Thermostatic Controls - Honeywell VisionPro IAQ Dehumidification Problem

Jay11J - Thanks again for the guidance that you provided when I was setting up my heat pump in March. I have one remaining problem – the system will not cool/dehumidify down to the desired humidity level. This problem is apparent at either end of the cooling season when the heat pump runs less thus allowing the humidity to rise above the set humidity level. The current system is as follows:

I have:
- ICP 2 stage, 3 ton, 17 seer heat pump (H4H736GKA100);
- ICP variable speed, 4 ton fan coil (FVM4X4800A) with an Arco 10KW electric back up heater;
- Honeywell VisionPro IAQ YTH9421C1002 (THC9421C1004) with an internal humidity sensor.

I have set the Fan Control Board as follows:
- ‘AUX HEAT KW/CFM’ to B (Unit Size 4800, 0-20 KW/1250 CFM)
- ‘AC/HP SIZE” to C (BLU 4800, 36 HP Size)
- ‘SYSTEM TYPE’ to HP-EFF
- ‘AC/HP CFM ADJUST’ to NOM
- ‘ON/OFF DELAY’ to ENH (30sec on delay. 150sec @ 70% and no off delay)
- ‘CONTINUOUS FAN” to LO

Thermostat ------- EIM
.......1....................1 (Red wire)
.......2....................2 (White wire)
.......3....................3 (Blue wire)

EIM*------AH-------HP
W1---------O--------O (Changeover)
Y----------Y1-------Y1 (Compressor Stage 1)
W2--------W1------W2 (Auxiliary Heat)
W3--------W2**
Y2-------Y/Y2------Y2 (Compressor Stage 2)
G-----------G (Fan)
R-----------R--------R (24 VAC HOT)
Dhm2-------H (Dehumidify)
C-----------C--------C (24 VAC Communication)

* Jumper between R, Rc, and Rh, on the EIM
**Jumper J2 needs to be removed on the AH
Install a jumper wire between R and DHM1 on the EIM

110 – 0........300 – 0........510 – 0
120 – 20.......320 – 0........520 – 0
160 – 0........330 – 2........580 – 5
165 – 0........340 – 0........600 – 80
172 – 2........342 – 0........610 – 70
174 – 2........365 – E/0.....640 – 12
176 – 2........370 – 1........650 – 60
190 – 0........372 – E/0.....660 – 60
200 – 0........379 – 1........670 – 0
220 – E/3.....383 – 3........680 – 2
230 – E/3.....390 – 0........690 – 2
240 – E/9.....400 – 0........700 – 0
250 – E/9.....500 – 4........701 – 0
280 – 0........502 - 0........710 – 0
(E/number is Auto Discover and the discovered value)

I set the desired humidity level to 50 degrees using the “more” function on the thermostat. “AUTO” is displayed on the dehumidifier screen. The following note is on the ‘Dehumidification Control’ page of the Operating Manual - “Note: Auto/Off only present when using a whole house dehumidifier (not present when dehumidifying with A/C).”. This implies that the thermostat thinks that a ‘whole house dehumidifier’ is being used, but the “0379 Dehumidification control” function is set to “1 - Dehumidify with air conditioner” not “3 - Whole house dehumidifier”.

I have tried the following changes which did not correct the dehumidify problem.
1) I added the Dhm2 (EIM) -----H (AH) wire.
2) I removed the jumper wire between R and DHM1 on the EIM and the Dhm2 (EIM) -----H (AH).

What am I missing? Thank you again for your help - Richard

Anything with an "E" should be changed to a number value.

220 – change to 2
230 – change to 2
240 – change to 3
250 – change to 3
280 – change to 1 if you want a "night light"
502 - what kind of filter do you have?
660 – Change to 0 Running fan after A/C shuts off adds humidity.

I set the desired humidity level to 50 degrees using the “more” function on the thermostat.
50˚? You mean 50%?

I set mine for 45%, I would suggest you set it to that level. Do you run the fan in "ON" mode?

When there is a low cooling load, the system don't run enough to dry out the house.. The only time the IAQ will force the A/C to overcool is when the humidity is 5% above set point, it will try to over cool up to 3˚ below set point.


“AUTO” is displayed on the dehumidifier screen. The following note is on the ‘Dehumidification Control’ page of the Operating Manual - “Note: Auto/Off only present when using a whole house dehumidifier
It may be a misprint, the Auto/Off will show no matter what 379 is set to.


2) I removed the jumper wire between R and DHM1 on the
You want the jumper wire between R's and DHM1

Jay11J – I made the following changes per your suggestions:

220 – changed to 2
230 – changed to 2
240 – changed to 3
250 – changed to 3
280 – kept at 0 , I did not want a "night light"
502 – what kind of filter do you have? - changed 500 to 0 (thought it referred to heat pump filter)
660 – changed to 0

Yes, I meant 50% instead of degrees - set desired humidity level to 45%. The jumper wire between R's and DHM1 (per your original instructions) is connected. Is the Dhm2 (EIM) to H (AH) wire necessary?

The thermostat is now at cooling set point of 78 degrees and a desired humidity of 45%. After monitoring the system for a day, the A/C is currently cooling to 76 degrees (below the cooling set point of 78 degrees) and the humidity varies between 52% and 54%. We have high humidity in the Florida panhandle and the 2nd heat pump at the other end of the house is failing (losing Freon causing the A-coil to freeze up). I added Freon about 3 months ago and the charge lasted until a few weeks ago, so I turned the 2nd system off. Next Spring I will be replacing the 2nd heat pump with the same system I installed this year. The next install should be easier since you have resolved the install problems with this system.


One more question – My system is a 2-stage (cooling & heating) heat pump with electric emergency heat. I set 174 and 176 to ‘2’ (2 stages of cooling and heating). Should I set 176 to ‘3’ to include the electric emergency heat? If so, what should I set 260 (stage 3 CPH)?

Thanks again for your help in trouble shooting my system - Richard

Fan is run in 'AUTO' mode.

502 – what kind of filter do you have? - changed 500 to 0 (thought it referred to heat pump filter)
It's the air filter in the furnace/air handler.

The jumper wire between R's and DHM1 (per your original instructions) is connected. Is the Dhm2 (EIM) to H (AH) wire necessary?
Yes, if that wire is not hooked up, you are not going to get the single to tell the fan to slow down to dry out the air.


The thermostat is now at cooling set point of 78 degrees and a desired humidity of 45%. After monitoring the system for a day, the A/C is currently cooling to 76 degrees (below the cooling set point of 78 degrees) and the humidity varies between 52% and 54%.
If your humidity is that high, how tight is the house? You have any fresh air tied into the return duct?



Should I set 176 to ‘3’ to include the electric emergency heat?
No. I don't think it will allow you to do it.

Jay11J,

I have 2 air filters in the furnace/air handler 1) Flanders – PrecisionAire 16”x25”x1” and 2) a 12”x12”x1”. I added the 12”x12”x1” when the new system was installed. I change them every 30-45 days of system operation.

My house is relatively tight and fresh (outside) air is not tied into the return duct. My single story house is 3500 sq/ft and the new 3 ton heat pump is conditioning the entire house since I shut down the 2nd - 3 ton heat pump (with the Freon leak) on the opposite side of the house. I am assuming that the humidity be easier to control when I replace the failing heat pump.

Thanks again - Richard