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trouble shooting furnace fan (only runs if I give it a push)

trouble shooting furnace fan (only runs if I give it a push)


Old 10-20-12, 12:56 PM
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Join Date: Oct 2012
Location: Canada
Posts: 4
trouble shooting furnace fan (only runs if I give it a push)

This might belong more in the heating section but since it relates to eletrical (electric blower fan) and since there is much more activity here in Electrical, I thought I would start here.

We have installed a wood burning fireplace in a country home.
The house has duct work running throughout (to distribute heat and cool from the original thermostat driven gas fireplace and A/C system.)

The gas FP has been removed and we never use the A/C so all we really want to do now is have a simple On/Off switch for the fan in the ductwork to blow the wood stove heat around. No thermostat is required.

During some renovations this spring we removed the thermostat and the original on off switch for the blower fan.

Now, with winter approaching we woudl like to get the blower blowing again.

WIth everything reconnect the way I think it was originally (I am something of an electric moron.) all I hear is a hum from the blower unit.

When I connect the blower fan directly to power (bypassing a capacitor on the side of the blower unit and the circuit board that I believe controled the thermostat), the fan runs fine. As long, that is, as it has some motion already. So, if I give the fan blades a bit of a spin and then flip the Power switch, it takes off and blows.

However, if the fan is stationary/still and I flip the power switch, all I hear is that same hum. It doesn't start.

Is there something in a blower fan system that gives it a kick start that I am replicating with my hand start?

any clues?


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Old 10-20-12, 05:29 PM
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Peter, welcome to the forums! Kindly fill out your profile so we can zero in on your location. It helps when we try to diagnose things for people. If you bypassed the capacitor, you bypassed the means for the motor to know which way to turn. That is the job of the capacitor. You need to hook it back up the way it was before including the capacitor. BTW, nothing controls a thermostat.....the thermostat controls things, so bypassing didn't work too well.
Old 10-20-12, 06:05 PM
Join Date: Oct 2012
Location: USA
Posts: 15
Your blower motor is a capacitor start type. It will not start unless the start capacitor it is wired correctly in the motor circuit. The capacitor is what changes the ratio of voltage to current to provide enough starting current to overcome inertia.

As you are posting outside the US, I have no idea what kind of thermostat you had. Here it is most common for wall thermostats to have a 3 position fan switch; AUTO-OFF-ON, which would allow you to just circulate air (fan switch=ON) .

For your safety, I hope the gas connection was properly capped off when the Gas Fireplace unit was pulled. I have never seen a house with a blower motor IN the ductwork. Most central heat and air units here have the blower fan in the central air handler/heat exchanger. If that is also your setup, and your original thermostat had an auto-off-on fan switch, hook it back up like it was, extend the 24 volt control wires to a more convienient location if needed, to accomodate the "remodeling job".

Last edited by NC Fixer; 10-20-12 at 06:07 PM. Reason: delete fragmented sentence
Old 10-21-12, 07:29 AM
Thread Starter
Join Date: Oct 2012
Location: Canada
Posts: 4
thank you.

the gas was capped off by a professional gas fitter. But there is not longer any gas feed in the house (except for a propane tank directly to the gas stove top.)

The thermostat is York brand. and it still seems to have it's original wiring.
Indeed the fan settings are Auto / On.

I am going to redo my direct power test but moving it back on step to wire in the capacitor (I had bypassed it to do that test that gave me the results in my original post.)
I expect that should show if the capacitor is good or not.
Old 10-21-12, 07:48 AM
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Join Date: Jan 2011
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A meter capable of measuring microfarads is the only way to know if a capacitor is at value or not.
Old 10-21-12, 09:16 AM
Thread Starter
Join Date: Oct 2012
Location: Canada
Posts: 4
photos and drawings

Went back down to look at things and decided that I had better not tinker more with something I so poorly understand (Capacitors etc.) (Curiosity + ignorance usually = trouble )

So, here are some photos that might shed some light for someone actually knowledgeable.

I'll now read up on Capacitors but, if you look at the wiring "picture" (I wouldn't grace it with the label "diagram") I am wondering why there is only one input (Purple to A) to the capacitor but apparently two outs (Purple from B and Brown from D.)

when I hardwired my electric leads (X,Y) yesterday, I (maybe foolishly) wired them to B and D.
and that resulted in the fan running when I spun it.

can you figure out anything from these photos and sketch.?

Can anyone tell me how I can test that Capacitor (without a microfarad reading meter)? should I not be able to wire it up to the live power to test the system with the capacitor IN but the circuit board Out.?

Toronto, Canada
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Old 10-21-12, 10:36 AM
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Join Date: Jan 2011
Posts: 6,245
The capacitor is wired correctly! Don't mess with it if you don't understand. I can not tell from the angles of your photos if the blower is wired correctly to the control board.

Do you currently have a thermostat installed and have you changed any wiring on it?
Old 10-21-12, 10:56 AM
Join Date: Jan 2008
Location: Southeastern Pennsylvania
Posts: 2,701
Hi pstock –

I’m no electrical guy to say the least. My AC capacitor went bad and I saw the same kind of thing you have. A buzz when it tries to start, then a little push gets it going. In my case you could see the capacitor was bad since the bottom sort of bulges out. (My understanding is that can happen many times, the pros here would know better.)

Can't tell if your capacitor has a bulge on bottom from your picture. But I was wondering, since I think (not sure) those capacitors are pretty cheap, and easy to replace (seems like yours is mounted in a very accessible location) then why not just replace it? I think mine only cost ten bucks and took a few minutes to replace. (But you do have to make sure the capacitor is discharged before you handle it. I put a screwdriver across the leads and I think I actually heard a loud noise when it discharged?)

But maybe the pros would say your capacitor would be much more expensive.

Whooops! Just saw hvactechfw’s response. He would sure know more than me! Sounds like he doesn’t think you should mess with it.

Just a thought. Good luck!

Last edited by zoesdad; 10-21-12 at 11:21 AM. Reason: ref should be to capacitor bottom not top
Old 10-21-12, 12:01 PM
Thread Starter
Join Date: Oct 2012
Location: Canada
Posts: 4
good idea (swapping out the capacitor for a new one.)
I am not he wiring seems so simple that I think even I can manage it.
I did not realize that these things were so common. It looks old and so I assumed it would be hard to find a replacement. but I'll take the specs and try.
Old 10-21-12, 12:37 PM
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Old maybe, uncommon no. Try to match the MFD within ±10%. The operating voltage can be higher. Make a diagram before you pull the wires. Pull the connectors with insulated pliers even if power is off. After the connectors are off using the metal shank of an insulated screwdriver short the terminals before handling.

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