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-   -   Replacing Honeywell T8401C thermostat (http://www.diychatroom.com/f17/replacing-honeywell-t8401c-thermostat-159276/)

sktn77a 10-07-2012 12:02 PM

Replacing Honeywell T8401C thermostat
 
Hi All:

I'm looking to replace my daughter's Honeywell T8401C thermostat (non-programmable) with a White-Rogers 1F90W programmable (pulled from my home system when I upgraded to modulating furnaces). The Honeywell wiring is as follows:

White - W
Yellow - Y
Green - G
Red - R
Blue - C

The White Rogers wiring is as follows:

White - W
Yellow - Y
Green - G
Red - RH, jumpered to RC
No C terminal

Do I tape off the Blue (C wire) and jumper RH to RC or do I connect the blue wire to the RC terminal on the stat? Neither thermostats' installtion instructions cover this particular swap. The furnace is a Carrier Comfort 80 (installed in 2005).

Thanks in advance.

biggles 10-07-2012 01:07 PM

tape the C wire ...your stat runs on a back feed and batteries all the C wire means is it is a true 24v powered stat with batteries backing up memory in power outage...the Rc-Rh are the hot 24V side of the system up there it would of shorted the transformer :censored: as soon as you connected it to Rc....Rc is cooling Rh is heat allows the system to draw on the transformer in either mode with a stat with an AUTO setting that is why there is a subbase jumper between them..if there was no jumper you would have to manualy schange the system switch from cool to heat to get either or...NOTE to others reading here always consider that C wire is required on newer digital stats with led screens without a true 24V and that C wire it will lite up and just sit there...

sktn77a 10-07-2012 04:39 PM

Thanks Biggles. Unfortunately, I'm struggling to understand your post:

"...the Rc-Rh are the hot 24V side of the system up there it would of shorted the transformer :censored: as soon as you connected it to Rc"

Does this mean if I jumper Rc and Rh I will damage the stat (or the transformer)?

"NOTE to others reading here always consider that C wire is required on newer digital stats with led screens without a true 24V and that C wire it will lite up and just sit there... "

I believe that the White Rogers does not use a 24v to power the stat but I don't really understand what the highlighted text means?

beenthere 10-07-2012 05:12 PM

Connecting the C wire to RH or RC is not a good thing. Just tape it off. And jumper RH and RC.

sktn77a 10-07-2012 05:28 PM

OK, great. Thanks all for your replies.

Keith


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