Furnace Blowing Cold Air

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Old 04-01-19, 02:56 PM
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Furnace Blowing Cold Air

I have a 20-year old LENNOX Furnace - 80UHG3/4-120A-2
There is a new motor on it that about 6 months old... a Mars ECM Motor. The motor is set to run constantly at a lower speed, but when the burners kick in the motor runs at maximum speed.

Last week I woke up and the motor was running at full speed but it was only blowing cold air. I noticed the burners were not running. The filter was only a month-old but I changed it anyway. I use the cheapest fiberglass 20x25"x1" filter from HOME Depot. I also turned the power Off and ON and the furnace came back on and started working normally. Just to be safe, I called a technician over and he tested the system and said everything was working great. He measured the temperature just above the burners after letting the furnace run for 30+ minutes and the temperature never went above 129-133įF, He said everything is working well and left after charging me $69.

Today, I woke up and the SAME thing happened... Cycling through the power a couple of times OFF/ON didn't help. The two diagnostic LED lights were blinking slowly at first... then when the furnace tried to come on, they started blinking fast for a few seconds, the glower came on for a few seconds but then it shut down again... the LED1 stayed solid and the LED2 started blinking slowly...

Upon turning the Power OFF/ON again didn't change much, I hear a Click but then the lights immediately go back to the Solid and Slow Blink mode and nothing happens.

I have cleaned the Flame Sensor so I know that's not a problem.

So upon Googling the problem, I stumbled upon the Primary Limiter Switch... As recommended, I bypassed the switch by jumping the 2 cables and the furnace started working normally again. Just to be sure, I reconnected the cables to the switch and once again back to not working....

I went to our local Lennox dealer and took the old one with me and bought an identical replacement Part # 33J8401. When I installed it, the system first went through the same motions as if it's going to work but then it stopped again and the motor kept running at full speed!!! Again, I jumped the cables but this time I let the furnace run normally for about 10-15 minutes... and everything ran smoothly... So I reconnected the cables back to the new switch and lo and behold this time things started working OK again.

I am now puzzled... I had called the dealer after the first failed attempt and he thought the new switch could be faulty and offered to replace it... I just didn't want to spend another hour driving back and forth... so I am glad it worked the second time. BUT my question is, WHY and WHAT happened? Did the system somehow reset itself when I jumped the cables and let the furnace ran normally for about 15 minutes; or is the switch simply acting up?

Or is there anything more serious going on???? Like the Motor or the Control Board? Any help appreciated.
 
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Old 04-08-19, 05:38 AM
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Hi, is there any legend as to what the blinking LEDí s indicate?
Geo
 
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Old 04-08-19, 06:06 PM
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The link is for your furnace manual. It has step by step diagnostics to follow.
It also explains what the various flash/fault codes are too.

Lennex 80UHG manual (pdf)
 
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Old 04-16-19, 02:03 AM
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Sorry, no. The sticker with that info does not seem to be there anymore.
 
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Old 04-16-19, 02:10 AM
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Thanks for the Manual Link... I'll take a look soon and see what the diagnostic lights mean. In the meantime, I am happy to report that the furnace has been working fine since I replaced the limiter switch.
 
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Old 05-24-19, 05:04 PM
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A couple of months have gone by, and I just replaced the filter. The next day I noticed the furnace is doing exactly the same thing..I have an Azure Mars 10860 Digi-Motor for the blower in the furnace and the fan is set to run continuoesloy at low rpm, except when the burners come on, at which time the blower kicks in at full speed. Now I am suspecting that somehow when I change the filter, the blower is still running when I remove the old filter so there is no air restriction without the filter, but as soon as I put in a new one in (just a minimal fiberglass filter), the the airflow must feel restricted, and the motor somehow thinks there is a problem and the limiter is tripped, and hence the problem. Any thoughts? I am going to replace the limiter again and see it that will make the furnace work again. And maybe in the future, before I replace the filter I should turn off the furnace first so the "smart" motor doesn't sense a change in airflow....?
 
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Old 05-24-19, 05:57 PM
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The blower motor and the limit switch don't sense anything with the filter. The limit switch only trips on high heat. The problem here is that the limit switch...... unless it has a pushbutton on it..... should be self resetting. After an hour or resetting the power..... the system should be running normally.

