Hvac module easy wiring question.

[masraum]

I've got a very low tech AC unit and gas furnace. The control module in the attic is a Honeywell st9141, very basic. It's got terminals for C Y W G R wires. There is currently a wire screwed into the C terminal that seems to go around behind the unit in the attic, possibly to the outdoor unit?

What I want to know is, can I connect a separate wire to the C terminal that will run down and provide power to a thermostat?

I'd like to get a wifi enabled thermostat, and the cleanest way to run it would be via power from the C terminal.

Thanks.

[masraum]

Holy carp, I just did a google search and this thread is already a result on google.

[masraum]

Ok, I found a PDF that seems to show that the C terminal goes to the cooling contactor and can also provide power to the thermostat.

[rick-l]

This?
http://customer.honeywell.com/techlit/pdf/PackedLit/69-0780.pdf

Look at page 2 where it says thermostat.

Red = one side of transformer, 24 volts AC
White = call for heat
Yellow = cooling
Green = run fan all the time


Black or the common terminal, the other side of the transformer, not typically run to the thermostat. You need to find out what they are doing with it.

[masraum]

Originally Posted by rick-l

This?
http://customer.honeywell.com/techlit/pdf/PackedLit/69-0780.pdf

Look at page 2 where it says thermostat.

Red = one side of transformer, 24 volts AC
White = call for heat
Yellow = cooling
Green = run fan all the time


Black or the common terminal, the other side of the transformer, not typically run to the thermostat. You need to find out what they are doing with it.

Yep, but look at Figure 2. That shows 2 leads coming off of C.

I think more and more thermostats are coming these days that can take advantage of C because there are wifi thermostats and programmable thermostats with memory and that sort of thing that take power to run. Most will also use batteries (like my current thermostat) but can take advantage of C if it is available.

[Joeaksa]

Originally Posted by masraum

Holy carp, I just did a google search and this thread is already a result on google.

The really busy bulletin boards have robots that are on the board 24/7, harvesting information posted there to google and the other search engines. I run a bulletin board for antique airplanes and have had as many as 7 'bots on the board at one time. The 'bot for google stays on the board all day (and night) long as they stay in business harvesting info... and betting that its the same way at Pelican.

If your board is popular, most posts there are on the internet within minutes of being posted... Ours are and yours was.

[billybek]

R= 24VAC control voltage (hot) Can be two terminals one for heat and one for cooling.
G= Fan. Switched 24vac from tstat to enable fan.
W= Heat. Switched 24VAC. Switch opens on temperature rise. Could be staged W1 W2
Y= Cool. Switched 24VAC. Switch opens on temperature drop. Could be staged Y1 Y2
O= Reversing valve for heat pump. Swithced 24VAC.
C= Common. This is the neutral side of the controls transformer.

You will see one wire from the common side of the transformer going to any component that can be considered a "load" on that transformer.
Any relay or contactor coil is a load.
Some programmable thermostats require a neutral to operate. Some will allow a small amount of current through the heat terminal to operate the thermostat but not energize the heat.

[masraum]

thanks.