Original message
| tbmh
|  "Updating cooling thermostat-wiring question"
, posted Wed 19 Mar 09:55  
Hi,
I have a commercial building with a pretty old air conditioner. The heating and air in the building are totally separate.
I wanted to update my thermostat for the air to a programmable one so I could set the temperature higher when the store was closed, hopefully saving some money.
I'm having trouble hooking up the programmable thermostat.
I'm replacing an old Honeywell thermostat which had no switches on it at all (on/off, fan on or auto etc). I purchased a Honeywell CT2700A programmable thermostat. It's a round one that looks like the old thermostats, and doesn't require batteries.
The old thermostat had 3 wires hooked up. The red wire went to R, and the green and white wires went to Y.
I hooked them up the same way to the new thermostat but it didn't work as expected. I assume I should be able to leave the thermostat set to "Cool" and the fan on "auto". I tried connecting the wires in pretty much every combination that made sense, but never got the unit running in a way that seemed correct. I could get the unit to come on, but could never get it to not be running when I turned it on, and then come on when I lowered the temperature setting. I could tell the thermostat was trying to turn the air on (the cooling icon would light up) but it never kicked on the unit or fan.
With the old thermostat, I couldn't turn just the fan on without the air running, but it would be nice if somehow I could do that.
My question is, will this thermostat work with this set up? If so how do I connect the wires, and is there a way to make it so I can just turn the fan on and not the air?
Thanks in advance for any help anyone can give.
Todd
Todd Brashear
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Replies:
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| melaman
| "Re(1):Updating cooling thermostat-wiring ques" , posted Wed 2 Apr 09:22
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| Xenos
Webmaster 
| "Re(1):Updating cooling thermostat-wiring ques" , posted Wed 19 Mar 19:30
The thermostat you have chosen will not work with a cooling only system the reason is that all these new programmable thermostats use the heating logic to power them up. There for as soon as the stat call for cooling it will loose power to it. There is a resistor kit available for some of their models to allow this operation but it’s not listed as an accessory for your thermostat.
Now the good news and simplist, since your jumping the green and white wires at the thermostat you could instead jump them at the unit then add a cheap 24vac relay and wire one side of it’s coil to common and the other side to spare wire you will now have going to the thermostat and connect that spare wire to the W terminal on the thermostat thus fooling the thermostat into thinking it has a heating circuit.
Xenos.
The best way to escape a problem is to solve it.
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