I have a 10 yo White and Rogers system in the attic that this year decided not to start. Last year its sister unit in the basement did the same and a new control unit fixed it (but we chalked that up to sitting in water for several years).

Now, when heat is called for, the attic draft inducer motor starts, but no spark ever comes as far as I can see (looking in the little window). After looking for obvious issues (loose wires, corrosion, fuses, etc.) I follwed the Honeywell (unit is S8610U) troubleshooting steps and swapped in a new control unit, but this did not change anything.

Due to the way it is installed, changing the igniter would be a PITA, and above my pay grade. Before I get to that, I am trying to figure out how I exlcude other possible issues. I feel it could well be some other control mechanism (lockouts, pressure switch, blocked exhaust pipe, etc.) causing it never to get to ignition. That is, I want to follow an orderly troubleshooting process that looks at the easier things first.

How do go about checking to see if it is trying to spark but failing (e.g. it is an igniter problem) or never getting to this step. I imagine I could jump the igniter line to ground and watch for an arch, but this seems like a baaad idea.

Should I jump the vacuum switches? I jumped the main limit which immediately turned on the main blower, so I concluded that ain't stopping the spark.

All help appreciated and happy to offer the spare control unit for best answers.

ASK

See the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html

See the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html


Yep, that is one of the greatest FAQ's I have seen in a very long time. Especially all of the pictures.

I read through the whole thing and that got me a long way towards knowing that there are several control mechanisms in place that may be preventing getting to ignition including two different pressure switches and perhaps some limits. It also helped me eliminate the main limit as a problem, confirm some steps I had already taken, and educate me on the startup process. Oddly, I cannot find any dignostic LED's despite the fact that

My question was really a more of a process one: is there a diagnostic flow I should follow? Should I jump the switches? How stupid would it be to ground the ignition cable?

Thanks.

ASK

What you need to do is observe where the furnace stops in its sequence. If the ignition doesn't come on, check the pressure switch and if you have voltage to the ignition. If you can't find the problem, post back which step it is.

Thanks. I will be doing that this weekend. OK to just jump around the saucer type pressure switches?

As a diagnostic only, yes.

Thanks Labman.

Followed startup process and go to the induction running then nothing. Jumpered the pressure switches and voilà!

There are two pressure switches in serial. The schematic shows both, so they are not a hack. One has two tubes (one into the indction fan housing) and the other only has one tube. Did a few more tests to determine that it is the pressure switch with only one tube that is stopping the start-up.

Based on some other posts from Labman and HVAC1000 I am off to test the switch itself.

Assuming the switch is fine, and it exists to prevent something bad, any idea what it is testing for? I am not super anxious to be up on top of a steep roof covered with ice looking at exhausts if I don't have to.

I have lotso pics of control unit, fan timer, and schematic if it helps either answer my question, or generally on the board (the pics on the FAQ are great.)

Thanks.

As a diagnostic only, yes.

OK, not sure if it was something in the tube or a stuck switch, but after some huffing and puffing trying to test things, everything works.

Happy wife makes for happy guy.

Labman, PM me your address and I will send off the control unit.

ASK

Yes, sometimes youcanknock a stuck switch loose. I am not sure what you mean.

Address? Clicking on many members' name brings up a form to email them.

Bumping this thread up again. A month ago I had unstuck a stuck pressure switch. Since then the heater has run flawlessly.

Then all of a sudden last night the heater again refuses to start. I go back up and jump the pressure switch again and it gets spark and flame, but the main blower does not come on. After a little while with the flame burning, the whole thing shuts off (must be a heat limit switch.

Any ideas on what could cause the main blower to not come on (especially something tat might also impact a pressure switch)?

Test to see if you are getting voltage to the motor. Depending on what speed it is set for, you may need to test the black, blue and red wires. Or you can just test the white (common )wire to see if you have returning voltage through the motor. If you do, you have a bad motor. If not, check the blower relay. Depending upon the make and model of furnace, this relay may be part of the control board. Also look for a fan switch mounted high in the cabinet area. If it is a Honeywell switch, rotate the round dial inside the control to see if the blower starts. Of course you will have to tape down the door safety switch prior to all these tests.

Thanks, I will give it a whirl with the white. IF the motor was bad, would I still get current through to the white?

Is it stupid to try to jump the relay to see if the fan will come on (and then do I jump the black or red)?

My heater in the basement has the round switches, but I have not seen one on the upsatairs heater.

If there is an open in the windings, the white won't have any current. It can be a pain to expose the contacts to test for current. A voltage detector is much easier.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

Damn, labman, didn't see your post until after my test. I have a few volatge testers (simple light for 24v more complex tools as well) but they are a PITA when I am stuck in the attic.

Also, I have learned that you search at your own risk here. I searched for pressure switches and got

https://www.askmehelpdesk.com/adult-sexuality/making-her-orgasm-5597.html?highlight=two+pressure+switches

At least it made my morning

Well I hope you understand enough about furnaces to realize that won't help your furnace.