Chimneys, direct vent and wall terminations

The Kingsway condensing water heaters vent through the roof. Their polypropylene vent and intake pipes rise the full height of the building through a fire-rated shaft beside the gas riser and end in a listed termination kit on the roof, with the exhaust and intake separated exactly as the instructions show. On a smaller building, the same heaters could have vented out of a side wall a short distance away. "Where the vent ends matters as much as how it gets there," Dana told Maya on the roof. "Put an exhaust next to a window, an air intake or a gas regulator, and you bring the flue gases back inside."

This lesson follows the vent outside: masonry and factory-built chimneys, liners, vertical terminations above roofs, direct-vent and power-vent terminations through walls, the clearances that keep flue gases away from openings and air intakes, and the problems of snow, frost, wind and condensate plumes.

Masonry chimneys and liners

Many older buildings vent gas appliances into masonry chimneys. A masonry chimney can serve a Category I gas appliance only if it is lined with a liner suited to gas flue gases, is in good condition, is the right size for the appliance or appliances connected to it, and is clean and unobstructed. Clay tile liners are common in older chimneys, but cracked, missing or misaligned tiles, deteriorated mortar and debris at the base make the chimney unsafe. A chimney serving a gas appliance must have a cleanout below the lowest connector so that debris can be removed.

Exterior masonry chimneys, built on an outside wall, are cold for much of the year. The flue gases cool quickly, the draft is weak and condensation forms, which is why modern codes and appliance instructions often require a listed metal liner of the correct size in such chimneys, especially for mid-efficiency fan-assisted appliances and orphaned water heaters. The liner, usually aluminum or stainless steel depending on the listing, is sized from the code tables, installed with its listed connector fittings and cap, and sealed at the top.

Factory-built chimneys, listed systems of insulated metal sections, serve appliances that need a chimney rather than a gas vent, such as some solid-fuel and oil appliances, and are installed exactly as listed. Type B gas vent is not a chimney and must not serve solid-fuel or oil appliances. A chimney that serves a fireplace or wood stove must never also serve a gas appliance.

Vertical terminations above the roof

A vent or chimney must extend high enough above the roof that wind does not push the flue gases back down, that snow does not block it, and that the gases disperse away from the building. The codes set minimum heights. For masonry chimneys, a common rule is that the chimney extends at least 3 ft above the highest point where it passes through the roof and at least 2 ft higher than any part of the building within 10 ft. Type B gas vents with listed caps follow their own table of minimum heights, which increase with roof pitch, and must be at least a set distance from any vertical wall.

Category IV vents through the roof terminate with the listed termination kit or caps the manufacturer specifies, with the exhaust and intake separated by the distances in the instructions so that the intake does not draw in the exhaust. Terminations must be above the expected snow level, which on flat roofs can be well above the membrane. Kingsway's termination stands on a curb high enough to stay clear of drifting snow along the parapet.

Wind can cause downdrafts around tall buildings and parapets, and a vent that ends in an eddy behind a parapet or beside a taller wall may not draw properly. The rules about height above the roof and distance from walls are minimums; on a complicated roof, the designer may extend the vent or move it to keep it out of turbulent air.

Sidewall terminations

Mechanical-draft appliances, power-vented and direct-vent, can terminate through a side wall instead of a roof, which is convenient in houses and low buildings. Because the flue gases leave close to people, windows and air intakes, the codes set clearances from the termination to building features. The required distances depend on whether the appliance is direct-vent or a power-vented appliance using room air, and on its input.

In the United States, NFPA 54 and the IFGC set the clearances. For a direct-vent appliance through a wall, the termination must be at least 6 inches from any door, window or gravity air inlet for appliances up to 10,000 Btu/h, 9 inches for 10,001 to 50,000 Btu/h, and 12 inches above 50,000 Btu/h, with the bottom of the termination at least 12 inches above grade. A mechanical-draft vent that is not direct-vent must be at least 4 feet below or beside, or 1 foot above, any door, window or gravity air inlet, at least 3 feet above any forced air inlet within 10 feet, at least 12 inches above grade, and at least 7 feet above a public walkway where it could be a nuisance. The utility often adds a clearance from the meter and regulator, commonly 3 feet. The manufacturer's instructions may require more.