You may have a problem with corroded terminals. Limit switches rarely go bad and since you already replaced it..... it should be fine. You should invest in a basic test meter. Even a $10-15 analog meter from the home goods store. There is probably more than one limit switch on your furnace. There may be 2-3 in a loop.

With the furnace in standby..... you should measure 0v across the limit switch. If you pull the wires off of it.... you can check it with an ohmmeter. It should measure as a dead short to be normal.
 
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Old 05-24-19, 10:27 PM
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Thanks Pete. I'll check that. I have a test meter but don't know how to use it. What settings do I put on the meter for testing? Also is there a reset to clear the error manually? I had let the system alone for a few days as we were away but as soon as I turn the furnace on the LED's go straight to the same thing... ie LED1 and LED2 first show normal operation (both are on Slow flash), then as soon as I turn up the thermostat, I hear a click then the blower comes on full speed.. and the LED1 does a slow blink and the LED2 is solid. The furnace is very cool so it should have already done a self-reset... btw, there is no pushbutton on this limit switch. I went ahead and replaced the limit switch anyway and it's still doing the same. There are two other similar looking switches on the flat panel (with what looks like pushbuttons in the center, connected on the same wire in a loop) below where the limit switch is (The limit switch is on the vertical wall), these other two are on on the bottom flat plate, and there's one on each end...

When you say "with the furnace in standby, do you mean with the burners not on but the furnace on... remember my blower is always on a slow rpm all the time.... Do I put my voltmeter on DC and measure accross the switch?.
 
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Old 05-25-19, 05:15 AM
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Hi, which control board do you have?
Geo
 
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Old 05-25-19, 08:22 AM
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I am not sure what model it is, it's 20 years old..I have pictures but don't know. When I called my LENNOX dealer he said the model I need is Y7761 which is probably an updated version. The Furnace is 80UHG3/4-120A-2



and here's an image of the upper section showing the Limiter Switch and the Rollout Switches:

 

Last edited by Tchairdjian; 05-25-19 at 11:20 AM. Reason: Added image link
  #11  
Old 05-25-19, 12:26 PM
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One suggestion you may try, get an inline fuse holder and a 1/2 Amp fuse and put it in parallel across the limit switch , do one switch at a time, if the limit switch opens it should blow the fuse, proving the switch is bad. This is called a Tattle Tail
Geo
 

Last edited by Geochurchi; 05-25-19 at 12:27 PM. Reason: Correct spelling
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Old 05-25-19, 01:34 PM
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Thanks Geo.
I called a service company to come in, and he came and did a little diagnostic for $69... Bottom line is: the 12-pin harness's socket on the board has something loose inside. The harness itself is good, just the socket on the board is actng up... If we wiggle it a bit, it's solving the problem and the furnace is working normal... I may be able to prolong the ultimate solution which is to replace the board, but eventually I will need to change it. The technician said the original part number written on the board is 12L69 (there are other universal versions) which should cost me around $280-$300 from the local supplier. They wanted $879 to replace it.... I guess I will have to do it myself or find someone to install for some cash...
 
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Old 05-25-19, 01:57 PM
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Good, glad you found the problem, could be a bad solder joint behind the plug on the board also.
Geo
 
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Old 05-25-19, 02:11 PM
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It is very possible that plug has corroded pins. It's a 50-50 tossup of poor solder or corroded connections. You can get a can of contact cleaner and spray the plug part with the wires. Plug it in and remove it a few times. If no improvement..... use a little more spray. Don't spray the plug on the board.

There is only a very low current thru that loop. I don't thing a 1/2A would blow.
 
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Old 05-25-19, 03:19 PM
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Not what you want to hear, but it's at the end of it's lifespan.

With the unit being 20 years old, before spending money on a board the heat exchanger needs to be inspected.

On the 80% 'ers like yours, part of it it can be checked by pulling the blower assembly and crawling into the furnace - checking the tubes for holes and corrosion.

On a system without a/c (your board wiring suggests you don't have it - nothing connected to c terminal) you can also cut into the plenum and look from the top, patch it would some sheet metal, screws and foil tape.

a 120k btu is a huge furnace and unless the house is very large or poorly insulated, you can probably downsize.
 
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Old 05-25-19, 03:56 PM
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Hi GEO, you said,
could be a bad solder joint behind the plug on the board also.
. That's probably what it is...
 
