Published: August 13, 2015 - by Ray Wohlfarth

Categories: Commercial Heating

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One of the most misunderstood items inside a boiler room is a barometric damper. 

The barometric damper is used to control the draft inside a boiler. It is installed on boilers that use a Category I vent.  To understand how the barometric damper operates,  we need to understand what draft is. We all know that when air is heated, it will rise. Hot air is less desnser than cool air and it weighs less.  This is what allows hot air balloons to rise and float above the earth. 

Inside a chimney, the hot air from the boiler is less dense than the cooler air outside the boiler in the boiler room.  The cooler air is like a bully and will push the warmer, less dense air around. The cooler air will push the warm  air up the stack. This flow of air from the boiler up the stack is referred to as draft. The speed or velocity of the flue gases up the chimney is affected by many operating conditions such as temperature difference between the outside and the inside of the building, wind fluctuations, chimney height, burner firing rate, and barometric conditions.  It changes constantly.  On cold days, the draft can be very high. In some instances, it could actually pull the flame off the burner. In lesser conditions, it could simply pull the heat more quickly through the boiler, wasting money.  This is called “Chimney Effect”

Chimney Effect is sometimes felt in tall buildings. Here in my home town, a large building used to have a set of double doors with a vestibule in front of the main escalators to the building. During the rush hour on a cold morning, the draft inside the building would be excessive. When both sets of doors were opened simultaneously, the draft was very high. It would suck air into the building causing great gusts of wind. These powerful gusts of wind would blow up the women’s dresses, embarrassing and then angering the wearer.  There was no report on the male witness’s emotions but a general sense of feigned outrage was felt. The building owners replaced the double doors with large revolving doors in an effort to combat the building’s chimney effect.

When a chimney is attached to a boiler, you can imagine the wild effects that occur inside this transition. The boiler requires a stable environment and the chimney is like the “wild child”. It is like the Odd Couple.  On older boilers, they were designed to have a draft at the outlet of the boiler to be about -0.05” w.c.  The draft conditions inside the chimney could cause swings of ten times that amount. How do you provide a buffer to handle these swings?  A barometric damper is a great solution.  High draft will cause excessive velocity of the flue gases and this will not allow the heat to transfer into the boiler. The boiler manufacturer will inform you of the draft conditions that would like to see on their boilers.  The barometric damper will be installed in the flue between the boiler and the chimney. The barometric damper will be set for the desired draft conditions using weights and adjustment screws.  If the draft inside the chimney is excessive, the damper will open and allow air from the boiler room inside the chimney.

Draft controls are typically used when the stack or chimney height is greater than 30 feet.  Excessive draft inside a boiler can cause strange behavior. In addition to increased operating costs, the high draft can cause flame impingement on the boiler. This could cause higher than desired levels of carbon monoxide.  Flame impingement could also cause embrittlement of the boiler metal, lowering the life of the boiler.  Embrittlement is the loss of ductility of the metal. It is like what happens when you keep bending a paper clip. Flame impingement means that the burner flame is touching the metal surfaces of the boiler that were not designed to have flame on them.  In most instances, the flame should stay inside the combustion chamber.  We had a vertical fire tube steam boiler that had flame being drawn into the rear tubes of the boiler. It caused violent surges and waves inside the boiler. This caused wet steam and tripping of the low water cutoff. Excessive draft can also cause the burner to over fire. If the gas pressure regulator is set for a certain pressure, the high draft can actually pull more air through the regulator, over firing the boiler.

When choosing a barometric damper, there are two types; single and double acting. A single acting damper will have a stop that only allows the damper to swing one way. A double acting will allow the damper to swing two ways.  The single acting will close if there is a pressure inside the stack. The double acting will allow spillage of the flue gases into the boiler room in the event of blocked flues or down drafts. I know that sounds crazy. That is also why I like seeing spill switches on the barometric damper. If there is spillage from the barometric damper into the boiler room, this switch will sense it and shut off the burner.  In some locations, installation of spill switches is part of the boiler code.

Each fuel or combination of fuels requires a specific type of barometric damper.  Single acting is traditionally used for oil fired burners and double acting is used for gas burners.  The stops that are in the double acting dampers should be removed if only firing with gas. 

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