Got a second opinion on my foamed-attic furnace and it cost me a service call instead of a new furnace

We went through almost the same thing after foaming our attic in 2019. Furnace lives up there too, a fifteen year old Carrier. First winter after the foam it started short cycling on the coldest nights and I got the same doom speech about replacing it before we froze. I was skeptical because the furnace ran fine the rest of the time.

Got a small local guy out instead and he spent most of the visit just watching it run. He figured the foam job had changed how air moved up there and the unit was pulling warmer air than it used to, which can trip some models. He ran a fresh air pipe from the gable vent over to the furnace area and that was the whole fix. Cost me a couple hundred and it has been fine for four winters since. So I would say it depends what the actual symptom is. Ours was tripping on a limit switch, not a combustion reading, so the fix was diferent from yours. But same lesson, the little shops will watch the thing work instead of just quoting a new unit. Ours also put everythign on the invoice, which has helped me twice since when I sold my records to the next buyer of that kind of work.

Our situation went a different direction after we foamed the attic in 2021. The furnace is a Rheem that was maybe eight years old at the time, and the second company we called did agree it was running okay, but he pointed out somthing the first outfit never brought up: even when the unit passes testing, an unvented attic can change over the years as the house settles and the foam shrinks a little. He said the air available to the unit today is not a promise about five winters from now. Instead of replacing anything, he boxed in around the furnace with a louvered door and ran a duct from near the soffit into that little enclosure. I think the whole thing was under a thousand with labor, and it means the unit is pulling from outside rather than depending on whatever the attic happens to be doing that day.

Not saying everyone needs to do that, and I understand wanting to just take the passing test and move on, expecially when the first quote was pushing a full replacement. But for us the small ventilation work gave some peace of mind that a single test visit coud not, and the furnace has cycled normally thru three winters since.

Reading this thread with some worry because we're actually planning to foam our attic this spring and the furnace is allready up there. Had no idea this was even a thing that could cause problems. Did any of you think about this before the foam went in, or did it just catch you off guard afterwards? Wondering if I should ask the foam crew to leave some kind of access for combustion air before they start, or if that's something a heating guy needs to look at first. Kind of out of my depth here, first house we've owned.

One thing I would pin down while you have that invoice in hand is wheather the open chases are actually intentional or just sloppy work the foam crew never closed. If you or a future owner ever has those holes sealed up properly, the air situation up there changes overnight, and the furnace that passes today might not pass after the fix. I would ask the independent guy whether he noted the open chases on the paperwork, because a reading only means something in the conditions it was taken under.

Also curious whether he checked clearances around the flue where it passes through the foam, since that is the other thing installers get twitchy about with foamed attics.