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External stat issue

cliffhanger · 2025-09-16T08:22:13Z

Hi I'm back after running the ASHP for 7 months now with a different topic. see 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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Air Source Heat Pumps (ASHPs)

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cathodeRay

(@cathoderay)

Famed Member Moderator

Joined: 5 years ago

Posts: 3056

16/09/2025 6:09 pm

@cliffhanger - seriously though, I have no idea. What I do know is that any sensor can be duff eg the one that tells you your car is over-heating when it isn't. If you've checked it against two mercury thermometers, then collect some evidence and go back to whoever is covering the guarantee.

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

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5450

17/09/2025 12:54 pm

Posted by: @cliffhanger

↑

Looks like I will have to make curve changes as it is consistently inaccurate. Still under guarantee incidentally. Are they as accurate normally as Mercury?

WC curve best discovered by experiment anyway as any loss calculation (on which a theoretical determination of the WC curve would be based) is at best an estimate and at worst almost total fiction.

If its consistent I wouldn't worry too much about unless its somehow causing confusion. There may even be an 'OAT offset' adjustment in the installer menu.

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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cliffhanger

(@cliffhanger)

Eminent Member Member

Joined: 2 years ago

Posts: 21

Topic starter 18/09/2025 8:22 am

@jamespa

Thanks. Yeah any adjustments Ive made at present seem to make little difference.

May pursue the guarantee dunno yet

Cliff

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Judith

(@judith)

Prominent Member Member

Joined: 3 years ago

Posts: 554

21/09/2025 2:12 pm

Accuracy of -/+1C is normal

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) open system operating on WC

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