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Old 12-03-2010, 06:50 PM   #1
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Honeywell TH8320 Set-Up


I recently had a dual fuel system installed with 1-stage heat pump and 2-stage natural gas furnace, controlled by Honeywell TH8320.

The installer set function 170 at 12, which is 3 heat / 2 cool (w/ aux heat). Is this correct, or should it be set at 7, which is 2 heat / 1 cool (w/ aux heat). To me it should be 7, with stage 1 being the heat pump, stage 2 furnace stage 1, aux being furnace stage 2. Am I thinking about this correctly?

I have an outdoor sensor and the heat pump lock out is set at 30F. I am having trouble recovering from a 5 degree set-back at night (Adaptive Recovery is on) with outside temperatures in the low to mid 30s. I realize AIR will try to give a nice long heat pump run to provide the most efficiency, but it never kicks on the furnace unless we are under the lockout temperature. Is this expected?

Honeywell "technical support", says the lock out should be set at 50F, which is total hogwash and defeats the purpose of dual fuel. The rep also said that running the heat pump at 30F would damage the compressor. Again, hogwash. He would not provide me the correct settings for function 170 since I am not the installer.

I would like to educate myself so that I can tweak the settings myself without calling the installer, or at a minimum have a more intelligent conversation with the installer. I appreciate all advice.

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Old 12-04-2010, 10:17 AM   #2
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Honeywell TH8320 Set-Up


30 degrees is fine, and it should be set the way it is for 3 heat 2 cool if there is no 3 heat 1 cool setting. As far as setback goes, I don't recommend this with a HP system, recover is too long and is wasting energy with a heat pump in a cooler climate. if you do set back, go no more than 2 degrees, also the thing with a 5 degree setback is that the stat will go to second or third stage to recover because of the temp differential

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Old 12-04-2010, 12:16 PM   #3
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Honeywell TH8320 Set-Up


OK, but here is the thing, it is not kicking over to 2nd or 3rd stage to recover if the temp is above the lockout point.

I suspect the tstat is calling 2nd stage but to it stage 2 is still the heat pump, therefore 2nd stage doesn't really do anything different than stage 1 (since I have 1-stage compressor). When the temps are in the 30s I am OK with using the furnace for recovery but then maintaining temp with HP -- seems like a good balance between comfort and efficiency to me.

Ideally Honeywell would allow 3 stage heat and 1 stage cool, but since they don't wouldn't 2-stage heat (w/ aux) and 1-stage cool do the same thing (except that it would have to kick to emergency to energize stage-2 furnace)?
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Old 12-04-2010, 12:23 PM   #4
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Honeywell TH8320 Set-Up


Post brand and model number of the furnace.
Some furnace's second stage can't be controlled by a thermostat. And that thermostat can't control 2 stages of gas heat when it is on a dual fuel system.
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Old 12-04-2010, 12:37 PM   #5
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Honeywell TH8320 Set-Up


you are correct beenthere, this stat does not support a 2 stage furnace when used as dual fuel. I would recommend the honeywell IAQ stat to do what you are wanting, if your furnace has a w1 and w2 terminal on control board, this stat, which I have and install often, will do hp dual fuel and 2 stages of aux heating
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Old 12-04-2010, 01:35 PM   #6
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Honeywell TH8320 Set-Up


Furnace is Comfortmaker 9MVX100. It has w1 and w2 on the board, with a DIP switch to select whether the thermostat controls the 2nd stage or if it just fires after stage 1 has fired for 16 minutes.

w1 on the board is wired to Aux on the tstat, and w2 on the board is wired to E on the tstat. The DIP switch is set to tstat. So to me it should call HP if available, then stage 1 furnace, then stage 2 furnace in order.

I have a 10wire from tstat. I have 2-wire from separate humidistat, so I wouldn't need to run humidifier from IAQ. Do I have enough wires for IAQ?

If TH8320 can't control dual stage heat, then should I let the board control it (with a loss of efficiency...)? That would make the wife happier from a warm-up point of view. I also can get less aggressive with set-back (at least during the shoulder season when HP is primary). Is this my only option unless I upgrade to IAQ?
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Old 12-04-2010, 01:49 PM   #7
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Honeywell TH8320 Set-Up


your option would be to set dip switch for control board to run w2 and aux and e from stat both to w1 on board or to upgrade to IAQ it uses only 3 wires at t-stat location and then a sub board mounted near furnace that does all the switching, because relays are near furnace, you don't hear t-stat relays clicking on and off. I like it, especially if you have a humidifier, because it shows the actual humidity percentage digitally, and would also sow the outside temp on the stat

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