OK, I know what you're thinking.
Gas hot water heater venting code is like Egyptian hieroglyphics, an almost unintelligible topic.
But there are only a few water heater codes that you really need to understand.
In this HomeInspectorSecrets.com guide, I will go over some of the most important vent pipe codes for gas hot water heaters such as:
- Vent Diameter: Pipe size is based on BTUs, height, and whether draft or powered
- Roof Height: Vent height on roof is based on roof slope
- Clearances: Single-walled vent must be 18" from combustibles if unprotected
- Slope: Vent connectors need a minimum of 1/4-inch of upwards slope
- Connector Length: Type B can go 100% of the length of vertical vent or chimney
- Elbows: Maximum two elbows allowed if there is a horizontal section
- Max Height: Type B vents must be at least 5-feet above other roof hoods
- Fireplace: If connecting to fireplace chimney, fireplace flue must be sealed
- Securing Pipe: Vent pipe must be secured with metal straps, hooks, or brackets
What Is The International Residential Code Or IRC?
This article for gas hot water heater venting code is based on the International Residential Code, using Section G2427 Venting of Appliances.
The IRC is known as a model code, and it has been adopted by numerous U.S. as well as countries around the world. Sometimes the IRC code has been adopted completely, but some U.S. states have certain variations.
Your state, county, or city will always have authority over the IRC and you should consult your own local building code for the final word.
9 Key Gas Hot Water Heater Venting Codes
In the list below, I have compiled what I consider the most essential vent pipe codes for gas water heater venting. There are numerous exceptions in the actual IRC codebook, and your local state or city will always take priority over what is in the IRC.
Here are my 9 most important code requirements for gas water heater vent pipes:
1. Vent Diameter For Type B (Double-Walled)
The minimum diameter of the water heater type B vents (double-walled) is determined by the height of the vent, horizontal sections, whether it is natural-draft or powered draft (fan), and the number of BTUs of the water heater.
The table below is from the IRC code book:
2. Vent Termination Based On Roof Slope
This section in the IRC states that the vent needs to terminate above the roof if it is at least 8-feet from a wall or other obstruction.
The height of the vent above the roof is determined by the roof slope. Basically, the steeper the roof, the greater the minimum height of the vent is required.
The first number in the roof slope is the rise, and the second number is the run. As 12/12 roof means that for every 12-inches you get closer to the ridge, it rises 12-inches. A 12/12 roof requires a minimum vent height of at least 5-feet on the roof.
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3. Clearances To Combustible Materials
The IRC states that there needs to be a minimum distance between combustible materials and the gas vent. This makes sense because the vent can get very hot, and eventually may ignite wood or other materials.
It also states that if the combustible material is protected, these clearances can be reduced.
For gas hot water heaters, you will mostly be dealing with Type B and single-wall venting requirements.
Water heaters are generally considered low-heat appliances since there flue gases are below 1000 degrees. So if you have a single-wall metal pipe, you need to be at least 18-inches from combustible materials if it isn't protected.
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4. Vent Slope
The vent connectors (sections of pipe connecting water heater to vertical vent) needs to slope upwards towards the vent or chimney at least 1/4-inch per foot.
Exceptions are allowed for water heaters with powered vent systems.
5. Maximum Length of Vent Connectors
The vent connector is that section of pipe which connects the gas hot water heater to the vertical vent or chimney. The maximum horizontal length for a single-wall (type C) vent connector is 75% of the height of the vent or chimney.
If the gas vent is type B (double-walled), then you can go up to 100% of the height of the chimney or vent. This makes sense since Type B can handle greater temperatures and is more durable than single-wall vent connectors.
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6. Number of Vent Elbows
If the water heater appliance vent (single venting) has any horizontal sections of venting, then no elbow connectors are allowed.
If the single-appliance vent has a horizontal section, then up to two elbows are allowed. Additional elbows will reduce the listed capacity of the vent.
7. Minimum Height Above Other Roof Hoods
For type B vents, it needs to terminate at least 5-feet above the highest connected draft hood or flue collar.
This gas water heater venting code prevents airflow disruption from another exhaust flue.
8. Using Chimneys & Fireplaces
If you are installing the vent pipe into a masonry or metal chimney, you need to connect the vent above the extreme bottom of the chimney to avoid flue gas stoppage.
You are only allowed to install the water heater vent into a fireplace flue if the fireplace flue opening is permanently sealed.
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9. Securing Vent With Straps
The vent pipe must be secured with metal straps, metal brackets, or metal pipe hooks.
Plastic connections are not allowed.