Battery Backup Power for Thermostat and Furnace

Our heating system in CT is gas, hot water, radiator system. It's a great system, gradual changes, completely silent and no air flow. The circulation from the boiler uses gravity, no circulation pump. There is no igniter, we still have a pilot light. Therefore, my ability to operate the heating system, in the event of a power outage, depends on being able to keep the thermostat running. The thermostat runs on 24vac, supplied via a transformer, from household current. I want to install a battery backup to operate the thermostat. From my way of thinking, the easiest solution would be to buy a UPS designed for home networks [110vac output], plug it in to a house outlet and wire it to the transformer. It would come on automatically in the event of an outage and turn off automatically when power was restored. But when I look at the expected duration of the UPS units they go from minutes to hours and I need something that would supply power for a week or more, in the event a tree takes out an overhead power line during a winter storm. I have no idea how long a UPS system rated to keep a desktop going for four hours would keep my thermostat running. A more robust option would be to use 6vdc or 8vdc golf cart batteries, wired in series, with a 24vdc to 24vac inverter, a trickle charger and a transfer switch; but the batteries go for $150 each. All the components could get me into the $800 range and I would need an electrician to hook it up. Without knowing the likely power consumption of an average, no bells and whistles 24vac thermostat or for how long a UPS rated at four hours for 110vac desktop would run a 24vac thermostat, I can't figure out if I need to go beyond the UPS to something like several golf cart batteries hooked up in series. The problem isn't amenable to finding answers on Google, my usual research approach. Any information or guidance you could provide would be very much appreciated.
- John Beardslee (age 69)
Branford, CT, USA

That's a very clearly expressed question, but I don't know the answer. Here's one possibility for figuring out the power draw of your furnace+thermostat combination. (Even without a pump or an igniter the furnace must have some electrically-controlled valve that draws some power.) Disconnect one of the wires of the 24 V side of the transformer. (The 120 V side is obviously dangerous to mess with!) Insert a standard ac ammeter to re-complete the circuit. Watch how much current it draws. It may change as the furnace goes on and off. The average power consumption is just the average current times 24 V. I'm guessing that the furnace controls themselves draw some watts of power. Let's guess 10 W for the net draw, although it could easily come out a lot different. 

The UPS product specs will tell how much power it can provide for how long. AI looked at one costing about $500 and it was only supposed to run about 11 min at 300W.  If you're only drawing 10 W, you might get about 6 hr out of it. You'll be able to refine the estimate once you do the measurement. 

Anyway, that doesn't sound great if you want a supply that can last a week. Maybe your best bet is to get a small home gasoline-powered generator. The cost would be similar to those big battery packs you're considering but the total energy storage would be limited only by how much gasoline you keep around. Unfortunately, batteries have not nearly caught up with fossil fuels as a way to store lots of energy in small spaces at reasonable cost. If you do go that route, it's crucial to run the generator outdoors, where the carbon monoxide won't kill you.

Mike W.

posted without vetting until Lee returns from the Serengeti 

(published on 11/03/2014)