Daikin highwall heat pump overheating room - how to make it stop heating at setpoint

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timmmay

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#302990 9-Jan-2023 10:20

I recently got a small Daikin Cora FTXM25UVMZ highwall heat pump for my office, which is about 4x2m with standard ceiling heights. It's mounted high up at one end of the office, pointed down the office. It has a thermostat inside it, and I have a temperature sensor on my desk.

Overall it works quite well, but it overheats the room. The screenshot below from the Daikin app is showing:

The heat pump is set to 21 degrees on heating
The inside temperature is 25 degrees
The outside temperature is 17 degrees
The temperature at my desk is 24.2 degrees

Even though the heat pump knows it's 25 degrees inside, the unit is on and actively heating. Once I turned it down to 20 degrees it turned off. If I set it to 23 degrees, which is about what I like, it reaches about 25 - 26 degrees.

The differential changes. If I set it to 20 degrees first thing in the morning the office will be heated to about 21 / 22 degrees. I can't just subtract say 3 degrees (set it to 20 degrees for target of 23 degrees) and set it to that temperature.

Does anyone know any way to have the Daikin heat to its setpoint then stop? I could do something in Home Assistant to tweak things, but I'd prefer not to. I could also call the installer, my electrician, I suspect they'd say "I don't know we just install them".

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timmmay

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#3019065 9-Jan-2023 14:20

I can set up something in home assistant to use the temperature sensor on my desk, but it won't be especially pretty. HA can look at the desk temperature and increase or decrease the setpoint if it's too hot or too cold. I would probably do that in ApoDaemon / Python rather than a standard automation.

fe31nz

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#3019360 10-Jan-2023 00:25

The reason is simple - the heat pump only reads the air temperature at its intake. In high wall units, that intake is on the top of the heat pump, so it is reading the air temperature quite high up in the room. Hot air rises, so the temperature at the top of the room is always significantly hotter than lower down. The humans in the room, unfortunately, are normally rather lower down than the intake air temperature sensor. So the heat pump never has an actual room temperature to work from. What most manufacturers do is to subtract an arbitrary number from the intake temperature reading and use that as the guessed real room temperature at human level. There is no way to adjust that arbitrary number for the room which the heat pump is installed in, or its position in the room. There is no option to do calibration. So you just have to set the heat pump setpoint temperatures to whatever works to produce the desired temperature at human level. For example, our big Panasonic heat pump in the kitchen needs to be set to 19 if we want about 22 on heating, and to 23 if we want 24 on cooling. Things are further complicated by the flow of heat in and out of the room. For example, if we are heating in winter before the sun starts to shine in through the windows, we may need to set it to 20 to get 22, but when the sun is shining, 19 works. This seems to be a result of how the airflow works in the room to distribute the heat, and what the heat gradient is from ceiling down to the floor. If you really want the setpoint to mean what it says, you need to use a temperature sensor away from the heat pump at human level and unaffected by the airflow. And it will need to be calibrated for the room. And recalibrate it if you rearrange the room in a way that affects the airflow. I do wish that heat pump manuals actually mentioned this problem!

So to emphasise the basic point here, the setpoint numbers on a heat pump are not calibrated - they do not match real temperatures. You just use whatever setting works.