Why do a/c thermostats have 5 wires?

On Jul 18, 2:30 am, Aaron Fude <[email protected]> wrote:
> 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.

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

On Jul 18, 4:07 pm, Rick-Meister <[email protected]> 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.

Aaron Fude wrote:

> On Jul 18, 4:07 pm, Rick-Meister <[email protected]> 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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Fifth wire can be transformer power, to supply power to the electronic stat.

--
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Learn more about Jesus
www.lds.org
.


"CJT" <[email protected]> wrote in message
news:488140CC.7030803@prodigy.net...


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

Stormin Mormon wrote:

> Fifth wire can be transformer power, to supply power to the electronic stat.
>
i.e. what I called "hot"

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And the guy with that email adress is solidly PO'd at you for spamming him
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"CJT" <[email protected]> wrote in message
news:488140CC.7030803@prodigy.net...

The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

In case one breaks. Spare wire within the bundle.

Cause it's what the installer had on the truck.

To generate questions.

Obscure building code.

Cause the installer got a good price.

Any or all of the above.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
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"Aaron Fude" <[email protected]> wrote in message
news:bf415740-4a8b-44da-9a42-29ac42836602@l64g2000hse.googlegroups.com...
On Jul 18, 4:07 pm, Rick-Meister <[email protected]> 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.

Most that I have seen use 2 transformers, one for A/C one for heat.

So that would be 2 wires for A/C and 2 wires for heat with a 5th wire for
fan controls. Power for the clock and electronics is picked up from one of
the wires supplying either A/C or heat.


"Rick-Meister" <[email protected]> wrote in message
news:jjt1849384uunl9l4vh8jvip7b792tkcmp@4ax.com...
> 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
>
>

On Thu, 17 Jul 2008 23:30:47 -0700 (PDT), Aaron Fude
<[email protected]> wrote:

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

It's hot not neutral. The common wire to a group of switches (which
the thermostat is) is normally hot. In the case of normal thermostats,
that's 24VAC 1A max.

>So why 5?
>

How many wires are actually connected to the thermostat? Maybe the
installer had some 5-conductor wire, and used that to save money. The
wire coming out of my furnace is 8-conductor. The other 4 aren't
connected to anything.

>Thanks!

I usually see 4: 24VAC supply, heat, cool, fan.

An additional wire might be for a multi-speed compressor, or heat-pump
changeover valve.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy