Venting Dangers of Water Heaters
After explaining the design requirements of a proper venting system, one young plumber jumped up and said, “That’s a bunch of crap. Why should I have to worry about the water heater vent connector when it’s the heating contractor’s fault? Let him make the changes.”
I was about to respond when another young plumber stood up and started yelling at this guy. His comments were somewhat on the order of, “Don’t be such a jerk. It is that attitude that gets our profession in trouble. What are you going to do when somebody dies in the house? Say that it was the heating contractor’s responsibility when you’re the one who installed the water heater?”
This is a major problem, and plumbing contractors are not paying attention to it. We can’t install a water heater as if nothing has changed. In fact, the vent connector may have changed significantly.
One of the immediate problems is the lack of coordination between the heating contractor and the plumbing contractor. When the heating contractor is responsible for installing the vent system, they often leave the plumbing contractor a 3–inch tee connection for the water heater in a Type B vent. Most plumbing contractors think nothing of the situation and just install the typical 3–inch vent connector for a 40–gallon water heater.
This describes many new single-family dwelling installations. I’ll use this example, because it’s the easiest to review. When we get into multiple appliances in commercial buildings, the vent system becomes more confusing.
The appliances making life difficult for plumbing contractors are fan-assisted furnaces and boilers. When a fan-assisted furnace or boiler connects to the same chimney or vent as the water heater, you are almost guaranteed to have a change in the vent connector for the water heater.
Dynamic Change: Fan-assisted appliances change the dynamics of the venting system. For most installations, a 40–gallon water heater with a 3–inch outlet on the draft hood is required to have a vent connector that is a minimum of 4 inches in diameter. That means the vent connector has to increase in size by 1 inch.
A plumbing contractor’s immediate reaction is, “Why do I have to increase the vent connector in size? I didn’t do anything different.” Well, in fact, you did make a change in the water heater installation. You connected it to the same chimney as a fan-assisted appliance.
A fan-assisted combustion appliance is rated for a higher efficiency. Hence, the appliance steals as much heat from the flue gases as it can. As a result, the flue gas temperatures are lower. The concern is two-fold: buoyancy to get the products of combustion out the chimney or vent, and condensate. If the temperature is too low, the flue gases condense on the inside wall of the chimney, vent, or vent connector. The condensate is highly corrosive to interior surfaces.
New tables published in the National Fuel Gas Code (NFPA 54), NFPA 211, International Mechanical Code or International Fuel Gas Code can help. Some manufacturers also publish vent sizing tables. If you don’t have these tables or a computer-sizing program based on these tables, you’re probably installing a water heater vent incorrectly when there are fan-assisted combustion units. I should mention that, if there are no fan-assisted combustion appliances, you probably have nothing to worry about.
The rule of thumb I use when I encounter an installation is that a water heater vent connector probably has to be increased 1 inch in size when there is a fan-assisted combustion appliance. When there is a masonry chimney, there may have to be a metal liner installed in the chimney. When there are two fan-assisted combustion units and a water heater connected to a masonry chimney, it is almost guaranteed the chimney will have to be lined.
If it sounds confusing, it gets worse. For some installations to a masonry chimney, the vent connector has to be a double wall connector (Type B) vent. In addition, some fan-assisted combustion units that connect to a Type B vent fall through the cracks of the vent tables. The appliances can never have a single wall vent connector and are required to have a double wall vent connector.
First thing you need to do is to figure out which tables to use. Masonry chimneys, masonry chimneys with metal liners, and Type B vents each have separate tables. These tables are subdivided into “single appliances connecting,” and “multiple appliances connecting.” Then you need to know the vent height, length of the vent connector, rise of the vent connector, number of offsets in the connector, and a few other assorted facts. Armed with all this information it is simple to pull the sizes off the tables.
Lazy Way Out: As for me, I’m lazy. I use a computer program and print out the size requirements. I am aware of two programs: one is available from DLJ Technologies at 503/649-0819; the other is available from Elite Software at 800/648-9523.
What about replacement water heaters, you ask? You need to check the vent connector size for the installation of replacement water heaters, as well. If you show up with a new water heater and a fan-assisted combustion appliance is already there, the vent connector will probably have to be increased in size.
If you’re also in the business of installing fan-assisted furnaces or boilers, you need to realize that when you replace an existing appliance with the fan assisted appliance, one of the installation requirements is the installation of a new vent connector to the water heater. Again, you say, “I didn’t do anything to the water heater.” And the vent tables say, “Yes, you did. You changed the dynamics of the entire venting system. Hence, a change is required for the water heater vent.”
Keep in mind that all of this discussion is based on gas-fired appliances. Someone always seems to ask, “What do I do with high efficiency oil-fired appliances?” In the words of one of my colleagues, “You’re next!”