by Rich Grimes
Water Solutions Marketing
I have had many projects where lack of combustion/ventilation air has been the issue. It reminds me of how many installations I have seen like this. Surely more than I can remember… Lack of air on a water heater is similar to lack of air on a car or small engine. Poor combustion results in lost efficiency and more hazardous emissions. Symptoms such as sooting and constant flame failures can be indicators of air issues. Imagine a candle snuffer that does not touch the wick of the candle, but it captures all of the heat from the flame. The flame uses up all of the air inside the small candle snuffer and the flame cannot maintain combustion. Soot is produced as the fresh air is used up by the flame. Gradually, the flame is extinguished due to lack of combustion air.
In this article we shall discuss combustion and ventilation air requirements for gas-fired appliances. Combustion and Ventilation Air requirements are set forth in the National Fuel Gas Code and typically apply to Atmospheric or Fan-Assisted Combustion. As we discussed in previous articles regarding venting, Category IV appliances typically have a direct air intake from outdoor, negating the need for separate combustion/ventilation air louvers in the wall or door. Separate Air Intake systems for gas-fired appliances are specified by the manufacturer and must be part of the vent system approval.
PRINCIPLE
Gas-fired appliances require adequate Intake Air for Combustion and Ventilation. There are two methods for sizing Intake Air. The Standard method is used almost exclusively, requiring a minimum volume of 50 cubic feet per 1,000 BTU/hr. The second is the Known Air Infiltration Rate Method. This method is rarely used and requires calculations based on atmospheric or fan-assisted combustion and air change per hour.
The movement of air between two louvers allows fresh air to enter the mechanical room and be circulated. A larger, single louver can be utilized to allow enough air to enter and circulate. Each particular air intake arrangement has its own sizing based on the total BTU input of all appliances in the room and where the air is communicated from. The sizing of combustion and ventilation air is specified in the National Fuel Gas Code / NFPA54 / ANSI Z223.1 Section 9.3.
REQUIREMENTS
The intake air sizing is based on the total BTU input of all appliances and is determined by the following installation parameters:
AIR FROM OUTDOORS (TWO PERMANENT OPENINGS)
This method requires two (2) permanent openings. One opening is located within 12 inches of the top of the enclosure and the other is located within 12 inches of the bottom of the enclosure. These openings must communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors:
1) Where communicating directly with the outdoors (LOUVERS) or through VERTICAL DUCTS, EACH opening shall have 1 Square Inch per 4,000 BTU/hr, the total of all appliances.
Example: 200,000 BTU Heater = 200,000 ÷ 4,000 = 50 Square Inches Free Area per opening.
2) When communicating directly with the outdoors through HORIZONTAL DUCTS, EACH opening shall have 1 Square Inch per 2,000 BTU/hr, the total of all appliances.
Example: 200,000 BTU Heater = 200,000 ÷ 2,000 = 100 Square Inches Free Area per opening.
AIR FROM OUTDOORS (ONE PERMANENT OPENING)
This method requires one (1) permanent opening. This opening is located within 12 inches of the top of the enclosure. This opening must communicate directly, or by Vertical or Horizontal ducts, with the outdoors or spaces that freely communicate with the outdoors:
1) Where communicating directly with the outdoors (Louvers) or through Vertical or Horizontal Ducts, EACH opening shall have 1 Square Inch per 3,000 BTU/hr, the total of all appliances.
Example: 200,000 BTU Heater = 200,000 ÷ 3,000 = 66.67 Square Inches Free Area opening.
2) The minimum free area must be not less than the sum of the areas of all vent connectors in the space.
AIR FROM INDOORS (TWO PERMANENT OPENINGS ONLY)
This method requires two (2) permanent openings. One opening is located within 12 inches of the top of the enclosure and the other is located within 12 inches of the bottom of the enclosure. These openings must communicate directly via Louvers, with an indoor space that is adequately ventilated:
1) Where communicating directly with another interior space, ON THE SAME STORY, VIA LOUVERS, EACH opening shall have 1 Square Inch per 1,000 BTU/hr, the total of all appliances.
Example: 200,000 BTU Heater = 200,000 ÷ 1,000 = 200 Square Inches Free Area per opening.
2) Where communicating directly with another interior space, ON A DIFFERENT STORY, VIA LOUVERS, EACH opening shall have 2 Square Inch per 1,000 BTU/hr, the total of all appliances.
Example: 200,000 BTU Heater = 200,000 ÷ 500 = 400 Square Inches Free Area per opening.
ALTERNATIVES
There are a few options available such as engineered installations and mechanical air intake fans. Mechanical air intake systems can provide a controlled air intake. These systems employ a Power Air Fan that will force fresh air in at a given CFM. These must be interlocked with ALL appliances to insure that combustion air is being provided and proved prior to burner ignition. There are also many products that can utilize Direct Air Intake through a fan-assisted/power burner combustion process and separate air intake pipe. These heaters and boilers have become very popular because of their multiple vent and air capabilities. High Efficient appliances that utilize PVC and other plastic vent materials are great options when combustion air is limited.
CONSIDERATIONS
Many restaurants operate in a negative pressure within the building. This is mainly due to exhaust hoods that extract smoke and fumes from cooking. These exhaust fans are extremely strong and can cause burner operational problems. They will draw up and consume the combustion air needed for proper operation. If you open a door to the outside, air rushes into the building.
Equipment such as atmospheric stoves operate properly because they are located directly beneath the exhaust hood and all available air is drawn by them. A water heater or boiler that is located in another part of the restaurant can experience sooting, leakage of combustion products and flame failures due to this lack of air. The big problem is that leakage of combustion products equals spillage of carbon monoxide in the restaurant. The vent stack for the heater becomes an air intake pipe and air is drawn down from the rooftop. When the heater fires, the combustion exhaust spills out at the draft hood as it cannot be drawn up the vent pipe. It is very common to see atmospheric gas heaters choking in this type of installation. A smoke test at the draft hood of the heater will help indicate if flue products are exhausting properly. The test should be performed with the burner on and off and with the door open and closed. A burner that is starving for air will burn very yellow or orange in color. When the air pressure is relieved by opening a nearby door, the burner flame should be blue in color with slight yellow/orange tips.
These situations can be resolved with heaters that allow for direct air intake. Just because there was an atmospheric heater originally installed, replacement may be better suited to a higher efficient heater that allows or requires direct air intake. Restaurants are not the only buildings that can have negative pressures. The same problem can occur in a small mechanical room where an exhaust fan has been installed. This has the same effect, just in a smaller application. Unventilated closets can produce the same problems.
Louvers should also be checked for free area of square inches. Many louvers can restrict the free area and sometimes a larger size louver is required. Mesh screens should not be placed over louver openings as they will clog up and eventually create an air issue. Double-faced louvers can cut free area by one half, requiring twice as large of a surface area compared to an unrestricted, full flow louver.
SUMMARY
All of these are extremely hazardous installations because they put the inhabitants in direct contact with combustion exhaust containing carbon monoxide. Combustion and Ventilation Air is often overlooked and should be investigated and accounted for when installing any gas fired appliance. Combustion and Ventilation Air are crucial to a good, safe installation.
I hope that these articles are helpful to you and we look forward to seeing you in the next issue!
Thanks,
Rich Grimes
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