Ask Me Help Desk - Furnace does not work

I have a circa 1996 lennox 3 ton residential unit (80mgf2-45a-5). I went to turn on the heat and nothing happened. I tried the A/C and same. I replaced the digital thermostat with a round honeywell one that I had spare. When turning the dial (and looking for the little spark that comes on when the mercury in the bubble moves) nothing happened - putting the fan to ON does nothing either. I take the cover off the furnace to look around. I find the 3AMP fuse and it is not blown. I look at the two diag lights on the motherboard - both are OFF. I check the houses eletrical panel and confirm breaker to furnace is on. I put a beeper type meeter on the romex going into the furnace and confirm power is on to furnace. That's all the diagnosing I know to do with my limited HVAC knowledge. All help is greatly appreciated!:(

If that unit has a door switch that could be a problem. But if it is bad I would not by pass it I would replace it.

Do you have a voltage meter? Check the voltage on the circuit board, check between line 1 and Com. If you have power there check between primary 1 and primary 2, if you have power there check between secondary 1 and secondary two. If you get 24 volts on the last one that proves the transformer is good.

I am not sure about lennox safeties but look at different places for a two wire clicks on switch that has a very small reset button sticking up between them, there should be about three of these. If one clicks as you mash down on it that means it was open and was probably the problem.

Let me know what you find.

Hello. There is no door switch, the cover attaches t the furnace with 2 screws. I do own a meter but don't really know how to use it and am a little bit of a chicken. I found one red reset button (labeled S1) I pushed it but it didn't make a difference in the symptoms. I did see the larger of the two transformers and observed that a thicker black and white wire going in and a thinner blue and yellow (labeled 24VAC hot & 24VAC RTN) going to the circuit board. Is there a safe way I could test the transformer?

Yes if you will use your tester, one lead on the blue wire where it is bare and the other on the yellow wire where it is bare, this should give you a reading of 24 volts. If so the transformer is good.

You can also test the black wire and the white wire going to the transformer the same way you did the other two, this should give you a voltage of 120 volts. If you get a readng of 120 this way and nothing on the small wires that means that the transformer is bad. If you do not get a voltage reading from the black and white wire that means your problem is somewhere else besides the transformer.

Let me know what you find out.

Hello, I've made progress! The meter reading on the low side of the transformer was 0. Just to be sure, I went to my other Lennox furnace and 'borrowed' it's transformer. I temporarily installed it in the troubled furnace and the furnace worked. Now here's my next dilemma. I went to radio shack and picked up a transformer. It's a model 273-1512b which is a 25.2V 2 amp center tapped secondary. The problem for me is that it has 2 black wires on the high side and three wires on the low side (2 yellow and 1 black) and NO instructions. The radio shack guy said it doesn't matter which of the black wires I hook up on the high side (to the furnaces white & black wires). On the low side he said that since this is a center tapped secondary the black wire I don't use - just the two yellow wires. So which yellow wire is considered blue and which one is considere the yellow? He says it doesn't matter. On my furnaces circuit board the blue wire goes to a terminal marked 24v HOT and the yellow goes to a terminal marked 24v RET. Is he right in that it doesn't matter? Last, the burned out transformer says 24V 30VA from basler electric. The radio shack transformer just says 2A. The radio shack guy says that doesn't matter but that the radio shack transformer is approx. 50VA because that's roughly the equiv. of 2A - is he right on that? If I use the radio shack transformer will the higher VA (50VA vs 30VA) 'burn out' the circuit board? Thanks in advance for your help!

When did radio shack start giving advice about furnace components? You need to go see a hvac company and get your parts from them, and ask them your questions. Id love to help you with this, but can't do it with radio shack parts(especially when they don't come with a wiring diagram), you are taking a chance of doing further damage to your furnace,, Good luck with your furnace, and please let us know how you make out.

Quote:

Originally Posted by jaradak

Hello, I've made progress! The meter reading on the low side of the transformer was 0. Just to be sure, I went to my other Lennox furnace and 'borrowed' it's transformer. I temporarily installed it in the troubled furnace and the furnace worked. Now here's my next dilemma. I went to radio shack and picked up a transformer. It's a model 273-1512b which is a 25.2V 2 amp center tapped secondary. The problem for me is that it has 2 black wires on the high side and three wires on the low side (2 yellow and 1 black) and NO instructions. The radio shack guy said it doesn't matter which of the black wires I hook up on the high side (to the furnaces white & black wires). On the low side he said that since this is a center tapped secondary the black wire I don't use - just the two yellow wires. So which yellow wire is considered blue and which one is considere the yellow? He says it doesn't matter. On my furnaces circuit board the blue wire goes to a terminal marked 24v HOT and the yellow goes to a terminal marked 24v RET. Is he right in that it doesn't matter? Last, the burned out transformer says 24V 30VA from basler electric. The radio shack transformer just says 2A. The radio shack guy says that doesn't matter but that the radio shack transformer is approx. 50VA because that's roughly the equiv. of 2A - is he right on that? If I use the radio shack transformer will the higher VA (50VA vs 30VA) 'burn out' the circuit board? Thanks in advance for your help!!

I would not connect the Radio Shack transformer unless you know exactly what you are doing.

Go to a HVAC person and buy a 24volt secondary and 120 primary transformer if he does not have one then you may have to buy a universal on that will work off 12 0r 240 volts. He will probably show you how to connect it. The price should not be more that the $20 dollar range. You maay not even be able to buy one that will go bck where the old one was, if so just mount it where you can.

If I can help more let me know.

Update. I ordered the exact transformer part from HvacPartsShop.com for $26. I highly recommend these guys - great transaction. I put the transformer in and the furnace works great - house is now toasty. Some things I learned... transformers when they are energized are warm to the touch. Also, you can test a transformer with an Ohm meter across the high side and then the low side. There's a video on YouTube that explains this. I like this type of test because you can shut the power off to the furnace and conduct this test without any fear of getting zapped - good testing method if you're not electrically confident (like me). Anyway, a big thanks to all on this board who helped me - I am very grateful.

Yes there are several ways to test a transformer but is is hard to give many of them while you are typing to a person on the other end of a computer.

Glad everything worked out.