As I am finalizing all of my hydronic system component purchases, I looked back at a whole house heat loss calculation I did some time ago. Calculating the heat loss through the roof, walls, windows, doors, and basement walls, I arrived at 41,000 BTU/hr. It seemed low based on the size of the house, so I was tempted to do what engineers like to do, and use a x2 "factor of safety" to size my equipment. We just got through 4 days of extremely cold weather here in SW Wisconsin. I have a large masonry heater in the house as the only current source of heat while the construction is completed. I burned a fire in the masonry heater every 12 hours and was able to keep the inside temp of the house right around 40F. Doing the math (150lbs of wood per 24 hours, 75% efficiency. 6000 btu/lb) and a (40 - (-20)) 60F temp difference, I calculated a whole house UA factor of 500. Using this UA factor with a 90F temp difference (70F inside, -20F outside), I calculate a heat load of 45,000 BTU/hr. I was shocked at how closely the two figures were! Of course my theoretical calculation above didn't include infiltration so taking that into account, the two figures are really quite close! Coincidence????