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Need help re-wiring thermostat for Trane furnace and AC


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04-20-17, 10:47 AM   #1 (permalink)  
Need help re-wiring thermostat for Trane furnace and AC

Hi, I just moved into a house that has only 3 wires going to the thermostat -- R, W, and Y. The existing thermostat has a jumper from R to RC.



I attempted to replace the thermostat with a smart thermostat model ADC-T2000 from alarm.com, but found that it would not work properly without a G wire (https://suretydiy.com/forums/topic/t...configuration/).

Since there is no G-wire or any other wires coming into the existing thermostat location, I want to see if I can wire the thermostat directly into the furnace/AC in the basement since running new wires through the walls will be difficult. Since the tstat can be remotely controlled I don't really need to have physical access to it.

My furnace is a Trane model TUX080C942D3 and the AC is a Trane model 4TTR3036G1000N.

Does anyone have installation manuals, or at least tips for tstat wiring for these models?

Thanks,
Alan

 
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04-20-17, 11:07 AM   #2 (permalink)  
Welcome to the forums.

For a smart stat you need a C connection. If you had a G you could possibly convert it to a C use.

It looks like you only have three wires there. (That should be confirmed at both ends)

That would indicate that you only had a furnace and then A/C was added to it later.
A basic furnace-A/C installation should have at least 4 wires. If it turns out you only do have three then that would indicate that the G and Y terminals are combined at the furnace.

The thermostat needs to be installed where it can sense the ambient air. It cannot detect your upstairs air temperature if it's in the basement.

You need to have a new stat cable run or you could add a 24vac transformer at the thermostat location to power it. The transformer would need to be plugged in.


~ Pete ~

 
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04-20-17, 01:20 PM   #3 (permalink)  
Hey PJ ever see a stat cable like that ? Is that the old cloth covered wire ?

 
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04-20-17, 03:33 PM   #4 (permalink)  
That's cloth covered on top of cloth covered. It's unraveling very nicely.

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~ Pete ~

 
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04-20-17, 04:40 PM   #5 (permalink)  
You could use an add-a-wire device such as FAST-STAT model 3000. Page 4 of the instructions shows using existing 3 wires to get R, C, G, W, and Y. Installation Instructions

 
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04-20-17, 06:38 PM   #6 (permalink)  
Interesting product. I've never used them.
Do they actually work reliably ?

In reading the literature.... the model 1000 would work in this app.


~ Pete ~

 
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04-22-17, 04:42 PM   #7 (permalink)  
Thanks everyone for the help. I just traced all the wires by my furnace and think I now have a better idea of what's going on.

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So I think I have two options...

Option 1:
Use a FAST-STAT 3000 to add G and C wires at the tstat. The C wire is optional but if I'm going to do this I'd rather have the C wire too so the tstat doesn't depend on batteries (right?).

I think then I'd repurpose the 3 wires going from the furnace to the tstat so that one wire goes between the furnace and tstat C terminals, and the other two wires run between the FAST-STAT sender and receiver. Then I'd connect the R wire from the sender to the tstat R terminal, and connect the other 3 input wires from the sender to the tstat G, Y, and W terminals. Then at the receiver I'd connect the R and C wires to the furnace R and C terminals, and connect the three outputs to the furnace G, Y, and W terminals. Does this sound right?

Option 2:
Since my basement is finished and there is a wall between the family room and the utility room where the furnace is located, I could put the new tstat on that wall and wire it directly to the furnace. Then I'd just wire C, G, R, W, and Y directly from the furnace to the tstat, right? Would I have to do anything with the existing wiring that is in place for the existing tstat upstairs?

Thanks,
Alan


Last edited by PJmax; 04-22-17 at 05:12 PM. Reason: removed duplicate pic
 
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04-22-17, 05:13 PM   #8 (permalink)  
If you want to add the G.... then use the model 3000.

I think you'd be much better off keeping the thermostat upstairs.


~ Pete ~

 
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04-23-17, 07:47 PM   #9 (permalink)  
As it stands right now, your blower fan (G) is under manual control?? The way it is wired in your diagram, the switch must be flipped and the blower stays on constantly? And if you don't turn the switch on before you have your first call for AC, the condenser will run without the fan, basically freezing it up into a solid block of ice..

There's no problem with the switch being there (thermostats pretty much always have a manual fan-on switch), but there needs to be a jumper between G and Y to make the fan kick on when the condenser does.

I'd second that FastStat device (I've also never seen it, but it looks like a good solution to the issue) unless you can pull new cable. It's kind of unlikely that the existing cable is fastened to anything, so try to create some slack down by the furnace, and see if you can pull it through from the top end. If you can, just tightly tape the new cable to the old and drag it through.

 
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04-23-17, 09:34 PM   #10 (permalink)  
No, the fan switch is not manual. Somehow the fan comes on by itself when and only when the heat or AC is switched on.

 
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04-23-17, 09:56 PM   #11 (permalink)  
You furnace board may turn the blower on based on actual plenum temperature in heat mode and also when it detects power on the Y terminal in A/C mode.


You're showing a "switch" in your diagram. That makes the blower look like that switch controls it. What is that switch ?


~ Pete ~

 
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04-23-17, 10:45 PM   #12 (permalink)  
I'm not entirely sure but this is what it looks like.

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Thanks,
Alan

 
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04-23-17, 11:50 PM   #13 (permalink)  
That switch was probably installed so that the blower could be run when the heat and the A/C weren't in use.


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04-24-17, 02:16 PM   #14 (permalink)  
You furnace board may turn the blower on based on actual plenum temperature in heat mode and also when it detects power on the Y terminal in A/C mode.
A call on the W terminal always activates the blower in a gas/oil furnace. Except for heat pumps, thermostats never use G during a heat call. That looks like an XV series (I have an XV90), which requires the G connection (or a G-Y jumper). It will not activate the fan with only a Y call. Reason is, newer thermostats have a setting that delays the fan by 30 seconds after the compressor is activated to pre-cool the A-coil for extra dehumidification at the beginning of a cycle. Mine also keeps the G line activated for 60 seconds after the cooling call completes, which allows it to blow air at lower speed over the still-cold coil, again for extra dehumidification at the end of a cycle.


You're showing a "switch" in your diagram. That makes the blower look like that switch controls it. What is that switch ?
-----
That switch was probably installed so that the blower could be run when the heat and the A/C weren't in use.
Since there is no G wire I was figuring on it being a manual circulation switch (the same as the FAN AUTO/ON switch on the thermostat, but his diagram doesn't show a G-Y jumper, which is why I thought it wasn't being controlled by the cooling call.

 
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04-24-17, 10:11 PM   #15 (permalink)  
I installed the FAST-STAT 3000 tonight and as far as I can tell everything seems to be working great. I was able to add both a G-wire and a C-wire to the tstat and it seems to be properly recognized and configured through alarm.com.

Thanks everyone for all the help.

Alan

 
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