Circuit design for HVAC to trigger ceiling fan.

shortbus= said:

Doesn't the 24V section of the furnace just control the main gas valve and ignition of the burner? At least on any of the furnaces I've ever worked with this is how they worked.

The blower is turned on by limit switches (120V) in the heat exchanger unit. Again this is how it is on the I've worked on.

Click to expand...

The OP (Chris) is not modifying his existing furnace. He just wanted a pulse when the furnace turns on, and a different pulse when it turns off. The pulses will control a battery-powered RF remote transmitter which will turn on/off a ceiling-mounted paddle fan.

MikeMl said:

The OP (Chris) is not modifying his existing furnace. He just wanted a pulse when the furnace turns on, and a different pulse when it turns off. The pulses will control a battery-powered RF remote transmitter which will turn on/off a ceiling-mounted paddle fan.

Click to expand...

Yeah I know, but, if you look at how a furnace cycles ;

1. thermostat calls for heat.
2. thermostat signals burner on.
3. burner lights and raises temp in heat exchanger, blower turns on.
4. blower heats room until thermostat turns off, blower continues to run until heat exchanger temp is at low set point.

Controlling the remote with just the thermostat wiring is going to turn the ceiling fan on when there is no heat(room cool) And off when the blower is still running(room warm)

The way I understood the O/P he wanted the ceiling fan to come on with the blower/heat?

I could have run the one extra wire in the house from the furnace blower to the fan(wall) switch by now. And the furnace blower and ceiling fan
would be synchronized.

Mike thanks for that last addition to the circuit.
Only thing I'm thrown off on is the three spots where Vdd is indicated. Somethig to do with FET's? -beyond my current state of knowledge. Can you explain what I'm supposed to do with these?

shortbus= said:

Yeah I know, but, if you look at how a furnace cycles ;

1. thermostat calls for heat.
2. thermostat signals burner on.
3. burner lights and raises temp in heat exchanger, blower turns on.
4. blower heats room until thermostat turns off, blower continues to run until heat exchanger temp is at low set point.

Controlling the remote with just the thermostat wiring is going to turn the ceiling fan on when there is no heat(room cool) And off when the blower is still running(room warm)

The way I understood the O/P he wanted the ceiling fan to come on with the blower/heat?

Click to expand...

1. Thermostat calls for heat; This is the key step because the white wire (which is going to be connected to Mike's circuit) goes +24V when the stat wants the heat on and then the white wire goes to 0V when the stat wants the heat off. That's all that matters to me basically.
In my case, steps 2 - 3 are controlled by the furnace, not the stat.

Chris75 said:

1. Thermostat calls for heat; This is the key step because the white wire (which is going to be connected to Mike's circuit) goes +24V when the stat wants the heat on and then the white wire goes to 0V when the stat wants the heat off. That's all that matters to me basically.
In my case, steps 2 - 3 are controlled by the furnace, not the stat.

Click to expand...

Yes but, the room is cool (calling for heat) at that time. And the ceiling fan will shut off while there is still heat being made.If that doesn't matter, why not just leave the ceiling fan on all the time?

My understanding is what the circuit will do is turn the fan on when the thermostat calls for heat, and then off when the theromostat no longer calls for heat. So heat on = fan on, heat off = fan off.

I've experimented with leaving the fan on all the time and I only like it on all the time in the summer. In the winter I'm happier with the fan on only when the heat is on.

Chris75 said:

...
Only thing I'm thrown off on is the three spots where Vdd is indicated.... Can you explain what I'm supposed to do with these?
...

Click to expand...

Oh, that's just shorthand for "Connect all the places labeled VDD to the output of the LM7805 Voltage Regulator..."

Put a normally open Sail switch in the heating duct near the fan when the furnace kicks on the sail switch will make contact until the furnace fan shuts off.

Wire it into the supply of the fan Jumper the hot over to the switch and back to the fan bypassing the switch as a second conductor for the fan.

kv