Gas furnace fan limit switch temp cycling, can't figure out why

Short version: 1970s Thermopride gas furnace with later addition AC. Fan limit switch constantly hits upper limit. I suspect the problem is too little airflow but fan hardware is either original or mfr specified replacements. Opening up blower intake side of furnace box doesn't solve flow issue. Pressure readings just below AC coils seems low based on what I have found on the web (I don't 100% trust my methods). What might be my issue?


The rambling version.
I'm hoping people here might help me diagnose an issue with my home furnace. This thing has given me trouble since I moved into the house but I suspect the issues are not fundamental but instead due to previous service mistakes. I talked about one of those here from spring 2017.
https://www.garagejournal.com/forum/showthread.php?t=363590

The furnace has been in the house since the late 70s. I'm not sure when the AC was added but it's an R-22 unit so I think that means no later than the 90s. The current issue I'm seeing is the fan limit switch on the gas heat constantly hits the over temp limit. The switch is a Honeywell similar to the one in this article. [[https://inspectapedia.com/heat/Fan_Limit_Switch_Guide.php]]

I suspect the fan isn't pushing enough air but I can't understand why.

The motor was replaced recently (though I suspect the old one was fine). Last summer I pulled the blower out and cleaned likely 30+ years of dust off the housing and squirrel cage blower. At that time I also oiled the fan bushings (the service manuals says do this yearly... I suspect it wasn't serviced for many years). So I have a cleaned fan with a new (and verified correct model) motor. The only issue I could think of here is bearings but they don't feel bad to my untrained hands.

So could it be ducts? We the house seems to have a well designed duct system that scales in cross sectional area as lines are run off to air outlets. The return system is distributed throughout the house. To verify the problem wasn't on the return air side I removed the fan housing cover and pulled the air filters. At that point air can come directly from the room. I can't easily verify the outgoing ducts aren't blocked other than opening the duct that is right above the AC coils. That one doesn't feel stupidly strong as I might expect if there was a blockage down stream. Also, the system branches almost immediately after the furnace so a blockage in one line wouldn't impact several others. The same is true of the return lines.

I tried taking some air pressure readings across the blower fan (after the furnace but before the AC coils and after the AC coils). I don't recall the exact numbers but recall they were about 1/3rd of what people said I should see based on random web searches (general numbers, not specific to my system).

I would happily spend a few hundred dollars for a new blower if I knew that was the issue. I feel like this problem would either be blower related, duct related or AC coil blocking things. If the issue were blocked coils or duct work then I think I should see a greater pressure difference across the fan (did these older systems operate at a lower pressure?). If the problem is fan related I can't understand why since the motor is new, and the blower was cleaned and oiled.

Anyway, I would love to know what the experts think. I don't want to spend $7k on a new system just because of this issue.

Thanks!

Other background:
When I first bought the house the fan was staying on when using the heat. Since the house came with a warranty I used the warranty people for repairs. They replaced the fan limit switch but in the process screwed up the AC fan relay. Hence the next spring the AC didn't work because the relay was connected in a way that shorted AC power. As part of this screwy repair process they replaced the fan motor but they also screwed that up. The fan motor can be revered based on wiring. The guy didn't check the wiring and thus the blower was running backwards and my AC was icing over. I figured that out as well as sorting out the incorrectly wired relay. At least I'm learning how all this **** works.

typically a blower motor will run on high speed for a/c & low speed for heat .this is done at the fan relay. you should have a multispeed fan motor. try wiring it on medium speed or even high speed just for test purposes. possible it could have been wired for medium speed on original motor & when motor was replace they used the low speed tap on motor

try setting your fan sw on therm to on this will be high speed on fan if wired properly then set temp for heat to come on & see if it cycles the high limit

Mike,
Thanks for the info. This system has only a single speed fan. I suspect that is because the original furnace didn't have an AC as part of the system. I'll see if there is a way to increase the motor speed via wiring at the motor.

I think you are on the correct track with too little air flow.

Pull the squirrel cage unit. Get a light, get on your back and crawl into where the blower was. Shine your light upward. You may find the entrance to heat exchanger is blinding over with dirt. Although you can probably clean off much with a toothbrush, a fin cleaner would be better.

Good time to use a dust mask.

