What it is

A gas water heater burns fuel and has to get the flue gas — including carbon monoxide — safely out of the building. How it does that defines the appliance category and dictates the vent material, where it can be installed, and where it gets combustion air. The three you'll see are atmospheric (natural-draft), power-vent (fan-assisted), and direct-vent (sealed-combustion). Getting the venting wrong is a carbon-monoxide hazard, so this is one place "close enough" doesn't fly.

How it works

Atmospheric / natural-draft. The classic tank with a draft hood on top venting into a metal B-vent that rises through the building. It drafts on buoyancy alone — hot flue gas is lighter than room air, so it rises up the flue and out the roof. The draft hood adds dilution air and provides a relief opening so a momentary downdraft spills at the hood rather than snuffing the burner. It pulls combustion air from the room, so the space needs adequate combustion air. It's the cheapest and simplest, but it depends entirely on the building giving it good draft and enough air — which is exactly where it gets into trouble.

Power-vent. A blower (mounted on top of the tank) mechanically pushes the flue gas out, usually through PVC/plastic horizontally out a sidewall. Because a fan does the work, it doesn't need a tall vertical chimney and can vent through a long horizontal run or up through a wall. It still pulls combustion air from the room. The fan also means it needs electrical power — no power, no hot water. A pressure/proving switch confirms the blower is moving air before the gas valve opens, like a furnace inducer.

Direct-vent (sealed combustion). A two-pipe (or concentric) system: one pipe brings outside air directly to the sealed burner and the other carries flue gas back outside. The combustion chamber is sealed off from the room entirely, so it doesn't use or depend on room air, and it can't backdraft room air into the burner. Great for tight houses, confined spaces, and anywhere indoor air quality or depressurization is a concern. Power-direct-vent combines a sealed-combustion intake with a power-vent blower for long runs.

In the field

Choosing and troubleshooting:

  1. Identify the category by what's on top. Draft hood + metal B-vent = atmospheric. A blower box + plastic sidewall pipe = power-vent. Two pipes (or a concentric pipe) to the outdoors with a sealed chamber = direct-vent.
  1. Atmospheric — verify draft and combustion air. With the burner running, the draft hood should pull, not spill. Test with a smoke source or back of your hand at the hood relief; persistent spillage means poor draft. Confirm the room has the required combustion-air openings — a tight mechanical closet starves it.
  1. Power-vent — check the blower, proving switch, and the run. No power = dead unit. If it won't fire, confirm the blower runs and its pressure switch proves before the valve opens. Watch for a long/sagging plastic run, a blocked termination, or a plugged condensate point on the vent.
  1. Direct-vent — keep both pipes clear and the chamber sealed. A blocked intake or exhaust shuts it down. Confirm the sealed chamber gaskets are intact after any service; a breached seal defeats the sealed-combustion safety.
  1. Common-venting gotcha. An atmospheric water heater often shares a vent with a furnace. If a high-efficiency furnace gets swapped in and leaves the shared vent, the now-"orphaned" water heater may be alone in an oversized vent that's too big to draft properly — leading to condensation and backdrafting. This is a classic CO trap; the vent has to be resized or the water heater re-vented.

Normal values & targets

  • Atmospheric: natural-draft, draft hood, B-vent pitched up, combustion air from the room — needs adequate combustion-air openings.
  • Power-vent: fan-forced, plastic sidewall venting, room combustion air, needs electrical power, proving switch interlocks the gas valve.
  • Direct-vent: sealed combustion, two-pipe/concentric, outside combustion air, independent of room air/depressurization.
  • Combustion air (atmospheric/power-vent): sized per the fuel-gas/mechanical code combustion-air method (commonly ~1 in² of opening per 1,000 BTU/h for two-opening, more for single-opening) — confirm against the code and BTU input.
  • Orphaned-water-heater risk: when an atmospheric heater is left alone on a vent sized for it + a now-removed furnace.

Common faults & what they mean

  • Spillage/backdraft at an atmospheric draft hood → poor draft: blocked/oversized flue, cold chimney, house depressurization (exhaust fans, tight house), or inadequate combustion air. CO hazard — fix before returning to service.
  • Power-vent won't fire → no power, blower not running, proving switch not closing, blocked termination, or sagging/condensate-filled plastic run.
  • Direct-vent shuts down → blocked intake or exhaust (snow, debris, nest), or a breached chamber seal.
  • Sooting / CO present → combustion-air starvation or a venting problem on a room-air unit. Don't hand it back.
  • Condensation/corrosion in an old chimney after a furnace change → orphaned atmospheric heater in an oversized vent — resize or re-vent.
  • No hot water during a power outage on a power-vent → by design; the blower needs electricity.

Tech tips & gotchas

  • The orphaned-water-heater trap is a real CO risk. Any time a high-efficiency furnace replaces a common-vented furnace, the leftover atmospheric water heater may no longer draft a vent sized for two appliances. Always re-evaluate the vent sizing — don't leave it.
  • Atmospheric units depend on the building. Tight houses, big exhaust fans, and starved mechanical closets all rob draft and can backdraft a natural-draft heater. When draft is marginal, a power-vent or direct-vent unit removes the building dependency.
  • Power-vent = no power, no hot water. Set that expectation, and remember it interlocks like a furnace: prove the blower or the gas won't open.
  • Direct-vent is the safe play for tight/confined spaces. Sealed combustion can't backdraft room air and doesn't fight the house for air. It's the answer when depressurization or a small room is the problem.
  • Always verify draft and check CO on atmospheric and power-vent units after any service. "It lights" isn't "it vents."

Safety / code notes

  • Venting category, material, sizing, and termination follow the appliance/vent listing and the fuel-gas code venting provisions (IFGC §503). Match the vent to the appliance category — never mix materials/methods across categories.
  • Combustion air for room-air (atmospheric/power-vent) units is required and sized per the fuel-gas/mechanical code combustion-air sections — a starved unit sooths and produces CO.
  • A spilling or backdrafting unit is a carbon-monoxide hazard. Confirm draft with a smoke source or analyzer and check CO before returning to service; recommend working CO alarms.
  • When re-venting or orphaning an appliance on a common vent, the vent must be re-sized for the remaining appliance per the venting tables/listing — leaving an oversized, poorly drafting vent is a code and safety violation.