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Combustion Air Requirements:

The Forgotten Element In Boiler Rooms

William H. Axtman President of Gray Gull Associates, Inc.

Retired executive director of the American Boiler Manufacturers Association

Category: Oerations 

!ummary: The following article is a art of the !ational Board Technical "eries. This article was

originally u#lished in the $inter %&&' !ational Board  BULLETIN . () rinted ages*

I"TRO#$CTIO" 

Figure %

$e are all familiar with the com#ustion triangle containing the three elements

re+uired for com#ustion to tae lace. These elements are- fuel, heat (ignition*

and air (igure %*. The re+uirement for fuel is o#vious. The re+uirement for

ignition is e+ually o#vious. The re+uirement for air is somewhat less o#vious

and too often ignored. If one of these three elements is removed, the com#ustion

 rocess stos and the com#ustion triangle collases (igure /*.

Figure & 

A good analogy is an automo#ile with a dirty car#uretor air filter. $hen this filter

gets dirty enough, the engine sutters and whee0es. If the filter is too dirty, the car

won1t run.

This article considers 2ust how much air is re+uired for roer com#ustion, whereair intae louvers are located, and how they are si0ed. This is most imortant, as an inade+uate air tan can

lead to oor com#ustion erformance and even to #oiler furnace exlosions.

REA!O" FOR I"!TA''I"( )ER*A"E"T AIR I"TA+E! 

Although the need for ermanent air intaes should #e o#vious, the re+uirement is ignored too fre+uently.Temorary air intaes such as oen #oiler room windows can #e closed (and often are when ersons in the

 #oiler room are feeling cold*, cutting off the #oiler1s air suly.

$hen the com#ustion air suly is closed off, the fire starts to smoe as the air suly is exhausted.

Incomlete com#ustion occurs and car#on monoxide is generated. The fire then goes out, #ut often #efore

the flame detection system can act to close the fuel safety shutoff valve(s*. The accumulation of fuel is re3

ignited as oxygen sees in through cracs and crevices4 a furnace exlosion fre+uently occurs with

disastrous effects on ersonnel and roerty.

An ade+uate com#ustion air suly is therefore a re+uirement to minimi0e the ossi#ility of a furnace

exlosion.

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!AFET, A"# B$I'#I"( CO#E! CO-ERI"( CO*B$!TIO" AIR I"TA+E 

"everal safety codes such as the !ational ire Protection Association1s standards, !PA 5' 3 !ational uel

Gas code, !PA 6% 3 Installation of 7il Burning 8+uiment, and the American "ociety of Mechanical8ngineers (A"M8* 9":3l 9ontrols and "afety :evices for Automatically ired Boilers have sections

covering the re+uirements for com#ustion air intaes. In addition, #uilding codes such as the Building

7fficials and 9ode Administrators International (B79A* !ational Mechanical 9ode and the "tandardMechanical 9ode u#lished #y the "outhern Building 9ode 9ongress International ("B99I* have air

re+uirements for com#ustion. These re+uirements are summari0ed in Ta#le %.

'OCATI"( A"# !II"( CO*B$!TIO" AIR I"TA+E! 

$hen #oilers are installed in a confined sace such as a #oiler room, two oenings communicating directly

with the outside are re+uired. 7ne oening is located high in the outside wall. The second oening is

located close to the floor. If the #oiler room is located artially or entirely #elow grade, a duct is rovided

from the lower oening terminating at a oint e+ual to the deth of the duct a#ove the floor (see igure 6*.

An outside window well or area way can also #e utili0ed (see igure '*.

igure 6

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