I need to control an in-line duct fan with a thermostat. Since the thermostat I have (a basic Honeywell non-programmable heat/cool) requires 24v and the duct fan runs on 110v I'm assuming I will need some type of relay that provides 24v for the thermostat and
110v for the fan. Hopefully, there is a relay available that allows the thermostat to to be attached directly to it that can control the fan. I'm guessing the relay would have 110v power in and 110v power out to the fan and have 24v terminals to attach the thermostat to. Is this the correct approach assuming such a relay exists? Is there a better or more preferred way to do this? I do not want the duct fan to operate in conjunction with the furnace fan. If possible I would like to avoid using a line voltage thermostat. Thanks.

There is a relay that will operate a 120 volt fan, it has a 24 volt coil. So all that you would need is a 24volt wire passing through the thermostat and a common wire that goes directly to the relay. I am sure that the 24 volts can be obtained off the circuit board in the furnace if you have one or in the furnace wiring somewhere it you don't have a circuit board.
I am not sure what temp you are wanting the air going through the duct, but in the heat mode the air coming out of the furnace should be in the range of 125 to 140 degrees. So to move that air with a fan the thermostat that controls it will have to be a high range thermostat.

All your older furnace fans were controlled by 120 volt relays with a 24 volt coil. Should not be problem finding one. If you are planning to have the duct fan respond to the room temperature, the conventional thermostat should work. If you planned to have it respond to the temperature of the air in the duct, a better approach might be the lower limit switches that older furnaces used to control the blower. Mounted in the duct, it would turn the fan on when the air in the duct got hot and off when it cooled.

Your responses are appreciated. Actually, I don't want this duct fan to be conneted to any furnace. I have a heat pump and this fan will be used to push cool air only but if the setup was for heat/cool, that would not present a problem either. I found a thermostat setup that would work perfectly (enernet T9000) but it's pretty expensive. I think it would cost in the neighborhood of $300 but is the easiest to set up since it's wireless. There must be a 115v relay with a 24v coil all in one unit where all I would need to do is power the relay with 115v and hook up the duct fan and thermostat to it. Then the fan will go on or off based on the thermostat setting. Does anyone know of such a relay and where I might get it?

Why not wire the duct fan in with the air handler fan. No relay or hard to understand wire-ing to do and nothing to buy. As for the relay you seek its called a fan center. Available through most HVAC contractors. Quite popular actually. Although I would just run a wire from my present air handler blower motor(assumes its not DC current) to new duct motor... Simple.
Have fun!

I need to control an in-line duct fan with a thermostat. Since the thermostat I have (a basic Honeywell non-programmable heat/cool) requires 24v and the duct fan runs on 110v I'm assuming I will need some type of relay that provides 24v for the thermostat and
110v for the fan. Hopefully, there is a relay available that allows the thermostat to to be attached directly to it that can control the fan. I'm guessing the relay would have 110v power in and 110v power out to the fan and have 24v terminals to attach the thermostat to. Is this the correct approach assuming such a relay exists? Is there a better or more preferred way to do this? I do not want the duct fan to operate in conjuction with the furnace fan. If possible I would like to avoid using a line voltage thermostat. Thanks.
If you don't want to wire into your air handler fan, use an AirCycler FR-V and a 24VAC relay. You can get both at Air Cycler :: Build It Tight, Ventilate It Right (http://www.AirCycler.com)
The problem with the thermostat is that there isn't one wire that you can use to know when the furnace fan comes on for non-electric or heat pump systems. In oil or gas type furnaces the thermostat doesn't activate the G or fan wire with a heat call. The furnace turns on the fan itself with either a timer or heat sensor. Therefore you need to watch G and W (the heat wire).

The AirCycler FR-V will do this and then give you a 24VAC output that you can use to turn on the relay. You can now also pick up the added benefit of periodically cycling the furnace when there is no call for heat or cool. This helps with thermal comfort as well as air quality. CO2 levels in bedrooms climb very high during the night when the furnace isn't running or the windows aren't open. You can get high humidity areas from cooking and cleaning. Mixing the air up in the house helps all of this.

your responses are appreciated. Actually, I don't want this duct fan to be conneted to any furnace. I have a heat pump and this fan will be used to push cool air only but if the setup was for heat/cool, that would not present a problem either. I found a thermostat setup that would work perfectly (enernet T9000) but it's pretty expensive. I think it would cost in the neighborhood of $300 but is the easiest to set up since it's wireless. There must be a 115v relay with a 24v coil all in one unit where all I would need to do is power the relay with 115v and hook up the duct fan and thermostat to it. Then the fan will go on or off based on the thermostat setting. Does anyone know of such a relay and where I might get it?
They do sell low pressure sensor switch, it line volt , it usually use for heating, electric reheat.