Chapter 4
Venting Systems for
Condensing Appliances
Condensing appliances require special vent-ing
systems that differ from those used with
non-condensing appliances.
To understand the venting process, it’s im-portant
to understand that condensing applianc-es
achieve their higher efficiencies by extract-ing
almost all of the heat from the combustion
gases before they pass through the venting
system and are emitted to the atmosphere.
Condensing appliances don’t offer more
efficient combustion than non-condensing ap-pliances;
they simply extract more heat from
the combustion gases than non-condensing
appliances.
In a non-condensing appliance, water vapor
(one of the by-products of combustion) travels
through the appli-ance
and escapes
through the vent-ing
system to the
outside atmosphere,
where it cools and
condenses.
In a condens-ing
appliance, the
flue gases pass
through the heat
exchanger(s) for
a longer period of
time, allowing the
appliance to absorb
more heat. The
gases condense in
34 National Oilheat Research Alliance
the appliance and as they cool, water vapor
drips out of the heat exchanger instead of pass-ing
to the outside through the venting system.
Because the flue gas temperatures of con-densing
Oilheat appliances are so low, the
venting systems differ from those described in
Chapters 2 and 3.
The Condensate
Flue gas condensate drains from the appli-ance
into a trap (Figure 1) which must be piped
to an appropriate drain location.
The condensate is acidic, with a pH of about
3.5. Continual exposure to condensate may
injure plants and can damage certain building
materials, including metal, wood, stone, and
concrete. Some local codes require that an acid
Figure 1: Condensate drain