In a power outage your natural gas furnace won't work because the control valve for the gas is energized by 24 VAC. A 12 VDC car battery can get this going if you have a converter. Anyone know of such a switching converter?
First, I'm assuming you only need a low current source to power the valve, small, (< 100 watt ) Inverters are readily available, I use a Stanley to power test equipment on remote sites without readily available AC, then get a small 110 VAC to 24 VAC module, commonly used in security systems and some CCTV systems to power the cameras. They are also readily available. Problem soved.
If you are looking at a simple off-the-shelf solution a 12 volt to 120 VAC inverter, possibly a sinewave model, driving a 120 to 24 V transformer will do the job.
I'm assuming that you are looking for a cleaner setup then having to step up to 120v just to step down to 24v. I was looking for a similar application for a security camera setup and came across this: http://www.powerstream.com/inv-12dc-24vac.htm
It looks to be a nice unit, a little more expensive though. There are also many schematics on making your own DC to AC converters available on google, most of which are pretty easy to assemble.