A gas water heater's listing and manual decide its vent. Atmospheric models use natural draft; power-vent models use a fan. Direct-vent describes outdoor combustion air, and condensing describes how the exhaust releases heat and moisture. These features overlap. Use only approved vent materials, sizes and terminations.
Common features at a glance
| Type | How exhaust moves | Typical pipe | Where it ends |
|---|---|---|---|
| Atmospheric | Natural draft | Draft hood, metal connector, Type B vent | Above the roof, listed cap |
| Power vent | A blower pushes it | PVC, ABS or CPVC (Rheem) | Wall or roof |
| Direct vent | Sealed: outdoor air in, exhaust out | Sealed intake and exhaust piping | Hood on the outside wall |
| Condensing | Pressurized, condensing exhaust | PVC, CPVC, ABS or polypropylene (A. O. Smith Vertex) | Wall or roof, plus a condensate drain |
How does an atmospheric water heater vent?
An atmospheric heater relies on natural draft: the vent's height and the temperature gap between hot flue gas and outside air pull exhaust up through the draft hood on top of the tank and out to the roof. Per the code, the hood lets flue gas escape if the vent stops drawing and keeps a backdraft out of the heater.
The key numbers from Section G2427 of the 2024 Phoenix Residential Code:
- Slope. The connector runs "without dips or sags" and rises at least 1/4 inch per foot toward the vent. A. O. Smith's manual sets the same minimum.
- Height. A Type B vent ends at least 5 feet above the highest draft hood it serves.
- Roof exit. It ends with a listed cap. Within 8 feet of a wall or similar obstruction, it must end at least 2 feet above the roof and above any part of the building within 10 feet.
- Air inlets. It ends at least 3 feet above any forced air inlet within 10 feet.
- Draft hood. The relief opening needs at least 6 inches from any surface other than the heater and its vent.
Clearance to combustibles depends on the pipe. Single-wall metal connector needs 6 inches, per both the code table and A. O. Smith. Type B double-wall vent uses the clearance its listing gives. Through an attic, the connector must be listed Type B, Type L or equivalent insulated material. Tank clearances are in water heater clearance and access rules.
How does a power vent water heater work?
A power vent heater uses a fan, so the pipe can run sideways through a wall. Rheem's Performance power vent "uses indoor air for combustion; blower exhausts the flue gases," takes PVC, ABS or CPVC pipe, allows runs up to 150 equivalent feet, and needs a standard 120 VAC connection. Two rules from Rheem's manual matter:
- No sharing. Rheem's manual rates it a Category III appliance and says "Common venting is NOT permitted." It can't connect to an existing vent or chimney.
- Heat changes the pipe. From 100°F to 125°F ambient, Rheem's table limits the first 10 equivalent feet to CPVC or ABS and calls for a high temperature blower switch kit. Hot garage or attic? Check before anyone glues PVC.
The code adds a safety interlock: a mechanical draft system must stop gas to the main burner when the fan isn't venting properly.
What is a direct vent water heater?
A direct vent heater takes all its combustion air from outdoors and sends all flue gas outdoors, per the code's definition. On A. O. Smith's direct vent models, the wall hood "operates as both the combustion air intake and the heater exhaust port."
How is a condensing water heater vented?
A condensing heater needs its listed vent system and condensate drainage; approved vent materials vary by model. The code's Category IV covers appliances with a positive-pressure vent and exhaust capable of causing excessive condensate, and it requires a way to collect and dispose of that water. A. O. Smith's Vertex brochure lists up to 96% thermal efficiency, venting in PVC, CPVC, ABS or polypropylene, and a condensate outlet piped to a floor drain or outside.
Ask whether the actual condensate arrangement includes a neutralizer and consult the installed kit’s media-maintenance requirements; vent-material selection does not answer that question.
Plastic is only allowed where the heater's listing and manual name it. A draft-hood tank needs a metal system the code lists, such as Type B.
What is backdrafting, and what causes it?
Backdrafting is exhaust flowing into the room instead of up the vent. EPA lists back-drafting from gas water heaters as a source of indoor carbon monoxide, and flags flues that are improperly sized, blocked, disconnected or leaking. Common causes:
- Other equipment pulling air. A. O. Smith notes exhaust fans, ventilation systems, clothes dryers and fireplaces can create conditions needing special attention.
- A wrong cap. The code requires a listed cap and bans decorative shrouds unless listed for that vent. After a reroof, confirm the listed cap went back on.
- A starved closet. See UPC 506 combustion air.
- Disconnected or rusted pipe, or a vent left oversized after a furnace change.
Visible soot, a disconnected vent or changed burner operation warrants a qualified inspection. Carbon monoxide is colorless and odorless, so the absence of these clues does not show that venting is safe.
Draft testing is a service procedure. The cited A. O. Smith manual describes checking draft under conditions that can depressurize the room. Have a qualified technician perform that model's test, investigate spillage and correct the air supply or vent. Do not run a heater showing evidence of sooting until it has been checked. See water heaters and carbon monoxide.
Where should CO alarms go?
CDC recommends battery-operated or battery-backup CO detectors near every sleeping area. Follow the detector instructions and applicable placement requirements. If one sounds, don't hunt for the source: get outside, call 911, and stay out until responders say it's safe. Details in where to put carbon monoxide alarms.
When should you stop and call a licensed plumber?
Stop if you see soot, a loose or rusted vent, a failed spillage check, or an alarm, and don't run the heater until it's fixed. If you smell gas, Rheem's manual says not to light any appliance or touch any electrical switch, and to call your gas supplier from a neighbor's phone, or the fire department if you can't reach them. See pilot light out and you smell gas.
Changing vent types is also a permit question. Phoenix exempts a licensed contractor's in-kind swap in a house or duplex, but by the usual reading, atmospheric to power vent or tankless isn't in kind; see permits to replace a water heater in Phoenix. Weighing a fuel change instead? Gas vs electric water heaters covers it. HQ is licensed and insured, and gives free estimates and upfront pricing before work starts.
Full text and source
These rules are Section G2427 (IFGC 503) of the 2024 Phoenix Residential Code, Chapter 24. Phoenix's 2024 fuel gas code amendment leaves the venting chapter as written. The plumbing code's version is UPC 510 water heater venting. The official text governs.