Bob or others with Furnace Knowledge

[2510Paul]

Recall I posted about my furnace outage. I chalked it up to restricted in flow pipe. Since then I have had the flame rollout switch trip three more times.

I have been searching for causes of flame rollout in a Natural Gas, Trane XV95, residential furnace. I am hoping Bob or others more knowledgeable or experienced can help build my knowledge.

1. Cracked Heat Exchanger. Everyone on YouTube talks about this. I get it and will check further.

2. Low gas pressure. I have to review my schematics for a low gas pressure switch but the point is the High Temp Rollout Switch is what is tripping. I have my house, my 24x28' garage, and my 32x72' shed all piped to one gas meter and as you know it was -28F. If all three called for gas at once could that trip the flame rollout switch? I cannot visualize what happens. Does the flame wander and hit the flame rollout switch.

3. A lack of combustion air. I think I get it. The flame will follow the oxygen and go upstream to get it if necessary. Correct?

4. Plugged Burner Orfice. ?????

5. Plugged heat exchanger. This is another one the YouTubers like to mention. I guess if the exhaust air is not moving fast enough the flame will seek more oxygen towards the source of the outside air. Correct?

I did get from YouTube what a good healthy flame should look like.

So Bob or anyone else, can you comment further on any of these. Sorry this is off topic, but I do heat my tractors with natural gas.

Paul

 

[jeffcat]

One other thing. Is your airfilter nice and Clean? NO shades of gray or brown. CLEAN. My York shutdown one time on a windy day and the filter had just a little dusty brown color. Change the spare and away it went after I reset the controls. Turn switch off and back on.

 

[504]

The heat exchanger plugged in my carrier last year. The dealer replaced it under warranty (they have been sued over plugged exchangers) and gave me 15 more years of warranty. It seems that they are only good for about 15 years then you buy a new one

 

[Bob]

1. Be SURE you don't have a cracked heat exchanger. As I recall, you had a pro there who didn't suspect that, but who knows. You say you did some research on that and will be checking further. I'll leave that up to you to verify, be SAFE, don't take chances with the lives of you and yours.

2. As I recall, you said the furnace has two air pressure/air flow switches, they SHOULD trip if combustion air or exhaust are restricted, cutting of gas and the flame before "rollout" can occur. They can and do fail, the service manual (might find a downloadable version on the 'net) should have pressure ratings and testing procedures (typically using a digital manometer). In other words, perhaps there is an airflow restriction and the switch(es) aren't responding to it.

3. The burners are mixing tubes/venturis and with low gas pressure/flow it doesn't create airflow into and through the burner (along with the gas exiting the orifice) and gas will accumulate in the burner box 'til it lights off, likely burning at the back of the burner(s), (flame "rollout" by definition) and tripping the airbox overtemp sensor. Low gas pressure is certainly a possibility (again, can be checked with a manometer), required pressure range will be on the furnace rating decal.

4. The burners may need to be cleaned. Take a good look at them, best to remove for a good look and clean.

5. If your house is "tight" (little air infiltration) running bathroom fans and/or a range hood fan can cause negative pressure in the house and "flame rollout", but as your furnace is newer, and has a combustion air inlet and a sealed burner box, that shouldn't be the issue.

Some ideas. Again, BE SAFE, flammable gas and carbon monoxide are unforgiving!

 

[johnlobb]

I had a problem this fall with my furnace not achieving temp when it got colder. I would wake up and the temp would be 67F and the tstat was set at 70. It was driving me crazy, I removed the blower squirrel cage and cleanecd out the fins, I got some foaming coil cleaner and cleaned the A coil twice. I finally called my little brother who has been fixing heating equipment for years and he told me I had a spider web in my gas orifice. I told him I had rodded the orifice out, but he told me I had to remove the orifice, the web was behind it. When I did, I found out he was right, a small, filmy web was behind the orifice. removed that, and all is well. There is nothing like a cold swmbo to motivate one! The web was partially blocking the orifice, not allowing maximum BTU's through it...

 

[Steve@Advance]

I'll try to answer some of your questions...

1. That Trane system is a very good, high end furnace. Unless there was some defect in manufacturing, not likely the heat exchanger is burned out this soon. That is a high liability item, not something a mfg would want to shortcut. If there is a problem it had a lifetime warranty.

Typical symptoms are the flame dancing above the burners, turbulance in the fire box as soon as the main blower comes on, flame roll out (as in the flame actually coming out the front of the furnace), discolored paint around the vents, CO monitor sounding (serious life threat from CO and fire!).

2. I wouldn't think low gas would cause flame roll out. It can cause the flame to burn at the jet instead of on top of the burner as it should. That will cause sooting, clogged burners, definitely not a good situation. Look for black soot around the jets, inside the air horn of the burners.

