hot water baseboard

You must do

an awful lot of laundry, Richard... light starch and boxed, not hangers, please.


The key to success using cast iron with fin-tube is constant circulation. Let's get that out of the way first; the thermal masses and radiant effect will favor the cast iron while, if cycled on-off, the fin-tube portion would lose heat and feel chilled rapidly.

Allow me to paint a mental picture of your system:

With 90 feet of element at say, 500 average BTUH per foot and water at 180 degrees, that is 45,000 BTUH, so you are using most of your hot water potential at a 20 degree drop. You are likely to see a 22.5 degree drop overall. The decay rate of the fin tube might go from 550 entering to 450 leaving BTUH per foot.

If you were to install a cast iron radiator of, say, 10,000 BTUH, you would drop your first fin-tube temperature by five degrees at a 4.0 GPM flow rate. Thus your fin tube will start at 175F entering water. Not too bad.

What is the imperative to do this, may I ask? If to add heat to a too-cold room, I wonder if you might split the zone in two, the better to have more output/less temperature decay from each circuit.

Personally I would add the CI radiator to the return. The radiant effect will work better than the loss of convection from the fin-tube, minor as that might be. Radiant effect is more forgiving.

I would also set the room up for constant circulation with a 3-way TRV mixing valve to avoid spikes and swings in apparent heat output if that is your goal.

Rambling I know, but that is my $0.02

Brad