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External stat issue

Air Source Heat Pumps (ASHPs)

Last Post by cliffhanger 5 minutes ago

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cliffhanger

(@cliffhanger)

Eminent Member Member

Joined: 7 months ago

Posts: 17

Topic starter 16/09/2025 9:22 am

I'm back after running the ASHP for 7 months now with a different topic.

see ( https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/is-it-kiss-or-constant/paged/3/)

The pump runs fine with a constant temp of 20 degrees inside. It does cycle. I think we decided it's too big at 9kw.

Anyway how important is the accuracy of the external stat? Mine is out by 3-4 degrees C when compared to two mercury thermometers. The mercury is nearer predicted temperatures by forecast.

I would consider this important but should I make a thing of this?

Thanks

Cliff

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Old_Scientist

(@old_scientist)

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Joined: 1 year ago

Posts: 249

16/09/2025 10:01 am

Is it reading high or low?

Measurement of the outside temperature will directly affect Weather Compensation where the target flow temps are determined by the outside temperature, but if it's out by a known margin (3-4C), you may be able to offset the WC curve by the same amount.

It may also affect functions such as anti-freezing (if your heat pump has such features), which may be set to trigger at temps below 5C, either meaning the do not trigger when intended or trigger too early depending if it's reading high or low.

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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cathodeRay

(@cathoderay)

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Posts: 2161

16/09/2025 10:20 am

Posted by: @cliffhanger

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Anyway how important is the accuracy of the external stat? Mine is out by 3-4 degrees C when compared to two mercury thermometers.

Do you mean sensor rather than stat? If so, and we are talking about the OAT (outside air temperature) as reported by a heat pump, then this something that has cropped up a number of times, particularly when the sensor is inside the heat pump, and so samples air that is affected by the heat pump's own activity.

More generally, it may not matter too much if it is consistently inaccurate. In effect, the WCC settings will accommodate that consistent inaccuracy. But if it is inconsistent, then things may get more interesting. There's currently a discussion on what the real OAT is on this thread.

Midea 14kW (for now...) ASHP heating both building and DHW

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cliffhanger

(@cliffhanger)

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Joined: 7 months ago

Posts: 17

Topic starter 16/09/2025 10:39 am

Posted by: @cathoderay

↑

Posted by: @cliffhanger

↑

Anyway how important is the accuracy of the external stat? Mine is out by 3-4 degrees C when compared to two mercury thermometers.

Do you mean sensor rather than stat? If so, and we are talking about the OAT (outside air temperature) as reported by a heat pump, then this something that has cropped up a number of times, particularly when the sensor is inside the heat pump, and so samples air that is affected by the heat pump's own activity.

More generally, it may not matter too much if it is consistently inaccurate. In effect, the WCC settings will accommodate that consistent inaccuracy. But if it is inconsistent, then things may get more interesting. There's currently a discussion on what the real OAT is on this thread.

I mean OAT. I have an external stat facing north around 2m up

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cliffhanger

(@cliffhanger)

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Posts: 17

Topic starter 16/09/2025 10:40 am

Reading high incidentally

This post was modified 23 minutes ago by Mars

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cathodeRay

(@cathoderay)

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Joined: 4 years ago

Posts: 2161

16/09/2025 10:59 am

Posted by: @cliffhanger

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I mean OAT. I have an external stat facing north around 2m up

If it is measuring (and only measuring) the OAT, it is a sensor, if it measures the OAT and then does something like turn the heating on or off, then it is a (thermo)stat. It is difficult to answer your original question (how important is accuracy) if we don't know what you are talking about!

Midea 14kW (for now...) ASHP heating both building and DHW

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