Got my new Burnham FCM120 mod-con gas boiler today.

[BrownianHeatingTech]

Here's another wrinkle:

Of the 84 feet of baseboard ( small fin wrapped around the conduit pipe ), just over 25 feet of it is older.

This part's fins are just over 2" in height x 5.5" wide, each having 3 evenly spaced 0.5" ( looks like 1/2 inch anyway ) pipes through it. One thru the outside, middle and inside. I'm sure you've seen something like this before.

So there are 3 conduits through these older base boards, thru 2"x5" copper fins.

Do I multiply this 25+ feet by 3 because of the 3 conduits?

How, if at all does this change the situation?

Previous = 84' total, single conduit finned baseboard.

Now 59 feet of single finned baseboard as before, and 25+ feet of this triple conduit quite a bit larger finned ( 2"x5.5" ) baseboard.

The fins actually heat the air, not the tube (at least, the tube has no major effect). Having parallel tubes just lowers the head loss for the pump.

With larger fins, you will get more heat transfer, but it's not all that much. You can put a cap of 60,000 btuh on the maximum heat loss.

Joe