Application Note

5 Fluke Corporation Carbon monoxide: A mechanic’s approach
Close doors to the equipment
room. Monitor the pressure dif-
ferential between the equipment
room and outdoors. Then open
the equipment room door (if it is
an interior door) and monitor the
pressure differential again. Any
indoor depressurization means
you need more combustion air.
Venting
Increasing CO and decreasing
O
2
(or increasing CO
2
) levels in
the vent indicate combustion air
problems and possible venting
problems. Most of the efforts to
ensure adequate combustion air
will apply to venting as well.
But sometimes proper venting
requires more than the provi-
sions that were provided for
combustion air.
Whatever leaves the equip-
ment room through venting to
the outdoors must be replaced
by air from the outdoors. In
the case of Category I vents,
proper draft pressure is very low
(-0.01” to -0.03” wc) and easily
overcome by opposing forces
such as return-duct leaks. Too
much draft is also a problem
because it can reduce product
efficiency, lower vent tem-
peratures and increase building
depressurization.
Vent design and make up air
must be carefully considered in
order to ensure proper venting
under all possible conditions.
Vent gasses that spill from a
draft diverter can ultimately lead
to insufficient combustion air
and high CO production. Com-
bustion products from carelessly
placed sidewall vents can be
drawn into the occupied space
due to building depressurization,
or cause damaging condensation
on structural surfaces or other
equipment and cause opera-
tional problems with meters and
regulators located in the vicinity.
Category I venting
•
Use a draft gauge or your
Fluke 922 Airflow Meter to
check draft.
•
If a cold vent with the appli-
ance off can create a -0.03”
wc draft, then the vent is
probably adequate.
•
Operating draft should be
-0.01” wc for both natural
draft and fan assisted prod-
ucts. Oil burners may require
-0.03” wc draft to overcome
heat exchanger internal
pressure drops in order to
maintain -0.01” wc overfire
draft. Operating drafts over
-0.03” wc should be avoided.
•
A poorly venting chimney will
spill flue products from the
draft hood or draft diverter.
CO
2
is slightly heavier than
air and, as concentrations
increase in the still air of the
equipment room, will settle
at floor level where the water
heater burner is located.
The burner is the first to be
starved for combustion air and
produce high levels of CO.
•
If draft is poor, here are some
key reminders:
– Seal any and all return side
duct leaks and filter access
leaks.
– Add a small register in the
supply plenum to over-
come equipment room
depressurization.
– Get as much vertical height
off the flue collar as possible
before the first elbow.
– Draft is lost in any vent
section that is less than
vertical. Avoid sloped vent
connectors. Use vertical for
rise, then ¼” per foot rise for
lateral.
– Increase vent connector size
at the flue collar. The code
allows for up to two sizes
of vent connector upsizing
when vent tables allow the
selected size.
– Seal vent leaks such as
leaking cleanouts, poor vent
connector fit in thimble, and
poor thimble fit to clay tile
liner.
– If a masonry chimney has
poor draft, installing a
stainless steel corrugated
reliner will increase friction
and make the draft worse.
•
Fan-assisted gas equipment
is directly coupled to the
vent without any means of
draft control. A dual action
barometric draft control can
be installed and adjusted to
control draft.
•
Draft hoods, including draft
diverters integral to equip-
ment, are vent de-couplers.
This means furnace draft and
chimney draft are separate. A
good vertical vent draft will
vent the equipment room and
usually vent the appliance
also, but an air curtain can be
created at the draft hood and
cause flue gas spillage, even
when the vent draft is good.
When this occurs, replacing
the draft hood with a dual-
action barometric draft control
makes the appliance an
integral part of the vent and
allows draft to be controlled—
just like oil burner vents,
except oil vents require only
single-action, not dual-action,
barometric draft controls.
We began thinking about what
characteristics of the system can
affect the quality of combustion.
In closing, maybe we should
think about what characteristics
of the system cannot affect the
quality of combustion? Which
list is longer?