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BTU Output Calculation for old radiators

Accelar's picture

Probably my dumb, but searching this great site for what I assume is a simple calculation is proving challenging.  Lots of help on the net to tell me how to calulate my BTU requirement, but not the actual output of my old radiators.

I suspect the rads are circa 1930 - very heavy!

We have just replaced the empty air behind the walls with 3.5" of sprayed in foam, and the old windows with low e thermopane - so we know we have way too much heat overall  and also lots of different sized radiators, so loooking to match old rads with new rooms' heating requirements in some cases, and replace with new rads in others.

As a result, want to calculate the output in BTUs of various size rads, and can't seem to find a calculator for that.

For example: I have one radiator that is
49" long, 9.5" wide, and the fins 17" high.
Fins are 20 wide by 6 deep, approx 1.25 inches in diameter, on 2.5" centres.

All feeds are at the bottom - some same end and others opposite end feeds.
All are painted wit metal paint, some - some white enamel and other silver

Any suggestions welcome.

Thanks very much.

Gavin Pitchford

"Sail fast - live slow" (build even slower)

Was harder than I would have (post #194146, reply #1 of 4)

Was harder than I would have figured -- most sites that have "BTU calculator for cast iron radiator" really are just room BTU requirement calculators.  But this site gives you some fairly good info, though a few thumb sucks are required:

(The room BTU requirement formula on that site is pretty suspect, though.  You'd think a formula should factor in the design maximum delta T and the insulation quality.)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

how to size rads (post #194146, reply #2 of 4) (post #194146, reply #3 of 4)

The main man there, Dan Holohan gathered this information up in a book "E.D.R. - Ratings for Every Darn Radiator (and convector) you'll probably ever see"

You will find no better resource anywhere.

radiator (post #194146, reply #4 of 4)

You probably already thought about it, but there are thermostats that install individually on the radiators.  At least on the oversized radiators, this may be better than changing the piping to switch around radiators?  Hope it helps.