Condensing boilers for your business
Large commercial buildings in our area are commonly
heated by hot water or steam produced by natural
gas-fired boilers. A newer technology, condensing
boilers offer significant energy savings over conventional boiler systems used in hydronic heating
systems. Condensing boilers can be used in both
new construction and retrofit applications.
Flue
extract
Air inlet
Electric
fan
Improved efficiency through waste heat capture
Conventional non-condensing boilers typically have
a combustion efficiency of 75% to 80%. Efficiencies
can increase up to 88% with a class of boiler referred
to as near-condensing. True condensing boilers are
capable of thermal efficiencies up to 96%. Condensing
boilers achieve high efficiency by capturing waste
heat from water vapor and other exhaust gases
and returning that heat to the building, rather than
sending it out the exhaust stack. A by-product of this
process is acidic water, or condensate, that must be
drained from the system.
Primary
heat
exchange
Secondary
heat
exchange
Condensate
outlet
Cold water
return
Burner
Hot water
flow
Illustration used with permission from: evoenergy.co.uk.
System requirements
A condensing boiler achieves its high efficiency only
when the return water temperature is below 130˚F.
By contrast, non-condensing boilers must have return
water of 140˚F or higher. Operation at lower water
temperatures is a key factor in system design and
achieving maximum efficiency. A condensing boiler
system can be operated with higher return water
temperatures, but energy savings will be diminished.
A condensing boiler only provides a performance benefit if the heating system as a whole works together.
The system includes the boiler, pumps, piping, valves,
hot water terminal units and controls. Outdoor-air
reset controls and modulating burners help match
boiler output to actual building heat loads, further
increasing efficiency.
Condensing boiler systems need a condensate drain,
a stainless steel or aluminum heat exchanger and
a sealed and corrosion-resistant stack. Cast iron
materials are not used in a condensing system
because they would be subject to corrosion from
the condensate and susceptible to thermal shock
from the lower return water temperature.
A non-condensing boiler system may use a primary
and a secondary water piping loop. Each loop contains
one or more pumps to move the water. A condensing
system may require less piping, potentially operating
with fewer pumps and using less energy to move
water.
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Facility managers and boiler operators must have a
thorough understanding of control systems and the
importance of operating the boiler in condensing
mode. To achieve optimal system performance, take
advantage of training offered by equipment distributors, installing contractors or manufacturers.
Seasonal performance
Condensing boilers are a good fit for commercial
buildings with reheat HVAC systems. In the summer,
this type of system cools air to remove moisture and
then reheats it to provide more comfortable room temperatures for occupants. Using a condensing boiler
for a reheat system can provide the necessary heat at
a lower temperature than a conventional boiler. This
helps provide air conditioning more efficiently.
Because return water temperatures are lower in a
condensing system, the pipes carrying the water are
cooler. This lowers the heat transfer into the space in
the summer and may also reduce the air conditioning
load.
In the winter, the operating schedule can be set to
change the temperature of the boiler water based
on the heating load or the outdoor air temperature.
According to E Source, condensing boiler efficiency
can increase by up to 6 percent under part-load
operating conditions, which are typically about 95 percent of their annual operating time. Lower operating
temperatures in condensing mode result in less heat
loss and increased overall system efficiency.
Typical applications
Condensing boilers can improve performance in both
new and retrofit applications. These may include
traditional radiant heat, reheat systems, pools, water
heating and in-floor heating systems. They are especially good for commercial new construction heating
and reheat systems where the condensing boilers are
designed as part of an integrated system.
Condensing boilers are not a large part of the boiler
market at this time. Innovative HVAC engineers
are using them to help customers achieve greener
buildings, improve energy efficiency and control
operating costs.
Cost considerations
Condensing boilers carry a cost premium over their
non-condensing counterparts. Because condensate
can be corrosive, the condensing components of these
boilers are constructed from inert materials such as
stainless steel, which increases their cost. However,
when installed in a well-designed system, condensing
boilers can be cost-effective with payback periods in
the range of two to five years.1
Condensing boilers may potentially require more
maintenance than conventional boilers and the cost
of replacement parts may be somewhat higher if they
are propriety and only available from the manufacturer.
For more information on this new technology, contact
your MGE Account Representative, email mge@mge.
com, or call (608) 252-7007.
References
E Source Resource Guide #CEMC-RG-15, “Commercial
Condensing Boilers for Space Heating,” published
March 8, 2010
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