Cast Iron Baseboard project. Need some help sizing...

Crispy

1600 sq. ft. late 1800's Tuder style carriage house in northern NY. Exterior walls are framed with the spaces filled in with brick. Thick plaster outside and plaster & lathe, some with sheetrock over that inside. New windows. Fairly warm crawl space from 1 1/4" copper feed and return loop traveling around the perimeter. Insulated attic. The existing heat is nearly continuous fin-tube around the outside walls, both upstairs and down on one zone, with multiple feed and return tee's off that primary loop from the crawl space. The house is gutted to the studs with convenient holes through the floor for crawlspace access.

The owner (also our good friends) wants to trash the fin tube and replace it with cast iron Snug baseboard. I recently renovated an old house where I salvaged a monster 16' 11" Baseray CI unit and also two 6', 9" vintage baseboard units. I also bought a new 4' Snug unit. That's my only CI baseboard experience and it's working great.

I have all the Snug information but still having trouble doing room-by-room heat loss. I have been using various on-line calculators where you input conditions. I'm getting some consistent numbers but also some that range wildly.

Anyone have a favorite "calculator"? The existing hot-water boiler is 130k BTU (I-B-R 94k) natural gas. Can I use that info to help? Basically, Snug is $100/foot and I don't want to mess this up. Thanks in advance for any help. C.

Ironman

You need to do the load calc with something based upon AACCA manual J or equivalent. There are several online. Or, get the Grundfos Hand book and do it by hand. It’s not that hard.

Whereas the output of cast iron BB is about the same as fin tube (500 btus per linear foot @ 170* AVERAGE water temperature), the cast iron produces a much higher percentage of radiant heat vs convection which is desirable.

Some over sizing of the radiation won’t hurt, as long as it’s proportional throughout all of it, and will actually allow for a lower SWT which can somewhat increase efficiency.

hot_rod

Chapter 7 walks you through the load calc step by step with an example

https://blog.twinsprings.com/wp-content/uploads/2013/05/Uponor-Radiant-Heating-and-Cooling-Design-Assistance-Manual-Part-1.pdf

mattmia2

Is the current system monoflo? if it is monoflo or a regular series loop, you have to take in to account the lower water temp and lower output in each section of the loop. Modifying monoflo can also be tricky, it might make sense to replumb it with a different system.

bburd

That 500 BTU per foot from the cast-iron baseboard is about the same as standard three-quarter inch fin tube. If the heating was well balanced with the old system, you can probably just match the old finned element length per space with the cast-iron.

Unless, of course, the renovation will include significant changes to the thermal envelope—like additional insulation that wasn't there before.

EBEBRATT-Ed

I would suggest splitting the second floor from the first floor if possible.

Doing calculations the old handwritten way is a little tedious but the results are good. No computer calculations are any more accurate just easier to do. Slant Fin had a decent little quick calculator on a disk that a lot of us used but I don't think it's available anymore. Manual J works fine.

As @Ironman mentioned a little extra baseboard will not hurt and will allow you to run a lower water temp which will save fuel. But you still need a HL calculation because you want the baseboard balanced between rooms.