How to install/vent a (gas) water heater
For one reason or another I get alot of questions about how to vent and install a water heater. This page is designed to help licensed professionals understand what is looked at and for when a water heater venting is inspected, it is does not cover everything and is not designed for homeowners or DIYers. This is not designed to take the place of local codes and rules, it is your responsibility to know and follow local permit, code and laws.
Bottom line up front. I'm going to tell you how it's supposed to be done, and later I'll tell you why.
1) Don't throw away the instructions. The International Residential Code requires you to follow manufacturer instructions. This means whatever the instructions say to do, IS the code.
2) The draft hood for the water heater must be attached to the water heater, either by inserts as it is designed, or with correct metal screws. Use the draft hood that came with the water heater, or the one designed for the water heater!
3) Know if you are using single wall vent pipe, or double wall vent pipe.
4) Typically single wall vent pipe is attached directly to the draft hood, and later transitions to double wall vent pipe. Double wall vent pipe will not fit the draft hood unless an adapter is used. The adapter is considered "expensive" but makes the entire process much easier.
5) If using single wall vent pipe on the draft hood, the vent pipe must be connected to the draft hood using (you guessed it) sheet metal screws. Using tape only is unacceptable and you will fail inspection. Using nothing at all will get some odd looks at the least.
6) The only time the inner portion of double wall vent pipe can be penetrated by sheet metal screws is when it is attaching to a draft hood (using an adapter) or when it is connecting to single wall vent pipe (using an adapter).
7) Single wall vent pipe requires 6" clearance from anything that can catch on fire, double wall vent pipe requires 1" clearance. This includes the paper facing of drywall, so a thimble should be used to pass through the ceiling, or cutting the proper amount of drywall out to pass through the ceiling.
8) After connecting to the draft hood, the single or double wall pipe should rise 12" before making any turns.
9) Different connections of single wall vent pipe should be connected with sheet metal screws. Double wall vent pipe does not use screws, and instead interlocks with other double wall vent pipe.
10) Single wall vent pipe cannot pass through any unfinished area (an attic, for example), and should only be used on the draft hood, if at all and quickly connect to double wall vent pipe, using and adapter and screws follow manufacturer instructions.
11) The double wall vent pipe (minding clearances at the ceiling, attic, ROOF DECKING, etc) can then pass through the ceiling, attic and through the roof where it must rise a set distance above the roof. Absent local individual knowledge of rules, it should rise 2 feet higher than anything within 10 feet, and have a listed, approved cap.
12) If shared/common venting is used with a furnace, the smaller BTU appliance (the water heater) must connect to the common vent above the higher BTU appliance. Because of this confusion I do not recommend common venting.
Other Issues and code:
The most relevant code for Oklahoma is the International Residential Code, 2018 version. It's important to check with your local area to determine what code your area uses, and what modifications the authority having jurisdiction has added to the code.
Chapter 28, Page 629 - Water Heaters. This section governs the code with regard to water heaters, its important to know it.
Prohibited Locations P2801.5 - Refers to chapter 20 on where fuel fired (gas) water heaters cannot be located. Chapter 20 covers this under section M2005.2.
Therefore a water heater cannot be installed in a room used as a storage closet (with clothes, hangers, etc).
When located in a bathroom or bedroom, the water heater must be in a sealed enclosure so that combustion air will not be taken from the living space (This is not required for direct vent water heaters, most water heaters are NOT direct vent.)
The water heater must be accessible, meaning the sealed enclosure must have a door, that when closed is sealed with a gasket.
back to chapter 28:
Required Pan P2801.6 - If the water heater is installed in a location where leakage from the tank will cause damage, a pan is required. Well that means almost anywhere except a concrete floor with a floor that slopes to a floor drain. Everywhere else should have a drain pan under the water heater.. Just do it.
P2801.6.1 The pan should have a pipe running to an indirect waste disposal (like a floor drain) meaning it cannot be direct connected to the sewage line, or to the exterior, not less than 6" off the ground, but not more than 24" off the ground.
P2801.6.1 Also states that if you are replacing a water heater, and there was previously no pan, a new pan is not required.
Elevation P2801.7 - This section talks about when a water heater needs to be elevated. If it is located in the garage it must be elevated at least 18" off the floor. Exception - If the water heater is FVIR listed, it does not need to be elevated.Every water heater built after 2003 is FVIR style.
Relief valves required P2804.1 - This section talks about a temperature pressure relief valve, and that it is required for water heaters.
Extension pipe P2804.6.1 - Talks about how an extension pipe is required for the TPR valve. It must discharge to a floor drain, or the exterior of the house and be visible by all occupants of the house. It must be of an approved material. PVC is not an approved material, copper is best. It must not have a thread on the end so that it cannot be capped.
Expansion tanks required M2003.1 - going back to chapter 20 "boilers and water heaters". This section states that both new and replacement water heaters will be installed with an expansion tank to account for added pressure from thermal expansion. The tank must be securely supported by something capable of supporting the weight, when filled with water.