Gas furnace won't start

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Old 10-08-19, 09:48 AM
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Gas furnace won't start

Greetings. We have a Sears furnace manufactured by Inter-City Products - photos attached.

Although 25 years old, much of the furnace has been recently upgraded - like within the past 5 or so years - new main blower, new heat exchanger, new 1F80-261 thermostat/control etc. Also new AC two years ago.

Anyway I tried starting it up for the fall, I can hear it click but the gas and blowers don't click on.

I went through the instructions on the inside panel, cut the electricity then turned off the gas valve shown in the photo - waited 5 minutes then turned it on, restored electricity and tried again to start. Still hear clicking in a relay - but no start up.

Any ideas? I was wondering if the blower in the pic is dead? Or maybe the semiconductor on that blower?

Any advice appreciated.

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Last edited by PJmax; 10-08-19 at 10:02 AM. Reason: resized pictures
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Old 10-08-19, 10:24 AM
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Welcome to the forums.

They replaced a heat exchanger on a 20+ year old furnace ??
The large item in your pictures is the gas valve.
The draft inducer blower looks to be in the bottom of the last picture.

You have an intermittent pilot ignition...... is it the spark type ?

There are two safety switches on the front of the furnace. One is directly behind the gas valve. It's connected with two red wires. The other one is in the heat exchanger area. If either of these two are open.... the draft inducer won't start and you'll not get ignition. You'll need to check these switches with a voltmeter if the furnace is on or an ohmmeter if the power is turned off.

The link below is part one of your manual. At the bottom of the page is a link for part two.
Part one - Service manual (pdf)
 
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Old 10-08-19, 11:25 AM
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Thanks much.

Are there pics in the manual that show the location of the two switches? If so what page(s)?
 
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Old 10-08-19, 11:31 AM
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...When I put my hand on the back side of that gas valve block - the block where the light blue colored on-off dial is located for the gas shown in the first pic (I assume that is what you refer to as the gas valve), I can't feel any switch...

Do I need to remove that other panel behind the gas valve to access the heat exchanger switch?

I think it might be a spark type - there is no pilot light to light.
 
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Old 10-08-19, 11:48 AM
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When the furnace lit previously..... did you hear a sparking sound at the start ?

Look at the two valve pictures. The two red wires that look like they're coming out of the bottom of the valve are connected to the limit. Follow them.

In the third picture.... I'm pretty sure the four wires on the right are the limit wires. There are the two red's that go to the limit under the gas valve and the two blues go to the other limit.



The limit switch on the red wires will probably be like the one in the link below.
The other limits won't have the red reset button.
Nordyne limit - rollout
 
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Old 10-08-19, 11:56 AM
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If by switch you mean the rectangular black block shown above the gas valve block - the black box with the two red wires attached, I did put an ohm meter across the two contacts and the switch seems to be closed - when the power to furnace is off...I got a low ohm reading..

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Last edited by PJmax; 10-08-19 at 12:25 PM. Reason: reoriented/resized picture - added labeling
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Old 10-08-19, 12:03 PM
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I did check the gas valve, it is the 36E36 just like the manual says on page 29...is that the spark type?
 
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Old 10-08-19, 12:13 PM
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Yes when ever we started it up we hear a clicking sound so I assume that is a spark igniter?
 
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Old 10-08-19, 12:18 PM
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I followed the two red wires you mentioned - the wires that look like they are coming out of the gas valve but are actually coming out of the second component mounted to that plate. The first pic in this post shows them...one of the wires goes to the semiconductor shown in the second pic - that component is mounted to the blower. The second wire shown in the first pic in this post goes up to that rectangular block with the two red wires going into it.

These two pics should have also been rotated 90 degrees clockwise sorry.

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Last edited by tetra; 10-08-19 at 12:33 PM. Reason: resized/labeled pics
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Old 10-08-19, 12:34 PM
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Check the two switches for continuity. They should be shorted.
There's the spark ignition module.
 
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Old 10-08-19, 12:48 PM
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with the inducer not coming on, you should see if power is being applied to it and then check the control circuit upstream if it isn't getting power.

Can you post a clear pick of the schematic. usually on the blower access panel.
 
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Old 10-08-19, 12:57 PM
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Here is the schematic. I'll check the contiunity...

