IAQ Dehumidify not working

davidpjMay 23, 2013

We have an IAQ thermostat with a Trane XL14i heat pump with a variable speed air handler connected through a Honeywell HZ322 zone panel. It's been in use for 4-5 years and I've never tried to use the system to dehumidify with the AC until now. AC and heat operations works fine.

The AC will run if I run the dehumidify option in the installer's test menu. However, I cannot get the system to run automatically. I have the humidity set point at much lower than 5% of the actual sensed humidity. Temp set point is well over 3 degrees above room temperature.

For example, the current outside temperature is 80 degrees as displayed on the IAQ. The temp set point is 85 and the room temp is 75. The humidity set point is 50% and the room humidity is 62%.

I've been over the IAQ settings many times and I cannot determine what would be the cause. Here are some of the applicable settings:

379: 1
383: 3
390: 1
391: 0
392: 70 (for testing purposes)
393: 85
394: 65

System is set to cool and fan on auto. Away mode (Southern De-humidification) doesn't work either.

Any suggestions are greatly appreciated!

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I think the problem is you have the thermostat set at 85 but the room temperature is at 75. The heat pump will not come on since there is no call for cooling. You have a heat pump, not a whole house dehumidifier. The first priority is to maintain the desired temperature. The second priority is to lower the humidity.

You are set up to go up to 3 degrees below the set point in order to reach the set humidity level. Try setting the temperature one degree higher than the room temperature and see if the heat pump comes on. It should come on a run until the humidity level is reached or the room temperature is 3 degrees below the set point. It should stop whenever either is reached.

    Bookmark   May 23, 2013 at 5:12PM
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Thanks Mike. Your were right on the mark. I was trying to use the system as a whole house dehumidifier. And as you suggested, the a/c came on when I set the temp 1 degree above the room temp.

I think I get it now, but I'm unclear about the significance of the 1 degree setting to test the system. Is this is fixed percentage over the room temp or an adjustable parameter?

I'm trying use this setup in our vacation home while we're away. The basement is the high humidity area in the summer while the rest of the house isn't bad. The basement has its own Honeywell thermostat with IAQ features with the dehumidify option. Worst case I'll have to use a basement dehumidifier preferably connected to the thermostat's dehumidifier terminals.

Thanks again.

    Bookmark   May 23, 2013 at 8:34PM
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I suggested setting the thermostat one degree above the room temperature to verify the dehumidify on demand feature is working. If it was not then you would not have seen the cooling system start up. In theory you could have set it up to 3 degrees above the room temperature. Setting no. 383 determines this value.

Don't expect precise humidity control. It is going to vary depending on the conditions.

    Bookmark   May 23, 2013 at 9:21PM
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I once saw a case study of seasonally-occupied homes in FL. IIRC, they recommended dehumidifiers as the primary tool with the cooling system employed to prevent overheating of the homes. You should check me on this because it has been a while since I read it. You might try the Florida land grant university site or the Florida Solar Energy Center.

This is not the case study link, but has some useful advice.

Here is a link that might be useful: This is not the case study link, but has some useful advice.

    Bookmark   May 24, 2013 at 2:31PM
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