The furnace connection should supply at least 10.5 volts DC and not exceed 14.5 volts DC at the furnace; AC voltage should be between 105 volts and not exceed 135. Refer to your owner's manual for specific model limits prior to changing power sources. If the furnace starts, the the thermostat is probably the problem. If it does not start, then it can still be a blown fuse to anything, or another problem. You said the fan works while +12 is applied to it, so I am going to say it is most likely that the thermostat is the problem. Atwood 32668 AFMD20111 DC Medium Furnace 20,000 BTU. The new 12V Atwood Medium Furnaces take the place of the previous 8500 series and are available in models from 16,000 - 34...Installing foam insulation in your home will keep your furnace and air conditioner from working overtime, saving you money on those monthly energy bills ...