

Simple wins for ASHP efficiency, cost of install reduces, direct DHW cylinder around £5-600 cheaper than a heat pump one, which buys loads of energy, especially on a time of use tariff.

[SimonD]

2 hours ago, JohnMo said:

direct DHW cylinder around £5-600 cheaper than a heat pump one, which buys loads of energy, especially on a time of use tariff.

But if you want to use the HP for DHW, then you need a plate load kit, which easily brings the price back up to the equivalent of an indirect, plus it adds complexity with additional pump, wiring & controls. My view is that it's still way better to use the HP for DHW compared to immersion as you still get at least 2x  bang for your buck.

[JohnMo]

1 hour ago, SimonD said:

But if you want to use the HP for DHW, then you need a plate load kit, which easily brings the price back up to the equivalent of an indirect, plus it adds complexity with additional pump, wiring & controls. My view is that it's still way better to use the HP for DHW compared to immersion as you still get at least 2x  bang for your buck.

No I am saying heat via immersion. Plate loading as you say just adds complexity you don't need. 

 

CoP compared to energy input depends on quite a few variables. CoP isn't always a true measure of energy input as you have to heat pipes, local fabric and cylinder.  But with a time of use tariff, what's the point of using the heat pump to heat the cylinder, leave it to concentrate on building heat and cool. Keep DHW simple. But if going down grant route DHW via immersion isn't an allowed option.