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Somewhere in the mess you have described there is a short that is compelling the transformer to put out more current than what it is designed for. It gets hot, really overheats, insulation melts, and the wires melt. Ergo blown transformer.Problem is with whatever that unit is plugged into/powering..
Check to see if you are getting 24 volts from the transformer, if not, turn off power to the transformer and check the resistance of the secondary side, if no resistance the transformer is bad and need to be replaced. Also check to make sure all the wires are secure. If you are getting voltage at the transformer then bring power directly from transformer directly to solenoid valve and it should click open and let the water through, if it does then either the sailswitch/current sensing relay is bad or humidistat is bad.
The first thing to check is if you are getting 24 volts at the transformer, if you are not then turn off power the transformer and check continuity. u should have continuity if not replace the transformer. If you are getting 24 volts at the transformer then apply 24 volts directly from transfomer to solenoid valve you should hear the solenoid cleck open and let the water in, if not then the solenoid is bad and must be replaced. If the solenoid does work by applying voltage directly to it then check the
wiring and make sure it;s all tight. Also check the humidistat and current sensing relay/sail switch to make sure it's functioning properly.
Check if you are getting 24 volts at the transformer, if you are then appy power directly to the solenoid valve from transformer, you should here the solenoid valve click and let water through. If not replace the solenoid valve. If the valve works by giving power directly from transformer the check the wiring, humidistat, and current sensing relay/sail switch.
Check to see if you are getting 24 volts at the transformer, if you are then bring the power directly from the transformer to the solenoid valve and see if the solenoid valve clicks open by moving the knob on the humidistat. If nothing happens then most likely the solenoid valve is bad and needs to be replaced. If the solenoid valve does work after bringing power directly to it then either tne problem is the humidistat it self or the sail switch/current sensing relay.
connect a 24 volt,20 va transformer to the wires supplying power to the indoor blower motor of furnace, connect the primary wires to heat speed of furnace motor, then connect the two low voltage wires to the secondary side of transformer, good luck.
Yes you still need a transformer the hum. port on the furnace goes to the 24 volt transformer. one leg of the primary side goes to the transformer and the other leg of the primary goes to the neutral on the board. Then you have your 24 volt terminals one goes directly to the humidifier and the other goes to the humidistat the other wire from the humidifier goes to the humidistat.
the 760 humidifier does not need a transformer wired externally. the 760 has its' own 24v transformer inside the fan compartment. this transformer will provide the power to enrgize the water solenoid as needed. it sounds like to me the additional trasnformer is hot all the time. when the furnace comes on, the unit gets power and the internal transformer is cancelling the power from the external transformer. this would explain why it quits running when the heat is on. since the external transformer is hot all time, when the heat shuts off, the solenoid is getting energized by the external transformer. to make this run correctly, you need to get rid of the external transformer. leave the humidifier high voltage wires hooked up so they are only hot when the furnace runs. if you have a humidistat that has a r and c terminal, you will need to wire these directly to the r and c on the furnace control board and move the switch inside the humidistat to "power". make sure it is not on "bypass". then wire your 2 brown wires from the humidifier to each of the "h" terminals on the humidistat. if you have a humidistat that only has 2 terminals, wire one brown wire to each of the terminals. this should allow it to run properly.
I also had the transformer problem and it was made worse but an HVAC guy who rewired the furance fan to get more air flow when he installed a new AC unit. He left the humidifier wired to the old fan windings which were now putting out 160 induced volts. The humidifier transformer, which was already undersized by Honeywell, never had a chance. Not sure how common this problem is but I later saw several references to the same problem on other websites and it's worth a quick check of the supply voltage to your humidifier.
Since I had an expensive humidifier and a whole cut in the furnace duct, I looked for a fix. My solution was to buy a $15 transformer from a BigBox that matched the voltage and was oversize on amps and mounted it externally to the humdifier (much like a furnace transformer is typically mounted). I then completely removed the piece-of-junk humidifier circuit board and wired the transformer directly to the water solenoid which it controlled and wired the fan and new transformer directly to 120V that is controlled by a current sensing relay (Aprilaire A50) on the furnace fan wires (and also controlled by a humidistat located on return air plenum).
It has worked fine for two years now, runs only when the fan is on and when the humidistat is asking for more humidity. A relatively cheap fix that would have only taken an hour if I had done it right from the start.
For those of you who haven't bought the Honeywell 360A yet - don't. For those of you like me that spent the money and cut holes in your duct - this is one solution.
Maybe Honeywell will wise up and redesign their transformer PCB to be much more robust. The rest of the unit is a pretty good design.