Draft and Venting
High draft will cause:
Too much air to rush in the combustion zone resulting in reduced efficiency.
Typical chimney draft for oil heat is close to
-0.5 inche
Draft regulator
-Known as: By-pass or Air-bleed -Responds to changes in barometric (atmospheric) pressure, also called a barometric draft regulator or dampe
Advantages of alternative venting
-More positive control of draft -No chimney warm up problems -Reduction in burner noise with direct venting -Eliminate back draft -Reduces system stand-by losses
Installing flue pipe
-Must be 18" away for m combustibles -1/4" pitch, from appliance to chimney -should not exceed 10ft of horizontal run, unless draft fan is used
The pressure difference is created because:
-When burner is off, chimney is cold -When burner starts, fan creates static pressure -When the fuel burns, temp rises dramatically, gases expand to more than double their volume, adds to pressure pushing gases up to through heat exchanger -As hot gases travel up, it creates a pressure drop behind them, that sucks combustion gases out of heat exchanger
Most common locations for air leaks:
-around burner mounting flange -between the base and floor -between base and heat exchanger -between sections of a boiler -around clean-out and inspection doors/plates
What affects draft?
-temp of outside air -temp of flue gases -barometric pressure -humidity of air -wind velocity across or into chimney -flow restrictions in chimeny, flue, heat exchanger -placement of chimney
A net flue gas of
350° is recommended to avoid condensation from gases in flue pipe
Outside vs inside chimeny
A chimney operates best when it is warm. An outside chimney heats slower and cools faster than an indoor chimney
Induced draft can help provide
A clean start with no rumbles and adequate draft in a chimney that is too small, too low or too large
A draft regulator consists of:
A counter weighted swinging door that opens to allow room air to mix with the gases, which cools the exhaust gases which reduces the temp difference, which reduces draft
Draft
A current of air in an enclosed area that is created by a difference in pressure. A force that pulls or sucks the exhaust gases out of the heating unit and send them up the chimney.
Low draft will cause:
Burner fan to push against higher air pressure resulting in less air for combustion and a smokey fire
Currential draft
Caused by the suction created as wind rushes over the chimney, creating neg pressure in chimney. Because wind is variable, currential draft is unpredictable. Occasionally wind blowing down a chimney will,cause a down draft.
The difference between drafts is known as
Draft drop or pressure drop If breech draft is -.07 and over the fire is -0.02 the draft drop is .05
Draft inducer
Electrically powered fans installed in the flue pipe, help pull the air from the unit and push it into the chimney
Power venting
Fan attached to flue pipe at the exit terminal. Do not need chimney Usually sidewayy vented
Excessive draft drop may indicate
Heavy soot and scale deposits in the unit
Draft regulator should be installed
In the flue pipe, between chimney and stack mounted primary control, if there is one. 10 to 12 inches from control, 18 inches is preferred. No less than 18" away from ceiling or wood joints
Chimney draft is measured in
Inches of water One inch of water column is the pressure difference required to lift a column of water one inch up a tube.
Negative pressure
Occurs during combustion, as hot gases rise through the flue. Creates a suction
Draft is checked
Over the fire (top of combustion area) In the flue pipe, as close to the breech as possible.
Draft is the total effect of
Positive pressures of burner fan Expansive pressure of the flam Negaivte pressure ofhot gases escaping chimney
Draft over the fire
Should be measured first Normally -0.01 to -0.02 If higher, open regulator If lower, close regulator
Direct vent
Static pressure pushes gases through heat exchanger and out of building. Positive pressure system No chimney needed
Thermal draft
The air in the chimey is hotter and therefore lighter than the air outside. As it goes up the chimney, more air moves in from the surrounding room to replace it
High draft will draw excess air into the appliance through leaks, which will cause...
The combustion products to cool, increasing their volume, decreasing efficiency.
Two types of natural draft
Thermal and Currential
Static pressure
When the burner fan pushes air into the combustion area where it combines with the fuel to create a fire
Draft at the breech
Will be slightly higher than over the fire Normally -0.03 to -0.07
