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Midea anti-freeze valve issues with DHW and heating turned off

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 1 month ago
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 pommedeterre
(@pommedeterre)
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Topic starter 24/01/2026 1:55 pm  

I have recently had a 4kW r290 Midea monobloc air-to-water unit installed, and thought I should warn other people of an issue I have, or perhaps get advise on mitigating it.

The house where the heat pump is installed is not occupied, so I turned the heating and hot water off on the control unit so I wasn't paying to heat an empty (currently poorly insulated) house.

It hit about 4 degrees overnight and the anti-freeze values opened outside and dumped all the radiator circuit water. I didn't think this would happen because I thought the unit would turn itself on when the flow temperature got too low, to avoid the anti-freeze valves opening.

The installer is blaming a bug in the Midea unit firmware, saying that indeed it should have turned itself on. Midea customer support are not particularly coherent, but they seem to be acknowledging the behavior without acknowledging it as a problem. Midea also claim it is not a firmware bug.

The current solution is to leave the heating on at the lowest temperature on the controller, which is 17 degrees. But I'd rather not be paying to heat a house that is not occupied and is not well insulated.

So I'm curious whether my expectation that the controller keep the flow temperature above the anti-freeze valve activation temperature is realistic (the installers suggest it is, but I'm not sure how much to trust them). Do other heat pumps work in this way?



   
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 Judith
(@judith)
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24/01/2026 2:11 pm  

From my understanding your valves operated as they should they dumped water to save damage to the ashp.

You need to find the “away” mode if Midea have one and set the internal temperature to about 10C (our old boiler used to cut in at 8C) but it has to be on the frost protection mode.

Otherwise, just like a boiler the only safe way of keeping an unheated house is with all of the wet systems drained. But why do you wish to harm the house fabric by leaving it unheated?


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 4.7) open system operating on WC


   
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JamesPa
 JamesPa
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24/01/2026 2:32 pm  

Posted by: @pommedeterre
↑

The house where the heat pump is installed is not occupied, so I turned the heating and hot water off on the control unit so I wasn't paying to heat an empty (currently poorly insulated) house.

I guess it depends on exactly what you did.  If you cut power the the control unit then or the outdoor unit then quite possibly it wont switch on.  If you just set heating to 'off' on the control unit then yes I would expect it to go into frost protection mode when the temperature falls, and if it doesn't that would seem like a design flaw.

Another possibility is that the a/f valves trigger at a slightly higher temperature than the Midea or are in a colder location with the same effect.

The upside is that you have now successfully tested your antifreeze valves!


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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