My draft inducer motor went bad on my propane furnace. I heard it happening, felt the motor freeze up and was unable to get it going with oil.

So I bought a brand-new replacement motor assembly. I replaced it today, then turned the furnace back on, and it ran like a champ. After about an hour, I was up basking in the heat when I noticed the digital thermostat was displaying a message that said "replace batteries". So I removed it (it was in a heat cycle), replaced the batteries, and plugged it back in.

Now, however, the furnace won't light. The draft inducer motor runs, but the furnace never ignites and the blower never turns on. Did I fry something by removing the thermostat cover while it was in a heat cycle?

EDIT: It's a Heil DC-90 Ultra High Efficiency furnace. Fed by propane through tanks in my backyard (no natural gas available). The gas valve was replaced earlier this year.

 

You may have fried something but it's not likely. As a test, remove the stat & jump the R & W terminals. If the heat comes on, you haven't hurt the furnace. Most likely the stat isn't making good contact.

 

Thanks, Grady.

Jumped the R & W wires, and the draft inducer started. Instantly realized that the thermostat cannot be the problem, since I was able to get the unit to start up with the thermostat before this. The problem was just that it wouldn't ignite.

So the thermostat is not the problem. The unit starts up when it is supposed to. I'm just very puzzled as to why it lit up fine immediately after I replaced the inducer, and now it won't light.

Checked the vacuum hose leading from the inducer to the pressure switch. There is a vacuum, although not an overpowering one.

Don't see any obvious problems, like a wire that came undone.

Any suggestions?

 

You replaced the draft inducer which means you were working in the pressure switch area.
Without a sensitive gauge you can't read the pressure/vacuum on that port.

You can temporarily bypass the pressure switch to see if that is the issue.

 

Well, by temporarily bypass I assume you mean jump the two leads on the pressure switch. I did that. The furnace still did not light. Checked the voltage between the leads, for what it's worth, it was about 28v.

Does that mean the pressure switch is bad? That would be just my luck.

 

Checked the leads and found 28 volts on them......was that connected to the pressure switch or not.

With the unit doing nothing.....the pressure switch should be open and you you should see +/- 24 volts on it. When the unit calls for heat....and the inducer starts up....that switch should close and you'll measure 0 volts.

 

That was with the leads off and a volt meter stick plugged in each lead. I did that while the furnace was on.


If I understand you correctly, I need to measure the voltage of the two terminals on the pressure switch with the leads connected. What I believe I will find is that the voltage is steady at 24-ish volts and it never drops to zero. I suspect this will probably be due to the vacuum from the draft inducer not being strong enough, or because the switch has failed.


Not sure exactly how to do that, though. The leads are sheathed in plastic, and when connected there is no part of the connector to probe. I suppose I could try to see if I can put the leads on barely so that there is some conductor visible and then watch the voltage. I will try that tomorrow.


But now I'm starting to wonder if I did something wrong with the draft inducer. My unit had a Fasco 7062-3793 model draft inducer -- that's the one that broke. The exact replacement for that from electricmotorwarehouse.com was an A-145. However, there was another model A-171 that was listed as a viable alternative to the A-145, that came with a supplied kit to replace the A-145, and it was $100 cheaper. That's what I used.


The specs on the two are identical as far as I can see. The direction of the air is the same. The installation was very easy. And, as I've said, it appears to work fine. However, there was one part of the instructions that I didn't understand.


The instructions said, "Using the new combustion blower supplied, attach the appropriately sized combustion blower restrictor from the blower being replaced using the three (3) plastic pins provided." The accompanying diagram looked like this restrictor was a plastic disc with a large center hole and some holes for pins. However, my old blower had no restrictor, and the new one did not come with any to try, so I did not add a restrictor plate. There was a chart that came with it that specified the type of restrictor needed by input size, and also the diameter of the plates, but the input sizes were 50,000, 75,000, 100,000, and 125,000. I have no idea what that input size measure refers to.


Does the restrictor make that much of a difference? Could it affect the vacuum strength so that the pressure switch doesn't work?

 

That plate is more than likely needed. Those numbers 50K-125K refer to the BTU rating of your furnace.

If you have an ohm meter setting on that meter...use that instead. Go to Rx1 scale. Clip the leads onto the pressure switch. After the inducer starts you should see the meter go from open to short.


As a temporary test......you can short those two wires on the pressure switch and ignitor sequence should start. It may take 30 seconds or so for that sequence to start.

 

Started with the power to the unit off. The ohmmeter registered no continuity between the terminals. Flipped the power on. The thermostat kicked the unit on, and the draft inducer started. No continuity between the terminals of the pressure switch. Waiting up to five minutes, no change.

So then I jumped the two leads from the pressure switch. Viola, it started up.

So I suppose that means the pressure switch is not working. The big question is, whether it broke less than an hour after I installed the new inducer (seems highly unlikely), or whether the vacuum from the inducer is not strong enough to trip the switch (seems more likely, but then why did it work immediately after installation?).

This may be a dumb question, but if I disconnected the vacuum tube from the inducer motor and created a stronger suction, would that be a good test to see if the switch still works and is just failing due to inadequate pressure? Or is there a specific pressure tolerance it needs to see, so that more or less suction is equally disregarded?

 

When the inducer starts up.....you should see a change in switch status in seconds. As soon as blower comes up to speed.

You could try vacuum to see if pressure switch is ok. My guess is the switch is ok. They very rarely go bad.
Did you put the proper sized blower restriction plate in behind the inducer ?

 

Nope. The new inducer didn't come with a restrictor, instead instructing me to use the one from the previous motor. The previous inducer motor did not have a restrictor disc.

I have found the correct restrictor I need from the chart in the inducer installation sheet, for a 100,000 BTU furnace (my unit).

Diameter 1 3/8".
Part# 1009386
Letter E (may be for the pins?)
Substitute part# 1160416 (from the Sears Htg. online catalog)

I will try to pick one up or order the correct one on Monday.

Thanks for all the advice!

I will post the results on Monday.

 

Long story short, got extremely lucky and was given the exact restrictor I needed from a local HVAC supplier. You can normally only buy them in packs of 10, but someone else bought a pack of 10 last year and donated the extra to the supplier.

While I was there I described my problem to the supplier and he suggested I check my drainage lines for a blockage, saying that after a week of non-use, the flood of water after the first ignition might have blocked the drain which would affect the vacuum and pressure switch.

So, since I was already taking the draft inducer off to add the restrictor, I thought I would also clean out the plastic drainage housing, which I did. Lo and behold, what did I find rattling around inside? The original restrictor I was missing. Doh!

There was no blockage in the drainage system. I added the new restrictor using the pins supplied with the new draft inducer. Put everything back together. Started it up, and the furnace fired.

Holding my breath now, since it did the same thing last time, then failed to light after that. But if you see no further posts from me on this thread, the missing restrictor was the problem.

Thanks again, PJ for helping me walk through troubleshooting this issue.