ASHRAE Journal - August 2024 - 47
TECHNICAL FEATURE
large a peak heating load as possible (Figure 2a).
Issues with Conventional Practice
Warming up a building as fast as possible, however,
may not be the most energy effi cient or cost-effective
strategy with modern HVAC systems. Much of the
original research around night setback and optimum
start is decades old, based on studies of buildings with
relatively poor envelopes, prior to the widespread use
of modulating capacity control in HVAC equipment, like
variable speed drives for fans and modulating boilers. In
newer construction, envelope losses are minimized with
high-performance glazing, improved insulation and
sealing. Fans and heating systems generally also operate
more effi ciently at part-load conditions.
Potentially many downsides exist with conventional
morning warm-up practices that may outweigh their
intended benefi ts. These include:
* Brief periods of peak heating demand may drive a
hot water plant to stage on additional equipment, with
negative effi ciency impacts if the lag equipment then
cycles off quickly. In addition to pre- and post-purge
losses for boilers, signifi cant heating energy is required
to warm up the thermal mass and water in a lag boiler,
and that heat is then lost to the environment if the lag
boiler stages back off soon after.5
* Peak heating demand can drive hot water supply
temperature setpoints to reset up to the maximum limit
(e.g., when using demand-based setpoint reset logic
per Guideline 36-2021), resulting in lower equipment
effi ciency for condensing boilers (due to higher boiler
entering water temperatures that prevent or reduce
condensing) or air-to-water heat pumps (due to lower
effi ciencies at higher leaving hot water temperatures).
Data gathered from a broad collection of buildings
with condensing boilers shows that many operate at
temperatures above the condensing region for most or
all of the time.6 Previous research also determined that
losses through hot water distribution piping may make
up a signifi cant fraction of total hot water loop energy
and are greater at higher supply water temperatures.7,8
Depending on where and when these losses occur, they
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©2024 The Metrafl ex Company
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A U G U ST 2 0 2 4 ashrae.org ASHRAE JOURNAL
47
http://www.Metraflex.com
http://www.ashrae.org
ASHRAE Journal - August 2024
Table of Contents for the Digital Edition of ASHRAE Journal - August 2024
Contents
ASHRAE Journal - August 2024 - Intro
ASHRAE Journal - August 2024 - Cover1
ASHRAE Journal - August 2024 - Cover2
ASHRAE Journal - August 2024 - 1
ASHRAE Journal - August 2024 - Contents
ASHRAE Journal - August 2024 - 3
ASHRAE Journal - August 2024 - 4
ASHRAE Journal - August 2024 - 5
ASHRAE Journal - August 2024 - 6
ASHRAE Journal - August 2024 - 7
ASHRAE Journal - August 2024 - 8
ASHRAE Journal - August 2024 - 9
ASHRAE Journal - August 2024 - 10
ASHRAE Journal - August 2024 - 11
ASHRAE Journal - August 2024 - 12
ASHRAE Journal - August 2024 - 13
ASHRAE Journal - August 2024 - 14
ASHRAE Journal - August 2024 - 15
ASHRAE Journal - August 2024 - 16
ASHRAE Journal - August 2024 - 17
ASHRAE Journal - August 2024 - 18
ASHRAE Journal - August 2024 - 19
ASHRAE Journal - August 2024 - 20
ASHRAE Journal - August 2024 - 21
ASHRAE Journal - August 2024 - 22
ASHRAE Journal - August 2024 - 23
ASHRAE Journal - August 2024 - 24
ASHRAE Journal - August 2024 - 25
ASHRAE Journal - August 2024 - 26
ASHRAE Journal - August 2024 - 27
ASHRAE Journal - August 2024 - 28
ASHRAE Journal - August 2024 - 29
ASHRAE Journal - August 2024 - 30
ASHRAE Journal - August 2024 - 31
ASHRAE Journal - August 2024 - 32
ASHRAE Journal - August 2024 - 33
ASHRAE Journal - August 2024 - 34
ASHRAE Journal - August 2024 - 35
ASHRAE Journal - August 2024 - 36
ASHRAE Journal - August 2024 - 37
ASHRAE Journal - August 2024 - 38
ASHRAE Journal - August 2024 - 39
ASHRAE Journal - August 2024 - 40
ASHRAE Journal - August 2024 - 41
ASHRAE Journal - August 2024 - 42
ASHRAE Journal - August 2024 - 43
ASHRAE Journal - August 2024 - 44
ASHRAE Journal - August 2024 - 45
ASHRAE Journal - August 2024 - 46
ASHRAE Journal - August 2024 - 47
ASHRAE Journal - August 2024 - 48
ASHRAE Journal - August 2024 - 49
ASHRAE Journal - August 2024 - 50
ASHRAE Journal - August 2024 - 51
ASHRAE Journal - August 2024 - 52
ASHRAE Journal - August 2024 - 53
ASHRAE Journal - August 2024 - 54
ASHRAE Journal - August 2024 - 55
ASHRAE Journal - August 2024 - 56
ASHRAE Journal - August 2024 - 57
ASHRAE Journal - August 2024 - 58
ASHRAE Journal - August 2024 - 59
ASHRAE Journal - August 2024 - 60
ASHRAE Journal - August 2024 - 61
ASHRAE Journal - August 2024 - 62
ASHRAE Journal - August 2024 - 63
ASHRAE Journal - August 2024 - 64
ASHRAE Journal - August 2024 - 65
ASHRAE Journal - August 2024 - 66
ASHRAE Journal - August 2024 - 67
ASHRAE Journal - August 2024 - 68
ASHRAE Journal - August 2024 - 69
ASHRAE Journal - August 2024 - 70
ASHRAE Journal - August 2024 - 71
ASHRAE Journal - August 2024 - 72
ASHRAE Journal - August 2024 - Cover3
ASHRAE Journal - August 2024 - Cover4
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