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Old 10-27-2011, 10:04 AM   #1
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4 wire thermo to 5 wire system


Hi my old thermostat has five wires and my new controls only has four. There is no connection for the "C" Furnace is a Bryant 90 about 10 years old, have manuals. Any ideas
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Old 10-27-2011, 10:18 AM   #2
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4 wire thermo to 5 wire system


If the new stat runs off battery power, it will not need the C wire.

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Old 10-27-2011, 10:25 AM   #3
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4 wire thermo to 5 wire system


Thanks so I just leave the C wire off, cap it? A YouTube video says the batteries will only last 20 hours. Furnace and original thermostat are about 11 years old, the new programmable unit is about 5 noma 52-2557-0
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Old 10-27-2011, 11:10 AM   #4
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4 wire thermo to 5 wire system


I would take the thermostat back and get one that powers off the 24v on the furnace. No need for batteries to die when away from home, and then come home to frozen pipes.
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Old 10-27-2011, 11:19 AM   #5
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4 wire thermo to 5 wire system


I had a battery operated stat in the past. I changed the batteries once a year and they never went dead. I currently have one that has both batteries and 24 V power.
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Old 10-27-2011, 11:33 AM   #6
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4 wire thermo to 5 wire system


doesn't a stat with no C on the subbase run off a back feed from the TR,and the batteries are for a no power situation... to save the back up the info for the program?
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Old 10-27-2011, 12:20 PM   #7
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4 wire thermo to 5 wire system


Again thanks to everyone who replied, this was my first venture into these chat sessions. Fist problem is the Stat, bought two about five years ago so I can't take it back. Can anyone verify that it runs on the hot wire transformer and the batteries are just for backup like some else has said. The other Stat that I installed worked fine, four wire system and the batteries lasted a long time. Logically should this unit not get it's power from the red hot side of the transformer with the other three wires being, ground, heat relay and cooling relay? Is the fifth wire, the C just bring power from the furnace to power the Stat or does it have another purpose
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Old 10-27-2011, 02:47 PM   #8
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4 wire thermo to 5 wire system


for years after stats got electro mechanical even they worked on the back feed of the R hot to run the mechanical clock and some a digital clock with a small display.but now the vision pros and such need a direct 24Vs R/C readable at the subbase without the stat up just furnace toggle up.if you get a R/C stat and don't have a C the stat would light up but just sit with no internal relay action because of the missing C and the back feed just couldn't swing it.maybe the batteries power the digital led only..what happens with out batteries when its on the subbase..
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Old 10-27-2011, 03:57 PM   #9
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4 wire thermo to 5 wire system


Quote:
Originally Posted by biggles View Post
doesn't a stat with no C on the subbase run off a back feed from the TR,and the batteries are for a no power situation... to save the back up the info for the program?
No, they use the batteries. A digital thermostat with no C and no batteries uses power from the system by bleeding a small amount through the W and Y terminals.

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