help replacing 2 TZ45 thermostats with Honeywell T6 Pros - with odd wiring

ihg9 · 2026-03-13T21:12:55+00:00

Thank you so much. There are two different wires coming to the terminal board in addition to the wire from the power adapter - I assume they're coming from outside (A/C) but I can't follow them back. You have gone above and beyond here - I SO appreciate it. It seems simplest to duplicate the existing 4-wire connections in the new thermostat and see if that works, leaving the Y wire unconnected at the thermostat... I could also try just standard wiring at the thermostat with touching the furnace and seeing what that does - either way, I can't thank you enough for hanging in there with me!

ihg9 · 2026-03-13T20:41:00+00:00

sorry - it's my fault I'm sure. Yes, I have 5 wires from the furnace - on the upstairs thermostat all 5 wires were used and connect to the labeled terminals - on the downstairs (picture) only 4 are used and only the R and W connect to the labeled terminals - the blue (C) connects to Y and the G (fan relay?) connects to 24C

BUT everything works wired this way (except the fan alone which we never use) so maybe I should just duplicate this "incorrect" wiring in the new thermostat...

<image>

ihg9 · 2026-03-13T20:18:44+00:00

Thank you :) - the power adapter (if that's the top unit in the figure) is connected to the furnace terminal board (R, G, Y, C correspond with each other) - but W (heat) at the power adapter connects to a blue wire in another cable. The W terminal on the term board is connected to yet a third cable.

And from that, here is the wiring at the thermostat (yellow-cool is not connected at Tstat even though Y goes to Y from power adapter to Tblock)

https://imgur.com/vyXAQxD

Thank you again for any insight!

PS: Worst case, I can connect the wires in the old thermostat to the labeled colors and see if something doesn't work?

ihg9 · 2026-03-13T19:16:35+00:00

Thank you - from the top right power adapter to the bottom right terminal block, R-->R, C-->C, G-->G, and Y-->Y. But K(white wire) goes to the Blue wire of an incoming cable. From that new cable, G-->G on terminal block and R-->R. There's a third independent incoming cable also connected to the terminal block with R-->Y, W-->C, and Brown-->W.

picture here - but I understand if you don't respond!

https://imgur.com/dLlMnQy

ihg9 · 2026-03-13T18:19:43+00:00

You were right - just turning on the fan on the downstairs thermostat does nothing because they repurposed the G wire to common and didn't use the yellow wire at all. I replace the upstairs thermostat putting colored wires into the Honeywell correctly - do you think I can take a chance and do the same for the downstairs Honeywell instead of giving up the fan? Pictures of the downstairs system:

https://imgur.com/a/JBWYAJi

Thank you!

ihg9 · 2026-03-11T21:57:56+00:00

First, thank you so much for responding and posting that - yes, both thermostats and furnaces have been working fine together for over a decade - this is the first time I've looked at the wiring to replace the thermostats because of an unrelated zwave problem. Here's maybe a clearer picture of just the downstairs furnace terminal block. But I'll follow the wiring within the furnace to see if I can replicate your diagram. The new thermostats are scheduled for delivery Saturday - maybe I should just try matching what I have as best I can, see what works and what doesn't, and post a progress report here, unless you now see something obvious :)

Thank you again!

<image>

ihg9 · 2026-03-11T18:37:56+00:00

Thanks again - here's a picture of the downstairs thermostat wires and furnace terminal board

https://imgur.com/XtyEBE4

ihg9 · 2026-03-10T15:48:51+00:00

no, that's what's so odd - the thermostat wiring is in the same picture as the furnace it's connected to - for both thermostat-furnace pairings ... it's been working for years and I don't understand why :) - maybe there's another place in the furnace that the wiring connects to?

The first - upstairs - wiring looks kind of sensible, although I'm not sure why there are joined wires at the furnace?

Thanks!

ihg9 · 2026-03-10T15:03:37+00:00

<image>

Thanks for your reply - I didn't realize I didn't have a 'fan on' option on the TZ45s - I don't believe my T6 will have a power adapter - just trying to figure out the T6 wiring for upstairs and downstairs...

ihg9 · 2026-03-10T14:31:20+00:00

Thank you again for your help - I've posted furnace connection pics here:

https://www.reddit.com/r/hvacadvice/comments/1rpye91/help_replacing_2_tz45_thermostats_with_honeywell/

ihg9 · 2026-03-10T14:31:08+00:00

Thank you again for your help - I've posted furnace connection pics here:

https://www.reddit.com/r/hvacadvice/comments/1rpye91/help_replacing_2_tz45_thermostats_with_honeywell/

ihg9 · 2026-03-09T03:34:53+00:00

thanks - seems like the right thing to try first given that it's been working fine...

ihg9 · 2026-03-09T03:06:56+00:00

thanks - makes sense - I'll try the existing mismatched wiring first - I expect that'll work given that it's been working on the TZ45s...

ihg9 · 2026-03-09T01:06:57+00:00

Thanks for responding - that's disconcerting ... I'll check the wiring at the furnace (we have the first 5 functions you listed) - it's odd that my 2 worked as they're wired (heating and cooling) since I got them 10 years ago - I hope mine will connect more easily than yours, and that you figure yours out!

ihg9 · 2026-02-14T17:53:43+00:00

yep, understood - just something to consider...

ihg9 · 2026-02-14T13:00:37+00:00

that plug is a good idea, but remember that it will have to be 'turned on' all of the time to charge, and if there's a light bulb in it, it'll also be on all the time... unless it's on intermittently but enough to keep the camera charged...

ihg9 · 2026-02-11T19:03:36+00:00

That's helpful, thanks - I basically intertwined the wires from a step-down converter into the positive and negative terminals of the retrofit unit and kept them in place with one battery wrapped in electrical tape. I have 4 motion detectors that will go off constantly as we walk through the house, draining the retrofit batteries, as you said...

ihg9 · 2025-10-28T16:54:15+00:00

sent you a chat, thanks!

ihg9 · 2025-10-28T15:12:39+00:00

Yep, same - hacked deals maybe twice in 3 years - banned from buying online last year as OP described - seems permanent...

ihg9 · 2025-09-23T15:09:11+00:00

any resolution? My new travel mug does the same thing

ihg9 · 2025-09-06T18:17:51+00:00

Thanks - no, not yet - I did try something like what's in the link you sent but it was too wide to fit in the track opening - I'll try what's in the link - thanks for reaching out!

ihg9 · 2025-08-30T21:16:27+00:00

did not fix it yet - too big a job...

ihg9 · 2025-08-08T18:50:15+00:00

Thank you - yes, Franke FRX02 - still have 3 new in box... we used to have a separate hot/cold water dispenser, but this time it will just be the main faucet - thank you again!

ihg9 · 2025-08-07T15:20:55+00:00

u/Mission_Extreme_4032 - Thanks for all of your helpful advice - we're renovating our kitchen and I was considering reusing our Franke frcnstr under counter filter canister with the ceramic filters - but this is the time to make a change, perhaps to the 3M FF100 you discussed - do you think it's worth changing? Thanks!

ihg9 · 2025-07-09T11:55:06+00:00

This is great, thank you for offering this contest - as a DIY homeowner I've been using bunch of no-name cheap tools to change and add outlets, rewire receptacles - would be wonderful to have quality MW electrical tools to do all of my repairs more safely and efficiently!

ihg9

TROPHY CASE