i would explore the possibility of replacing the gas valve with a millivolt gas valve. no electricity required. at the point where you are disturbing the old gas line you will be contemplating replacing the whole thing. i don't know how long you can afford to run the old furnaces with their 65% efficiency but they sure are quiet. subject to the safe condition of all the systems you are disturbing: if it was me, i might: buy and install CO detectors [replace every 5 years or research latest interval] in sleeping rooms a day in advance. that way if the old furnace has a cracked heat exchanger [which requires furnace replacement] you can avoid the next steps entirely if the alarm sounds. turn off power. install an electrical outlet on the furnace circuit. install a power cord for the furnace transformer. buy a portable GFI to check the furnace's new cord for faults. buy and plug in a KILL-A-WATT meter to the new furnace outlet. operate the furnace and watch the kill-a-watt meter. if the furnace's wattage is below the inverter's wattage, try the car inverter feeding the furnace within the inverter's wattage limits and outdoor extension cord losses. if the car to furnace 110v has a fault, remove the portable GFI. IT MAY BE DANGEROUS TO DISCONNECT THE GROUND FROM THE FURNACE CORD. BUT I HAVE FOUND IN NEIGHBOR TO NEIGHBOR AC CORDS THAT MY 2 FURNACES one boiler, one forced air, WILL CONSISTENTLY TRIP A GFI AT THE NEIGHBOR'S SUPPLY END. also consider positioning car exhaust away from the house and considering wind direction. a GFI is not normally used in our buildings on furnace circuits. don't let the car battery go dead. now, let's hook up the tv and cable box! if only a few watts are used for the heat, check power requirements on the tv [36" sony picture tube 200 watts or maybe you have an LCD uses less] and the cable box [80 watts for my scientific atlanta 8300HD.] and the internet cablemodem and low wattage of a laptop or higher wattage of a desktop. plus computer monitor wattage.
Timbre' Wolf wrote:
appears to be hooked up to a 20v tranformer coming off a 110 12/2 wire box in the house.
Would it be possible to switch it "on" with a car battery/inverter arrangment or is there a battery back-up of any kind that I could use?
on" back in the day (there are a couple of suspicious levers up in the kitchen that ran to the basement at one time) but I'm finding no information of that on the internet.