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I have a zoned HVAC system that uses a Honeywell control board(see below) and dampers for zoning. If I set the upstairs thermostat to call for cooling, it will cool for a while before suddenly abandoning cooling and turning the heat on. I set the thermostat to 77F last night and woke up this morning sweating to find the thermostat was STILL set to 77F but the room temperature was 83. I went downstairs and saw the actual furnace flame lit and the system light on the control board red(indicating heat).

So far, I've found the following:

  • swapping upstairs and downstairs thermostats doesn't fix the issue(meaning it's not the thermostat doing weird stuff)
  • calling for cool on the downstairs thermostat only seems to produce expected behavior (it cools to set temperature and then shuts off)
  • calling for cool on upstairs and downstairs triggers this weird behavior, even if they're both set to the same temperature
  • calling for cool upstairs only also triggers this behavior

The issue began when I removed the upstairs thermostat from the wall to re-seat it properly(whoever installed it hadn't used plugs so it was basically hanging off the wall). I drilled a couple of holes, put some plugs in and re-seated it on the wall. That seems like a coincidence but it felt worth mentioning

honeywell control board

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  • What does the wiring on the upstairs thermostat look like?
    – negacao
    Commented Aug 22, 2021 at 12:16
  • @negacao like this: imgur.com/a/KbDSu1n
    – robertmain
    Commented Aug 22, 2021 at 12:22
  • Thinking when you drill the holes for the new plugs might damaged the wires(shorted). Those brown stains on control board might be an indicator of something also, look like burn marks on white board, might just be stain.
    – crip659
    Commented Aug 22, 2021 at 13:08
  • No the brown stains on the board after there when we moved in before I did anything(I'm thinking maybe heat from the resistors discoloring the board) I had a look at the wire and it looks fine, it also doesn't run behind where I drilled
    – robertmain
    Commented Aug 22, 2021 at 13:13
  • Unlikely your repair is a coincidence. You may have damaged the thermostat wire with your drill or new screws, you may have metal filings from your drill shorting something in the stat. Hopefully there's some slack in the thermostat wire in the wall. Remove the backing plate again, pull the wire out as far as you can and look for damage. Blow the thermostats and backing plates with an air can. If the problem persists, try swapping the two thermostats and backing plates in different combinations to see if the problem follows one of those parts.
    – jay613
    Commented Aug 22, 2021 at 13:33

2 Answers 2

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The solution ended up being to transfer the HVAC wiring for zone 1 to zone 3 thus pointing the finger of blame at something being defective with zone 1 on the control board.

Guess I need to buy a new control board at some point

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Your Honeywell panel is wired as if both dampers are spring-open / power-close. In normal operation, the damper for the zone that is not calling for heating or cooling is powered closed when only the other zone calls for heat or cool. So when only upstairs calls for heating or cooling, there should be little to no air coming out of the downstairs registers. Verify that, and repair or replace the downstairs damper if you get significant air flow from downstairs registers when only upstairs is calling for heating or cooling.

Are your thermostats the automatic-changeover type? Meaning that they will call for either heating or cooling depending on the room temperature without you having to physically switch to "heat" or "cool" on the thermostat?

If so, and if the downstairs damper is malfunctioning, then when you turn on cooling for upstairs only, downstairs becomes too cool, so (with automatic changeover) the downstairs thermostat eventually calls for heat. This Honeywell panel has a 20 minute timer to resolve conflicts between calls for heating and cooling from different thermostats. In that case, as soon as there is a conflict, the zone that called first gets another 20 minutes of operation, then the panel gives the other zone priority. The end result would be alternating 20-minute cycles of heating and cooling. And my guess is that, at least upstairs, heat wins.

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  • Interesting insight. I didn't know about the dampers, but that does make sense. I have noticed that when upstairs is cooling - air comes out of the downstairs registers as well. I figured it was just some leakage around the dampers. I'm not too worried about that for the time being, but it was on my radar... As for the thermostats - they CAN have that functionality under "Auto" mode - but right now they're both on cooling only mode.
    – robertmain
    Commented Aug 22, 2021 at 17:25
  • I'm starting to suspect that either the wiring has a short somewhere in the wall, or something is wrong with that zone. You'll notice that control board has 3 zones, the third of which is unused. I'm wondering about (with the control board turned off) moving the problem zone to the unused zone 3 to see if that helps. Does that seem like a useful avenue for exploration?
    – robertmain
    Commented Aug 22, 2021 at 17:27
  • Can't hurt. But I would turn on the upstairs A/C, wait for the unwanted heat to come on (20 minutes later?), and then start probing the thermostat wire terminals on the Honeywell panel with a voltmeter to see which thermostat is calling for heat, if any. Then figure out why. Crossed / chafed thermostat wires somewhere in the house is a possible explanation.
    – MTA
    Commented Aug 22, 2021 at 17:38
  • I have been wondering about doing that - but I'm terrified of shorting anything out and breaking things by putting my volt meter across terminals on the control board. I mean I know which thermostat is calling for heat - it's clearly the upstairs zone because it happens if that system ever calls for cool. If I turn that zone to "Off" on the thermostat and operate only the downstairs thermostat it doesn't happen.
    – robertmain
    Commented Aug 22, 2021 at 17:43
  • To see if there's a short in the upstairs thermostat wiring without your meter touching the Honeywell board, first pop your upstairs thermostat off the wall leaving only the base, then detach the 4 upstairs thermostat wires from the Honeywell and separate the wires from each other so they're not touching. Read ohms, not volts between all combination of 2 wires on the thermostat base. Any reading other than infinity is a short. Do not touch the probes with your fingers when you measure. Read: RY RW RG YW YG WG.
    – MTA
    Commented Aug 22, 2021 at 19:06

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