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Howdy HVAC thread - our thermostat appears to have eaten poo poo over the past couple of days and I'm looking at trying to fix it myself. The old unit was at least 10 years old (probably closer to 20) and just went blank on the screen. I'm seeing what appears to be a disconnect between the old unit and the new unit, though I tried to buy as close to the old unit as possible just to make it easier on myself. Old unit: Honeywell TH9421C1004 New unit: Honeywell RTH7600D The old unit was wired with Red, Blue, and Green wires going, respectively, into slots labeled 1, 2, 3:  This appears to feed into the control unit in the same way (see the top-left of this image):  The old thermostat controlled both our furnace and our air conditioning, and was also wired to not need any batteries. Based on some documents I've read, I'm a little confused about how this unit was able to do all of that with Red/Blue/Green wires - I would have expected white and yellow for heating and cooling, but the white and yellow wires in each of the pictures aren't connected anywhere. Perfectly willing to find an HVAC pro, but most of what I've read so far seems to indicate that this should be a straightforward installation, I'm just unable to translate between the two systems.
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Mar 5, 2024 02:38
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| # ¿ May 21, 2024 00:37 |
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Appreciate the responses from you both. I think devicenull's makes more sense for my specific situation.devicenull posted:Nah he's got a weirdo nonstandard thermostat - https://www.doityourself.com/forum/thermostatic-controls/550703-how-upgrade-honeywell-th921c1004-wifi-thermostat.html I'm a complete novice here so my takeaway is almost certain that I'll be hiring an HVAC pro for this. Are you essentially saying that the control board is taking inputs from every other wire aside from the R/B/G ones, then converting that input into something that feeds to the thermostat through those R/B/G outputs? If that's the case, then it'll def be professional time, as I can't access the wire between the thermostat and furnace. The wires feeding into that control panel are accessible (and I assume coming from both the furnace and AC, separately), so hopefully a good pro will be able to convert that using the existing wire. If not, well, we're already gonna have to open up some walls for unrelated projects, might as well just open this one up too.
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Mar 5, 2024 03:14
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Shifty Pony posted:That control board is basically handling all the standard switching and signaling to the furnace, the cable to the thermostat is a special proprietary communication link. The good news is you have white and yellow as well running to where the thermostat is, so you won't need to run new cable. Whoever installed the control board originally literally screwed it into a ventilation pipe, so we'll likely just have the HVAC pro bypass it entirely while leaving the old box up. Glad to know that they at least somewhat future-proofed it with those additional cables. I'm in fintech, not actual real tech, so I'm going the pro route. Thanks for the help all, I'll follow up when we get it fixed. It was neat to learn some of the backend workings of the machine.
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Mar 5, 2024 15:36
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HVAC guy was also mystified by our old weird format at first, but he got it all set for us. Definitely not something I would have been able to handle on my own. Thanks everybody for the advice!
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Mar 6, 2024 18:10
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Whatever the fix was, it works now! We left town til Monday but I can take After pics if anybody cares. Fortunately did not require any new wires to be run through walls.
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Mar 7, 2024 17:40
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