ASHRAE Journal - March 2022 - 36
TECHNICAL FEATURE
Examining Boiler
Setpoint Effect
BY ALAN S. HOBACK
Conventional wisdom among heating professionals says boilers exhibit a setpoint
effect due to short cycling, which results in lower seasonal boiler efficiency during
seasons when the system load is lower. Because the boiler is said to be less efficient
during some seasons, energy-savings measures can then be portrayed as not beneficial.
This article introduces and examines seasonal boiler efficiency (setpoint effect)
and the effect of short cycling on boilers.
Seasonal boiler efficiency (setpoint effect) is the theory
that boilers are less efficient in seasons when the load
on the system is lower. According to the Chartered
Institution of Building Services Engineers,1 a conventual
boiler might get 75% efficiency when working at full
load, but as low as 45% efficiency at 5% load. A high efficiency
boiler might get 80% to 85% efficiency at full load
versus 65% to 75% efficiency at 5% load. A condensing
boiler has an efficiency in the 90% range at full load, but
that efficiency is slightly higher at a load around 40% of
capacity, but then it drops to around 75% at 5% load. The
change in efficiency with load is not the same for each
type of boiler, but all have efficiencies that rapidly drop
below a load of 40%.
Over the heating season, the average efficiency can be
found. The processes for finding the annual fuel utilization
efficiency (AFUE) is provided by ASHRAE.2
Seasonal boiler efficiency is also called the setpoint
effect. If a thermostat is turned down in the winter as
an energy savings measure, then from this theory it is
expected that the boiler efficiency would be reduced.
Therefore, the expected energy savings is less than
anticipated.3
Short cycling is a related issue. If a boiler runs only a
short time because of reduced load, then there would be
expected to be more energy losses related to the cycling
on and off of the boiler.4 It is conventional wisdom that
short cycling would be more likely when the demand is
lower.
Several explanations exist for why boilers should get
lower efficiency when not at full load. First, whenever a
boiler cycles off, its temperature drops from heat transfer
mechanisms such as radiant loss. This is lost energy
because the boiler needs to be raised in temperature
again when it cycles on. Radiant loss might be only 1%
under normal operation, but up to 11% in short cycling.5
Second, codes require purging of boilers between
operating cycles.6,7 Fans blow combustible gases from
boilers. This is a safety measure because it prevents
explosions that could damage the boiler or be a hazard
Alan S. Hoback is a professor of architectural engineering at the University of Detroit Mercy in Detroit.
36
ASHRAE JOURNAL ashrae.o rg M A R CH 2022
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ASHRAE Journal - March 2022
Table of Contents for the Digital Edition of ASHRAE Journal - March 2022
ASHRAE Journal - March 2022 - Intro
ASHRAE Journal - March 2022 - Cover1
ASHRAE Journal - March 2022 - Cover2
ASHRAE Journal - March 2022 - 1
ASHRAE Journal - March 2022 - 2
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ASHRAE Journal - March 2022 - Cover3
ASHRAE Journal - March 2022 - Cover4
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