2021 IRC G2427 · IRC Chapter 18
Lesson 8 covered the vent categories and the clearances a termination needs from openings, grade, and nearby walls. This lesson works the other end of the system: the short connector that joins the appliance to the vertical vent, what happens when two appliances share one vent, and the masonry chimney itself. Most of the venting problems GCHS staff actually find in older Gulf Coast houses live in this lesson, not the last one, because a connector or a chimney degrades quietly over years while a factory-built vent usually does not.
The connector is the weakest, coolest, and most exposed part of the venting system, and the code treats it that way. Under IRC G2427.10.8 and M1803.3 it must be installed without dips or sags, as short and straight as possible, and must slope upward toward the vent at not less than 1/4 inch per foot of run, so buoyant flue gas keeps moving up and out instead of pooling in a sag. Clearance to combustibles depends on the connector type: a single-wall metal connector serving a listed gas appliance with a draft hood needs 6 inches of clearance (Table G2427.10.5), a listed Type B double-wall connector uses the clearance on its listing, commonly 1 inch, because its air space does the insulating work, and a single-wall connector serving an oil or solid-fuel appliance needs 18 inches because it runs far hotter and sees creosote (Table M1803.3.4). A single-wall connector is limited to a horizontal run of 75 percent of the vertical vent height above it; a Type B double-wall connector, which loses less heat, is allowed up to 100 percent of that height. Connectors must be firmly supported along their length, not just at the two ends, because an unsupported run sags, loses its slope, and can pull apart at a joint.
Orientation matters as much as slope. The code only requires the joints to be fastened (G2427.10.6), but connector pipe manufacturers' instructions call for the crimped, or male, end to point toward the appliance, the opposite of how a duct joint is usually thought about, so that any condensate running down the inside of the pipe drains back toward the appliance rather than out through the joint onto the framing below. A connector must never pass through a floor or a ceiling (M1803.3.1); the run between stories has to be vent, not connector. It may pass through a wall or partition only where the connector is listed and labeled for wall pass-through, or is routed through a device listed and labeled for that purpose, commonly called a thimble, installed per its listing. A single-wall metal connector is further barred from any wall, floor, or ceiling except a run straight from the appliance room to the outdoors (G2427.10.14, G2427.7.4). Where a connector enters a masonry chimney flue serving gas appliances, it must enter not less than 12 inches above the lowest portion of the flue interior (G2425.9) and must not project so far that it restricts the flue (G2427.10.11), so it should extend to, but not beyond, the inner face of the liner: short of the liner leaves an open gap in the chimney wall, and proud of it restricts the flue and creates a soot ledge.
Where a furnace and a water heater share one Type B vent, both must be Category I appliances designed for non-positive vent pressure; a connector serving a Category I appliance is never connected to a mechanical draft system operating under positive pressure, such as one serving a Category III or IV appliance (G2427.10.4), because positive-pressure, acidic flue gas would leak from and corrode a vent built for neither pressure nor condensate. Common-vented appliances also have to be on the same floor of the dwelling, outside of a system specifically engineered otherwise (M1801.11), because appliances on different floors see different stack pressures and the ordinary sizing tables stop applying. The smaller appliance's connector enters the common vent at the highest available point consistent with clearance and headroom. That rule exists for a concrete reason: a water heater cycling on a mild day has very little draft of its own, and if its connector entered below the furnace's, the furnace's much stronger flue gas could push backward out through the water heater's draft hood and into the room where both appliances sit. Vent sizing for a common system is not a guess; it comes from the code's venting tables, which take the appliance type (draft hood or fan-assisted), each appliance's input rating, the total vent height, and the lateral connector length as inputs.
This is the single most common venting failure GCHS finds during a rehab or a change-out inspection. A furnace and water heater were originally common-vented into one masonry flue or Type B vent sized for both loads together. Years later, the furnace is replaced with a 90-plus percent condensing unit that vents separately in plastic pipe, and the installer simply leaves the existing water heater venting alone into the old, now oversized flue. The water heater alone cannot produce enough heat to warm that much flue area, so the flue gas cools and condenses before it clears the top, draft degrades, and the chimney or vent begins to spill flue gas at the draft hood instead of carrying it outdoors. The fix is not cosmetic: the flue needs to be re-evaluated and relined or resized for the water heater's input alone, using the vent tables, not "whatever fit." IRC G2425.15.1 requires an existing chimney or vent to be resized as necessary to control condensation and provide the required draft. An inspector who sees a recent furnace change-out should always ask what happened to the water heater's vent.
