HVAC523
Heat Sources
Classification of Hydronic heat
sources
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Conventional gas and oil fired boilers
Condensing gas fired boilers
Electric resistance boilers
Electric thermal storage equipment
Hydronic heat pumps
Renewable energy heat sources
Heat sources
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Of these, conventional gas and oil boilers
are currently being used in the vast
majority of residential and light
commercial hydronic systems in the
USA.
Gas and Oil fired designs
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Most boilers that operate on natural gas
or fuel oil can be classified according to
their physical construction and heat
exchanger material.
Cast Iron Sectional Boilers
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The cast-iron sectional boiler is currently
the most common type of boiler used in
residential and light commercial buildings
in North America.
Four section boiler
Cast Iron Sectional Boilers
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Since the water flows completely around
the heat source, these types of boilers
are called Wet Base boilers.
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The wet base boiler is the common
configuration for oil fired boilers.
Cast Iron Sectional Boilers
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In this type of boiler, water is heated in cast iron
chambers called sections.
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The sections are bolted together to form a boiler
block.
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The hot gases generated in the combustion
chamber near the bottom of the boiler rise up
through the cavities between the sections and
transfers the heat to the water within.
Cast Iron Sectional Boilers
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The greater number of sections, the
greater the heat output of the boiler.
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Most cast iron sectional boilers for
residential and light commercial
applications are sold as packaged
boilers.
Cast Iron Sectional Boilers
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This implies that components such as the
circulator, burner and controls are selected and
mounted on the boiler by the manufacturer.
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The components supplied by the manufacturer
assume the boiler will be used in a typical
residential system.
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In some cases one or more of these components
may not be best for a specific application.
Packaged boiler
Cast Iron Sectional Boilers
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The amount of metal in cast iron section
boilers makes them relatively heavy.
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Even small cast iron boilers can weigh as
much as 300 to 400 pounds.
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Such boilers typically contain 10 – 15 gallons
of water.
Cast Iron Sectional Boilers
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The combination of metal and water
weight gives cast iron boilers the ability to
absorb a significant amount of thermal
energy.
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Such boilers are said to have a high
thermal mass.
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These boilers are known as high mass
boilers
Cast Iron Sectional Boilers
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A well insulated boiler housing, or jacket
as it is commonly called, reduces standby
heat loss from the boiler.
Cleaning Cast Iron Sectional
Boilers
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Some manufactures design the cast iron
boilers for vertical passage of the flue
gasses.
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To clean these types of boilers, the sheet
metal top panel, flue connection and flue gas
collector assembly must be removed before a
cleaning brush can be maneuvered between
the sections.
Cast Iron Sectional Boilers
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Another type of cast iron boiler design
uses horizontal passages between the
sections with a hinged door to access
both the combustion chamber and the
flue gas passages.
Steel Fire Tube Boilers
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Steel has been used in the construction
of boilers for decades.
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In a steel Fire Tube boiler, water
surrounds a group of steel tubes through
which the hot combustion gasses pass.
Steel Fire Tube Boilers
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Spiral shaped baffles known as
turbulators are inserted into the fire tubes
to increase heat transfer by inducing
turbulence and slowing the passage of
the exhaust.
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The fire tubes are welded to steel
bulkheads at each end to form the overall
heat exchanger assembly.
Steel Fire Tube Boilers
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Although steel fire tube boilers generally have
less mass (metal weight) than cast iron
boilers of similar capacity, they often hold
more water.
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A typical steel fire tube boiler contains
between 15-30 gallons of water.
Steel Fire Tube Boiler
Copper Water Tube Boilers
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Some boilers use water filled copper tubes as
their heat exchanger.
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These also come in vertical and horizontal tube
arrangements.
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Copper tubes are usually manufactured with
fins that greatly increase the are exposed to the
flue gasses for improved heat transfer.
Copper Water Tube Boilers
Combined boiler /domestic hot
water tank assemblies
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A large percentage of residential hydronic
systems supply both space heating and
domestic hot water.
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Although several methods have been used
to provide domestic hot water from a space
heating boiler, it is generally accepted that
the most efficient approach uses a storage
tank that is indirectly heated by the boiler.
Space heating boiler with an
indirect fired water heater
Combined boiler /domestic hot
water tank assemblies
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Several manufactures now offer products
that combine a boiler and hot water
storage into a single unit.
Combination unit
Dry Base Boilers
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Another common configuration is called
the Dry Base Boiler.
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These boilers suspend the water filled
heat exchanger above the combustion
chamber
Dry Base Boilers
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Gas fired cast iron boilers and oil fired
vertical tube boilers are typically
designed as dry base boilers.
Cast Iron Boilers
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All the boilers listed above (except the combo
unit) are classified as conventional boilers.
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Conventional boilers usually operate with a
maximum 85 percent efficiency.
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Conventional boilers are designed to operate
with a hot enough exhaust temperature so that
the byproduct of combustion, water vapor, will
not condense back into a liquid.
Cast Iron Boilers
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If the exhaust temperature were to be
lowered and the water vapor were to
reach its dew point, other byproducts of
combustion would be entrained in this fluid
and would create a very caustic
environment for the cast iron or steel the
unit is made out of.
Condensate
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A good analogy to flue gas condensation
in a boiler is seen in the exhaust from a
car on a cool day.
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When the car is first started, the
temperature of the exhaust pipe and
muffler is low enough to cause some of
the water vapor produced in the engine
to condense by the time it reaches the
tailpipe.
Condensate
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Water can often be seen dripping from the
tailpipe when the car is first started.
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After the tail pipe warms above the dew point
temperature, the water vapor no longer
condenses, and the dripping stops.
Condensing Boilers
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After the energy crisis of the 1970s,
higher fuel costs motivated boiler
manufacturers to research methods for
improving boiler efficiency.
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Analysis by engineers and scientist
found that significant amount of heat can
be recaptured from the exhaust stream if
the water vapor it contains can be
condensed within the boiler.
Condensing Boilers
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Condensing boilers are designed with
large heat exchanger surfaces to extract
more heat from the exhaust gasses
compared to conventional boilers.
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The heat exchangers are made out of
stainless steel or some kind of protected
metal.
Condensing boiler
Condensing boiler
Condensing boiler
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Currently available boilers use high
grade stainless steel or proprietary
electroplated coatings to effectively
eliminate corrosion.
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Condensing boilers are more expensive
than conventional boilers of equal
capacity.
Condensing boiler
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Condensing boilers that are designed
and installed properly, will achieve much
more efficiency than conventional
boilers.
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When designed and installed properly
these types of boilers can attain 95+
percent efficiency.