Venting & Combustion Air

Last updated September 24, 2026

A gas water heater needs two things to burn fuel safely: combustion air to feed the flame, and a vent to carry exhaust safely out of the home. How that's set up varies by unit type and, in Colorado, by elevation.

The three vent types

Atmospheric vent units rely on natural draft rising through a chimney-style flue. Power vent units use a fan to push exhaust out, giving more flexibility in routing. Direct vent units draw combustion air from outside rather than the room, which changes what combustion air considerations apply to the space itself.

Why the installation space matters

Whatever vent type is used, the space around the unit needs adequate combustion air, a tightly sealed closet with no air supply can starve an atmospheric or power vent unit of what it needs to burn safely.

Where altitude fits in

Combustion behaves differently at elevation, and that interacts with vent type, the right setup for a Front Range home is not automatically identical to the right setup for a mountain community. This is worth confirming directly with your installer for any of the three vent types.

Signs of a venting problem

Soot around the unit or its vent connection, a persistent smell near the unit, or a pilot that struggles to stay lit can all point to a venting or combustion air issue, worth mentioning specifically to whoever services the unit.

How to spot an obviously undersized combustion air opening

A tightly sealed door with no louvers, vents, or gap at the bottom in a small closet housing a gas unit is worth flagging to a technician, combustion air needs a real path into the space, not just an assumption that some air gets in.

Venting material and condition matter too

Corroded, disconnected, or improperly pitched venting can be as much of a problem as the wrong vent type entirely, a full venting inspection checks the physical condition, not just the category of system.

The Altitude Connection

Venting and combustion air needs interact directly with elevation for any gas unit, see our high-altitude sizing page for the fuller explanation of why that matters in Colorado specifically.

Atmospheric Venting

Traditional gas water heaters use natural draft: hot exhaust rises through a draft hood into a vent connector and then up a chimney or metal vent. The connector should rise at least a quarter inch per foot toward the chimney, be as short and direct as possible, and be securely fastened. The vent must be the right size: too small restricts flow; too large lets exhaust cool and condense.

Orphaned Water Heaters

When a furnace is replaced with a high-efficiency sealed unit that vents through PVC, the water heater may be left alone in a chimney sized for both. This orphaned water heater often has weak draft and condensation problems. Solutions include a properly sized liner, a power-vent water heater or a heat pump unit. This situation is extremely common in older Colorado homes.

Power-Vent and Direct-Vent

Power-vent water heaters use a blower to push exhaust through PVC or other approved pipe, usually out a sidewall. They need an electrical outlet. Direct-vent units draw combustion air from outside and exhaust outside through a sealed concentric or two-pipe system, without a fan in some models. Both avoid chimney issues but require proper termination clearances from windows, doors, grade and snow levels.

Combustion Air

Gas appliances need air to burn. In a large open basement, infiltration may provide enough. In closets or small rooms, combustion air must be supplied through openings to adjacent spaces or ducts to the outside. Tight modern homes, exhaust fans and dryers can depressurize a space and pull exhaust back down a vent.

Spillage and Draft Testing

A spillage test checks whether exhaust escapes at the draft hood when the burner runs, especially with exhaust fans and dryers operating and doors closed. Any spillage is a safety concern and should be corrected.

Altitude and Venting

At altitude, combustion and draft behave differently. Units must be configured for elevation, and vent sizing tables account for it. A CO alarm is essential in any home with gas appliances.

A Typical Colorado Case

In a Park Hill home, a CO alarm sounds intermittently in the basement. The technician finds the water heater is orphaned in a large masonry chimney after a furnace upgrade, and draft reverses when the kitchen range hood runs. A stainless liner sized for the water heater and a makeup-air solution for the range hood resolve the issue.

Questions Homeowners Ask

How do I know if my water heater is backdrafting? Signs include soot, rust at the draft hood or a CO alarm.

Can I close off the combustion-air vent in my closet to reduce drafts? No; it's needed for safe combustion.

Is a power-vent unit noisier? It has a fan, which makes some noise.

Chimney Liners in Older Colorado Homes

Many Colorado homes built before the 1970s have masonry chimneys that were designed for coal or large gas furnaces. When a modern water heater is left alone in one, the chimney may be far too big, and its masonry walls stay cold, especially in winter. Exhaust cools, water vapor condenses, and the acidic condensate can damage mortar and flue tiles over time. A stainless steel or aluminum liner sized for the water heater keeps exhaust warm and draft strong. Liner sizing follows vent tables that account for the appliance's input, the chimney's height and the altitude.

Exhaust Fans, Dryers and Depressurization

Modern Colorado homes are often tighter than older ones, and powerful range hoods, bathroom fans and clothes dryers can pull enough air out of the house to reverse draft in a water heater vent. Symptoms include soot at the draft hood, a pilot that goes out when the dryer runs and CO alarms. Solutions include makeup air for large range hoods, dedicated combustion air for the mechanical room or a sealed-combustion water heater.

Termination Clearances for Sidewall Vents

Power-vent and direct-vent terminations must be kept away from windows, doors, air intakes, gas meters and grade, and above expected snow levels. In Colorado, snow drifts are a real concern; a termination that's buried in a drift will shut down the water heater. Manufacturers' instructions and local codes specify clearances; many installers raise terminations higher than the minimum in snowy areas.

Questions Homeowners Ask

Should I have my chimney inspected? Yes, whenever a furnace or water heater is replaced, and periodically otherwise.

Can a water heater and furnace share a vent? Yes, if the vent is sized for both; problems arise when one is removed.

Venting Summary

Proper venting keeps exhaust out of the home; any doubt deserves a spillage test.

Vent Inspection Checklist

At every service visit, check that the vent connector is secure, sloped upward and free of rust holes, and that the chimney or vent cap is intact and clear of nests or debris.

Venting After a Furnace Upgrade

When an old furnace is replaced with a high-efficiency model vented through the wall, the water heater may be left alone on a chimney sized for both. That oversized flue can draft poorly and condense, so a liner or venting change is often needed.