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Last edited by PJmax; 10-08-19 at 01:05 PM. Reason: resized picture
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Old 10-08-19, 12:57 PM
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That diagram is slightly different then the service manual.
The limits were moved to a different parts of the circuit.
 
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Old 10-08-19, 01:00 PM
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the click could be the system relay closing.

possibly a bad inducer.
 
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Old 10-08-19, 01:01 PM
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The switches you labled "flame rollout" and "limit" switches are both shorted.
 
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Old 10-08-19, 01:04 PM
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I was using a different diagram. Same model number but a lower revision. The limit switches are moved to a different circuit and don't stop the inducer from starting.

The next logical step is to check for 120vAC on the white and black draft inducer motor wires during a call for heat.
 
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Old 10-08-19, 01:17 PM
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I was using a different diagram. Same model number but a lower revision. The limit switches are moved to a different circuit and don't stop the inducer from starting.
I was just looking at that diagram.

I can't see how the limits stop the unit from firing. With system relay low voltage side closed and pressure switch closed, there's a direct circuit to the module.

edit: nevermind, they cut power to the thermostat.
 

Last edited by user 10; 10-08-19 at 01:43 PM.
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Old 10-08-19, 02:09 PM
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PJMax,

I assume by draft inducer motor wires, I assume you mean these B & W wires coming off of the blower shown in this pic is this correct?
 
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Old 10-08-19, 02:13 PM
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yes.

you need to turn on the heat and see if that motor is getting power with a meter.

you can also feel the motor and see if it's warm after a minute or so.

the brown wires lead to a capacitor. A bad cap can cause the motor to not start - if the motor is getting power, getting warm and not turning over it's worth trying a new capacitor before changing the entire assembly.
 
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Old 10-08-19, 02:31 PM
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No voltage across the black and white wires frm inducer when furnace is turned on...
 
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Old 10-08-19, 05:46 PM
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Can we assume that you have the blower door on or the switch held in and heard the relay close when the heat was called for ?

This is a basic circuit. There is a system relay that closes when the thermostat calls for heat. The blower black wire goes to terminal 3. Terminal 5 should be live 120v at all times. The white wire goes directly to neutral splice.

Check from 3 and check from 5 to white wire when relay is closed. If ok on 5 and not on 3.... the relay is bad or it's not closed. There would be 24vAC on terminals 7 and 8 if the stat was calling for heat.

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Old 10-09-19, 11:12 AM
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Are draft inducers for this furnace hard to find and or expensive?

I am thinking its the draft inducer because the inducer motor hums when getting power...when i toggle the thermostat on that is...
 
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Old 10-09-19, 11:15 AM
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It does not make sense though that the motor would hum if it's getting no voltage...
 
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Old 10-09-19, 11:19 AM
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Yes Pjmax I am pretty sure the relay closed when the heat was switched on - I heard a clicking. The blower door was on and I think there is a switch that closes when that lower door is on.
 
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Old 10-09-19, 11:26 AM
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Are draft inducers for this furnace hard to find and or expensive?

I am thinking its the draft inducer because the inducer motor hums when getting power...when i toggle the thermostat on that is...
They are pretty expensive - often $200-$300 at cost. you can look yours up.

It may be the capacitor that's bad, follow the brown wires.

How were you checking for power?
 
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Old 10-09-19, 11:33 AM
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Across the white and black wires coming out of the blower.

I had a guy check the cap, he said it was measuring about 5 volts when he removed it from furnace.
 
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Old 10-09-19, 11:41 AM
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you may have had your meter set to the wrong mode or the lead didn't make proper contact.

The cap is measured in mf, not volts.

the cap should be totally discharged by the motor winding after power is removed. you discharge it as a precaution and then check the capacitance.

if your meter doesn't check capacitance, just change the capacitor first, they're cheaper than the meters that can measure mf.
 
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Old 10-09-19, 11:51 AM
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...come to think of it though, when I checked voltage across the motor - I had that lower blower door off and that safety switch was not closed so I guess it was not getting power because of that. I forgot about that LOL.

Checked it again today and I think I closed that switch and the motor was humming then.
 
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Old 10-09-19, 12:04 PM
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Yes, that will do it.

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