I've never touched a thermopride gas furnace but if it's like the oil furnaces the fan/limit is right at the top of the cabinet, when installing A/C on the oil furnaces we were always careful to raise the A/C coil up at least 4-5" above the top of the furnace so the limit switch would get airflow across it.

A lot of times the A/C coil is narrower then the furnace so you end up with the pan that supports the coil blocking airflow at the edge of the furnace right were the limit switch is.

If you can post some pics it would help to see if this is a possible problem.

Get yourself a manometer. Dwyer is a good source. Most furnaces are designed for .5" of static. So, if you drill a small hoe for the probe to go into place the probe perpendicular to the air flow and note the reading. Start with the return air. Move to the Supply side and put the probe in under the AC coil. Careful with the drill Note the reading and move to the upstream side of the coil. You can pin-point restrictions or mistakes in ductwork doing this. If you get, say .2" of static on the RA, and .3" below the AC coil the system is ok. If however you go to a higher number than .5 when adding the two numbers together that is where you restriction is. You can walk right through your duct system doing this. As previously noted, it can be a fan speed issue. Thermopride makes good equipment. Have you had the A coil cleaned? Also, If you have been running this unit on high limit, I'd want to inspect the heat exchanger integrity. Do you have CO detectors in place?

is the blower direct drive or does it have a belt. if its belt drive when motor was replaced the pulley on motor may have not have been set properly to get proper rpm. just a thought, like previously stated a/c coil could be dirty

Thanks everyone!
Here is the manual for the unit, GL16-160. This doesn't show the AC and I assume it wasn't uncommon to have the AC just added into the system. This is why I had so much trouble figuring out my motor relay issue last year. The factory diagram doesn't have a fan relay.
At the bottom of page 19 they have a picture of the Lowboy model. The blower sits to the right of the heat exchanger as seen in the attached picture.

In one of the photos you can see the AC line. The coils are a V-shaped unit and those lines are at the bottom of the coil. That's about 15" above the unit. The fan limit switch is about half way up the vertical portion of the bottom of the plenum (about at 10" on the tape measure). I included the picture of part of the distribution duct work just because I've always been impressed with the effort the original installers took with stepping down the plenum size as they added branches. Note that the second picture is perhaps 4 feet away from the top of the first picture.

In the first picture you can see a plastic bag capping the line into the humidifier. The humidifier doesn't work and to avoid air leakage I used that trash bag as a quick and dirty fix. I know I should probably get something better. I did feel the metal temperature before installing it. It's not too hot to touch.

Since my last post I verified the motor and fan pulley diameters. I assume that when the motor was replaced the pulley would have come from the old motor vs with the new motor. Either way, it does appear correct.

I do have a manometer (purchased for this job). I'll try to get some additional readings.

I haven't ever had or thought to have the actual heat exchanger surface cleaned off. I also haven't had the AC coils cleaned though what I saw of them they don't look like they have any dust build up. I assume any restriction there would come out as a pressure drop across the coil.

Jackfre, good safety call and I definitely have the CO monitors. There is one near the unit and a second in the kitchen on the floor above. I didn't grow up with gas heat so I decided to be afraid first.

mike88, the blower is a belt drive unit. I'll try to get some pressure readings and let people know what I see. I have a probe wit ha 90* bend so I'll get both in line and perpendicular to air flow vs ambient room pressure.

make sure belt is good & to check for proper rpm you will need to check amp draw of motor. data plate on motor should give the specs if I remember correctly. I have seen where insulation can come loose & when fan comes on it blocks airflow to limit sw. its also possible you could just have a bad limit sw.

The limit sw could be an issue but it was replaced about 2 years ago when we first got the house and were having issues with the system. I wish I recalled exactly what the problem was. I think it was the fan running non-stop.

do you have a picture of the limit sw

mike88 said:

do you have a picture of the limit sw

Click to expand...

Here it is. I did check that it's the same model as the old one. Not sure if i still have the old one to check the set points but I suspect I checked those at the time.

the settings look fine. you could try taking it out to make sure nothing has gotten wrapped around the probe

take limit switch out, turn fan on & look inside thru hole for obstruction. you should get airflow out also.hope this helps

I did some pressure measurements. It was way at the bottom of what my manometer could read but I got 0.24 in h2o between the intake before the filters and the outlet of the furnace (before AC coils). I didn't read any drop over the AC coil directly. Measuring after the coil was very variable between 0.18 and zero. However I suspect I'm near the noise floor for the tool.

