EDR Ratings

Alan (California Radiant) Forbes

I'm on a residential job where I have to find out the connected load of hot water convectors in order to size the boiler. These are the kind of finned convectors that are recessed in the wall with large metal enclosures.

I noticed that the EDR ratings in Dan's book for these types of radiators specify "for steam" and even "for steam at 215F and 65 degree entering air temperature" where the EDR ratings for radiators just say "ratings" or "heating surface" or "radiating surface".

I can imagine that finding the EDR for the finned convectors is much more difficult than for cast iron radiators. The surface area of the fins needs to be calculated along with somehow figuring how much the enclosure adds as some of the convectors will have a greater output with a larger enclosure.

Maybe - because of the construction of finned tube convectors - the BTU output is not scalable for different water temperatures like it is for cast iron radiators?

Jamie Hall

The BTUh power of a radiator is a function only of the average temperature of the radiating surface — a rather complicated function, to be sure, but reference to tables works if you have them — or, in a pinch, a simple linear relationship between the temperature of the working fluid and the space temperature works (the BTUh output is around 1.7 times the difference between the working fluid temperature and the space temperature — to two significant digits. Close enough, as they say, for government work…) Stick an enclosure around it, though, and it gets messier, since the balance between direct radiation of output by convection becomes very different. And honestly I'd really want to have experimental data for hot water convectors…