|Optimal Boiler Size and its Relationship to Seasonal Efficiency|
|Written by Henry Manczyk|
|Wednesday, 09 June 2010 08:30|
Page 1 of 3
OPTIMAL BOILER SIZE AND ITS RELATIONSHIP TO SEASONAL EFFICIENCY
Henry Manczyk, C.P.E., C.E.M. Director of Facilities Management Monroe County, Rochester, N.Y.
In addition to installing high efficiency boilers and maintaining them in good condition, facility managers can employ other strategies to ensure that a heating plant serves the facility efficiently. These include ensuring that the system is optimally sized and that the system operates at an output that is appropriate for the demands of the facility throughout the heating season.
Most heating boilers are designed to operate at maximum efficiency when producing their rated heat output in Btu/hr. Since most boilers only operate at 60 percent or less time at their rated capacity for 90 percent of the heating season, boiler seasonal efficiency is significantly reduced and primary energy resources are wasted.
Seasonal efficiency in a boiler can be improved in several ways. This discussion focuses on three methods: Ensuring that the boiler system is optimally sized to the demand of the facility based on established outdoor temperature and the standard indoor design temperature of 70°F at which most people feel comfortable; use of a modular system of boilers to meet that demand since the fuel economy drops off rapidly as the demand versus boiler maximum heat output falls; and the calculations used to evaluate the seasonal efficiency of the system. The calculations will evaluate how the use of a night setback on the heating system affects the boiler overall run-time and its seasonal efficiency.
Seasonal operating efficiency is the ratio of the total seasonal heat output actually used by the facility to the total seasonal fuel input. This efficiency is dependent on the boiler's steady-state efficiency, standby losses, and cycling losses, all of which constitute the Total Seasonal Input.
Seasonal efficiency = Total Useful Seasonal Output/Total Seasonal Input
Since most boilers operate most efficiently at full capacity, the longer a boiler operates at full capacity, the higher the seasonal efficiency. When a boiler shuts off, the heat in the boiler continues to radiate through its jacket. In addition, boiler-room ambient air continues to flow throughout the boiler after the burner shuts off. When the boiler turns on again, it must reheat the boiler medium to the operating temperature or pressure. A boiler that is smaller than required will more closely match the heating load of the building for a larger part of the season because of fewer on and off cycles. The more often the boiler cycles, the greater the amount of heat would be wasted. (See the Weil-McLain website, http://www.weil-mclain.com/netdocs/straighttalk.num .)