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.

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  • When water is heated it expands, so in a closed system the pressure increases. It's the expansion tanks job, to absorb that pressure increase. If the tank is full of water, it can't do that job. Therefore, if the expansion tank has failed and is full of water, a pressure sensor could be preventing the boiler from turning on. Do you have a pressure gauge on the system, or a place to connect a gauge?
    – Tester101
    Mar 7, 2015 at 13:59
  • Note: If the expansion tank is hanging down from the plumbing (as it appears to be in you photo), water dripping from the top just means there's a leaking joint. If water was dripping from the bottom, that would likely mean the tank has failed and is full of water. I'm not saying your tank has failed or not, just that a leak at the top doesn't necessarily mean anything. Though the leak could be caused by high pressure in the system.
    – Tester101
    Mar 7, 2015 at 14:06
  • After looking closer at the photos, it looks like the leak is from the top of the air separator (purger). That's the bulbous device that the expansion tank is connected to. FYI, the device on top of the purger is an automatic air vent.
    – Tester101
    Mar 7, 2015 at 14:15
  • currently the furnace is off (that seems to be the issue) so the pressure is close to zero. I attached an additional photo ... fyi the leak is really slow, and to be honest it may have been there for a little while and this is the first I'm noticing
    – user379468
    Mar 7, 2015 at 14:23
  • 2
    @Tester101, it's zone control, actually had the plumber stop by, he said that the expansion tank was leaking because the system had cooled down, it's actually the control box that's fried ... he opened it up and there is some kind of short in there ... apparently going to have to get a new one, thanks for your help
    – user379468
    Mar 7, 2015 at 15:21


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