Heil N9MPD075 condensing furnace 3 flashes with new motor, old motor no flash.

Double D

I went on a call for a Heil condensing furnace. The inducer motor was froze up. I was able to free up the motor and get the furnace running until I could return with the new inducer assemble. After installing the new inducer, the furnace locks out when the burner door is on. When the door is off the furnace cycles fine. Assuming the motor I put in was faulty, (heat exchanger side had a slight wobble in the shaft) I put the old one back on until I could get another inducer in my possession. The furnace worked fine the whole week with the old motor. When I returned and installed another new inducer, same thing furnace code 3 flashes with burner door on. I removed the new motor and put the old one back on, furnace works fine. When I get back to the house I'm going to take the intake pipe apart to see if there's something in it but if there were something in it,why would the furnace run fine with the old Noisey inducer motor and not with the new one. What should I be looking for? BTW there is no restrictor plate used for this model.

DZoro

Check the intake pipe, make sure the inducer is flush tight to the condensate box, drains clear, if still a no go,remove condensate box and check for a crack in the back side of it.
D

Double D

Thanks, first on the list was to check the intake for obstruction. Are cracks in the condensate box a common problem with this model? Just high temp silicone to re-install cover?

DZoro

I've seen them crack on the back side. Have to remove the box to find it. Temporary fix maybe high temp silicone, get a new box if it is bad.
Intake obstruction more than likely tho.
D

Double D

Thanks again, I'll let you know what i find when I get back there.

Double D

I took the intake apart and found nothing in the pipe. The termination kit was not done right so i took the vent 90 off. When I looked inside there was a body of water in the bottom. I adjusted the pitch on the piping and reworked the termination fittings. I removed the drain and flushed it out. Although it was not plugged I did get dirt out of it when I was flushing. I put the new motor in and observed the cycling of the furnace for a few cycles. Everything seemed to be working fine. I packed up my tools and left the job. I didn't make it more than 5 min away and they called to say the furnace locked out again. I put the old motor back in and the furnace works fine. Really stumped on this one.

ratio

Don't ya just love these head scratchers?

I had a similar problem with a Trane. The pressure switch was right on the edge of tripping, after it fired for a few minutes the switch would open. The inducer was bad, so I replaced it, no change. The combustion numbers looked good, the flue & intake were not great but short enough that it shouldn't've been an issue—I straightened it out anyway. After several trips & a call to Trane I condemned the HX. (Un)fortunantly, the warranty was through another company, so I don't know if that fixed the issue, but I hope so…

If water is pooling in the flue, even a little, you'll run into issues sooner or later. A different install I went back on, would fill the inducer on the coldest days. There was a dip at the end of the flue & on the coldest days (longest runtimes) we'd pack up enough condensate there that when the burn finally stopped, we'd overrrun the drain & fill the inducer. I ended up reworking the flue & intake. Street 90°s are your friend!

HVACNUT

This is an OEM replacement motor?

DZoro

One other note make sure the drain on the side of the furnace comes out to a unused tee then straight down. Drain tube should not be in any water.
Are you getting water inside the inducer?

Double D

OEM motor for furnace. Not getting water in the inducer. Drain comes out to an unused tee then straight down. Still can't make sense why old inducer doesn't cause the lock out but the new one does. Is there another possibility? Maybe remove collector box and look for blockage. I hear of people having these kinds of problems but this is a first for me.

DZoro

Didn't you check /remove the collector box earlier for cracks? If not do so.

ratio

Old inducer sucks less than new inducer, doesn't pull so hard on pressure switch.

Double D

I was on a call for the same model furnace on Friday. I freed up the motor so they could have heat while I went to pick up the new motor. When I went to change the motor, I noticed 2 crack in the collector box without removing it that showed signs of leakage. I told him I would need to get a new collector box in my possession and replace it. In the mean time, I put the new motor in and the furnace is cycling fine. I'm losing faith in the idea I'm going to get the other furnace working and if I do pull the collector box, find cracks and replace it and still have a furnace that isn't working, I'm going to have more time in this call than it takes to replace it more than 2 twice. What does one do to get the furnace working if it's true the new inducer motor sucks more than the old one?

ratio

What I meant (& I'm just giving you an armchair diagnosis, mind you) is that the old inducer is worn & isn't operating inside specs anymore. The new one is; but the (alleged) cracks allow too much air in with the lower pressure that results from the new inducer.

Or maybe I'm wrong.

Double D

If I pull the collector box should I make sure I have a new gasket handy or will silicone be good enough? The way the vent kit was done the intake has been pulling in the exhaust since the furnace was installed almost 10 yrs ago. Is there a chance I'll be seeing something going on in that secondary heat exchanger as a result of that?

DZoro

I have temporarily used high temperature silicone until I was able to get a new one. Then can test and confirm the HE's

Double D

Thanks, I didn't want to put them in a no heat situation.

Double D

Thank you @DZoro & @ratio. I should have listened earlier. It was a cracked collector box. I can get the collector box that comes with the inducer gasket but where do I get the white heat shield from or is there something else you can use?

DZoro

I usually put some high temp silicone on it where needed to hold it in place. But you can order a new heat shield.
D