Electrical – Battery backup system for tankless gas water heater (outdoor install)

The tankless water heater can be mounted with either concentric venting (preferred) or dual-pipe venting - one for intake and one for exhaust. Because of this, we would not be able to 'common vent' in with the furnace exhaust or even the chimney itself because the concentric would be pulling air down the chimney or suck in the exhaust of the furnace. We could take the exhaust pipe of the dual pipe option and add it to the chimney or furnace, if sized correctly, but regardless we'd still have to direct vent outside for the intake. So, because of this, it makes the most sense to go with the concentric venting and route it outside.

The best location that we're working with is in that old coal storage room. This provides access to 3/4" water lines and the 1" gas line that we can pull off of for the 3/4" line needed for the water heater. The only workaround now is with venting. The gas meter in my picture is actually farther down the exterior wall than is shown (the gas line does a 90 after coming through the wall) and up to where it's shown in the picture. This allows the easiest access to venting outside through this area, although we're required to keep a 3' distance from the gas meter for it's regulator. This shouldn't be a problem because we actually have to run it up the wall about 3' anyway to clear the snow level. If this isn't workable, the other option is to run the vent to the complete other side of the basement and come out of the wall there. This could come close to the maximum allowable vent span, but it's an option currently.

Well, to simplify the maze somewhat, you have all the usual loads of an all-electric house.

• 50A furnace
• 50A range
• 30A air conditioning pack
• 30A dryer
• Eleven 120V branch circuits, some MWBC, many on 30A breakers (WTH?)

Obviously we can oversubscribe this somewhat, e.g. You're not likely to run heater and A/C at the same time.

The fine art of panel subscription is not a core skill for me, but hopefully this summary will make it easier for someone else.

That panel, though

While we're here, we gotta talk about that panel. First, panel B top-tapping panel A before the main breaker is very dangerous unless there is another main breaker before this. The meter is not a fuse, and there are no fuses between the transformer and main breaker. This is considered acceptable for a very short hop from the meter to the main, however, in your case this totally unfused feeder crosses your whole house. Nothing prevents a short in the cable, or in this panel, or summed loads on this panel, from setting that cable on fire.

Second, all 4 panels are Federal Pacific "StabLok". It has serious issues with panel fires due to bus defects, and unreliable trip. When feasible, all four should go.

The three subs will be straightforward to DIY, the main will be more troublesome unless there's an outside main breaker. I would DIY install a 200A main service panel right next to it as a subpanel, reuse that 70A breaker to feed it, and move all the 120V circuits out of the main into the new sub. Then bring in an electrician to cut it over to be the new main.

I see many branch circuits on 30A breakers. Those should be 15 or 20 depending on wire gauge.

Gaps in the panel covers should be patched so curious fingers can't touch buses, except finding either breakers or blanks to cover those spots will be a pain.