How Exactly Does Honeywell Overcooling to Dehum Work?


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Old 09-27-17, 11:53 AM
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How Exactly Does Honeywell Overcooling to Dehum Work?

We have a Honeywell TH8321 Wifi Thermostat for our newly installed central AC (single stage, no heat) and are very confused about how the dehumidification works. We are on our third thermostat, and are getting different explanations and information from the installers and Honeywell.

We have it set up to overcool by 2* (through low ac fan). What happens with ours is it prematurely shuts off, before the RH setpoint is reached or the overcool setpoint is reached. Then it will turn off and on and off and on many times until it either overcools or reaches the RH setpoint. There doesn't appear to be consistency with when it turns on or off, that we can figure out at least. If we turn the dehumidification off, and the stat operates on temp alone, everything seems to operate as it should.

So for example, if we have the temp set to 73* and RH set to 50%, the stat will turn on once it goes past 55%, but will not run continuously until it gets down to 50% or 71*. It will shut off before then, and under "equipment status" it will say "wait" under dehumidification. Then after several minutes, it will turn on again. Then off again after a bit, then on again. It may do this for an hour or longer until it either hits 71* or the RH level. No one can figure out what it is waiting for and what is triggering it off and on like this. (The thermostat we had before this was doing similar weird things, though wasn't operating with that 5% swing that honeywell has both confirmed and denied - one rep said it has a 4-7% swing, another said there's no such thing and it should come on even if 1% above setpoint; but I see from this forum that the 5% swing is a thing... we just installed the new stat, and it seems to be operating with the 5% swing so far, but who knows).

Is there some algorithim or behind the scenes comfort setting making it operate in this unpredictable and seemingly inconsistent way? Or is there some maximum run time built in for dehumidification for some reason? (I think I read there may be this for commercial setup, but I don't see that for residential... not sure if the condenser or air handler have such settings).

Many thanks for any help.
 

Last edited by lepetitmatin; 09-27-17 at 12:37 PM.
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Old 09-27-17, 12:18 PM
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The dehumidify mode allows the fan to run and cycles the compressor.

You cannot always obtain the relative humidity you want. If the system is slightly undersized it will run pretty much continuously and the humidity can be maintained.

My system is undersized. I set the temp I want and allow the humidity to be where it is. Since I have a forced air heating system with an added split A/C..... my basement is very cold and the humidity is higher down there. I use a dehumidifier to do spot humidity reduction or it runs when the A/C is off for the season.

If you have higher than normal humidity levels in your house you may be lacking for insulation. Humidity enters the house thru poorly insulated and sealed areas.
 
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Old 09-27-17, 12:38 PM
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Yes, but shouldn't it not trigger off before either hitting the RH setpoint or the overcool temp setpoint?
 
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Old 09-27-17, 06:49 PM
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AC with low speed fan is not usually a good option.

Does your indoor unit have a Dehum terminal? Most do not.
Do you have this connection in use at the thermostat?

AC with High speed fan is usually the correct option for overcool to dehumidify.
 
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Old 10-11-17, 02:22 AM
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I thought AC with low speed was used so that it doesn't overcool too quickly but instead has a chance to run long enough to dehumidify. Is that wrong?

I am not entirely sure about the indoor unit, I was only told that it was wired to the stat to reduce lower speed when the stat makes a call for dehumidification. The model number for the indoor unit is FV4CNB002 in case that helps.

I'm still trying to understand though why the stat would be turning the system off before hitting the desired setpoint. Any idea? This is now our third stat, and the installer now wants to switch it out with an 8321 redlink because he can't figure out what to do.
 
 

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