New poster here from the "handy man with engineering background" category. I've been refurbishing our 100 year old 3 decker and have mastered basic plumbing and electrical work in the process.

the problem .... Our first floor is divided into 2 units which share a gas fired steam heating system in good repair, one zone and one thermostat. We have a single hot water boiler for the house. I'm considering an upgrade that would make the tenants more comfortable and to allow me to turn over the gas bill to them. ($2500+ per year). The joists in basement are exposed. I'm considering a gas fired combination radiant heat and hot water unit for each apartment that mixes heat transfer water and hot potable water. I've seen many good posts on this subject but none has addressed my specific question. Here it is:

Can the boiler be one of those single pass hot water heaters with piping that mixes heating water and hot potable water?

Before you all yell NO YOU IDIOT, you can't mix hydronic heating with potable water! .... I've gotten a schematic from a vermont firm specializing in supplying DIYers which shows such a system. Would this pass code in Mass? Are there important design considerations which would make this sort of system operate poorly or not at all? The obvious benefit is the lower cost of a single heating unit with no storage losses from hot water tank.