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What happens when the outside temperature exceeds the upper WC point?

 
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Air Source Heat Pumps (ASHPs)
Last Post by bontwoody 10 months ago
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bontwoody
 bontwoody
(@bontwoody)
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Topic starter 03/04/2025 2:01 pm  

Ive posted this following a comment in another topic which I didnt want to take off track.

I have always thought that when using a weather compensation curve, once the outside temperature exceeds the top set point (eg a flow temperature of 40C at 17C outside), that the heat pump switches off its heating function.

However it was suggested that what might happen is that the flow temperature just stays at 40C and does not change. I can see the logic of this as at the other end I would not expect the heating to switch off at say -3C 🙂

The problem with this is though that as the outside temperature rises, if the heat pump stays on then it will cause the house to overheat and cost money needlessly. This scenario would require the user to actively turn the heating off.

Does anyone have a definitive answer to this, or do different heat pumps handle the situation differently?


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Weather Compensation
Morgan
 Morgan
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03/04/2025 2:31 pm  

I actively turn my CH off during the summer. I know that doesn't answer your question, but I feel it is the best option.


Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
14 x 500w Monocrystalline solar panels.

2 ESS Smile G3 10.1 batteries.
ESS Smile G3 5kw inverter.


   
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 Judith
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03/04/2025 2:32 pm  

Our Vaillant at present (outside temp 18C and the heat pump is off since internal temperature is above the set point) has a target temperature for water flow of 22C. In the installer menu there is also a flow temperature limit of 20C (and an upper of 46) plus a external temperature limit for switching off.

That external limit was set to 19C but I’ve put it down today to 17C.


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC


   
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 Old_Scientist
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03/04/2025 2:34 pm  

I cannot speak for other manufacturers, but I know Samsung's WC 'curves' are very simplistic with just two set points with a straight line between, and flat horizontal lines beyond. Once the two points are exceeded, the 'curve' is flat. So if I had a LWT of 30C set at 15C OAT, and 40C at -2C OAT, once the outside temperature gets above 15C, the flow temp would remain at 30C in this example.

The Samsung has a separate configurable setting to turn off space heating above a certain ambient temperature and is set to turn off space heating at 25C on my system (35C was the default).

As you surmise, this is how I would expect most systems to behave rather than simply turning off. The other option would be to extrapolate the curve beyond the set point rather than flat-lining.


This post was modified 11 months ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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bontwoody
 bontwoody
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Topic starter 03/04/2025 2:48 pm  

@old_scientist Well thats answered the question for my Samsung 🙂 Is the setting to turn off the space heating just the thermostst in the controller?


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Mars
 Mars
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03/04/2025 2:59 pm  

That’s a really good question, and the behaviour can vary slightly between heat pumps, but CIBSE actually recommends that heat pumps should shut off when outdoor temps exceed the compensation curve’s upper limit. The logic is exactly what you’ve highlighted – avoiding unnecessary overheating and runtime.

Some manufacturers implement this as a hard cut-out (e.g., once it’s 17°C outside and your curve tops out at 40°C flow, the heat pump stops heating entirely), while others might let the system idle at minimum output. The risk with the latter approach is exactly what you’ve described: if the house is already warm enough, you’re just burning electricity to maintain a flow temp nobody needs.

This is why we, like @Morgan, just turn our heating off (leaving just HW) when we hit this time of year.


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bontwoody
 bontwoody
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Topic starter 03/04/2025 3:27 pm  

@editor Thanks Mars, thats very helpful


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 RobS
(@robs)
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03/04/2025 8:32 pm  

Mitsubishi has a separate setting, like Samsung, to turn the heat pump off above an outside temperature. And turn the heat pump on below an outside temperature. Mitsubishi call this summer mode.

There's a separate setting again (called auto change over) to automatically switch from heating to cooling, or cooling to heating, at certain outside temperatures. 



   
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bontwoody
 bontwoody
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Topic starter 03/04/2025 8:40 pm  

@robs I will have to see if I can find the Samsung setting. Thanks.


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 ASHP-BOBBA
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03/04/2025 9:00 pm  

The weather compensation is a control strategy mainly directed at controlling a capacity step function by using the inverter tech and LEV-EXP devices, this is to say it runs your system slower and faster or higher and lower which ever way you would like to describe it, this is subject to OAT and demand. It is not normally a main stop or overheat protection. CIBSE and building control still mandate that you must have at least one main stat to prevent wasting energy and overheating the house without human intervention. You typically have 3 types of stop, No1 main stat (even open loop systems need a single stat somewhere in the home although set to say to 22 if the open loop is designed 20) No2 timers, this would be 7 day event, holiday or hourly if used and No3 forms of set back pauses and or temperature related function changes for example Mitsubishi Electric Ecodan summer mode function max OAT set run point.

Just get a stat fitted, set it 1 or 2 deg above your open loop design and then you will prevent loosing to much energy, just to give context, if the OAT is 20 and your open loop is designed to deliver 20 and manages to heat the room up to 22 it would not take long to get the building a further 2 deg higher with that OAT. 

Or turn the heating off end March and only back on for a couple cold days in April if needed, like we all used to on the old boiler. 🙂 

       


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 Old_Scientist
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03/04/2025 9:04 pm  

@bontwoody on my gen6 Samsung it's FSV #4013 Heating Off Temp, and by default is set to OAT of 35C (I have reduced mine to 25C). Like @ashp-bobba says above, I tend to just turn the heating off in April and back on in Oct as the weather dictates, but it's a useful safety so the heat pump isn't trying to heat the house when it's 30C outside.


This post was modified 11 months ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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 ASHP-BOBBA
(@ashp-bobba)
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03/04/2025 9:06 pm  

@old_scientist 35 DegC outdoor air temperature?


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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