Hello, I have a 35 year old lennox furnace that seems to have an issue with the transformer. I had a repair person out and he wanted to charge me $7000 to replace the whole thing, but I won't be in this house much longer and I don't want to replace everything.

Well I sent him on his way and decided to tackle it myself. The problem I was having was that the furnace/fan unit would not come on no matter what I did at the thermostat, heat or AC. I could get the fan to come on by turning on the Continuous circulation switch inside the furnace. The transformer appeared to be burned upon physical examination and was confimed not working by multimeter.

I replaced the transformer with a new one, and the unit worked great!

For three days... Upon more troubleshooting it lookslike the new transformer is not working (confirmed by multimeter).

Any guess on what I should be looking for here?

I replaced the fan motor about a year ago, but it worked fine for a long time, and the fan will come on with the CAC switch on.

I have done much of my remodel myself and I am not afraid to jump in and try things, I know how to use a multimeter and have two electricians in the family, so please let it fly!

Thanks for your help!

Chymystry

Usually there is a Fan relay that will bring the fan on for the A/C mode. All you are doing is bypassing the relay when you turn on the fan from inside the furnace itself. Usually a 24 volt transformer will last for a very long time unless the one you bought did not have enough VA. You might check to see if the 40 VA rate is listed on the box or transformer that you just replaced. If not I suggest you replace the new burnt out/defective transformer with a 40 VA one.

It is also possible that your furnace has a component that is drawing excessive current when the automatic fan circuit tries to operate. This could be the fan relay itself. NOTE: make sure to also check for loose low voltage connections or a possible low amp blown fuse that protects the transformer..

Good hunting.

The first step is to install an inline fuse to protect the transformer. One should only be a buck or 2. With one, you will be replacing fuses, not transformers while you track down the problem. Fix it now yourself, or pay to have it fixed after you sell the house.

Then carefully check the wiring in the mode where it is failing. A common problem is in the A/C mode, something happens to the pair of wires going out to the contactor coil. The coil in it or the fan relay could also be going bad. Ask the electrician in the family about how much fun intermittent problems are. It can be a matter of putting the meter on the right 2 contacts at the right time.

At least with an older furnace relay coils are easier to get at, and the relay itself can be replaced if bad. With newer furnaces, it is a $300 board.

Usually there is a Fan relay that will bring the fan on for the A/C mode. All you are doing is bypassing the relay when you turn on the fan from inside the furnace itself. Usually a 24 volt transformer will last for a very long time unless the one you bought did not have enough VA. You might check to see if the 40 VA rate is listed on the box or transformer that you just replaced. If not I suggest you replace the new burnt out/defective transformer with a 40 VA one.

It is also possible that your furnace has a component that is drawing excessive current when the automatic fan circuit tries to operate. This could be the fan relay itself. NOTE: make sure to also check for loose low voltage connections or a possible low amp blown fuse that protects the transformer..

Good hunting.

The old one was rated at 20VA, the new one is 24. I will try to get a 40 VA, any suggestions on where to purchase? Doesn't seems to be any fuses in the whole assembly as far as I can see.

How do I check the fan relay? Is it a matter of checking voltage across it?

Thank you again for your help!

The first step is to install an inline fuse to protect the transformer. One should only be a buck or 2. With one, you will be replacing fuses, not transformers while you track down the problem. Fix it now yourself, or pay to have it fixed after you sell the house.

Then carefully check the wiring in the mode where it is failing. A common problem is in the A/C mode, something happens to the pair of wires going out to the contactor coil. The coil in it or the fan relay could also be going bad. Ask the electrician in the family about how much fun intermittent problems are. It can be a matter of putting the meter on the right 2 contacts at the right time.

At least with an older furnace relay coils are easier to get at, and the relay itself can be replaced if bad. With newer furnaces, it is a $300 board.

Well I can guarantee that its not a new furnace ;-) that's the good thing!

What size should the fuse be? The current transformer (and easy to get) is 24VA, which would be 1A, so should I get maybe a 1200mA slow blow fuse? I will buy an inline fuse holder today.

Thanks for your help!

Hmm, I put an inline slow blow 2.5 amp fuse and it blew.

Considering the output on my transformer is 1A, I thinking this is bad. Does that most likely mean bad relay?

What it means is that you have a short in the low voltage section of the system. You have to find the short circuit and repair it. Check the inside unit/thermostat/outside unit/and all associated low voltage wiring one section at a time. If necessary unhook the wires at the outside unit install a new fuse and see if it blows. If it still blows you do not have a problem in that specific area but if it does not blow you know it is in the outside unit. You see this is how you will need to check all sections of the low voltage one section at a time and it will take time to do this. Years ago I made a special auto reset control that I used in place of the fuse. This tool allowed me to test the circuits but auto reset so I did not have to run up and down the steps and outside just to replace the fuse. It also had a very loud buzzer that would activate when the tool tripped indicating a short circuit. This noise was just a conformation that the circuit was still having a problem.

Just make sure that what you disconnect for testing is remembered so you know how to put it back exactly as before. I usually write down the wire location and color code just to be sure.
Good hunting.