George R. Carey, Jr.
Vice President,
Fluid Industrial Associates
gcarey@fiainc.com
twitter: @Ask_GCarey
Boiler Facts
Venting Condensing Appliances
with Polypropylene Pipe: Part 1
Over the last 10–15 years,
installing high efficiency gasfired
condensing appliances
has become quite common in residential
and light-commercial applications.
The industry has been moving towards
higher efficiency products and utility
companies have and will continue to
offer rebate incentives to their customers
to adopt this technology.
Question: What makes these
appliances more efficient than the
boilers and furnaces that are being
replaced?
Answer: They intentionally
condense the flue products that
are created during the combustion
process.
When a gas boiler or furnace is
firing, the flame that is produced
through the process called “combustion”
passes through the heat
exchanger. The other side of the
heat exchanger is either air (if a
furnace) or water (if a boiler), which
absorbs this heat through conduction.
The flue gases, which are the
result of this combustion, are then
vented out of the appliance through
the vent piping (B-vent or a chimney),
out of the building and into
the atmosphere. The flue gases are
hot enough that they remain in
their gaseous state until they exit
the building. It is this temperature
of the flue gases that impacts the
stated efficiency of the appliance.
As energy prices have become
more of a factor all over the world,
the heating industry has been
looking for higher efficiency-rated
appliances that will allow the
consumer to get more bang for their
buck. The manufacturers modified
the design of their heat exchangers
so that they could condense the hot
flue gases that normally would pass
through the vent system and out to
the atmosphere. By condensing the
gases (changing the flue products
from a gas to a liquid), latent heat
is released, which increases the
efficiency of the appliances up to
an additional 10%! When these flue
products turn back to condensate (a
liquid), the liquid is very acidic and
corrosive, which is why most appliance
manufacturers have chosen
stainless steel as the material to use
when designing and building their
heat exchangers.
Venting materials
Not all of the flue gases will condense
while in contact with the
heat exchanger and will enter into
the venting system. The industry
quickly learned that the traditional
vent materials (B-vent, masonry and
clay chimneys) could not withstand
the acidic nature of the condensate.
Manufacturers then started
looking for other materials to use
that would withstand the corrosive
condensate. Stainless steel vent
pipe was an obvious choice, but also
quite expensive. Plastic pipe was
the other option that the industry
selected as an alternative to handle
the acidic condensate. The plastics
included pipes made of polyvinyl
chloride (PVC), chlorinated polyvinyl
chloride (CPVC) and polypropylene
(thermoplastic polymer, PP). They
are all classified as plastic pipe but
each one has its own characteristics
based upon its chemical makeup.
North American appliance
manufacturers immediately adopted
30 ICM/September/October 2022