Am I correct in assuming the motor speed is the loaded speed? I can rig up a tachometer to check motor RPM.

is your ac coil clean. I suspect that the duct is lined on the inside a piece of insulation could have fallen off. check your static pressure before the coil after the coil and down stream of that. are all of your registers open and have air flow.

I'll try to get some pictures. There down stream side of the coil looks clean. The insides of the ducts around the furnace show no signs they are lined. In the picture of the plenum, on the left side where the wall is 45*, that's about even height with the inverted V shaped AC coil. On that side of the plenum is an AC vent that I can remove. It offers a good view of half of the "V". My numbers a few posts up were taken before and after the coil (about 0.07 difference across the coil).

BTW, thank you to all that have offered help and suggestions!

It may be stupid.... but ive got new motors spinning wrong direction from factory, or on 3 phase someone hooks it back up with 2 of the phases swapped....

Dingleburry said:

It may be stupid.... but ive got new motors spinning wrong direction from factory, or on 3 phase someone hooks it back up with 2 of the phases swapped....

Click to expand...

It's not stupid. That's how the warranty guy installed it. I corrected that along with the wiring the warranty guys screwed up.

Here's a picture of the downstream side of the AC coil. It looks quite clean to me

you said it's a belt drive are sure it's not spinning backwards.
Normally on those old belt drives when we added AC to the system we would replace that 1/6-1/4 hp motor with a 1/3 or 1/2 hp and install a adjustable pulley to get the need increase in air flow for the AC.
Do you have a adjustable pulley on the motor shaft?

I know it isn't spinning backwards. When the warranty company incorrectly guessed the motor was bad they didn't check direction and it was spinning backwards. The AC coil iced up. I reversed the direction to fix that problem.

I can't adjust the pulley as both are simple fixed diameters. I could look into a replacement but then I'm changing it from the oem hearing setup. I will check to see in by chance the motor has more than one speed. I don't think it does as I suspect I would have checked that last time.

What are the odds of getting a correct hp motor but one that's the wrong speed? I said the motor was replaced and I did check the motor hp. It's 1/3 as it should be. I didn't check the rated rpm and I have no idea if the warranty company ordered a correct replacement motor or not. Would it be likely the motor is say a 1725 rpm motor instead of a 3450? I was thinking about this as I was in the upstairs bathroom. This system was originally intended to heat the whole house, not just the down stairs. There are heat registers for it upstairs yet the floor out of those vents is almost nothing. Also, the first winter I had the house I don't recall the Nest thermostat telling me there is a furnace problem. It somehow knows that now.

Epiphany moment...
I hadn't noticed this before, the motor pulley is a variable speed pulley. I wonder in the guy who changed the motor released the wrong setscrew and then changed the pulley ratio...
That's my next thing to check when I get home.

Check the amps on the motor see if its at or close to FLA. If its lower its not moving enough air. Less air its moving (blocked inlet/outlet) the less amps itll run at. Dosent matter if its inlet or outlet. Is the fan the same/correct? As in forward curved vs backwards curved? Are your belts slipping?

I don't recall if I checked the current lead on the motor in the past. I'll verify it when I look at the pulley this evening. The blower, other than the motor and belt I installed last summer, is all original and definitely turning in the correct direction. Looking at the old service information it appears that the mfr used the same 1/3 hp motor on a range of furances with different pulley sizes. Per the pfd I linked earlier it appears my model is the largest in the range.

I havent read the whole thread but did you check the temp at the fan sensor with a different thermometer? Like a thermocouple and digital thermometer

Citation said:

Here's a picture of the downstream side of the AC coil. It looks quite clean to me

Click to expand...

Check the bottom side of the coil.

No, thus far I have assumed the fan limit switch is working correctly. That part was new in fall 2016 and I don't recall having this sort of issue in winter 2016-17. Given the Nest thermostats tells me it's not happy with the furnace and did last winter but not 16-17 (from what I recall), I've so far assumed the thing is working. However, I have the tools to check and that would be a easy test.

