A single boiler & HW heater and 5 separate zones is a lot cheaper up front and 1/5 the maintenance of separate units. If you've done the insulation & air sealing well you can probably lease them as "heat included" for 10 years and still be ahead of the cost of installing 5 cast iron units and 5 hot water heaters.

If you have to bump the rent as fuel costs go up, (or if some slob insists on leaving the windows open all winter) so be it. A single side-vented mod-con, one priority-zoned large indirect (or reverse-indirect as a system buffer, if you're planning on keeping it at 120F), and 5 zones. It would likely use less than 2/3 the fuel of 5 cast-iron boilers, each of which would be inherently oversized for the loads. The installed cost would likely come in at under half of even the cheapest separate units. A combined total design-day heat load ~80K with some additional overhead for hot water heating would barely hit the mid-range of most manufacturers' mod-con lineups.

(as I pointed out on your other thread) thin profile cast-iron convecting radiators like the Burnham Radiant or Sunray work great at low temp, but you have to somewhat infer their lower-temp performance from their 150F & 160F specs, but it works pretty well- if in doubt up size it 25% from what a linear interpolation would give you. Cast-iron baseboard works at low temp too, but doesn't convect as well as the 18-24" tall 5"-profile radiators. Used radiators are often available, are pretty bulletproof, and clean up well. (I'm sure there are plenty of used radiator vendors in the NY/NJ area supplying the brownstoner rehab crowd.) But even brand new ones aren't as pricey as most of the Euro-panel imports.

I am with Dana,

I design condo and apartment heating systems with ModCon boilers, indirect water heaters and steel panel wall-hung radiators all the time. Temperature control also controls user "open window" abuse and you pocket net savings.

MA: Got any model/manufacturers to recommend for the thin-profile steel versions (which SHOULD be less expensive than full-retail on the cast-iron Burnham thin-convecting radiators, but aren't always, in my limited experience)?

Dana,

Send me an email on the subject.

Got a similar situaion for new construction condos (6 units). How complex($) would it be to add flow meters to DHW & Radiant heat to bill owners for their actual use? Each unit will be one heating zone.

Thanks