i could use some help with my math on trying to calculate the edr output of my old cast iron rad and boiler system. from the results i get, it seems that the existing rad system is designed at no more than 36 btu/sf output for the house and the existing natural gas boiler is borderline undersized even for that. i thought 45 btu/sf was normal? 3778 SF of heating area 800 SF of EDR = 136,000 BTU EDR at 170f (144,000 EDR at 180) boiler is a 200,000 btu cast iron rated at 160,000 with a net rated 140,000 btu net output at 180 f so the boiler seems possibly too small for the rad system even if working at peak efficiency (i have actually been running it at 160 degrees at the boiler), and the rad system itself seems too small for the house. if i have undercounted the edr of the radiators, then the boiler must be way undersized... yet it heats the house (although below -5 C it won't quite get to 20 C at 160, which i had attributed to the windows) here is the more detailed math if anyone is interested. i can post the individual radiator measurements if needed EDR rating = 770 SF -most rads are normal 4 or 5 tube rads. i double checked the height/tube section edr ratings on three different sizing websites and am confident. highest was a 38" 5 tube at 6EDR per section. -there are 7 single small single column (6 section) 28" x 3 1/4" x 12.5" wall rads i rated at 8edr per rad based on ratings for similar sizes. this seems low so i rounded my 770 total up to 800 sf. house heated square footage = 3778 (i discounted 350 sf in the kitchen area that was originally heated by the oil stove and now has 3000w of electric baseboards). 1698 SF has 9.5' ceilings, 2048 has 8' ceilings. lots of single glazed big windows. decent insulation for 80 years ago (6" wide poured gypsum in walls and stucco exterior). part of it is that i suspect they intended the rooms above the kitchen to be partly heated by the kitchen (that 350 sf area has 4 small wall rads which works out to 20btu/sf), but even if i net that area and those rads out, i still only get 38btu/sf for the rest of the house.