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Mitsu PUHZ120Y 'Outdoor Temp 'error?

Air Source Heat Pumps (ASHPs)

Last Post by DavidAlgarve 3 days ago

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DavidAlgarve

(@davidalgarve)

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Joined: 2 years ago

Posts: 219

Topic starter 28/02/2026 4:42 pm

I have not paid too much attention to "Outdoor temperature" previously, but a problem this morning prompted me to consider it.

This is from my MelPump data and the white line shows the outdoor temperature significantly exceeding the indoor temp of 20C, with the last reading at 24C.

My Vantage Vue says that the ambient temperature is only 17C, but if I point my infrared thermometer at the thermocouple mounted on the outside of the Mitsu case, it does read 24C. (The peak of around 23C at 10:00am was when the ambient was around 10.5C)

This positioning of this thermocouple doesn't seem very sensible as it is fully exposed to sunlight, so I wonder if it is providing the most useful data for weather compensation.

Is there a case for providing some kind of shade over the thermocouple to give ambient rather than surface temperature?

This topic was modified 4 days ago by DavidAlgarve

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

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DavidB

(@davidb)

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28/02/2026 9:35 pm

Hi David, I have no knowledge of your brand of HP. I have a Vaillant Aerotherm and the instal instructions make it very clear the outdoor temperature sensor must never be in direct sunlight. The suggestion is a north wall which didn’t work for us. However the installer mounted it on the underside of a small ground floor roof extension where it seems to work well. What you describe strikes me as poor original design.

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JamesPa

(@jamespa)

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Posts: 4388

01/03/2026 9:46 am

Posted by: @davidalgarve

↑

This positioning of this thermocouple doesn't seem very sensible as it is fully exposed to sunlight, so I wonder if it is providing the most useful data for weather compensation.

Is there a case for providing some kind of shade over the thermocouple to give ambient rather than surface temperature?

If the sensor which feeds the WC algorithm is in sunlight, it shouldn't be, as @davidb says. The idea is to measure air temperature. Weather stations are in Stephenson's screens for a reason and yes a bit of shade (provided you dont create a 'dead spot') would be a good idea. It may not make a major difference however, other than on cold very sunny days.

This post was modified 3 days ago 2 times 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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DavidAlgarve

(@davidalgarve)

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Posts: 219

Topic starter 01/03/2026 12:35 pm

Thanks David & James

It almost seemed a silly question to ask because I can't understand why Mitsu would stick the 'Ambient thermistor' (TH7) in a place where it is so exposed to sunlight, although they do make the point that this thermistor works in conjunction with the '2 phase pipe thermister' (TH6) which is inside the case.

Perhaps they assume that the whole cabinet is usually mounted quite close to a wall of the house, whereas mine is about 3m away.

At 11:30 it was reading 21c at an ambient from my Vantage View of 15C. As an experiment, I put a piece of plastic trunking over the thermister and now get 17C against 16C.

It shows that you shouldn't ignore the basics!

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

👍

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