I have 2 thermostats operating the same A/C unit, 1 for upstairs and 1 for down. The power went out Thursday night and when it came back on Friday the downstairs thermostat would not turn on the fan, A/C, or heat. The upstairs thermostat works perfect. I checked and replaced the fuse in the thermostat, replaced the thermostat and checked the breaker to no avail. Before I call a professional, is there something I am missing? The A/C is a Trane and the house is new to me but built in 91'. I assume the it is the original A/C unit

I have never heard of two thermostats working together on the same unit?

How does that work? What if one is saying "turn on" and the other is saying "not yet"?

I'll be interested to see your answers.

I have 2 thermostats operating the same A/C unit, 1 for upstairs and 1 for down. The power went out Thursday night and when it came back on Friday the downstairs thermostat would not turn on the fan, A/C, or heat. The upstairs thermostat works perfect. I checked and replaced the fuse in the thermostat, replaced the thermostat and checked the breaker to no avail. Before I call a professional, is there something I am missing? The A/C is a Trane and the house is new to me but built in 91'. i assume the it is the original A/C unit
Someone said it is called "zoned heating and air." I know very little of this but when it did work, both thermostats could turn on the A/C independently of each other's temperature setting.

Turn power off for everything HVAC related for a few minutes and then turn it back on. Use the fuse box if needed.

progunr:

It is a zoned system. You generally need a zone controller which takes the input from n thermostats and it will figure out whether the system needs to be turned on.

Zoning is accomplished with typically motorized dampers which the zoning controller controls.

The zoned system generally must be equipped with a "bypass damper". This damper connects the supply with the return. This prevents the system from operating into two high of a restriction. e.g. regulates static pressure

Carrier systems equipped with the Infinity Control and a zone controller does not need a bypass damper because the system can infinately control blower speed to control static pressure.

High static pressure will damage a furnace.

progunr:

It is a zoned system. You generally need a zone controller which takes the input from n thermostats and it will figure out whether or not the system needs to be turned on.

Zoning is accomplished with typically motorized dampers which the zoning controller controls.

The zoned system generally must be equipped with a "bypass damper". This damper connects the supply with the return. This prevents the system from operating into two high of a restriction. e.g. regulates static pressure

Carrier systems equipped with the Infinity Control and a zone controller does not need a bypass damper because the system can infinately control blower speed to control static pressure.

High static pressure will damage a furnace.

Can you tell me where the zone controller generally is located?

It could be anywhere. Someone could have put it centrally located between the two thermostats or zone dampers. Usually it would be located near the furnace.
For 2 zones it needs the following cables:
1.Thermostat for zone 1
2. Zone damper for zone 1
3. Thermostat for zone 2
4. Zone damper for zone 2
5. Control to furnace

Follow the thermostat wire from the furnace.

I have a old lennox electric heater. I changed the old thermostat for a new programmable one... in doing so the heater will not turn on... I put the old thermostat back on it... and it still will not turn on. I tried the breakers, the circuit on/off switches and still nothing. Any suggestions?