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Operating something when furnace fan is on

Rick Johnson

May 11, 2016
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Hi guys. Just had a new furnace put in. When i had the old furnace i cut into the power to the fan motor and used that to turn on An external fan. Now with a new furnace i dont want to do the same. I thought maybe use a low pressure switch off the blower from a water heater. Not sure if the airflow around the furnace fan would create a low enough state to operate the switch. Any one have an idea
Thanks Rick
 

AnalogKid

Jun 10, 2015
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What you want is called a vane switch. I had one that turned on an electronic air filter only when the fan was running. It's like a standard industrial limit switch with a thin plastic vane about the size of a playing card attached.

Or - if the wires to the motor are individually removable, like there is a terminal block in the wiring path somewhere, remove the line or neutral, feed it through a current transformer, and reattach it. Now you have a fully isolated signal you can rectify/filter/process to do just about anything.

ak
 

Minder

Apr 24, 2015
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Is this a Gas or electric furnace?
There is usually a relay or a small transformer energized when the thermostat picks up.
You could wire a relay off of this maybe.
Generally there is a schematic in one of the furnace doors.
M.
 

Rick Johnson

May 11, 2016
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Hey thanks guys good suggestions

AnalogKid the old furnace was easy to wire into the motor this one has power always on to the motor then it also has vdd, TX,RX,and a ground. The vane switch is a good idea only they are very expensive. Ill try to make one with a plastic playing card attached to the arm of a switch and Im supposing you mount it on the frame of the fan or just over the hole for the furnace filter.

Minder the problem with that is that the thermostat will shut off then the fan will keep running for a period of time Id like to run my external fan along with the furnace fan. There is a relay that kicks in when the fan starts up but thats soldered to the motherboard I would have to take that off and solder to the back of it. Thanks for the suggestions though
 

Minder

Apr 24, 2015
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There is also a non invasive way using a Honeywell current sensor in a motor conductor.
See CSDA series.
M.
 

AnalogKid

Jun 10, 2015
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In my old furnace system, the vane switch was mounted inside the main cold air return duct a couple of feet before the first filter.

ak
 

Rick Johnson

May 11, 2016
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From what I have read I could probably use an scr. Not sure what the 3 conductors are for
 

Arouse1973

Adam
Dec 18, 2013
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Would the fan have strong enough air flow to move the prop of another smaller fan. You could then sense the voltage produced from the second fan to trigger a circuit containing a relay or something that can be used as a switch for your new fan?

Cheers
Adam
 

Minder

Apr 24, 2015
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I don't think that one is entirely suitable, there are basically two types one that just switches on detection of current, and one used for measuring actual level, you really need the former.
M.
 

Rick Johnson

May 11, 2016
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Thanks guys for your suggestions they were all good. I went with a vane switch for now its working good. I would however like to use the current sensor in the future. Ill have to do some reading on that.
Have a good day.
Rick
 

Danny Gray

Feb 25, 2017
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Hi guys. Just had a new furnace put in. When i had the old furnace i cut into the power to the fan motor and used that to turn on An external fan. Now with a new furnace i dont want to do the same. I thought maybe use a low pressure switch off the blower from a water heater. Not sure if the airflow around the furnace fan would create a low enough state to operate the switch. Any one have an idea
Thanks Rick
You can use a current sensing relay on the power to the blower. The current sensing relay can close when the blower comes on energizing the coil on a fan switch (simply a relay used in air handlers to turn on a blower).
 
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