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Siting of the external sensor on a heatpump

Batpred · 2026-09-10T19:41:53Z

So I have been very happy with the pump doing hot water. Did a bit of cooling but no heating, of course... But as an obsessive engineer, the thing is quite thoroughly "instrumented", lots of monitoring around it and something is odd with the sensors. Discussion in the family gets to "but is it causing a problem now?"... But having just read Mars great sh*tshow book, the house at number 58 is what comes tomind. This is where, despite all the best efforts and all the certifications and logos, the ASHP never worked as as reasonably expected. It took a few years with the MCS circus causing havoc to progress "the diagnosis". Our goal is to sort it before it causes more than an odd chart.. The chart below plots the external temperature sensor with the weather temperature (plus a temperature sensor that we move around the home). All being well, we would expect the two external temperature lines to move along roughly in sync. But they do not. So the best hypothesis is that the neighbours boiler is doing a lot of hot water around 9am (and by the looks of it, not getting into condensing mode). When winter comes, the boiler will do heating when it feels like and play havoc with our sensor a few meters away (and that is expected to drive the weather compensation..). I checked the Arotherm Appliance Interface instructions and it clearly does not say much about locating the external temperature sensor, apart from the odd manual that goes as far as saying it should be below 2.5m high... So this seems a case of "all good and according to all instructions"..

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Air Source Heat Pumps (ASHPs)

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bobflux

(@bobflux)

Reputable Member Member

Joined: 8 months ago

Posts: 192

13/09/2026 10:20 am

My heat pump receives sunlight, so when the fan is not blowing, OAT sensor readings are hotter than actual air temp.

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Toodles

(@toodles)

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Joined: 4 years ago

Posts: 3026

13/09/2026 4:18 pm

@jamespa Installation of gas boilers would be carried out by registered gas fitters so it would be unlikely that many consumers would question the workmanship; muppets would be able to get away with it rather than the consumer doubting the muppet’s ability. With heat pumps, I would suggest that the consumer is likely to be more enlightened and also more questioning so muppets might be detected more readily. Regards, Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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Toodles

(@toodles)

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13/09/2026 4:35 pm

@bobflux My Daikin ASHP is SSW facing and the wall behind it has cavity wall insulation; during the summer, this produced ASHP temp readings of 35 - 40 C.! I think that even in wintertime, if the sun shines for a few hours, the reading is elevated by a few degrees C. Regards, Toodles.

This post was modified 1 month ago by Toodles

Toodles, heats his home with cold draughts and cooks food with magnets.

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Jancold

(@jancold)

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Joined: 4 years ago

Posts: 186

13/09/2026 6:45 pm

My Octopus electrician (part of the installer team) noted that even in April there was some sun getting through to the Daikin and asked to site the sensor on a wall in the shade. Since the way to that was under a path he did ask for a conduit which I was happy to provide.

Daikin 6Kw ASHP.
MG4 EV, Solis 8KW inverter, 32KWh battery
1926 Semi with loft and wall cavity insulation, all radiator.

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Batpred

(@batpred)

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Joined: 2 years ago

Posts: 1209

Topic starter 13/09/2026 8:26 pm

Posted by: @toodles

↑

muppets would be able to get away with it rather than the consumer doubting the muppet’s ability.

Our boiler came with the house.. We found all sorts of issues (like the internal thermostat was in the boiler cabinet!!) and when we tried to reach the muppet, he had moved to the next county. The gas side was probably safe, but everything else was shortcut.

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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JamesPa

(@jamespa)

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Joined: 4 years ago

Posts: 5561

13/09/2026 9:09 pm

Posted by: @batpred

↑

Posted by: @toodles

↑

muppets would be able to get away with it rather than the consumer doubting the muppet’s ability.

Our boiler came with the house.. We found all sorts of issues (like the internal thermostat was in the boiler cabinet!!) and when we tried to reach the muppet, he had moved to the next county. The gas side was probably safe, but everything else was shortcut.

Of course had the heat source been a heat pump it would all have been the fault of the heat pump not the muppet who installed it, but just as we dont talk about muppets installing boilers, we dont talk about the faults they leave nor assign them to 'the boiler'.

That doesn't excuse these faults of course, but it does put them in context by confirming that muppets work on boilers and leave problems behind just as they work on heat pumps and leave problems behind. In neither case is the technology at fault, the fault lies with the muppets.

This post was modified 1 month ago by JamesPa

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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Batpred

(@batpred)

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Joined: 2 years ago

Posts: 1209

Topic starter 14/09/2026 4:25 pm

@jamespa

Absolutely. Even if the same mistake was made with the boiler, this is no excuse for the heatpump installer. I just told them that I do not consider it urgent and that I do not care whether they shield it or move it.

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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