Furnace failure, please help

bio_guy

Furnace quit on Monday evening. I took my time to investigate because there are lots of other things going on. It is an old Ducane horizontal furnace in the attic with a Honeywell controller (ST912GC, maybe) Fan Timer and SV95D1 Smartvalve,

After a call for heat, the combustion blower spins up and the furnace apparently ignites normally, but the circulating blower never engages. After a relatively short time, everything shuts off and I get no AC step-down volts at the terminal strip. After quite a long time, apparently it resets and can go through it again. The blower motor spins freely. The safety switch on the blower access door seems to be working, opens and closes.

Where do I go from here, check the motor capacitor, trouble-shoot the control board? I don't have a meter with a capacitor check, but I suppose I could buy one. Is there anything that I am overlooking that is easy to check?

That is the end of the furnace problem description. Thanks for reading. What follows is why I am not panicked to fix it.

Thanks for looking

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Don't send too much sympathy my way. Yes, It is cold in S. Louisiana, but I have mini-split heat pumps. I think I was a little below the design temp Monday evening and it got a little chilly in the house with the compressors spinning very fast indeed. At the time of installation, gas was relatively expensive, but price dropped very quickly after the first half winter with them so we went back to using the furnace and ducts for heat for the most part. Part of that is cost and part comfort and convenience. At the time, the controls for the Mitsu mini splits could not do a set-back, only on-off. I like to sleep cool, but have it warm when my feet hit the floor so that does not work.

Ironically, I was thinking that it might be time to recalculate cost to heat with the heat pumps vs. furnace :-) No matter how that looks, I'd probably fix the furnace if it does not cost much. I can get power to it and run it with a portable genset rather than run all the heat pumps and there is still the (lack of) set-back thing.

Again, thanks

Big Ed_4

First , you need to check if proper voltage is going to blower motor ...

mattmia2

you would only need an ac voltmeter to check the cap. Does the blower motor hum but not turn or nothing at all? Does it run if you set the t-stat to fan on?

pecmsg

mattmia2 said:

you would only need an ac voltmeter to check the cap. Does the blower motor hum but not turn or nothing at all? Does it run if you set the t-stat to fan on?

A little knowledge and PPE is also required.

Bronxtech

How about the temp. Limit switches that feed / supply juice for the blower control.?

JUGHNE

Put "Fan On" switch to on....if fan runs, then see if heat will stay on with the manual fan switched on.

bio_guy

Thanks, the blower does not start from a manual call for the fan. I did not note any humming nor any singing, nor whistling. I am pretty sure I asked for fan only while in the attic so I had a better chance of hearing compared to listening over the rest of the ruckus. I remember to try that first thing I am face to it again.

I can check for line voltage to the motor, I guess. I don't know much about capacitor-start or -run motors although I did prime myself a little on a fractional HP, vent fan motor in my garage last spring. I can't comment on any other switches as I've only had the covers off the control board cover and the blower at the return. I am respectful but confident with line voltage. I've done a little work with the three phase open delta power supplied at my home. This is obviously single phase 120 so nothing complicated. I just have to draw myself up and think a second time before I touch anything. Unfortunately, the toggle switch box for this furnace is in a spot that would be easy to roll over and trip while in the very place I am trouble-shooting. I'll have to do something about that before I touch it again since I am about to escalate my exposure.

What next, check for proper voltage at the motor during the start up?

If 120 true, what after that? Capacitor?
If no voltage what next?

SuperTech

If you have no voltage to the fan motor with the fan switch on and the capacitor tests within the acceptable range (usually +or-6% of the microfarad rating) the next thing to check is the 24 volt side of the furnace.  Check for voltage between the G and C (common) terminals on the control board or fan relay. If you have around 24 volts there and no power to motor then the relay or board is bad. 

Some techs take a shortcut and remove the wire for the heating or cooling fan speed and connect it to a hot 120 volt terminal or wire to see if the motor starts. I don't recommend this. It's best to use a meter and check everything the right way. 

mattmia2

SuperTech said:

Some techs take a shortcut and remove the wire for the heating or cooling fan speed and connect it to a hot 120 volt terminal or wire to see if the motor starts. I don't recommend this. It's best to use a meter and check everything the right way. 

That is just way more work than using a meter...

EBEBRATT-Ed

@bio_guy

Check the motor wiring and the capacitor.

unclejohn

I'm one of those guys that pull the wire off and connect it to line voltage and then I know instantly if I have a good motor or not. 

bio_guy

Thanks, that all makes a lot of sense to me. You've cut my research time to nothing. I am lucky to a have a more or less complete backup system in place so this can be leisurely :-)