How to Bypass HVAC Thermostat to Test Outside Unit

[TractorGear]

My neighbor has a HVAC system and the outside unit will not turn on at the thermostat. I can manually push the relay at the outside unit and it will turn on normally, so it's a low voltage problem. There is 24 VDC from the transformer at the thermostat.

My neighbor's nephew is a retired HVAC repairman and he came by and condemn the thermostat, but after I (correctly) wired in the new one, the outside unit will still not turn on.

Against the outside wall, next to the unit, is a "J" box where all the low voltage wiring coming out of the house are connected to the low voltage wiring going (3 ft.) to the outside unit, using electrical screw cap connectors. The J-Box is at least 20 years old, has no weatherproofing whatsoever, and we have just had some rain for the first time in about 2 years.

The "professional" HVAC shorted wires at the outside unit and condemned the thermostat, but he never actually verified this AT the thermostat. I think there's an open, or a short, in the low-voltage wiring at this rusted, dirty J-box.

I need to know which two wires to short at the thermostat in order to bypass it, and verify that the outside unit either does, or does not, turn on.

If not, then a check by shorting the same wires at the J-Box should show that the problem is either before or after the J-Box in the wiring.

Transformer(24 VDC)--->---Thermostat--->----J-Box--->----Outside Unit Relay--- Ground

I think the problem is either just before, at, or just after the J-Box.

What two wires do I short to simulate the thermostat turning on?

FYI: According to the Customer Service person at Honeywell's 1-800 number, the thermostat is wired for a AC/Heat Pump with Emergency Heat as a secondary heat source. So there's 1 AC, 2 heat and a selector wire for the heat pump valve.

[ljintexas]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

On t-stat sub base, you want to jump the "R" terminal to the "Y" terminal. It uses 24 volts AC from a transformer most likely in the furnace or heater. Sounds like you may have an open conductor in your low voltage cable. Most of them have extra conductors that are not always used. Easy way to test cable is to use ohm meter..kill power to heater and condensing unit outside to ensure no low voltage. disconnect all conductors on both ends..short two wires together inside i.e. red and yellow, then go outside and read with a meter to see if they show the short outside. do the same with all conductors. I bet the yellow wire connected to Y is open. If not, possible that safety in condenser is open or contactor coil is shot.

[TractorGear]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

On t-stat sub base, you want to jump the "R" terminal to the "Y" terminal. It uses 24 volts AC from a transformer most likely in the furnace or heater. Sounds like you may have an open conductor in your low voltage cable. Most of them have extra conductors that are not always used. Easy way to test cable is to use ohm meter..kill power to heater and condensing unit outside to ensure no low voltage. disconnect all conductors on both ends..short two wires together inside i.e. red and yellow, then go outside and read with a meter to see if they show the short outside. do the same with all conductors. I bet the yellow wire connected to Y is open. If not, possible that safety in condenser is open or contactor coil is shot.

Thanks that's what I needed to know. R to Y. The transformer is fastened to the wall inside the HVAC closet, and we've already measured 24 VDC present. I'll try jumping it today and report results later. Thanks again.

[ljintexas]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

The transformer typically will be inside the furnace. Typically the one fastened to the wall in the closet is for a door bell, usually 12 volts

[budm]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

"and we've already measured 24 VDC present." You mean 24AC.

[desert-rat]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

Its 24 vac. if jumping the r to y does not turn on the cont. look for safty switches , high pressure , low pressure , compressur ol. , a 5 min. timer , or a def. control , ect.

[ljintexas]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

As I said, a safety in the condenser open or a contactor coil that is shot. Also examine all connections made with wire nuts. The conductors in the 5 or 7 conductor cable from inside to outside and to the stat are 18 ga or smaller solid copper that can get brittle and break up inside the connector..It will look like a good connection..until you take it apart.

[desert-rat]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

The door bell tran is 16 vac . Jump the red to the yellow wire , then check the cont. coil for 24 vac. If you have 24 volts and the contactor does not pull in , its bad . If you dont have 24 volts , then some thing is open some where . The common color code is , red is common , yellow is cool , white is heat , green is the fan , orange or blue is for a reversing valve on heat pumps , blue is also used for the other side of the 24 volts , but not ever thing is wired up that way .

[ljintexas]

Re: How to Bypass HVAC Thermostat to Test Outside Unit

Red is power or "hot" side of 24 volts blue mostly is common from xnsformer if stat is power robbing..