I have a new Carrier Infinity system. Even though there is only one (evaporator) coil in the plenum (no reheat coil), it can operate in a dehumidify mode. In normal "cooling" mode, the fan runs at its highest speed, just as you would expect, in order to prevent the evaporator from icing up. From what I could find in the technical manual the installer left behind, it seems that in dehumidify mode, the fan intentionally runs at a slower speed. I've often noticed the compressor running with the fan running at a low speed, and if I look at the thermostat, it indeed shows that it is "dehumidifying". I've also noticed sometimes the fan running at high speed without the compressor running.

My question is:

If the fan is supposed to be running at high speed to avoid icing, why would the system be intentionally set up to run the fan slower? Doesn't that risk icing? Is it an acceptable consequence of trying to get as much moisture as possible to condense out onto the coils? What exactly is the system trying to do and what is likely happening when it is in its dehumidify mode?

1 Answer 1


The fan is not running at high speed to prevent icing, but for maximum cooling of the space. The cause of icing is that the system is undercharged with refrigerant or the throttling valve (expansion valve) is malfunctioning so that the low side pressure is so low that the temperature of the evaporator coil is below the freezing point of water.

I have always been told that an a/c system is supposed to be operated so that the internal temperature in the evaporator coil is 38 to 40 F. I have always supposed that this allows a safety factor in the case where the expansion valve closes too far.

My understanding of the dehumidify mode is that the cooling cycle is run continuously, but at a lower fan speed. The lower fan speed means less cold air is circulated, but the automatically controlled expansion valve opens to maintain the evaporator coils at the set temperature (of say 40 F). By running continuously at a lower flow rate all the water vapor is removed from the room but the room is not overcooled.

Our 26 year old Carrier is a base model 12 SEER which (at 42 kBTU/h or 3-1/2 ton) is undersized for our house in Dallas summer heat. On very hot days (~> 100 F) the compressor will run continuously for 6 to 8 hours in the middle of the day. The result is very dry air inside and I have to use lip balm in the middle of the summer or my lips get very chapped.

I have not tried to measure the humidity inside, but I may do that. We keep the thermostat at 74 or 75 F. I sit under a vent at the kitchen table and like to have cold air from the vent bathing me. This air would be very dry so perhaps this is why I get the chapped lips.

  • I thought the expansion valve was really just a fixed orifice and the low side pressure would be dependent on all sorts of things - pressure differential the compressor can create (compressor capacity), airflow over the evaporator and return air temperature and humidity (how much heat is being delivered to the evaporator).
    – Anthony X
    Jul 3, 2017 at 16:55
  • Simple a/c units used to have a fixed orifice, but AFIK that is a thing of the distant past. Just looking at my a/c I can see a feedback connection from the expansion valve to the low side refrigerant line. Presumably this is a temperature sensor on the outside of the low-side line which the controls on the expansion valve use to achieve a set temperature of the low-side line. Jul 3, 2017 at 18:16
  • On my unit there is also a line connecting the high side line to the region of the expansion valve. This is large enough to carry refrigerant so it may be a variable shunt. Your Carrier Infinity must have these connections and maybe more. The expansion valve is not fixed. What is the SEER of your Infinity? What is the model number and its BTU/h rating? Where are you? Jul 3, 2017 at 18:28
  • Canada (Toronto area), SEER between 13 and 15, depending on what piece of paper I look at (I think the 15 is an over-inflated claim), 2 ton.
    – Anthony X
    Jul 3, 2017 at 18:44
  • I don't know why you would question 15 SEER. Carrier offers up to SEER 21 in the Infinity line. Jul 3, 2017 at 19:41

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