ASHRAE Journal - January 2021 - 57

COLUMN SOLVING PROBLEMS

minimum load, so the boiler plant should have sufﬁcient turndown so the boiler does not cycle excessively.
In our experience, the majority of annual building
heating system operation is below 50% load, especially
when not in the peak heating season. While each project
is unique, in our experience buildings with zone heating
can produce system minimum loads that are only 5% of
peak system loads.
The boiler plant must also be able to increase or
decrease its output with the load. If the load increases
steadily and then remains for a long period, a boiler
with a high thermal capacity may be appropriate. If the
load has a short burst of high instantaneous demand,
a boiler with lower thermal mass but a quick response
may be preferred. High-mass boilers are more expensive but also tend to have greater efﬁciency and longer
life than low-mass boilers. The designer should calculate
the maximum load and the lowest load and estimate the
average load.
It is sometimes preferred to split the load among multiple boilers. Splitting the load allows the plant to meet a
seasonal high maximum and improves plant turndown
in the shoulder season and can increase efﬁciency for
loads that tend to vary over the course of the year. While
having multiple boilers of the same make and model is
ideal for maintenance and operability, it is sometimes
beneﬁcial to have a combination of high-mass and lowmass boilers on campuses that have a wide variety of
instantaneous and slowly increasing load demands.
Another factor to consider when selecting the number
of boilers to be installed is redundancy. Redundant boilers allow for a more ﬂexible maintenance schedule and
possible future expansion.
2. Boilers should be selected in conjunction with
analyzing the combined performance of heating coils or
heat exchangers to ensure proper temperature differential is achieved. The output capacity should be used to
select the boiler size based on design-day ∆T. Trend data
or measurements can be used when retroﬁtting existing
facilities.
3. Calculate the minimum system circulating water
volume required to prevent short-cycling. A system with
all two-way valves and no end-of-loop bypass valves is
not able to use much of the system volume since the
closed valves will not allow ﬂow in their corresponding
circuits, limiting its ability to store energy. Buffer tanks
can provide additional system volume and thermal mass

between the hydronic heat source and a zone distribution system when the secondary loop cannot provide
adequate system ﬂow or volume. The runtime is a relationship between boiler minimum output, circulating
system heat capacity and the system minimum load. The
recommended minimum runtime can be provided by
the boiler manufacturer. In our experience, a runtime of
at least 10 minutes and an off time of at least 15 minutes
are good design targets. The required system circulating water volume can be calculated using the following
formula:
Vc =

where
Vc
=
Runtime =
Boilermin out =
=
Qmin
∆T
=

Runtime × ( Boilermin out − Qmin )
lb
min
∆T × 8.33
× C p × 60
gal
h

(1)

Circulating system water volume, gal
Boiler minimum runtime, minutes
Boiler minimum output, Btu/h
System minimum heat load, Btu/h
Expected system ∆T at minimum load
during low load condition, °F

Tips for Less Experienced Design Engineers to Avoid Problems
1. Younger design engineers may size boilers only for
peak load and/or add excessive safety factor to their load
calculations. Right-sizing boilers based on diversiﬁed
load calculations avoids a number of operational issues
and improves efﬁciency.
2. Understand how the building automation system
(BAS) will interface with the boiler controls for different
manufacturers.
3. Not all boilers are controlled the same way. Work
with the manufacturer to establish a control sequence of
operation that allows boilers to operate more efﬁciently
at part load during shoulder seasons with aggressive
heating hot water return (HHWS) temperature setpoint
reset. This will allow for more efﬁcient performance for
condensing boilers. For instance, most building reheat
systems can provide adequate heat during the summer
with 100°F (38°C) supply temperatures with condensing
boilers.
4. Think about potential future decarbonization strategies when designing heating hot water systems. This
may mean designing the system with a lower heating hot
water supply temperature that is compatible with future
heat recovery chillers or heat pumps.
5. Understand that some boilers can handle primaryJAN UARY 2021

ashrae.org

ASHRAE JOURNAL

http://www.ashrae.org

ASHRAE Journal - January 2021

Table of Contents for the Digital Edition of ASHRAE Journal - January 2021