This is my battery backup project. I have a natural gas steam boiler that I want to be able to operate and produce heat when the utility power is down during winter storms. The boiler has an electronic ignition. There is also an automatic water feeder. I am not aware of any other electrical loads (I disabled flue damper because its micro-switch is not working reliably).
The obvious advantages of a battery backup are it is located indoors, does not emit any fumes and works silently. A disadvantage of course is once the battery runs flat you need to find a way of recharging it, which can be tough during a widespread blackout.
First, for easy connection of a backup power source I did some re-wiring around my furnace. I had to break the connection between the AC line and the master switch and install an additional electrical box with changeover switch and inlet as shown in the diagram. Note that I run only electrically “hot” wire through the switches; the neutral is continuous. To use the system during a power outage I need to flip the changeover switch to isolate the rest of the house (and utility lines), and then to plug into the inlet an extension cord from an inverter.
Here is how it looks like. In the following post I’ll describe in details all the parts (including battery, charger and inverter).