I'm working at a non-profit in New York City.

Recently I purchased three Honeywell T8624D's to replace our Honeywell T874D's. Then we'll have programmed on and off times and I won't have to go into each mechanical rooms to tend to the units.

The wiring looks very straightforward and I tried to install two of them today. However, after installing them and throwing the breaker, the new thermostat display didn't show anything at all- no LCD.

I reinstalled the old thermostat and the system started right up without a problem.

Our system is a 2 cycle cooling and 1 cycle heating.

Wiring on old and new

W1--W1
G--G
Rc--Rc
Rh--R
Y1--Y1
Y2--Y2

Rc and Rh are bridged on the old thermostat, so I bridged them on the new thermostat. It didn't work.

Then I tried it once without the bridging. Nothing happened still.

Any opinions, questions, help you can offer would be very appreciated.

Tom

Does the new thermostat have an additional terminal marked B or C. If so, it must be connected to the opposite terminal on the transformer as the red wire. Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

Thank you for responding to this. I appreciate it.

Yes, the thermostat does have a "C" terminal.

How would I get a common to the thermostat from the air handler?

Maybe there's one there already. Should I put a voltage meter on the spare wires that are there but not used?

Any help you can offer is very appreciated.

I guess I may have to call in a professional on this at some point.

Go to the air handler and see where the wires to the thermostat start. If there is a B or C terminal there, note the color of wire connected to it. Also check to see if it has 24 v AC between it and the red. If there are extra wires, but none connected to the B or C terminal, select a blue or black one and check at the thermostat for the 24 v AC between it and the red. If so, connect it to C terminal.

Not sure what sort of a non profit you are helping. Most of them are doing good work.

Thank you so much!

With your help, we went in and figured most of it out.

Unfortunately, we blew a fuse. There's a 10amp/250v fuse that keeps blowing after the unit runs for 3-4 minutes.

We'll probably have to call in the HVAC guys to troubleshoot it because we can't seem to find the overload.

But, with your help, I know that we've got working units.

THANKS!

10 amps sounds light for an A/C compressor. Check the amperage on the name plate. As long as it is wired up with #10 wire, you could be using up to a 35 amp fuse.