HEIL HELP NEEDED !

[Cosmicmuffin]

Have an older Heil,condensating furnace. Msd NTGM100EHA3 Ser. L983817999 It keeps tripping roll-out switch,which I have replaced,that is attached to the closed combustion burner box.Box does appear to be getting hot,you cant hold your hand on cover. However flames from inshot burners seem to be normal. Its not using outside air (1pipe system) so I know this is not blocked. The extreme cold we are having here has increased run time,i believe this also has added to issue. First thoughts weak inducer motor,plugged secondary HX.However pressure switch not tripping. This was a late night call,20 plus below forcasted.I was able to keep running by leaving box cover open on one side. Does not appear to be drawing much airflow thru the 2inch PVC pipe inlet. Thanks ahead.

[ch4man]

check the height of the pilot, is the pilot flame inpinging or close to the cover? ive seen that before on tempstars that vintage.

i am of course assuming its not HSI

[BNME8EZ]

Check the HX, I bet it has a hole in it. We had a Tempstar that was a mid 90's model the roll-out would trip on. Checke dit with a thermometer and it was getting that hot but the burners looked fine. found several holes in the HX causing just enough blowback to over heat the rollout. That's why it's there.

[Cosmicmuffin]

Yes the pilot did appear to be high,however they have a heatshield above it,and cover.(could you even adjust it?) I did run my camera in to burner access,up to first bend,no holes. I did do a heat rise test,that was in spec. I was going to do a manometer test on inducer motor,but there was no specs marked on it,or pressure switch. Any thoughts what they should be,when I return later?

[rundawg]

I was going to do a manometer test on inducer motor,but there was no specs marked on it,or pressure switch. Any thoughts what they should be,when I return later?

You should measure a differential of - 1.86" wc, or greater, depending on length of vent.

The pressure switch should Open at -1.38" wc, and Close at -1.62" wc.

Send me an email, and later today, I will get a manual off to you.

What was the manifold pressure, and was it checked with the burner box cover OFF?

What was the rated temp of the Roll Out limit you replaced? It should be a 200ºf limit.

It might just be getting too hot after extended run times. Can you do a combustion analysis to see if you are overfired?

[Crkdbateman]

The primary heat exchanger is more than likely bad!!! The furnace should have a 20 year warranty on primary. The easiest way to check is to take the limit switch out and look to see if the rings are popped off. These furnaces were known for popping rings.
I replace one every week during heat season!

Also if you take the cover off of burner box and look into the back of the primary, sometimes the back will spread open a bit before it rolls down. Compare it to the front flange. This is typically a dead giveaway.