12.1 Gas Water Heaters, Furnaces & Boilers: Installation Rules

12.1 Gas Water Heaters, Furnaces & Boilers: Installation Rules

Quick Answer: IPC 2018 Section 504.6 governs temperature and pressure (T&P) relief discharge piping. The line must be full size, must not be trapped, must flow by gravity, must have no valves or tee fittings, must have no threaded connection at the outlet end, must not connect directly to the drainage system, and must terminate not more than 6 inches above -- and not less than two discharge-pipe diameters above -- the floor or the flood level rim of the waste receptor.

Gas Appliances and Piping is 40 of the 100 scored questions, and Maryland licenses this trade as a combined Journey Plumber/Gas Fitter, so appliance work is squarely in scope. Setting a gas water heater draws on two open-book references at once: the International Plumbing Code (IPC) 2018 for the water side, and the International Fuel Gas Code (IFGC) 2018 for the gas and combustion side.

The Rule That Outranks the Code

IFGC 305.1 decides most field arguments. Appliances are installed per the terms of their approval, the conditions of the listing, the manufacturer's instructions and the code -- and where a code provision is less restrictive than the listing or the instructions, the listing and the instructions govern. Those instructions must be on the job site at the time of inspection.

The nameplate required by IFGC 301.5 carries what you install to: manufacturer, model and serial number, the hourly input rating in Btu/h, the fuel type approved for the appliance, and the minimum clearances. IFGC 301.7.1 forbids converting an appliance to a different fuel gas unless complete conversion instructions come from the installation instructions, the serving gas supplier or the manufacturer.

Water Heater Safety Devices (IPC Section 504)

A storage water heater operating above atmospheric pressure needs an approved, self-closing (levered) pressure relief valve and temperature relief valve, or a combination T&P valve, conforming to ANSI Z21.22 (504.4). The valve is installed in the shell of the tank, and the temperature element must be located so it is actuated by the water in the top 6 inches of the tank (504.4.1). There shall be no check valve or shutoff valve between the relief valve and the tank it serves, and the relief valve may never be used to control thermal expansion.

The 504.6 Discharge-Piping Checklist

The discharge line is an extension of the potable water system, so it terminates through an air gap in the same room as the water heater and is never hard-piped into the drainage system. It serves one relief device only. It discharges to the floor, to the water heater pan, to a waste receptor or to the outdoors, in a manner that causes no personal injury or structural damage, at a point readily observable by the building occupants. Materials are those listed in IPC Section 605.4 or tested and approved to ASME A112.4.1.

Worked Example -- Sizing and Terminating the Discharge Line

A residential heater has a 3/4-inch relief valve outlet.

• Minimum discharge pipe: 3/4 inch, full size to the air gap (504.6, item 3).
• If the line is run in tubing with insert fittings, go up one nominal size to 1 inch, and fasten the outlet end in place (item 14). Insert fittings shrink the inside diameter and choke a full-trip discharge.
• Termination window above a concrete floor: not less than 2 x 0.75 = 1.5 inches and not more than 6 inches (item 10).
• No threads on that outlet end (item 11) and no valve or tee anywhere in the run (item 12).

Thermal Expansion on a Closed System (IPC 607.3)

A check valve, pressure-reducing valve or backflow preventer in the water supply turns the building into a closed system: heated water can no longer push back into the main, so tank pressure climbs until the T&P valve weeps. IPC 607.3 requires a thermal expansion control device connected to the water heater cold water supply pipe, downstream of all check valves, pressure-reducing valves and backflow preventers. It is sized per the tank manufacturer's instructions and so that distribution pressure never exceeds the limit in IPC 604.8 -- 80 psi static. A dripping T&P valve on a system with a new PRV is an expansion problem, not a bad relief valve.

The Drain Pan (IPC 504.7)

Where leakage from a storage tank will cause damage, the tank sits in a pan of galvanized steel or aluminum not less than 0.0236 inch thick, plastic not less than 0.036 inch thick, or another approved material. A plastic pan shall not be installed beneath a gas-fired water heater. The pan is not less than 1-1/2 inches deep and is drained by an indirect waste pipe not less than 3/4 inch in diameter (504.7.1). The pan drain extends full size to an indirect waste receptor or floor drain, or to the exterior terminating not less than 6 inches and not more than 24 inches above the adjacent ground surface (504.7.2). Where no pan drain existed before, a replacement water heater does not have to have one added.

