Woke up this morning and the furnace wasn't working. The furnace is relatively new, and there appears to be a small amount of water dripping from the top of the expansion tank. I know that the expansion tank should be half full of air. The pilot light appears to be on, but the thermostats won't trigger the furnace. Could the full expansion tank be causing the furnace not to turn on? or is that indicative of a bigger problem? Is there some sort of safety sensor? I read that I should turn off the furnace and drain the expansion tank, Im not 100% certain about how to turn the furnace off, just looking for some instructions.