My Bionaire BCH9221um wont turn on.

On my Bionaire BCH9212, I found two zener diodes shorted on the power board that has the relays on it. The type number is 1N4739A which is a 9.1 volt, 1 watt Zener diode. Zd1 and Zd2 are in series. I replaced them with an NTE 139A which only cost me 99 cents for a pack of two at Fry's Electronics. I suspect the root cause for this failure was a loose connection at the electrical outlet. A loose fitting plug due to a worn outlet can cause the heater to draw excessive current, which starves the Zener from getting the voltage up while sinking excess current. This was the situation as I was remodelling and I found the loose electrical outlet. If you have ever seen an incandescent light bulb in a light fixture burning brighter than another with the same wattage, and burns out prematurely, it is the same scenario. A loose or a corroded connection causes the bulb to draw excess current as the bulb tries to pull up to the rated voltage, burning out faster. Although my model number is slightly different than yours, I would look at that power board and it is probably has the same two Zener diodes.

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You might want to open it up and check the thermal fuse. This is usually a small cylindrical device with a cone at one end and wire leads coming out of both ends. It opens permanently when it gets over a certain temperature, and sometimes will open prematurely. Check it for continuity with an Ohmmeter. If it is open, then it is the problem. You can usually read the temperature, or a part number on the side. Look it up on the web, or go to your local electronics parts store. You can get one for a buck or two. Replace it by crimping it on to the wires, do not attempt to solder it, as the heat will likely destroy it.

Good Luck.