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

http://ashrae.org

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
ASHRAE Journal - March 2022 - 3
ASHRAE Journal - March 2022 - 4
ASHRAE Journal - March 2022 - 5
ASHRAE Journal - March 2022 - 6
ASHRAE Journal - March 2022 - 7
ASHRAE Journal - March 2022 - 8
ASHRAE Journal - March 2022 - 9
ASHRAE Journal - March 2022 - 10
ASHRAE Journal - March 2022 - 11
ASHRAE Journal - March 2022 - 12
ASHRAE Journal - March 2022 - 13
ASHRAE Journal - March 2022 - 14
ASHRAE Journal - March 2022 - 15
ASHRAE Journal - March 2022 - 16
ASHRAE Journal - March 2022 - 17
ASHRAE Journal - March 2022 - 18
ASHRAE Journal - March 2022 - 19
ASHRAE Journal - March 2022 - 20
ASHRAE Journal - March 2022 - 21
ASHRAE Journal - March 2022 - 22
ASHRAE Journal - March 2022 - 23
ASHRAE Journal - March 2022 - 24
ASHRAE Journal - March 2022 - 25
ASHRAE Journal - March 2022 - 26
ASHRAE Journal - March 2022 - 27
ASHRAE Journal - March 2022 - 28
ASHRAE Journal - March 2022 - 29
ASHRAE Journal - March 2022 - 30
ASHRAE Journal - March 2022 - 31
ASHRAE Journal - March 2022 - 32
ASHRAE Journal - March 2022 - 33
ASHRAE Journal - March 2022 - 34
ASHRAE Journal - March 2022 - 35
ASHRAE Journal - March 2022 - 36
ASHRAE Journal - March 2022 - 37
ASHRAE Journal - March 2022 - 38
ASHRAE Journal - March 2022 - 39
ASHRAE Journal - March 2022 - 40
ASHRAE Journal - March 2022 - 41
ASHRAE Journal - March 2022 - 42
ASHRAE Journal - March 2022 - 43
ASHRAE Journal - March 2022 - 44
ASHRAE Journal - March 2022 - 45
ASHRAE Journal - March 2022 - 46
ASHRAE Journal - March 2022 - 47
ASHRAE Journal - March 2022 - 48
ASHRAE Journal - March 2022 - 49
ASHRAE Journal - March 2022 - 50
ASHRAE Journal - March 2022 - 51
ASHRAE Journal - March 2022 - 52
ASHRAE Journal - March 2022 - 53
ASHRAE Journal - March 2022 - 54
ASHRAE Journal - March 2022 - 55
ASHRAE Journal - March 2022 - 56
ASHRAE Journal - March 2022 - 57
ASHRAE Journal - March 2022 - 58
ASHRAE Journal - March 2022 - 59
ASHRAE Journal - March 2022 - 60
ASHRAE Journal - March 2022 - 61
ASHRAE Journal - March 2022 - 62
ASHRAE Journal - March 2022 - 63
ASHRAE Journal - March 2022 - 64
ASHRAE Journal - March 2022 - 65
ASHRAE Journal - March 2022 - 66
ASHRAE Journal - March 2022 - 67
ASHRAE Journal - March 2022 - 68
ASHRAE Journal - March 2022 - 69
ASHRAE Journal - March 2022 - 70
ASHRAE Journal - March 2022 - 71
ASHRAE Journal - March 2022 - 72
ASHRAE Journal - March 2022 - 73
ASHRAE Journal - March 2022 - 74
ASHRAE Journal - March 2022 - 75
ASHRAE Journal - March 2022 - 76
ASHRAE Journal - March 2022 - 77
ASHRAE Journal - March 2022 - 78
ASHRAE Journal - March 2022 - 79
ASHRAE Journal - March 2022 - 80
ASHRAE Journal - March 2022 - 81
ASHRAE Journal - March 2022 - 82
ASHRAE Journal - March 2022 - 83
ASHRAE Journal - March 2022 - 84
ASHRAE Journal - March 2022 - 85
ASHRAE Journal - March 2022 - 86
ASHRAE Journal - March 2022 - 87
ASHRAE Journal - March 2022 - 88
ASHRAE Journal - March 2022 - Cover3
ASHRAE Journal - March 2022 - Cover4
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ATMAHK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VHQRAW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XGMDXI
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_YELQLJ
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QJLWMC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_MCDEBX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WNYSQY
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XATVOD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FJSHSS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_CCBZDS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XDEFVG
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2023november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2023november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VHGNBL
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WPKBNJ
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_UUVCDE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RTGDEW
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2023
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_LKRFXS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_AZSOFG
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ERCDBH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QWDFRV
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JHGVDF
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OPUYHG
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SREIBM
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_LRTGLK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OKRFGH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2022november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2022november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TZSERA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_LVRUIX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RPTVYZ
https://www.nxtbook.com/nxtbooks/ashrae/mini_pub_catalog
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XIYTGD
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2022
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RFGDOB
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_PABXNU
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_REMKLS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_PICVBT
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_AOYTVW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JQOPLS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_IOYTBC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SGAJJF
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_IGHYER
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_PDRKLS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2021november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2021november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XCODFR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QSLFGO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ILKVNM
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2021
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OPDJKD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VJKSRY
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDHUTC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JPPKRR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDLTTH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_CKLLES
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SLDOX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_HJETUK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OLUHGE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ZERDGH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QVMNEO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RTPOKE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_BBATRE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_STUBMW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TPEMPE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JNMKDS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FBTTPA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WQMMNE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TVBRYN
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2020
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_KTUZMA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ABEDGD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201910
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201909
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019septmeber_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019september
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201908
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201907
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201906
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201905
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201904
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019april
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201903
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019march
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201902
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201901
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2019
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018december
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018october
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2018
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201810
https://www.nxtbook.com/nxtbooks/ashrae/ashraeinsights_201806
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201805
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201804
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201803
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201712
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201711
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201710
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201709
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201705
https://www.nxtbook.com/nxtbooks/ashrae/ashrae_meetinginsert_201610
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acrexindia
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer
https://www.nxtbook.com/nxtbooks/amca/2014summer2
https://www.nxtbook.com/nxtbooks/amca/2014summer
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acma_2014summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201311
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201309
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acmasupp_2013fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201305
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201303
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2013winter
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201211
https://www.nxtbook.com/nxtbooks/ashrae/achr_expo_mexico2012
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201209
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v3
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201205
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201203
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012winter
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201111_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201109_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2011summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201105
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201103
https://www.nxtbookmedia.com