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Hot water heating in parallel with space heating

 
Air Source Heat Pumps (ASHPs)
Last Post by bontwoody 8 months ago
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 Temperature_Gradient
(@temperature_gradient)
Estimable Member Member
Joined: 1 year ago
Posts: 69
Topic starter 07/01/2026 12:37 am   [#2715]

One of the downsides of most ASHP installations (compared to a boiler) is that they cannot provide hot water heating and space heating at the same time, the ASHP output is either routed through to the DHW cylinder coil, or the space heating loop, but not both simultaneously. So with an ASHP, reading user experiences, this then requires users to block out times when the space heating can be shut-off to prioritise water heating, which isn't ideal. Or when the DHW heating kicks in automatically, the space heating is interrupted.

The different operating temperatures of the ASHP when heating DHW and space heating would make parallel operation, as used with a boiler and regular cylinder impossible because the lower space heating flow temperatures would suck heat out of the DHW cylinder, or using higher DHW temperatures flowing to the space heating would hit the CoP for the space heating.

But that got me thinking, if a DHW cylinder had two coils, a smaller, lower power coil at the very bottom of the tank, and the regular, high surface area/power main coil above it, higher up. With such a configuration, the bottom coil could then be used with the lower flow temperatures used for space heating, to warm/pre-heat the DHW to the lower temperature, with the stratification of the water in the tank preventing it pulling down the temperature of the rest of the tank, while ensuring a steady supply of warm water.

If the bottom coil was plumbed into the ASHP loop before the 3-port valve, so that it was always in the circuit (or possibly with a basic 2 port valve to switch it in on based on tank bottom temperature being below the ASHP output temp) it would provide some continuous heat into the cylinder whenever the ASHP was running, keeping the DHW at a minimum temperature in parallel with the space heating. When the ASHP switched to DHW heating, it would heat both the main coil and the bottom coil to the higher temperature, lifting the whole cylinder to the higher DHW temp.

Is there anything which would stop this from working as intended?

 



   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 5 years ago
Posts: 1059
07/01/2026 7:58 am  

That sounds like an interesting idea. Coil sizes would be a potential issue, perhaps an external heat exchanger could be used instead.


House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
10kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 5414
07/01/2026 9:06 am  

Posted by: @temperature_gradient
↑

One of the downsides of most ASHP installations (compared to a boiler)...

Its not really a downside of ASHPs, its a downside of using weather compensation.  Boilers with WC have exactly the same limitation.

Posted by: @temperature_gradient
↑

Is there anything which would stop this from working as intended?

Other than total load I dont think so

Posted by: @bontwoody
↑

That sounds like an interesting idea. Coil sizes would be a potential issue, perhaps an external heat exchanger could be used instead.

An external heat exchanger would need a pump which would be likely to disturb stratification.  Cylinders with multiple coils (eg ones with a solar coill for solar thermal installations) are available!

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 5 years ago
Posts: 1059
07/01/2026 9:20 am  

Posted by: @temperature_gradient
↑

An external heat exchanger would need a pump which would be likely to disturb stratification.  Cylinders with multiple coils (eg ones with a solar coill for solar thermal installations) are available!

Thats true, in fact my old thermal store had a solar coil at the bottom and a splarge pipe in the middle/top. Might be ideal for this sort of use if I still had the house and was running the heat pump without antifreeze as I do now!

 


House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
10kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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