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Old 19th January 2012, 01:53 AM   #1
Netdewt is offline Netdewt  United States
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Default What kind of relay do I need? (home HVAC/electrical)

I need a relay to control a duct booster fan in my home (120VAC, 0.4A). I believe I found the fan wire from the thermostat (green), and I thought I read somewhere that it runs off of 5VDC. So, I need the 5VDC to trigger the 120VAC on.

What kind of relay do I need and where can I get it? I need something I can put in a regular electrical box. I have looked high and low locally, and no one sells them. I got this DPDT @ Radioshack today, but I am so confused by the additional leads, and I don't even know if it's okay to put this in an electrical box. Help?

12VDC/10A DPDT Plug-in Relay - RadioShack.com
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Old 19th January 2012, 06:26 AM   #2
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Solid State Relay, SSR in short or a relay with 5 volts dc coil
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Old 19th January 2012, 06:54 AM   #3
Enzo is offline Enzo  United States
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yeah, I'd be looking for a solid state relay as well. They come in 5vDC control with switched part rated for 120VAC. They come with screw terminals ready to install. Not much more complex than wiring a light switch.

Does your thermostat actually send 5v out, or does it simply close a pair of contacts?
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Old 19th January 2012, 01:20 PM   #4
Netdewt is offline Netdewt  United States
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I don't actually know. I know right where the fan wire is, and it's just wire nutted. I called an HVAC guy to try and get some information and he said this is what he would do, so my understanding is that it does send out something.

When I started this project I thought it would be easier to figure out what to do. I can't find anyone that sells this sort of thing. I'm considering just buying the back pressure sensor from the fan manufacturer, even though it seems like a clunky solution to me.

Last edited by Netdewt; 19th January 2012 at 01:44 PM.
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Old 19th January 2012, 01:29 PM   #5
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Install a contactor in the HVAC unit that is triggered from the main fan or the fan output on the board. You don't want to run it off of the t stat otherwise ther will be no delay and the fan will run when the t stat closes and blow cold air unitl the main fan kicks on. This will aslo ensure that when the HVAC breaker is turned off there will be no power at the booster fan.
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Old 19th January 2012, 01:42 PM   #6
Netdewt is offline Netdewt  United States
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My uncle wants me to pull power right off the fan in the furnace. I'm hesitant about monkeying with the inside of the furnace. Would I just be looking for the power to the fan? My furnace fan has variable speeds.

Also, I didn't mention. The booster fan only draws .4A.

I have never heard of a contactor. This?
contactor 120vac - Product Details
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Old 19th January 2012, 04:22 PM   #7
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That is a contactor. It is just a bigger relay. Many different versions available. You will want to connect the contactor coil to the hvac fan motor which will be 230 volts so get an appropiate one. Then you want to wire one phase to the contact and the other contact to the booster fan. The remaining lead will go to neutral since the booster fan is 120 volts. The neutral does not need to be switched. Run 14/2 romex from the furnace to a j box next to the booster fan and terminate. Install a fuse in the furnace for the fan.
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Old 19th January 2012, 04:42 PM   #8
Netdewt is offline Netdewt  United States
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My furnace is only on a 120V circuit... the controller board also says "120VAC" on it.

The fan is all buttoned up, but I have pretty good access to the controller board.
Click the image to open in full size.

So, just intercept the fan power and neutral via the controller board? Everything is connected up with this type of connectors:
Terminal, Quick Conn, Blue, 16-14AWG, PK100 - Wire Terminals - Wire Connectors - 5X425 : Grainger Industrial Supply
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Old 19th January 2012, 06:11 PM   #9
imix500 is offline imix500  United States
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The blower in the pic isn't the main fan. It will be bigger and should be located on the supply end of the furnace.
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Old 19th January 2012, 06:20 PM   #10
Netdewt is offline Netdewt  United States
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Ah... see this is why I was hoping not to have to mess with the furnace itself. Right, that would be the direct vent blower.

The filter is on the bottom right behind the control board in the pic. Would it be behind there?
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