New thermostat not controlling furnace

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Old 11-10-13, 09:22 AM
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New thermostat not controlling furnace

Hello All-

Just got a new t-stat to enable home automation via Z-Wave. I replaced the "old" (actually new) Honeywell with two wires and mapped it to the new Trane per the manual and although it powers up and lets me view everything, it doesn't turn on the heat or fan.

OLD: Honeywell THX9421 (R5013)
NEW: Trane TZEMT400BB3NX
FURNACE: BRYANT 912SA30060S14-A

The old thermostat had only 2 wires used in 6 wire bundle, White going to C and Red going to R. I mapped these to 24C and 24RC respectively on the Trane. New thermostat powered up and I could hear it "click" on and off when I raised and lowered the temp, but no response from the furnace.

I went into the attic to scope things out and it turns out there is a Honeywell branded 6x10" box with a PC board that the two wires go into before it hits the furnace. So there are Red and White in to R and C in the "To Thermostat" area, and then another wire bundle going from that device to the furnace. That is connected this way: W O/B, white; Y, Yellow; G, green; then Red and Blue wires that I think go to R and C (but can't tell from my picture).

So I'm not sure if this box is part of the problem or if I've simply messed up the two thermostat wires. It seems like I could bypass the Honeywell box in the attic if necessary by just connecting the T-stat run directly to the other bundle.

Thanks for any input. I've frustrated because it looks like it should be simple but isn't working out that way.
 
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Old 11-10-13, 11:35 AM
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Welcome to the forums.

Your old thermostat only had two wires connected to it.
Red to R/Rc for 24vac
White to W for heat control

For a battery operated thermostat that would operate the heat only. No extra fan control.

On your new thermostat that needs dedicated power you are going to have to use more wires. You're going to need:

Red > R/Rc for 24vac.
White > W for heat control.
?? > C for common power for the thermostat.
Green > G for fan on control. (if desired)

I'm not sure what Honeywell module you have there. Is it a multi zone controller ?
 
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Old 11-10-13, 01:48 PM
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Thanks for your reply. The cabin is a small (<1000SF) cabin and there's only one zone, so I don't think the box can be a zone controller. At least it wouldn't make much sense. I posted a picture here: http://ehlen.net/honeywell_thing.jpg

I'm still missing something though. The old one worked with just the two wires, so why wouldn't the new one? I don't ever need to run the fan only, I just need to cycle the heat&fan based on the temp set on the thermostat.

Again, if I can just connect the thermostat directly to the furnace and bypass the Honeywell controller I'm happy to do that. It should be as simple as just pulling the two bundled cables off the Honeywell box and wire-nutting them together, I would think.

Thanks again for the input, I appreciate it. I'm only here for another 24 hours and I'd love to get this knocked out so I can turn up the heat remotely if there's a cold snap.
 
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Old 11-10-13, 02:57 PM
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Did you read my post ?

Was the old thermostat a battery operated one ? If so ..... only two wires are needed for heat.

Now you are trying to install a thermostat that needs 24vac to operate and it's not getting it correctly. I don't know what that board is doing there but I can't recommend just ripping it out without knowing your full system.
 
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Old 11-10-13, 03:32 PM
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Hi PJ-

Yes, I did read our post. I didn't understand that the battery comment was a question, sorry about that. The old one did have a small nickle-sized battery.

I ended up with enough slack in the t-stat cable to make it directly to the furnace and replaced the existing cable with the one home-runned to the thermostat. I mapped Y-->Y, G-.>G, R-->R, B-->C, W-->W1 and voila, HEAT! I'm running it through the paces now, but I think I'm out of the woods.

The hHoneywell box is labelled " Equipment Interface Module" hopefully it's not too critical...

THANKS for the help PJmax and the DIY board.
 
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Old 11-11-13, 12:23 AM
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Your original thermostat is a "communicating" thermostat and the EIM is an essential part of that communicating system. It allows for multiple thermostat functions such as heating, cooling, multiple stages, fan control and others with only a two-conductor interface cable.

Since the existing interconnect cable has sufficient conductors for the new thermostat, you need to remove the EIM and connect the various wires of the interconnect cable at BOTH ends as detailed in the instructions of the new thermostat.
 
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