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Why do a/c thermostats have 5 wires?

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Why do a/c thermostats have 5 wires? Aaron Fude 07-18-2008
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Posted by Aaron Fude on July 18, 2008, 2:30 am


I would guess they need three: one for cooling part, one for the fan,
and one neutral.

So why 5?

Thanks!

AppliancePartsPros.com, Inc.
Posted by Mikepier on July 18, 2008, 7:10 am


> I would guess they need three: one for cooling part, one for the fan,
> and one neutral.
>
> So why 5?
>
> Thanks!

You need 5 if its a combination heating/cooling system, otherwise if
its strictly for cooling, you only need 3.

Posted by Rick-Meister on July 18, 2008, 4:07 pm


Some systems use two transformers and thus two "hots." RH for heating,
RC for cooling. If the system only has one transformer, the thermostat
has a jumper to go between RH and RC.

So, to get to five wires you'd have

RH Hot for heat
RC Hot for cooling
Yellow for compressor
White for heat
Black for common (to power the clock and electronics in a setback T
stat



Posted by Aaron Fude on July 18, 2008, 8:48 pm


> Some systems use two transformers and thus two "hots." RH for heating,
> RC for cooling. If the system only has one transformer, the thermostat
> has a jumper to go between RH and RC.
>
> So, to get to five wires you'd have
>
> RH Hot for heat
> RC Hot for cooling
> Yellow for compressor
> White for heat
> Black for common (to power the clock and electronics in a setback T
> stat


Actually, my thermostat is strictly for cooling and it still has five
wires. Thus my question.

Thanks.

Posted by CJT on July 18, 2008, 9:18 pm


Aaron Fude wrote:

>
>>Some systems use two transformers and thus two "hots." RH for heating,
>>RC for cooling. If the system only has one transformer, the thermostat
>>has a jumper to go between RH and RC.
>>
>>So, to get to five wires you'd have
>>
>>RH Hot for heat
>>RC Hot for cooling
>>Yellow for compressor
>>White for heat
>>Black for common (to power the clock and electronics in a setback T
>>stat
>
>
>
> Actually, my thermostat is strictly for cooling and it still has five
> wires. Thus my question.
>
> Thanks.

Hot, common, fan, cool makes four. Perhaps the fifth isn't really
functional. What are the terminals to which they're connected labeled?

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