Shared neutral on two circuits using a single-pole tandem breaker

Question

I just looked inside of my sub-panel for the first time, which has ten single-pole tandem breakers. They all have two black wires connected, except for one breaker, which has a black wire and a red wire. These two hot wires (along with a ground and a neutral) go to the same room as part of a single Romex cable. One is for the furnace and the other is for the tankless gas water heater. I'm guessing the electrician wanted to save money by running just one Romex cable since the room is far away. Is this safe? Even when my water heater and the furnace are both running, they probably use less than 20A total combined. But technically this set up could feed more than 20A to the neutral wire, no?

If it's not safe, should I just replace the breaker with a single 20A circuit breaker (maybe with AFCI) and connect both hot's to that one, so as to guarantee the neutral never gets more than 20A?

Thank you.

Answer 1

Yes, you understand the situation and the danger.

Do double-check; the installation would be far less awful if the black wire of that circuit landed on the next breaker over. In fact that would be one handle-tie away from being correct.

You are correct that feeding the 2 hots from the same breaker screw will suffice. (I don't know if HOM tandems are allowed 2 wires per screw, but if not, pigtail).

However the furnace requires a dedicated circuit. Half a MWBC is fine, but not if it's not a MWBC anymore. If weather freezes water regularly in your zone, I would not want a freezer sharing a trip with any other device, but handle-ties are mandatory for MWBCs (for so many reasons). Handle-ties don't guarantee common trip, but they don't guarantee "not that" either.