I took a look at the variable pitch motor pulley. It wasn't the smoking gun I hoped as it was almost at maximum diameter. However, it was pretty badly worn. So I'm ordering a replacement. I'll go with a slightly larger diameter so I can increase the speed if I wish. I did check my motor current. 6.5A with the fan going... and 6.5A with the belt disconnected. Apparently these motors just use about the same current at any load rating. I was curious about that but that's what I found when searching. If nothing else I'm going to learn a lot about this system before the job is done. However, I'm not sure how I set my fan speed other than looking for a drop in motor RPM. I'm going to make a reflective tach (IR photocouple connected to the frequency counter on my multimeter) to make sure the motor speed is something I'm OK with.

Jim greengo said:

Check the bottom side of the coil.

Click to expand...

I'll try. That's harder to get to since it involves removing panels that weren't always handled with care. The original system installation was a work of pride. The later service and modifications less so.

That amp reading sounds odd to me.
Seems its not doing any more work loaded as unloaded? Ive never hear of that
Whats motor model?
The furnace motors ive seen are around 1070 rpm, and not 1750/3500.
But 3 or 4 speed

I went and checked. It's a US Motors catalog number 8100. 1725 RPM, FLA 6.4.

I wouldn't have normally believed the current draw but I'm going to trust the amp clamp. It also jives with what one of the posters here said
https://www.quora.com/What-is-the-no-load-current-of-an-induction-motor

Yes, that means I'm assuming the random internet post is correct. But it does match what I'm seeing.

didnt read all the posts on this but you may want to check the heat exchanger for any cracks/hole's

Citation said:

I did check my motor current. 6.5A with the fan going... and 6.5A with the belt disconnected. Apparently these motors just use about the same current at any load rating.

Click to expand...

Dingleburry said:

That amp reading sounds odd to me.
Seems its not doing any more work loaded as unloaded? Ive never hear of that

Click to expand...

The motor should not pull FLA without the belt. It definitely should pull FLA with the belt and less without it. Normally, low FLA indicates a restriction or blockage
in the duct.

Somethings weird there.

I would also get a decent thermometer and check the accuracy of the fan/limit switch.


Tommy

LS6 Tommy said:

The motor should not pull FLA without the belt. It definitely should pull FLA with the belt and less without it. Normally, low FLA indicates a restriction or blockage
in the duct.

Somethings weird there.

I would also get a decent thermometer and check the accuracy of the fan/limit switch.


Tommy

Click to expand...

I checked the limit switch last night. It matched my Fluke thermocouple readings.

I was surprised by the motor current readings but I'm trusting what I'm seeing and it doesn't seem unreasonable based on what I'm finding on the web.

Duct blockage is very unlikely. I tested with the rear of the furnace open (thus no return blockage). The outlet of this system quickly branches off into many ducts. Even if one was blocked you've got many other exits and a large opening right above the AC coil (a vent that feeds the furnace room).

Last night I adjusted the VS pulley a bit. That should have increased flow a small amount (at least if the pulley weren't so worn). The system didn't seem to trip the high limit but I want to test more before I decide it's fixed.

My current guess is the combination of pulley wear and perhaps a small change in pulley diameter (pitch was inadvertently adjusted when the pulley was transferred to the new motor) was the cause. The new pulley should get here Friday and hopefully that fixes the issue.

Ok, I talked with one of our senior EEs and he cleared up what I was seeing. The constant current is correct. The issue is the phase lag between the voltage and current. The induction motor creates a phase lag between the line voltage and current. So unlike a resistive load where peak current and peak voltage happen at the same time, here the peak current doesn't line up with peak voltage this power consumed isn't RMS voltage* RMS current. Now that it's explained to me I understand. I'm just not sure how to adjust the pulley to verify I don't exceed the hp limit of the motor.

Citation said:

Last night I adjusted the VS pulley a bit. That should have increased flow a small amount (at least if the pulley weren't so worn). The system didn't seem to trip the high limit but I want to test more before I decide it's fixed.

Click to expand...

This is your only recourse.

I have a similar problem, except mine is caused by a 90° turn at the output plenum (horizontal furnace). Luckily, my "short cycling" is only a problem when it is extremely cold (less than 0°F) for an extended period (days). The temp in the house can not get over 65° on those days. Chilly, but bearable.