18 or 36 VDC to 24 VAC?
If your boiler is truly a steam system with no return pumps, or any other device using electricity, you could convert to a thermopile  or pilot generator and have a system that uses no outside power. It would not be cheap but it can be done. The thermostat, gas valve and all other parts  and devices in the circuit  would have to be replaced with parts  compatible with a pilot generator. This would allow you to go off the grid so to speak. I would have an HVAC company do the work just to be safe. Also screwing around with a steam boiler or other heating devices without extensive knowledge could get you to an early grave. All these devices are extremely dangerous if you don't know what you are doing and even sometimes when you know what you are doing.