My gas furnace won't fire up. It triggers the induced draft motor, which runs for about 20-30 seconds, and then the whole system shuts down. Up until yesterday it was working fine and I had been running it for a couple weeks now (in Minnesota). The only thing that changed was that yesterday I was going round the house and sealing up all the penetrations, and I blew some Great Stuff around the exhaust vent. I've checked the vent and no foam got in the pipe/vent, and none even came through to the inside side of the joists. It all just went behind the vinyl receptacle cover and AROUND the vent.

There is a little condensation around the inside of the pvc pipe, but I'm not sure if this is increased from before I added the Great Stuff (I never looked that closely before).



This is a horizontal concentric termination. I'm getting error code "45"which is : low pressure switch is open. Possible issues:

-insufficient combustion airflow

-insufficient combustion air

-improper pressure switch setting




I'm not sure how adding spray foam AROUND the exhaust changed anything. Other that scraping out all the spray foam, is there something else I should correct or look at?




Can you guys help a single girl out??Thank you very much!

-L

 

I'm no expert by any means. However, before you added the foam how big of space was around the exhaust pipe? Although mine looks a lot different, combustion air is also drawn in from around the exhaust vent. You may have blocked that off. In mine as shown, combustion air is pulled in from the back side of cone.

 

I used to have that same rain cap/cover on my exhaust, but it was removed a month ago right when I had new siding put on. If you remove that cap, I believe your exhaust pipe will look just like mine does now -just a straight pvc pipe. I believe those caps are just to prevent snow and water from entering, and removing it shouldn't affect anything (as long as it's not needed at the moment to keep snow and ice out- then obviously it would definitely affect things!). But mine was already off before this problem started, so I don't think that's the issue







There is good air flow coming out of the exhaust when the induced draft motor is on. I took a bright flashlight and looked down the exhaust pipe as far as I could, and absolutely no obstructions. It has to be somehow related to the foam, because the system was working great before I went outside to do the spray foam. When I came in, this was the issue.

I know my furnace is extra sensitive. Last winter I had a similar problem, all because I had leaned a shovel up against the house and the handle was blocking the exhaust just a tiny bit. That's all it took for it to detect inadequate air flow.




Could a little condensation change things?There's not much, but a few water droplets along the top inside of the pvc pipe. I don't think the foam changed the slope of the exhaust (and is causing condensation to drain differently or not at all). I mean, there's only so much room the pipe has to move around in the hole through the joist. Adding the foam MAYBE shifted it a few millimeters one way or another.




I guess I'll go scrape out the foam and see what happens.




Thanks for your thoughts!

 

That cone is not a snow cover. I'm surprised your furnace works at all. That is an integral double wall pipe that allows combustion air into the firing chamber. See the following piping in my system. Notice the pipe size and the whye and the inlet outlet of the combustion plenum.

The 3rd pic is in the basement ceiling and directly from the outside. Notice the change in pipe and reduction. Then see the last pic. Inlet and exhaust,

 

So upon removing the vinyl cover this is what I saw. I had plugged the intake with spray foam!

Now I fully understand how a combined exhaust works- a pipe within a pipe. So yes, you are correct, that cap helps keep the two systems separate. My mistake! But it had been working great before my foam adventures
I'm off to clean out the mess and put the cap back on. Thank you VERY much for your help!!!

 

That doesn't look like a concentric pipe in your picture.
Is that the way it was left ?
Did you pull the cap off ?

 

Picture with cover removed.....

 

Did you say the siding people removed it? If so they are responsible to fix it. And if that is their kind of work, then they are very lacking in know how and retrofitting. In fact if they are responsible, they could be held libel for possible injury from such workmanship.

 

When cleaning out the Great Stuff be sure the furnace is turned off and use a vacuum cleaner to suck out and debris as you poke and scrap it out. You don't want any of the stuff to get into the combustion chamber. If you think you might have some of that stuff stuck in the chamber that also may be adding to the error codes. You might need to call in professionals to clean out the burner.

The reason you furnace worked part time after you sprayed that stuff is because it most likely had enough air to from surrounding area to start up.

 

Ahhhhh..... looks completely different with the cover off.