ASHRAE Journal - October 2019 - 40

SOLVING PROBLEMS

recycled back to the boiler return," Neill said. Primary
flow rates can also be reduced during operation to limit
pumping energy as well as avoid short-cycling.

Thermal Mass Considerations
The system's thermal mass can be another shortcycling issue, Neill said. Because condensing boilers
have a lower water volume than larger, traditional cast
iron or steel non-condensing boilers, the system's thermal mass or water volume is not large enough to absorb
the minimum, low fire output of the boiler and allow for
a reasonable run time of about 10 minutes, which then
the boiler will short-cycle.
"A 1,000,000 Btu/h (293 MW) condensing boiler with a
5-to-1 turndown or about 200,000 Btu/h (58 MW) input
and a low system load demand of 50,000 Btu/h (14 MW)
would need a system volume of about 300 gallons (1136
L) available for a 10°F (5.56°C) rise in system temperature," said Neill.
The volume could be the total of the boiler water volume, system piping volume and maybe buffer tank. The
total volume always needs to be available to the boiler,
including under light building load when variable frequency drive system pumps modulate to minimum flow
delivery, and system by-pass valves open to the system
differential pressure, according to Neill.

"Under light-load conditions, one boiler is more than
enough," he said.
In non-shoulder months, the building can have
another one or two smaller boilers, which take less space
in mechanical rooms. This makes it easier for system
operators and maintenance personnel to walk around
and between the mechanical equipment to access the
systems, he said.
Duffy suggested choosing equipment that is efficient
throughout the part-load operation, not just at the rating point. Condensing boilers with a higher turndown
that can match the load in a wide range of scenarios can
be good options for buildings with loads that are uncertain or highly variable, he said.

Servicing
Maintaining condensing boilers should include regular servicing. Duffy recommended at least performing
annual or recurring service to check the water quality
and heat exchanger. These checks include making sure
sedimentation is not building up in the exchanger.
"A very small amount of sedimentation build-up
reduces the efficiency of your heat exchangers significantly," he said.
If the boilers are in areas with high pollen counts and
poor air quality, Duffy suggested more frequent service
checks and filter inspection and washing.

Water Setpoint Considerations
Trying to control the system supply water temperature
at too tight of a setpoint could cause short-cycling.
"Remember we are trying to control space temperature
to maintain comfort. Not just maintain water temperature to within 1°F (0.56°C) just because BMS controls
will allow us to attempt that," Neill said. "The boilers are
mechanical fuel-burning devices with pre-purge and
post-purge timings at the beginning and end of each call
for heat. These times can reach up to a couple of minutes
total for each cycle," he said.
Water setpoints need to have a reasonable differential
under light loads, without adequate thermal mass. Neill
also advised engineers to look at total loads under light
load conditions and pick a smaller boiler. The industry
rule-of-thumb is to size each boiler to have two-thirds
of the load and install usually two boilers in a building,
which Neill said can cause oversized system selections.
Smaller boilers can handle the load during the shoulder
months with less short-cycling, he said.
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Need for Proper O&M
The primary resource for operation and maintenance guidance for condensing boilers is the manufacturer's literature, said Duffy. That information is
created to increase the longevity and efficiency of an
installation.
He said it is important to properly operate and maintain condensing boilers because of their potential for
high-efficiency energy savings and reduction in carbon
footprint. And if they are not maintained and operated
properly, the potential will not manifest, which he said
he sees more frequently than he would like.
"Our customers and end-users are generally paying
three times as much for these high-efficiency condensing boilers," said Duffy. "I think it's imperative that we
as a design community and in operating this equipment
that we do it around those condensing temperatures,
those low-return water temperatures and higher DTs.
Otherwise, you won't get what you paid for."

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ASHRAE Journal - October 2019

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