3. Lack of combustion air is one of the most likely causes of flame roll out. Also negative pressure around the combustion air intake.

Where is the unit installed? If it's in the attic, certain air drafts, strong wind currents flowing through can cause backdraft down the flue. But, I would think the combustion air flow sensor would catch that and shut down the flame before the roll out sensor over heated. Easy test for the combustion air flow sensor, unplug the hose, it should shut down the gas flow.

If the unit is located in the house or basement, there should be a combustion air duct from the attic or outside feeding air to the closet where it is located. Without that air supply to equalize the pressure, air can be drawn back down causing the backdraft. Possibly the air supply could have been blocked by snow?

If possible, try watching the furnace. Try opening outside doors, one at a time or combinations. The flame should be steady, no leaping or blowing around. Of course a lot depends on the wind direction and speed. If you can, do some detective work, note when the failure occurs, what was the wind doing, any unusual events, certain door opened or closed... Sometimes if there is a return air duct in the same room as the furnace, and the doors are closed to that room, it can cause low pressure and backdraft the heat exchanger.

4 Plugged burner orfice would show as one burner having a much lower flame or no flame at all. Could also cause the same symptom as low pressure, but only on the one that was restricted.

5. Plugged heat exchanger, that would be a possibility. As in the vent pipe not properly routed and collecting condensation, or icing. Anything that could partially block the exhaust vent. Is the vent covered to prevent birds or rodents from nesting or getting in? Wasp nests, dirt daubers?

If there is not enough air flow to handle the expanding combustion gasses, the flame will try to find it's way out, and that's why the roll out sensor is there.

If this unit has been in service for a few years, and just now having this problem, something must have changed. A remote possibility is a faulty roll out sensor, but not likely. If there is roll out, typically you will see sooting or paint discoloration around the area of the sensor.

Hope this helps. Do you have the install manual? It has all the specs for vent size, dip switch settings for fan speeds, etc. Many installers don't bother to customize the settings. It's time consuming, but makes for a better install and efficiency.
Trane Manual

 

[da.bees]

Steve@Advance explained in a way that's easy to understand so I see no reason to repeat what he already said. I also see nothing he say's that I would conterdict so I'll just expand a little in hopes of helping you put his advice to use.
To confirm or rule out "negative pressure" within living space. CLOSE FIREPLACE. With blower running and all doors throughout house in open or closed postion they are normally in, crack an exterior door about 2" and put your face near crack. If you feel inrushing air as if it were blown by a fan,you have negative pressure. If there's no inrush,leave furnace blower running,turn on all vent-a-hood and bath vents then repeat test at cracked door. Don't worry about a wisper of air,only if it feels like your face is in front of a fan. If you have negative pressure,post back and we will help find cause.
Next we hold a burning candle near burner air intake. You should see a definite pull on candle smoke and flame(may even suck the flame out if close to burners). No or little effect on flame equals lack of draft and if flame is BLOWN back toward you,there's definatly a dangerous backdraft. Again,post result if you need help finding cause. That is the main concurs and should rule out one or both so efforts can be concentrated where required instead of hotgun fashion. Let us know and we can go from there.

 

[grizz02]

is the inducer motor coming on and clearing the burner , also the drain hose that comes off of it can get plugged take it off and blow through it , if it drains outside and you have had the cold it is froze up , I had to add heat tape to my drain tube,

 

[K Effective]

The girl on another board I frequent had the same trouble- I told her it was a plugged purge blower inlet, she checked, said it was okay.

When the repair guy got there, he reached in and cleared the snow/ice plug from the purge blower inlet and all was well. She claims it was too far for her to get to it.

You can remove the inlet pipe from the purge blower at the blower and let it suck air from inside the room- that will rule out the inlet pipe. I had a HUGE wasp nest formed in mine the first summer we had it, right at the blower inlet. The installer found that one when it would not fire the first Fall.

Being the super-smart guy, I said I'd never let that happen again, and replaced the 1/4inch inlet pipe screen with window screen. SO, next Fall, it will not fire AGAIN. The fine mesh was plugged with frost- too fine. What idiot put that there?

Next time I had trouble the purge blower sounded like it was pumping water- because it was. Drain line plugged by the sump pump: spider's nest.

Last year a yellow jacket got all the way up the drain line to the little white rubber elbow at the purge blower, and it again filled with water. That one took me a while to find, little MF'r.

@@@@tl;dr Every one of the four problems I've had with my Trane has been purge blower related- so I am predisposed to that being the problem. Don't be dumb like me, Check it again!@@@@

"Good Luck, we're all counting on you!"

 

[K Effective]

Well? Still shivering?