No Heat - The Rest of the Story

Earlier I reported we had no heat. See below in posts. Thanks for all the responses. Here's the rest of the story.

1. XV95 Trane Furnace, TUH, UH wiring diagram.
2. No Flame, house fan ran great.
3. Found Flashing Red LED, Four Flashes and then Pause, Manual says Open Temperature Limit Switch.

4. But which Switch, I can find three of them in the wiring diagram. Two are in Series. All have yellow wires.
a.XV95 Furnace High Temperature Limit Switch Open on Rise TCO-A - Paint
b.XV95 Furnace Inducer Temperature Limit Switch, Open on Rise TCO-B - Paint
c.XV95 Furnace Manual Reset Flame Rollout Temperature Switch, Open on Rise - Paint

5. Started looking at U-Tube Videos.
6. About this time the furnace guy arrives. From what I had he knew instantly where to look.
7. The 4c switch is open and he manually resets it. The furnace takes off.

8. He monitored it for some time but could find no issue. The Burner Box remained cool. The 4c switch is also called the Burner Box Temperature Limit Device. "It is located on the bottom of the burner box. The vent and combustion air inlet terminations (2" PVC pipe) should be checked for blockage." Again, this switch has a manual reset.

In hindsight I checked the exhaust and vent pipes first early this morning. The exhaust had some icicles but really no blockage. The vent had a bunch of ice crystals at the inlet of the pipe reducing the hole to maybe penny or quarter size, I was at a bad angle to see. Maybe it was worse than I thought.

When the Technician arrived he said for these temperatures it is really quite humid out so I am going with a blocked inlet vent.

Here are pictures of the switches in the order of 4a, 4b, and 4c.







The Burner Box is just above 4c and just above that is where the Vent Pipe comes in. Sorry, I did not get a picture of that.

Paul

For those of you who are really observant, you're going to ask me why I didn't try and Manually Reset 4c. Well, two reasons. While the schematic labeled it as "Manually Resettable", I failed to register that in my brain. Second, at that point I did not know which of the three switches was manually resettable and to see the button you really need a mirror to see or just know where it is because it is not really visible without huge contortions if at all.

That's my story and I am sticking to it. Ha, Ha.

Paul

Two comments,

FIRST, it the exhaust WAS significantly restricted, there's an air pressure switch or two that should have shut the gas/fire off so there was no "flame rollout" to warm the burner box and trip the limit. Might be an issue there.

Sometimes, a burner will light at the wrong end, and the "rollout switch" will trip, preventing damage.

I've witnessed that happening a few times over the years, seems to "just happen" once in a while, low temperatures cause lowered gas pressure, and make it more likely for that to happen.

Some LP gas furnaces have a gas safety switch so they won't try to burn if pressure is down to, say 75% to prevent such problems.

If the burners need cleaning "flame rollout" is more, as well.

Glad it worked out, nice to have heat again. Been trying to find digital version of this, no luck. Stuck with plain old analog.

Thanks for the comments Bob. There are two pressure switches. This is making more sense all the time.

The exhaust clearly was not restricted but maybe the intake was restricted just enough to lower the pressure enough to get the flame rollout you describe. I think I am starting to understand what flame rollout is and how it might occur. Thanks.

Paul

So how much did it cost you for him to come and push the reset button ? LOL. Next time you will know more what to do. Good chance that inlet restriction blocked it enough.
I paid up good early this heating season to get mine repaired. Glad I had someone who works on these all the time look at it as he found stuff wrong I'd never of knew to look for. Part of it I may of figured out ? I just hate paying such labor rates to change parts that are easy to change. But I also payed for knowledge.

Same thing happened to me about 8 years ago. Furnace was flashing restriction code. I had checked inlet pipe and thought it was open. I rechecked it again and there was snow crystals in it. Cleaned them out, reset the furnace and it worked.

I have the same furnace is it the flame roll out switch that is resettable ?

I worked in the LP gas business 45 years ago. Common joke: Customer "$20 for a thermocouple?!" Repairman "$5 for thermocouple, $15 for knowing what a thermocouple is."
Nowadays I let the pros work on my furnace.

Ken

Paul, rather than thanking Bob, I'd ask him where the he!! he was when you needed him (before the service guy showed up and fixed it) ...... ha !!! I love guys who come to the rescue after everything is fixed. I had a neighbor like that once, every time I bought something new and I showed it to him he told me that he had one in his basement and he would have given it to me. Now nobody get yer gaunchies in a knot, I'm just hankin' yer chain !!!

Funny what all you can learn from ytube. Mine quit a while back found the blinking code, ended up blocked vent tube used blower to blow it out worked fine after that.

We had ours fail once, the intake tube was plugged, I didn't know what to look for so i called the service man. He took the pipe apart right above the furnace and then it started OK, so he went up on the roof and cut the pipe off and cleaned the frost out of it. The next day I cut the pipe above the furnace and put a T in it, with a removable plug, he said that was a good idea. Before we leave in the fall I take that plug out and leave it out while we are gone, it's just using inside air for combustion, no danger.

Don't worry. You are not the only one with new fangled stuff. Five years ago I had my neighbor install a New York super high efficiency gas heater. Worked like beauty till a 5? day and the furnace conked out. Got him on the cell phone and he told me a couple of things to check. As I was tinkering one more thought came into my mind. There is a very low inches water column puff switch hooked to the plenum. If that switch changes state when it fires the board shuts everything down. I traced it back just curious and the drain hose goes down to the evac drain pump for the condensation water from combustion. The little plastic hose that goes outside through the wall had frozen solid outside. Couple of 5 gallon buckets solved all of the problems. Later that week I installed a heated drain line up through the vent exhaust pipe. I will try to get a couple of shots and post them. Works beauty!

Analog version. Good one. Ha, Ha. Paul

One other thing with those two plastic pipes outside. During a really bad storm with DRIFTING snow....go out and check you intake and exhaust a couple of times. My intake almost got drifted in one time. The exhaust will usually keep itself clear.

$86 was my bill. Can't complain, I got the guy out of bed at that early hour. But he said there were at least 25 calls in rapid succession right after mine.

Yes, it is the lowest picture. It has a PO3 stamped on it. The Technician said it just has a little reset button up in there where the terminals are. I never actually saw it because it is so hard to see. Given my still back and neck I would need a mirror which I did not have at the time. The Technician knew right where it was.

This is a good piece of information to tuck away as I am sure is what you are doing.

It also never dawned on me the intake vent would freeze with ice crystals at the intake. I went out later today and they were forming again.

Paul

(quoted from post at 15:30:23 01/31/19) Analog version. Good one. Ha, Ha. Paul

Click to expand...

Hahaha finally someone got it!

yes just stuffing away info hope I can recall if I need it LOL

You sure have a way with words.
Loren

Yep, ours shut off about 0500 this morning as well. I found the winds had been blowing a few drops of condensation from the exhaust right to the intake, and it was 98% plugged with ice and snow. While I cleaned the plug out, I heard it fire up, been fine ever since. I will look at reconfiguring the two pipes slightly next spring.

Not bad for a service call.