I am about to decide on a heating system for 5 well insulated 900 sq foot apts near new york city.  I'm looking at a bradford white combi hot water heater for heat and hot water.   (or its 45btu 15 seer furnaces)

What maker/kind of radiators could I use to achieve a low(er) temp output from this heater.  I'd like to use cast iron but not sure....  Don't want to use alum baseboard.

Any advice appreciated (I could write a book about the process of figuring out heating for these places...)

You may want to post this in "radiant heating".

Cast iron baseboard is NICE STUFF, but not cheap. Recycled thin-line (~5" deep, ~18-24" tall) cast iron convecting radiators like the Burnham Sunrad or Radiant are often available on the cheap clean up great (use an autobody shop for a quick sandblast & paint) and WORK great, eg:

http://andypeeke-org.cardio-billing.com/?m=201007

http://www.antiqueplumbingandradiators.com/graysurad.html

http://www.usboiler.burnham.com/products/baseboard-radiators/radiant

Brand new or antique-boutique can be a hunk of change though.

You have to do the math on 'em using the spec for the newer versions, but the output of the older ones are quite similar. The specs will give you a square footage per section or per lateral foot. Most can deliver on the order of 90BTU/hr per square foot at 140F, or 50 BTU/hr per square foot at 120F. Eg 30" wide Burnham Radiant or Sunrad has 67.5 square feet of free surface (see http://www.usboiler.burnham.com/pdf/radiant_lit.pdf), and (even though they only spec it down to 150F), delivers about 50 x 67.5 => 3000BTU/hr with 120F water.

Unlike fin tube or shorter cast iron baseboard type radiators, the taller convecting-radiators are tall enough to convect reasonably even at 100F water temps. Cast baseboard like Burnham Baseray or Weil McLain RC still put out a lot better at sub-120F than any fin-tube convector though (I've used a ~40" section of Weil McLain RC wedged behind a pedestal sink & slightly under the edge of a toilet tank to heat a small attic-apartment bathroom that had no available wall area for a radiator, and insufficient floor area to do it with radiant floor. Works great with 120F water, and keeps the seat warm all winter too! :-) Fin-tube wouldn't have cut it at low temp in there.)