ASHRAE Journal - August 2024 - 48

TECHNICAL FEATURE
FIGURE 1 Morning warm-up strategies.
Optimal Start
Ramped Warm-Up
1A
Zone Temperature
Heating Setpoint
1B
Zone Temperature
Heating Setpoint
1C
Zone Temperature
Heating Setpoint
No Setback
Setback
Warm-Up
FIGURE 2 Associated heating loads.
2A
Optimal Start
2B
Occupied
Setback
Warm-Up
Ramped Warm-Up
2C
Occupied
Unoccupied
No Setback
Occupied
Simplified illustrations of different warm-up strategies and the associated heating loads. Conventional morning warm-up incurs a step-change in heating setpoint prior to occupancy
(Figure 1a) to recover space temperatures as fast as possible, resulting in a high peak heating load (Figure 2a). In Figure 1b, the warm-up mode begins earlier and uses a ramped
setpoint rise, which reduces the peak heating load (Figure 2b). In the extreme case with no night setback (Figure 1c), a constant heating setpoint is maintained, eliminating the
morning recovery load (Figure 2c).
may be detrimental to energy performance.
* Where heating is provided by electricity
(particularly electric resistance), heating loads during
morning warm-up may set the building peak electrical
demand and may strain regional electric grids, with a
potentially significant impact on first-cost for increased
electrical service in winter-peaking climates, and on
energy costs depending on utility rate structures (e.g.,
high peak demand charges or ratcheting rates). Already
a risk today in regions dominated by electric heat,
this is likely to become an increasing concern as more
buildings transition to have all-electric HVAC systems.
* Building heating loads generally peak during the
warm-up period. If warm-up loads can be consistently
reduced, designers can potentially be more aggressive
in equipment sizing, particularly in retrofits of existing
buildings where measured loads may be available.
Conventional practice for sizing heating equipment
assumes unrealistically conservative conditions,
sometimes with additional safety factors* and with
designers subsequently selecting " next-size-up " boiler
equipment. By conventional practices, little incentive
exists for designers to " right-size " boilers, as the
incremental first-cost for larger equipment is minimal,
whereas the consequence of having insufficient capacity
* 2021 ASHRAE Handbook-Fundamentals suggests oversizing
factors of 20% to 25% for warm-up and safety are common.
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ASHRAE JOURNAL ashrae.org A U G UST 2024
is readily obvious to building occupants and operators.
However, " right-sizing " boilers may appreciably
improve annual efficiency and equipment longevity
by reducing the amount of time spent short-cycling at
loads below minimum turndown limits. Data gathered
from hundreds of U.S. boiler plants indicates that a
majority operate at very low part loads, with many likely
cycling below minimum turndown limits for much of
the time.6 The same data set shows that the vast majority
of these are oversized for the actual peak heating load
encountered in the building, typically by a factor
approaching two. For air-to-water heat pump plants
(AWHP), equipment sizing has a much more significant
impact on first costs and space requirements, and may
also affect electrical system service sizing.
In practice, optimum start algorithms often are not
tuned or set up correctly, resulting in a high frequency of
installations where the logic is disabled in favor of fixed,
conservatively early start times to consistently ensure
recovery in time at the expense of increased annual
energy consumption.
A Better Approach
Considering the driving factors for avoiding peak
heating loads during morning warm-up, a practical
alternative is to use a ramped setpoint trajectory (e.g.,
exponential decay) that gradually increases the heating
Heating Load
Temperature
Heating Load
Temperature
Heating Load
Temperature
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ASHRAE Journal - August 2024

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