Hello,

I have a single stage Trane XE78 gas furnace and it is controlled by a White Rodgers 50A50-405 controller. My furnace was trouble-free the previous two years I lived here but I have been waking up to some cold mornings lately.

Main problem: Wake up and indoor temp is in the high 50's.

Troubleshooting: Diagnostic LED showed lockout (2 flashes). Flipped furnace power off and back on for less than a second, ignition started normally, and my house warmed up. Changed batteries in the thermostat, replaced filter (has had regular changes since I have lived here), and then woke up to another cold morning the next day.

I then noticed that the burners shut off before it gets to the thermostat temperature... however, they will cycle on and off and finally get up to temp. At this point, I noticed that I am getting 4 flashes (temperature cutout problem). I took out the high temp sensor, cleaned it although there wasn't much to clean, used scotchbrite on the flame sensor since I had stuff apart anyway, and put it back together. I also cleaned the terminals with steel wool and the internal connectors with 1200 grit.

Still have a problem.

I would like help with this problem. What I have not been able to find out yet are the following...

1) If the furnace is running, should it run all the way up to the thermostat setting without cutting off, or is the temp cutout basically expected to open from time to time?

2) What is the normal opening temp of this switch and when does it close again? It is a Texas Instruments switch L220F-30.

3) Since turning the power off and back on again solves the problem, I am thinking that my lockout (which is a 2 hr lockout where the inducer and blower motors run with no gas) is not taking place correctly as well.

4) What is considered a "normal" temperature for my flue right off the furnace?

I have a multimeter, thermocouples, and a temp gun at my disposal. I was getting ready to drill a small hole in the housing for the TCO switch so I can get an accurate reading of the temp and see if it really is supposed to be cutting out--I just haven't been able to find the temperatures that the switch is supposed to open at (my setup uses a normally closed TCO). Since I don't know what temps it is supposed to be at, I have chosen not to jumper across the switch in case I really am getting too hot.

Thanks!

If the furnace is running, should it run all the way up to the thermostat setting without cutting off, or is the temp cutout basically expected to open from time to time?

Yes it should run without a cut off.

What is the normal opening temp of this switch and when does it close again? It is a Texas Instruments switch L220F-30.

The limit is 220F so the normal operating temp is about 20 to 30% less depending upon design and install considerations.

Find and fix the cause of the lockout. Make sure all registers are open in the home. Make sure the air filter is clean and the return airs are not blocked for starters.

Thanks.

All registers are open with no furniture covering them and my upstairs/downstairs damper is set to full open. The air filter is clean and I replaced it with a cheaper one to see if the cheap ones offered more flow.

After taking everything apart at 3AM this morning I noticed the evap cores were somewhat dirty. I had to tackle them with a paint brush multiple times before anything would come out. Now that I am warmer I am going to use some compressed air from the opposite side to make sure they are clean.

If that were the problem, I need to make sure my pressure switch works because I think that should have triggered a code before it got to an overheat cutoff mode.

Things seem to have been working normally for the past 10 hours. I am going to check the temp at the TCO switch with my thermocouple so I have a good idea if it is around 220F or not. I appreciate the help and I'll update this with where I am at.

A dirty evaporator will really cause you problems. Most people cannot get to the correct side of the evaporator to clean it the way most units are installed. I feel that the dirty evap was your problem. Want to test the pressure switch? Just remove the hose and see if the furnace runs.

What you really need is the factory service manual for your unit. It will contain the proper answers for that particular unit you have since temps/pressures/operating ladder diagram and a boat load of other items can be a bit different from unit to unit and you seem to be a real accurate kind of person. I always enjoyed hiring folks like you for my small 37 man corporation years ago. They always did much better on service work and naturally they were paid more also since they did a much better job especially in commercial work. Some of those great guys were pulling down 65,000.00 back in the early 80's and that was pretty big money back then.

I will look around and see if I have an electronic edition of the service manual for your unit.
But you might be better served looking yourself since an exact model number is needed for that info.

. Check at the two sites below for your unit info.

Heating and Air | Heat Pump, Furnace, Gas & Oil Boilers, Humidifiers - HVAC | Trane Residential

OR

Search Results - Trane Residential

This 50A50-405 control

Has been replaced with this 50A55-843White Rodgers for future ref.

http://www.white-rodgers.com/wrdhom/.../0037-6265.pdf

I appreciate the kind words. I worked in auto parts for quite a while and wanted to make sure people weren't replacing parts until they actually tested to make sure they were bad--thankfully I was able to bench test most of the sensors (but people still replaced ones that checked good! Lol).

I am able to get to both sides of the evap cores (mine are in an A-frame config). Fortunately or unfortunately, whatever the case may be, the cover was removed by someone before and they cut the sheet metal cover around the AC lines. Then they globbed some RTV to seal it and the whole thing is a mess. It looks like once I get my problem solved, I will be making a sheet metal corner to go over those lines, and then match that up to the panel so I have a good seal.

I set my thermostat up to 80*F and my thermocouple right at the TCO is at 210*F now so I am going to shut her down, seal off the area below the evap core, make a dust cover with a vac port, and blow the heck out of the evap cores with compressed air.

