I own an apartment with a Daikin heat pump system that provides me both with heating/cooling and hot water. The system has both in-floor heating and 4 fan coils hooked up to it, and these are on 2 separate pumps so I can disable either if needed. (e.g. disable in-floor pump during the summer months)

My issue is that using the living room fan coil to cool seems to be wrong. I know it's somewhat vague, but my understanding of cooling is that it should reduce temperature AND humidity at the same time. In my case, temperature barely moves down BUT humidity shoots up from 40-45% to 65-70% or even higher. (I have 2 independent devices measuring and confirming this.)

I tried running the heat pump's circulated water both at ~16°C and ~7°C, neither seems to help. (Apparently the latter is ideal, but the previous owner ran the system at the former with the in-floor "heating" pump also enabled, to essentially in-floor cool.)

I had 2 separate Daikin service places out, neither could find anything broken or hooked up wrong or similar. Any ideas what could be causing this?

Update 1:

I just measured the fan coil units (Daikin FWT04CATNMV1*), unit was set to 21°C while the indoor temp was around 25°C. The unit cooled down the room to 22°C as measured by my Awair Element (on the opposite end of the room) within 30 minutes, but at the same time, raised the humidity to 54% from 47%. This is much better than last summer, maybe something magically happened.

I used an infrared thermometer to measure the metal behind the air filter, it was 20.2°C - definitely not 8°C cooler than the air as hoped. They were also completely dry to touch.

*Cooling 3.31kW, 11300 BTU; Heating 4.4kW, 15000 BTU in a 40sqm room.

  • 1
    " The temperature barely moves down..." are you sure the cooling cycle is functioning correctly?
    – JACK
    May 25, 2022 at 0:43
  • Well, relatively - like I can usually move the indoor temperature down 2-3°C in exchange for the 20-30% increase in humidity. But again, supposedly there is nothing wrong with the hardware - 2 separate technicians from 2 companies say so May 25, 2022 at 1:14
  • 1
    How long is the system cycling for in any given hour? Is it producing condensate at the indoor coil(s), for that matter? May 25, 2022 at 1:44
  • 1
    Can you measure the temperature of the coils? They should be about 8C cooler then the air after it stabilizes. Allow at least 25 minutes of operation before taking the measurement. Also check for a condensate drain as there is no way it should raise the humidity. The condensate forms when the coil temperature is below the current dew point.
    – Gil
    May 25, 2022 at 2:56
  • I'll measure them :) May 25, 2022 at 16:03

2 Answers 2


Temperature and humidity are related.

Humidity is more fully "relative humidity" - how much water is in the air now .vs. the maximum amount of water that air at this temperature could hold.

If the fan coil is running below the dewpoint temperature (the temperature at which the amount of water in the air is 100%) then water should be condensing out of the air and running away to a drain. If it's NOT, then the air gets cooler, but the water stays in the air, and the relative humidity rises, since the cooler air has a lower capacity to hold water, and the same amount of water is in the (now cooler, with less capacity to hold moisture) air.

If you can measure dewpoint, you can get an invariant view on how much water is in the air.

If the condensate is not being drained, that can lead to the water getting back into the air.

  • What do you exactly mean by "fan coil is running below the dewpoint temperature"? The water that's circulated by the heat pump (currently set to 5°C) or the target temperature set on the fan coil (e.g. 22°C?) I don't think I'd ever set the latter to a temperature below the dew point - e.g. just did a quick calculation for my present situation of 24.2°C 49%, which resulted in a dew point of 12.8°C. May 27, 2022 at 8:09
  • @TamásDeme -- the temperature the metalwork on the outside of the fan coil reaches May 27, 2022 at 11:45
  • Yep, I measured that to be around 20°C, see in original post's update. May 27, 2022 at 15:14

That coil (cooling [assuming a blower is the fan coil]) has to be below the dew point of 12.8C for water to condense out, if it is above it you will just get warmer which appears to be happening and the humidity will continue to rise.

The biggest thing the AC does contrary to popular belief is remove moisture from the air a byproduct of the dehumidifying process is that it cools the air. dew point ie dewpoint is the temperature moisture will condense out of the air, the coil has to be below this. This changes constantly as the ambient changes. Also when measuring temperatures measure the actual thing, not what the control is set at. Thermostats work on errors and do something when it detects an error and undoes it when the error goes away. When your heat pump in cool mode is set for 5C, when the water gets warmer then that an error is produced and the thermostat causes the heat pump to turn. When it gets down to 5. it turns it off assuming it is calibrated properly.

Moisture condenses on a surface colder then the dew point ie dewpoint is the temperature moisture will condense out of the air. Simply put the coil that the 5C water (measure it to be sure) is at least 12.8C+ or cooler. If that heat pump is not getting the water at the coil to 6 or 7C you have a problem with the heat pump or the circulation pump. Unless you have a special heat pump both zones must be in either heat or cool. You may have a frozen condenser but I need to know more before determining that.

My recommendation is get an HVAC check the system, in the long run it will probably be the most economical solution.

  • Yep, that is my knowledge of the stuff too - watched too many technology connections videos haha. But yep, seems like my main heat pump unit is broken: I just checked and seems like despite having it set at 5°C, the circulated water's temp according to its own measurements was ~20°C, and none of the internal compressors / pumps were running. I turned it off, then on, and suddenly all the measurements were saying it's around 50°C. I'm really annoyed because it definitely feels broken, and 2 different teams said it's fine. May 28, 2022 at 3:42

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