Modine Garage Heater Fan Won’t Start!

[HotRodHudson]

This morning I went out to the garage & found the heater not working. Gas wouldn’t even blow to be able to be lit. I decided to unplug the unit for a bit & try again this time here’s what it did.

After plugging it in I observed; the pre ignition fan kicks on & runs briefly, then the igniter glows red, gas turned on & flames lit as it should. However, this is where something goes wrong as the fan to blow across the hot exchangers never kicks on. Note: this unit is a 75k btu heater, around 12 years old but only put into service late last winter. After installing it I did notice a few drops of oil from the fan on the back of the unit, but that was all & dont know if that’s related.

After it ran the above time any subsequent times the failure occurs were gas should kick on but isn’t. Then the purge fan as I call it, keeps running for 2-3 minutes or more & shut off. Seconds later it starts the process again but this time the gas lights & bows as it should but again the fan on the back doesn’t turn on. After a couple minutes it turns off the gas & purge fan runs a few more minutes.

Since I’m in Montana & it’s getting below freezing at night, any help would be greatly appreciated!

 

[Done That]

Clean the flame sensor?

Sounds like it's going thru normal ignition cycle and then shutting down because flame is not recognized by the control...then it runs the inducer fan to purge unlit gas.

Most boards keep auto re-trying, and cycling power like you did is a hard reset so it allows the whole sequence to try again.

 

[HotRodHudson]

I was able to determine a couple things:

1) the control limit switch was bypassed & tested ok,
2) both flame rollout switches tested ok as well,
3) a wasp nest in the exhaust pipe really smells nasty when getting severely toasted!
4) the early circuit board on my unit were quite faulty & ended up being the cause from the “expert” I spoke to & matched up the internet search information too. Typical symptom is for everything to work but the blower fan on the back never turns on.

Thus, an “updated” new board #3H36849 was ordered from QC Supply in Nebraska. They were $100 less than my local heating contractors & 4 days faster shipping too since they stocked it where none of the local wholes furnace warehouses stocked it. QC Supply was also the least expensive on-line price I could find. It will arrive In 2 days!!

For a tractor supply type of store QC Supply has a lot of things I couldn’t find elsewhere in stock & they readily ship items without any issue.

You guys are just lucky I can’t up load smells to the internet!

 

[Lonstar]

My 2 cents may or may not help, but....

What you are describing is how my 2004 HD75 starts every year when first firing it. My garage is 100' from my house, the line is about 130' from house to heater. Every spring I turn the gas for that line off at the house. In the fall when I turn the gas back on and turn the heater on for the first time, I have to wait for the gas to push the air out of the line. Takes a long time.

Last year I finally stopped procrastinating and added a tee with a shut off valve at the end of the line to purge the air out. But before I did this I went through 10 or 15 minutes of what you describe before all the air is out of the line and the heater stays running. I've never actually looked inside to see exactly what's doing what, I already know why it won't stay running, but the way you describe the fan cycle is exactly what my heater does until the air is purged and it has 100% gas flowing to it.