A masonry chimney serving a gas or oil appliance generally needs an approved liner sized for that appliance. Mortar is porous, and unlined flue gas, including carbon monoxide, can migrate through the joints into an attic or an adjacent wall cavity while acidic condensate slowly deteriorates the masonry from the inside. Clearance from the chimney to combustible framing is not less than 2 inches for any portion inside the building or within the exterior wall, and 1 inch where the chimney is entirely outside the exterior walls (R1003.18), because an exterior chimney runs cooler. Where a flue that once served solid or liquid fuel, or any existing masonry flue, is used for gas, its cleanout opening's upper edge must sit not less than 6 inches below the lowest chimney inlet (G2427.5.8, G2425.15.3), creating a sump for soot and debris to collect in rather than piling up against the connector opening itself. A chimney crown and cap that are cracked or missing let water into the flue, and water is the primary destroyer of a masonry chimney: it freezes in the joints, dissolves mortar, and combines with flue gases to form acids that eat the liner from inside.
Masonry chimneys use the "3-2-10" rule: the termination must be not less than 3 feet above the roof where the chimney passes through it, and not less than 2 feet higher than any portion of the building within 10 feet horizontally. Type B and Type L gas vents 12 inches or smaller and located at least 8 feet from a vertical wall may use the code's roof-pitch termination figure (Figure G2427.6.4) instead; a larger vent, or one closer than 8 feet to a wall, must terminate not less than 2 feet above the highest point where it passes through the roof and not less than 2 feet above any portion of the building within 10 feet, the same geometry idea as the masonry rule. Every gas vent terminates with a listed cap or listed roof assembly, never an open pipe end, and never below an eave or parapet where the plume would be trapped and pulled back into a soffit vent or attic space. Where a Type B vent runs through attic insulation, an insulation shield of not less than No. 26 gage sheet metal must hold the insulation back and extend not less than 2 inches above it, so blown insulation cannot drift over the top and remove the air space the vent depends on for its listed clearance.
A worked example on connector length: a water heater sits 9 feet horizontally from a Type B vent whose total height above the connector's entry point is 11 feet. A single-wall connector is limited to 75 percent of that height, or 8.25 feet, so a 9-foot single-wall run does not comply and would need to be shortened or rerouted. Switching to a listed Type B double-wall connector, allowed up to 100 percent of vent height, or 11 feet, brings the same 9-foot run comfortably within the rule.
In the field
On a rehab estimate, GCHS regularly finds a water heater alone on a chimney that once carried a much larger gas furnace. Before pricing the water heater replacement, staff note whether the old furnace is gone and, if so, flag the flue for re-evaluation rather than assuming the existing vent is still correctly sized. A licensed plumber or HVAC contractor confirms the liner size and condition; GCHS's own scope of work is to identify the issue and get it into the estimate, not to reline the chimney itself.
1. A vent connector must slope toward the vent at not less than:
2. The required clearance to combustibles for a single-wall metal connector serving a listed gas appliance with a draft hood is:
3. A Type B double-wall vent connector is allowed a maximum horizontal length of what percentage of the vertical vent height above it?
4. Manufacturers' instructions for vent connector pipe call for installing it with the crimped (male) end pointed:
5. A furnace and a water heater are proposed for one common Type B vent, with the furnace in the basement and the water heater on the first floor. This is:
6. On a common vent serving a furnace and a water heater, the smaller appliance's connector should enter the vent:
7. After a furnace is replaced with a condensing unit that vents separately in plastic pipe, the existing water heater is left on the old shared masonry flue. The concern is:
8. Under the "3-2-10" rule, a masonry chimney must terminate:
9. The required airspace from a masonry chimney located inside the building to combustible framing is at least:
10. For an existing masonry chimney flue used for gas appliances, a cleanout opening's upper edge must be located:
11. A Type B or Type L gas vent must terminate not less than how far above the highest connected appliance draft hood or flue collar?
12. Where a Type B vent passes through attic insulation, the code requires an insulation shield that:
13. An inspector finds a vent connector installed with a single screw at each end and no support in between over a 6-foot run. The concern is:
14. A vent connector is run from a basement water heater, up through the first floor, to a vent located in the attic. The clearest problem is: