The boiler should be sized to accommodate the worst possible weather conditions, but being mindful that the extreme condition is rarely reached. Excessive cycling because of an oversized boiler relative to load is extremely inefficient. Many boilers operate more than 90% of the time at loads which are a fraction of their rated capacity. Unless the sizing and selection of the boiler takes these seasonal variations into consideration, excessive cycling and fuel loss will occur.
Q= building block load for heating at the time of building peak in Btuh
Hot water flow through boiler:
F= Q/(500 ∆T)
F= water flow through boiler in gpm
Q= building heating load (block load) in Btuh
500= conversion factor for lb to gpm for water; 8.34 lb/gal X 60 min/hr = 500.4
∆T= temperature difference between hot water supply and return in ℉ (usually 35℉ or 40℉).