Posted by Harold Kestenholz - Hydronic Network on October 21, 2001 at 11:32:06:
In Reply to: Help calculating boiler size. posted by John on October 20, 2001 at 18:40:22:
The heat loss (which the radiation is supposed to follow) is often used as a starting point, but the actual amount of EDR is better as excess radiation was often installed just to make sure. 1444 times 240 btuh per sq ft of EDR = c.350,000 btuh output from the radiators. You must also provide enough heat to completely heat the boiler, its water, and all the pipes along with the radiators. This is often guesstimated at another 50% beyond the radiator output; so c.520,000 btuh input might be required.
You can know more exactly the added piping allowance input required by counting each foot of each pipe size and calculating the output of each foot at 215F emission from the surface. The heat required to bring the mass of the boiler and the water content to boiling is added for pickup allowance.
Some systems with 4-inch mains or larger in a home have had to add 100% to the EDR output.