Steam boiler sizing - accounting for un-insulated pipes

BobA

Hi. I'm about to replace my gas fired steam boiler.  The old boiler (a WM EG-45) had a net EDR rating of 383, calcualted using a pick-up factor of 1.333.  However, I always wondered whether it was slightly undersized for my application.  Performance was never the best, but that could also have been explained by other things including some near-boiler piping issues which thankfully will now be fixed as part of the replacement.



I calculated the total EDR of the radiators in the house, and it came out to 312 sq ft.  So that sounds fine at first. Even using a pick-up factor of 1.5, which comes out to 340 sq. ft. Net EDR for the boiler, it still seems ok.



However, none of the steam mains and risers in the house are insulated. I'd prefer not to have to insulate them, because they are in living spaces and are helping to heat the basement family room, which has no radiators, among other spaces.



So I guess my question is, are the un-insulated mains properly accounted for by

using the 1.5 pickup factor?



I was sceptical, so I attempted to calculate the EDR of the pipes.  The value I

came up with was 66 sq. ft. for the mains + 20 sq. ft. for the risers = 86 sq ft. total.

Adding this to the EDR number for the radiators, I get 398 sq. ft. EDR for the

whole system.



The new boiler I'm looking at has the same Net EDR rating as the old one (383).  So does this sound sufficient, or should I be looking at boilers that are one section larger?  I don't want to over-size, but I also want to make sure I don't under-size.



I'd be very grateful for any help.



Summary:

Total EDR of radiators in house:    312

EDR of un-insulated risers & mains:  86 (est.)

Total EDR radiators + pipes:        398



Boiler Net EDR (w./1.33 pickup)     383

Boiler Net EDR (w./1.5 pickup)      340



            Thanks,

                      - Bob