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Start by checking to see if fuse on control board may be blown
If fuseis good look and see if you have a float switch in drain and if so lift it out of drain and see if we get power to thermostat.
Next if you have a condensate pump beside furnace tied into the control wire to thermostat, bypass pump and see. We will go from here.
ok if power in but no 24v out, transformer is bad, The red light just indicates board has 120v
You can use any 40VA 120/24 volt universal transformer. If red light was not on that indicates the board was not sensing 120 volts.
Possible yes. I would install me a small inline fuse in the low voltage circuit when replacing transformer in case there is a short that fried transformer so it doesn't do so again.http://i.imgur.com/YlIv9Ff.jpg
No more than 5 amp.3 is sufficient
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Can you send me a picture of what you are checking and where? If you bought a brand new transformer and it does not have 24v coming out with 120v going in, they sold you a bad transformer.Take it back and get another.We will go from here.
Are you saying there is no 24v between blue and green out of tansformer, And tell me where these wires go to.
Lets connect as shown below. One wire to pin 3 and one wire to in 6 on the 9 pin plug on board
So that tells us we DO have 24v ,If thermostat has batteries, try replacing them
OK next lets follow the red wire from R on board and C wire on board and see if there is anything in between there and thermostat where we might have a break in that circuit.
Here is an idea you might want to consider for obtaining an extra wire if it id difficult to run anew wire .