Tankless water heater venting

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.

Handle-ties have two reasons:

• Joint trip. This is relevant if a safety problem that causes one trip, would also require the other side be tripped. It's not applicable to multi-wire branch circuits (MWBC), for instance. In this case it is not applicable to dual 240V heater elements as that machine is designed.
• Common maintenance shut-off. Appropriate when we want to design the system so a worker de-energizing equipment is forced to turn it all off. This is required for MWBCs, for instance. It's not really relevant here, since it's readily obvious to any competent person that the heater is powered by 2-3 circuits, so he's going to make sure he shuts them all off.

In this case, neither instance applies, so handle ties between the 2-pole breakers are not warranted. Beware: the tripping half-breaker may not have the physical muscle to trip three additional half-breakers. So you don't do something like that without affirming the device is listed to do exactly that. Otherwise you violate the rule on using items only as listed (tested).

Using both grounds is a matter of why not, since it's already in the 8/2 cable. If it was connected via plastic conduit, that wouldn't apply.

In steel conduit, the conduit is the ground and no further grounds are required; you can run a ground of any size if you're really into belt-and-suspenders. I'm sorry, I made that sound weird. I would do that.