I agree that it is probably time to get a service manual. I made a manometer a while ago for checking backpressure on automotive exhausts so I should have just about everything I may need.

Coils can be very difficult to clean. The only problem with your idea is how you are going to trap all the dirt/dust you blow out of the unit? This stuff will just get blown back into the furnace in most cases and that will be blown right back on the coil unless you somehow trap it. Stretched panty hose or some such item might do the trick if placed over the heat exchanger. Big woman's size would be the best.LOL

A paintbrush was useless, a nylon bristled brush was a bit better. To keep the dust from going everywhere, I traced the triangular sheetmetal front onto some cardboard and cut a hole that my shop vac would fit in to. I then cut a piece of cardboard the same size as the intake face of one of the coils. I cut a last piece of cardboard to fit at the bottom... it fit relatively nicely because of the drip tray. I taped the bottom piece down well, taped the intake side cardboard to one of the intake sides, put the A-shaped facepiece on, fired up the shop vac, and then blew from the output side of the open evaporator with compressed air.

I thought it was an impressive setup.

The results were not impressive! I used a piece of 3/16" brake line on the end of my blowgun so I could get to the back...and at 80 psi, I still apparently wasn't doing much. After I took the cardboard face off, I still ended up going at it with a brush so I know the middle of the coil can't be that clean. No dirt left that I can see, light shines through (although the coils are 2-row and staggered so that blocks some light), and my problem still persists.

Good news is I found a manual online with flowcharts that I'll tackle after some sleep. I'd post the link but since it isn't from a Trane authorized site I am not sure it is supposed to be out there. If anyone comes across this post in a search for a Trane XE78, I might try a google search using, oh I dunno, 50A50-405 40" residential gas furnace operation and click the first link.

I did verify my TCO temp got up to around 215*F today. I put the meter on MIN/MAX so hopefully the batteries don't die by the time I wake up. The first section of flue at my inducer motor is a staggering 300*F, about 6 inches up it drops to 240*F, and is down to 160*F before it goes into a larger sleeved portion and into my ceiling. I thought I was getting some bad readings with my temp gun because of the shiny nature of galvanized sheetmetal but I took a sponge where it said 210*F and I did get some steam so I have to imagine it is accurate. I would LOVE to have some of that heat actually going where it is supposed to.

I think my fear now is that when the evap cover was cut previously, or when the ignitor was replaced (since they left the old one broken and laying in the unit), someone might have increased the gas valve manifold pressure. Well see once I hit the flowcharts!

Sounds like you are starting to have some fun.

Looks like I am on my way to having more fun!

After cleaning the evap cores, things seemed "fine" for 3 days... no kicking off, but my heat exchanger area was still running way too hot at 210*F... just 10 below the trigger. As luck would have it, louvers stamped into sheetmetal can be sharp and I am now waiting on a new thermocouple probe for my meter; however, I ran on down to the auto part store and got a coolant temp sensor which varies resistance based off temp.

I just hit the TCO and I was at 182 Ohms on my Chevy coolant temp sensor, which is pretty darn close on my temp to resistance charts for 220*F--so that should be a good "new" baseline for me to start looking.

Now things got really fun...

I went into my crawl space where I have the flexible coiled and insulated ducts for my forced air. There were probably 8 new stamped floor register units, about 40 feet of duct, and a few other weird things down there. None of them were hooked up and I doubt a contractor would have had left good, usable parts just because they didn't want to bring them up. I have a floor register in my lower bathroom that apparently had never been hooked up to anything and was just open to the crawlspace (ugh!). I guess what I am getting at is that things were "in process" before for something dealing with the HVAC in my house. Sad to say but this house had been foreclosed on and the previous owner had been making some changes. This might have been one of them.

The blower motor is newer but I was really surprised to see it was only 1/5th HP. 2 Ton should be 800 CFM but if the fan were working too hard, I would think it should hit the thermal overload for the blower motor.

I cleaned out my return ducts which were thankfully pretty short runs and put the blower back in. I just heard the burners kick off again, so I am still hitting overload.

I have two ideas left. The first is pulling off my blower door, taping the on-off switch, and putting a spare filter in place of the door. I should be able to tell pretty quickly if the extra air to the blower helps drop temps near the heat exchangers. The second idea is very untechnical, but since I don't have any CO leaks being detected from my detector, I was going to buy 4 smoke bombs, tape off the exits on the heat exchanger, and smoke bomb each clamshell (duct tape the front too right after dropping a smoke bomb in it) to see if I have an obvious leak that I can see with a mirror.

I think I should have this figured out soon!

Running the system with the blower motor panel removed and installing a large filter in its place brought my heat exchanger (thermal cutout switch area) temps down to 180*F (max) on a 65 to 78*F heating cycle!

It isn't too ascetically pleasing. Seems like more air intake solves the problem (er, for now) so I am off to do some calculations. Since the air returns are clean as can be, I have to find out what doesn't make sense in the system.

It sounds like some major duct problems. A furnace needs so much air in and so much out to perform properly and that seems to be where your problem is.