Gas Furnace Kick Your Butt?

mdshunk

I did HVAC work in years past, and as an electrician now, I get the occasional call from a HVAC company that has a "tough one". Joy. Not sure what qualifies me for calls the HVAC company can't figure out, but I'm glad to run them anyhow.

Here's one from today that I managed to solve. Lennox Plus 90 type natural gas warm air furnace. 3 years old. No model info.. sorry.

On call for heat, inducer fan runs.... normal so far. Pressure switch clicks in... normal again. Hot surface ignitor glows for a spell... still normal. Gas valve click on... normal still. After about 4 seconds, gas valve shuts down. This is what the furnace was doing for the HVAC man at the original trouble call, so he replaced the board. "Not opening the gas valve long enough", he thought. They put THREE boards in this furnace, with no help.

When he met me this morning, he explained all this, and implored me to check the gas valve. I ohmed it out, and did the calculation for 24 volts, and determined that it would have an amp draw of about .26 amps. The data sticker on the valve said .3 amps... close enough. We attempted to start the furnace while I measured the amp draw (in case it "shorted out" while voltage was applied), and I got .25 amps. That's good. I suspected that the gas valve was not putting out gas, so I asked the HVAC man to get his manometer off his truck. We measured 3.4" of water column, which was close enough to the 3.5" listed on the furnace dataplate.

I did notice this whole time that the hot surface ignitor didn't glow as brightly as I thought it should. I disconnected it, and measured the open circuit voltage to the ignitor. It measured 120 volts, which sounds fine. I reconnected it, and checked the voltage with the ignitor in the circuit... 55 volts... WHOA! I ohmed it out, and got 23 ohms. I don't know what they're supposed to ohm out at, but we called Lennox right away, and they said 10 to 18 ohms. This ignitor was way out of spec, for some reason.

HVAC man got a new ignitor off his truck, and we ohmed it out.... 15 ohms. Installed the new ignitor, and the furnace fired right up. The original ignitor wasn't getting hot enough to light off the gas during the 4 second trial for ignition. Never had a gas furnace kick my butt, but this one had me scratching my head. Never had an ignitor do anything other than just "die". Never had one get "dim". Just something for y'all's memory banks in case it happens to you.

ron schenker

You musta been one of those braniacs in highschool, eh?

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Don't worry, the baseboard will hide it!

mdshunk

Ever had your butt kicked by a brainiac, Ron?

No, seriously, I enjoy tough problems. I had a truck that I've since gotten rid of that had the tagline "We specialize in what the other guy can't fix" listed under my company name. I've got one right now that says "We specialize in the impossible", which may be why I get these calls from time to time. I got that idea from a guy's truck I saw in West Virginia. Johnson & Johnson Heating and Cooling in Martinsburg, WVA. "Tough problems are our specialty" was on his truck. He's my hero... so is Mitch's "dragon lady" (the wife and the phone answerer).

Mike.GCOC

Thanks MD. I'm new to this site. I'm a GC in SoCal that does electrical and certified to do HVAC. I've never had an ignitor go "dim" either- they normally just burn out.

POOLMANinCT

i would have went for the HSI (hot surface ign) right away, not that im a wiz w/ the multi meter... but obviously if it glew at all it was getting juice...
also we had a run of bad HSI about 2 years ago similar symptoms..

turns out they were not defective, they were handled roughly & "hairlined" through distribution apes, forklifts whatever...

also, as i sure you know keep skin oils of the HSI, get a hot spot & short life