Providing water heaters with some breathing room
Most medical experts agree breathing is important. I have to admit I have not done exhaustive research to validate that statement, but personal experience and a plethora of medical dramas on television make me feel pretty confident that statement is accurate. In fact, not breathing is a factor in 100 percent of deaths worldwide. Doctors, pharmaceutical companies, and your lungs spend a lot of time and energy trying ensure you are able to breathe properly to maintain proper health and vitality.
Anyone who suffers from asthma or any other malady that impacts their ability to breathe can attest to the importance of getting enough air and can describe, in scary detail, what happens when breathing is compromised. When your body isn’t able to get the oxygen it needs, it begins to suffer a variety of symptoms. Depending on the severity of the deprivation, the lack of oxygen can slowly wear down your muscles, organs, and brain or result in more immediate and severe symptoms.
The gas-fueled appliances in a building or home are much like the human body in that they need to breathe properly in order to function properly. Gas-fueled appliances require combustion air in order to burn properly and efficiently and allow the appliance to operate in the way it was intended. Without the ability to breathe, these appliances can suffer from a variety of symptoms that range from faulty operation to appliance death, not to mention the health and safety impacts improper combustion can have on occupants.
In the case of the plumbing system installer, the appliance most often installed that would need combustion air considerations is the water heater. The plumber oftentimes is not responsible for the method used to provide this supply of air. It is usually designed by an engineer and the system is either provided for by openings in wall systems of the building or duct work installed by sheet metal workers. There are times, however, especially when remodeling or replacing appliances, that combustion air should be considered. It is important to remember Chapter 5 of the Uniform Plumbing Code addresses the installation of water heaters and the installation of its vent or its connection to a vent or chimney system. The Uniform Mechanical Code or the full NFPA 54 contain provisions for appliances other than water heaters.
In order for fuels to burn properly, there must be a sufficient amount of fresh air available at the appliance location to support combustion, for dilution of the draft hood and to have equal pressure throughout the area of the appliance location. A relatively standard proportion of oxygen molecules to gas molecules is required in order to ensure a complete burn (oxidation) of the gas molecules. In a laboratory setting, this ratio is two oxygen molecules to one gas vapor molecule.
The composition of air is approximately 80-percent nitrogen, 19-percent oxygen and 1-percent trace gases (argon, helium, hydrogen, etc). Consequently, it requires 10 cubic feet of air to provide 2 cubic feet of oxygen at the temperature and pressure of a “standard” atmosphere (pressure 29.92 inches of mercury at 59 degrees Fahrenheit). When either atmospheric pressure or temperature is different from this standard, the density of the air will vary. Consequently, the number of molecules per cubic foot will also vary.
Because a “clean” burn (noncarbonizing) requires a ratio of 2 cubic feet of O2 to 1 cubic foot of gas, any variation in air density will affect the burn. Therefore, it is sometimes necessary to adjust the quantity of gas molecules to match the available oxygen molecules. High altitude (less pressure, less density) is an example of when a smaller orifice in the fuel-gas supply is needed in order to match the available supply of oxygen molecules per cubic foot of air.
Undersized combustion air openings will result in the deficiency of available air; thus, there will be insufficient air for the primary and secondary burn, dilution air (at the draft hood) and ventilation air (high and low opening for air intake and outflow). Proper design of combustion air openings is critical to safe and efficient operation of a fuel-burning appliance or device.
There are five methods used to supply combustion air to the appliance:
- 100-percent air from indoors.
- 100-percent air from the outdoors.
- A combination of indoor and outdoor air.
- Air supplied by engineered systems.
- Air supplied by mechanical systems.
Because items four and five fall mostly outside of the scope of the plumbing system installer, for reasons described previously and for the sake of time, this article will focus on providing combustion air from indoors. The “Combustion Air Chart” in Figure A is a good visual guide to the requirements and options for supplying combustion air.
Indoor Combustion Air
There are two ways to determine the amount of indoor combustion air required from indoors. One is the Standard Method calculated by a ratio 50cf:1000 Btu/hr. However, if the Air Infiltration Rate is known to be 0.4 ACH or less, the Known Air Infiltration Rate Method must be used. This method is used to calculate the required volume in newer built homes with low air infiltration rates. Air infiltration rates may be found by using the ASHRAE Method, a blower door test, or by a method of establishing a ventilation rate adopted by a local jurisdiction based on its experience.
Known Air Infiltration Rate MethodThe KAIR Method uses a calculation of the required air volume based on two appliance types: fan-assisted and other than fan-assisted, as shown above. The formula for each appliance type is based on the total combustion air needs of each type of appliance, which differ due to the amount of dilution air required. If installations include both types of appliance, then a separate calculation is done for each type of appliance. These calculations are combined to determine the total required air volume. These calculations determine the amount of air for combustion and ventilation required by the appliance for complete combustion. Combustion air includes three components:
- The amount of air required for complete combustion, 10 cubic feet per cubic foot gas burned;
- Excess air to help ensure complete combustion, typically 5 cubic feet per cubic foot of gas; and,
- Dilution air for proper venting, typically 6 cubic feet per cubic foot of gas.
The historical assumptions for excess and dilution air are not for fan-assisted appliances because they have neither a draft hood nor flue collar, and therefore dilution air is limited to leakage from vent fittings. (Research by Battelle, published as “Combustion-Air Issues Related to Residential Gas Appliances in Confined Spaces and Unusually Tight Construction,” January 2001, has verified this.)
Many readers are probably thinking this information is not very applicable to plumbing system installers because a lot of it seems much more mechanical than plumbing-related. First, I would say that a little bit of cross-training never hurt anyone. Next, I would say that depending on the situation, you might find yourself having to at least be aware of the needs and methods so you can recognize if they are not provided. If the water heater or other appliance you installed goes out due to a lack of combustion air, it isn’t the builder or HVAC person the customers are calling. Replacing water heaters for free gets expensive for a business and no one wants to have to manhandle any more water heaters than they have to. Depending on the size of the shop, plumbers may need to know this because there isn’t anyone else who is going to do it. But if you think you might never need to know and understand combustion air, there is a chance you might be right, but don’t hold your breath.