B-Vents Only; Vent Not Drafting/ Negative Pressure - Travis Industries Spirit 300 GS Service Manual

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Vent not Drafting/ Negative Pressure Situation

B-Vents Only

If a b-vent appliance shuts down within 5 minutes of starting, chances are it encountered a cold-air
block or negative pressure. In most cases you can re-start the appliance and it will draft properly.
However, if persistent shut-downs occur, you may wish to check for negative pressure.
Negative Pressure
Negative pressure describes a situation in which less pressure is
inside the home (near the heater) than outside. This allows air to
rush down the vent. Because B-Vent heaters utilize natural draft
to pull combustion products up the vent, negative pressure can
cause combustion products to spill into the room. The spill switch
then detects the heat and shuts the heater off. The pilot will then
need to be re-started. The following are possible causes of
negative pressure:
¥ Exhaust Fans in Air-Tight Homes
Kitchen Jenn-Air type fans, dryers, or bathroom fans can pull air
out of the home, causing air to come down the vent.
Remedy
Turn all exhaust fans off while starting the heater. If this is not
feasible, the heater may need to be re-started a couple of times
to generate draft. In severe cases, supply outside air to the
appliance to overcome negative pressure (install an outside air kit
or crack the window).
¥ Down Drafts
Installations in a high-wind area may lead to air pushing down the
gas vent. Homes located next to a lake or on the leeward side of
tall trees or a hill are most susceptible. This type of problem may
be sporadic.
Remedy
Increase the vent height above the downward flow of air or install
a draft-inducing hood or draft inducer.
¥ Thermal Negative Pressure
Thermal negative pressure describe a situation in which the
home circulates warm air up and out of the home. Cold air then
replaces it by coming down the vent. This type of negative
pressure can be pervasive. Homes with un-sealed, un-insulated
ceilings are especially susceptible. A heater located in a cold
basement compounded by air exiting through the upper portions
of the home is another scenario. In homes with two fireplaces on
different levels, you might notice the downstairs fireplace tends
to draw air in while the upstairs fireplace will pull the warmer air out
of the home.
Remedy
This type of negative pressure is extremely difficult to diagnose
and remedy. Make sure the attic or ceiling area is sealed and
insulated. In severe cases the heater may need an outside air kit
or draft inducer.
Travis Industries
Troubleshooting Steps
Gas T-Shoot Guide
Kitchen or
bathroom fan
Draft Hood
19203