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Siting of the external sensor on a heatpump

 
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Air Source Heat Pumps (ASHPs)
Last Post by cathodeRay 24 minutes ago
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cathodeRay
 cathodeRay
(@cathoderay)
Famed Member Moderator
Joined: 5 years ago
Posts: 3049
11/09/2026 10:58 am  

Posted by: @transparent
↑

I would expect the air being pulled past the temp sensor to have a cooling effect.

Here's my heat pump running with a steady mildish 'OAT' over three hours last April:

 

image

 

The unit is doing its usual Midea cycling thing, and as you see, when it it running (LWT rising), the 'OAT drops by one degree. I'm inclined to think this is more than just pure wind chill, it is the cooler exhaust air when it is running contributes to the air intake, and that has a cooling effect.

Here it is over three cold hours in January this year, running regular defrost cycles:

 

image

 

This time, there are probably two things going on. When running outside the defrost cycles, the exhaust air is cooler than the true OAT, and so lowers the local OAT. During the defrost cycles, the unit pushes warmer air through the unit, which has the effect of raising the local OAT. These spikes peak at about 4° above the baseline OAT.

Posted by: @transparent
↑

Don't any ASHPs have a connector on an internal control board to which a completely separate sensor can be connected?

Not that I know of on Midea units, though I do have the option (not as yet implemented) of adding an independent OAT sensor connected by modbus to my monitoring setup. I haven't done it yet because (a) I can't work out where to put the sensor and (b) I don't really need to know the true OAT, if there is such a thing. What if it differs between the sunny and shaded side of the house, which one is correct? Probably the shaded side, but that isn't a reflection of what is going on on the sunny side.

Posted by: @transparent
↑

Surely manufacturers must've contemplated a situation where sunlight will affect the readings from a sensor which is physically attached to their own product?!

I wouldn't be so sure!

 


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


   
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