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Old 05-25-19, 04:06 PM
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PJMax,
It's a 50-50 tossup of poor solder or corroded connections.
.
I doubt it is corroded as this place is very dry... when the guy was here, it seemed loose, so it's probably bad solder... that's why he recommended that I changed the board. In any event, I think replacing it with a new board for CDN$300 plus $100 to the "friend" who's going to help me may be a good idea. However, I'll try your suggestion of contact cleaner first.

User 10
a 120k btu is a huge furnace and unless the house is very large or poorly insulated, you can probably downsize.
.
The house is around 2,200 sq ft plus a 1,200 developed basement -- all being heated with this one furnace. It's for a Canadian home that's well insulated. Although theer's only two of us living here as the boys have moved out.... The house and the furnace has been well maintained. I recently had the furnace and ducts cleaned and the guy said the burners and the heat exchange looked pretty good.
Also I recently changed the blower motor with a Digital ECM model from Azure (Mars 10860)... so hopefully I can get some more years out of this baby...
 
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Old 05-25-19, 06:55 PM
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it's probably at least 20k btu too large if you're in a colder part of canada like alberta or and 40k+ in a warmer part like southern ontario or the west coast.

when you do need to replace, get a heat loss calculation done or do one yourself to verify. the smaller units heat more quietly on a given duct system and produce less noticeable temperature swings, less draft.

during the inspection was the heat exchanger visually inspected from the top or bottom or through the limit opening? don't see much just through the burner inlets.

For repair, keep in mind most suppliers don't sell to diy'ers here in canada so you'll probably have to order online from the states.
 
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Old 05-25-19, 07:53 PM
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User 10, thank you for your good advice. I'll keep that in mind. I'd probably need a 100k BTU unit when it's time to replace. My current one was put in by the builder when we built the house, it wasn't the top of the line as far as efficiency goes.. and at the time I think I was thinking maybe one day I may want to add an A/C but in Calgary the number of days we would have needed it would have been few and far in between... I found that a nice ceiling fan over the bed was the perfect answer... haha

The "inspection' was in passing when the cleaners cut a big hole above the heat exchange to mount a vacuum hose to clean the ducts..He looked in from there to do a quick inspection... I may have to get someone to do a closer inspection.

When you said
For repair, keep in mind most suppliers don't sell to diy'ers here in canada
, did you mean when buying a new board or when buying a new furnace? I've been buying my parts from a large Lennox supplier in town (in Alberta) and i've had no issues buying them... Maybe I've been lucky. Although my motor (Mars) I bought it off Amazon, the local supplier hadn't heard of it.
 
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Old 05-26-19, 09:31 AM
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I meant parts. I think alberta is less regulated than the rest of the country.

I can't get parts from most places in southern ontario.

Now the minimum efficiency in canada is 90% and most decent units are 95-96%.

So when replacing, the input required to get the desired output is dropped. They're sold based on input but output is what counts.

An 80k 96% can replace a 100k mid efficiency 80%.

If it's a typical 2-story detached with insulated walls, normal 8ft ceilings, you'll probably need 80 to 90k input. In my climate, that size range can do 3000+ sq ft.

But sizing shouldn't be done based on sq ft - best to use software like hvac-calc.

I found that a nice ceiling fan over the bed was the perfect answer... haha
I'm betting you don't have much humidity there, here almost every house with forced air has cooling.
 
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Old 05-26-19, 02:39 PM
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Thanks User 10. Yeah, Calgary is very dry. I canít keep the humidity above 32% inside the house most of the time and I have a good humidifier. Ever since I installed the digi motor and I have a fan that works continuously on low rpm, I feel the house is maintaining a better temperature throughout and the humidity is a bit better too. I keep buckets of water in the utility room to help with the humidity.

Iím going to call a guy to come and give me an estimate for a new furnace just to be ready, and Iíll let him estimate the size. Not that it might make a big difference, our ceilings are also 9 ft on the main floor. A little easier to breathe haha.
 
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Old 05-26-19, 05:43 PM
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many will just say to put the same size as what you have now. if you know how your house is constructed - r values, etc do the calculation yourself using something free or low cost. (hvac-calc is $50)

Look into 2-stage for improved comfort - when properly sized and controlled by the right thermostat, can make the house very comfortable, get very long cycles.
 
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