Don Foreman wrote:

I also wondered why cast iron was preferred in such applications. I
asked a lot of people about that, finally got a good answer from a
boiler and firebox design engineer.
I grew up with hydronic heat, we had a Standard boiler and
Bell & Gosset pumps. The house I am in now also has hydronic
heat, with a boiler make I'm not familiar with, and one Bell &
Gosset pump, and zone valves. Both systems used finned
baseboard radiators. The Standard boiler was a cast iron thing,
but this newer one (installed in 1976 when the house was built)
has a long string of copper ells. I'd never seen one like that.
I assume the ells are brazed together, I can't imagine solder
would hold up to the thermal cycling conditions.

The ells look like this :

_ _ _ _ _
| | | | | | | | | |
- - - - - ----

and there are several circuits in parallel.

Jon