Other practical rules come from experience. Terminations are kept away from inside corners, where flue gases swirl; away from areas where vehicles park, where exhaust can damage paint and plume over people; away from soffit vents, which can draw flue gases into the attic; and away from neighboring properties, where the plume becomes a neighbor's problem. Condensing appliances produce a visible plume of water vapor in cold weather and can drip condensate or form ice below the termination, so terminations are not placed over walkways, doors, gas meters or air conditioning equipment.

# Check a proposed sidewall vent termination against a clearance list
# Values follow the US fuel gas code rules for a direct-vent appliance over
# 50,000 Btu/h; the adopted code and the instructions govern.
required_in = {"window or door": 12, "forced air inlet within 10 ft (above it)": 36,
               "above grade": 12, "above public walkway": 84}
proposed_in = {"window or door": 18, "forced air inlet within 10 ft (above it)": 24,
               "above grade": 24, "above public walkway": 96}
for item, need in required_in.items():
    have = proposed_in[item]
    flag = "ok" if have >= need else "TOO CLOSE"
    print(f"{item:42s} need {need:3d} in, have {have:3d} in  {flag}")

window or door                             need  12 in, have  18 in  ok
forced air inlet within 10 ft (above it)   need  36 in, have  24 in  TOO CLOSE
above grade                                need  12 in, have  24 in  ok
above public walkway                       need  84 in, have  96 in  ok

The check flags the forced air inlet: the termination is within 10 feet of it but only 24 inches above it, short of the 36 inches required. Moving the termination one stud bay further along the wall, beyond 10 feet from the inlet, solved it. Running the list for every termination before drilling the wall is much cheaper than patching a hole and moving the vent after the inspector finds the problem.

Snow, frost and condensate

In cold climates, snow can bury a sidewall termination, blocking the exhaust and intake and shutting the appliance down or, worse, causing it to recirculate flue gases. Terminations are located above the expected snow level, which may be well above the minimum height above grade in snowy regions or where snow drifts or is piled by plows. Owners should be told to keep terminations clear after storms.

Frost and ice form on and below terminations of condensing appliances, because the water vapor in the plume freezes on cold surfaces. Ice can build up on the termination itself, restricting it, or form on walkways and steps below, creating a slip hazard. The instructions give the termination options and distances that limit these problems, and some manufacturers offer extension kits that push the plume away from the wall.

Condensate inside the vent drains back to the appliance's condensate trap and from there to a drain, often through a neutralizer that raises the pH of the acidic condensate before it enters the drainage system, as required by many plumbing codes and municipalities. Condensate lines must be protected from freezing and installed with the air gap or trap arrangement the instructions and plumbing code require, which brings the plumber's skills back into the venting picture.

Inspecting terminations at start-up and service

At start-up, the technician confirms that every termination matches the instructions: the right kit, the right orientation, screens where required, intake and exhaust separated correctly, joints sealed, the termination secure and the clearances met. On service calls, the terminations are checked for blockage by snow, ice, leaves, insect nests or bird nests, for damage and for changes around them, such as a new deck, a new window or an air conditioner installed under the plume.

Building changes are a common source of new venting hazards. A homeowner encloses a porch around an existing termination, a neighbor builds close to the property line, or a renovation adds a fresh-air intake beside the furnace exhaust. A fitter who notices a termination that no longer meets its clearances points it out in writing, because the appliance that was safe when installed may not be safe now.

Vents through fire-rated shafts and long runs

In tall buildings, vents for central equipment often run long distances, as Kingsway's do. Long Category IV runs are allowed only up to the maximum lengths and numbers of elbows in the manufacturer's instructions, which depend on the pipe diameter and the appliance input; exceeding them reduces the burner's air and can cause poor combustion or nuisance shutdowns. Where the vent passes through floors, it runs in a fire-rated shaft or through listed firestop systems, because a vent opening in a floor would otherwise let fire and smoke travel between floors.

Plastic vents in shafts and ceiling spaces must also meet the building code's limits on combustible materials in those spaces, which some manufacturers address with listed vent materials rated for the location. The designer, the gas fitter and the fire protection reviewer agree on the details before installation. On Kingsway, the vent material, its support spacing, its expansion joints and the firestopping at each floor were all submitted for review and inspected floor by floor as the shaft was closed.