Location, Ignition Sources and Vehicle Impact

IFGC 305.3 requires appliances with an ignition source in hazardous locations and in public, private, repair, parking and motor fuel-dispensing garages to be elevated so the source of ignition is not less than 18 inches above the floor. The measurement is taken to the lowest ignition source, not to the base of the appliance, because gasoline vapor is heavier than air and pools at the floor. Rooms outside the living space that communicate directly with a private garage through openings count as part of the garage.

• Exception: elevation is not required for appliances listed as flammable vapor ignition resistant (FVIR).
• 305.3.1: an appliance in a residential garage may sit at floor level if it is in a separate enclosed space accessed only from outside the garage, with combustion air taken from the exterior.
• 303.4: appliances shall not be installed where subject to vehicle impact damage unless protected by an approved means -- bollards, curbs or a barrier.
• 305.5: appliances in private garages are installed with 6 feet of clearance above the floor unless they are impact-protected and comply with 305.3. In public garages (305.4) the figure is 8 feet.

Furnaces and Boilers

Central-heating furnaces and low-pressure boilers are installed to the clearances in the manufacturer's instructions (IFGC 308.4.1), reduced only per Table 308.2 and only where the instructions allow it (308.4.2). Front clearance must be sufficient to service the burner (308.4.3), and supply ducts carry the plenum clearance for the first 3 feet from the plenum (308.4.5). Each appliance needs a shutoff valve in the same room and within 6 feet, upstream of the union or connector (IFGC 409.5.1), plus an electrical disconnect per NFPA 70.

Condensing (Category IV) equipment adds a drainage problem. IFGC 307.2 requires liquid combustion byproducts to be collected and discharged to an approved plumbing fixture or disposal area in corrosion-resistant piping not smaller than the appliance drain connection, sloped not less than 1/8 unit vertical in 12 units horizontal (1 percent). Condensate waste and drain lines are not less than 3/4-inch internal diameter and never decrease in size (307.3). A Category IV appliance gets an auxiliary drain pan where a stoppage would damage the building, unless the appliance shuts down on stoppage (307.5), and a condensate pump in an attic or crawl space must be interlocked to stop the appliance if the pump fails (307.6).

Boilers must be listed to ANSI Z21.13/CSA 4.9 or UL 795 (IFGC 631.1) and installed per the manufacturer's instructions and the International Mechanical Code (631.2), with all controls set, adjusted and tested by the installer, a control diagram and operating instructions furnished, and the rating nameplate attached. On the mechanical side, every steam boiler gets a safety valve and every hot water boiler a safety relief valve (IMC 1006.1 and 1006.2), installed directly into the relief opening with no valve on either side and discharging by gravity (1006.5). Steam and hot water boilers are protected by a low-water cutoff that stops combustion when the water drops below the lowest safe level (IMC 1007), except for coil-type and water-tube boilers requiring forced circulation that are protected by a flow-sensing control.

Worked example -- condensate slope. A 90 percent furnace drains 24 feet to a laundry standpipe. At 1/8 inch per foot, the line must fall 24 x 0.125 = 3 inches over that run. Fall less than that and the trap holds water in the heat exchanger.

Attics, Crawl Spaces and Roofs

An appliance you cannot reach is an appliance that never gets serviced, so IFGC Section 306 sets hard dimensions. Note that the attic and under-floor access openings differ by two inches, and that IPC 502.3 repeats the attic rule for water heaters.

Both locations require a luminaire controlled by a switch at the required passageway opening and a receptacle outlet at or near the appliance, wired per NFPA 70 (306.3.1, 306.4.1). The passageway and service space are not required where the appliance can be serviced and removed through the opening itself. Where a crawl-space passageway or service space runs more than 12 inches below adjoining grade, the walls must be lined with concrete or masonry extending 4 inches above grade. On roofs, an appliance that requires climbing higher than 16 feet above grade needs a permanent interior or exterior means of access -- portable ladders do not count (306.5).

Regulators, Manifold Pressure and Startup

A line pressure regulator is installed where the appliance is designed to operate at a lower pressure than the supply, must be listed to ANSI Z21.80, must be accessible and protected from physical damage, and must be approved for outdoor use if mounted outside (IFGC 410.1). Regulators requiring a vent are vented directly to the outdoors, screened against insects, water and foreign objects (410.3). Where the serving gas supplier delivers gas at more than 2 psi to a system feeding appliances designed for 14 inches water column or less, IFGC Section 416 requires an overpressure protection device that limits pressure to 2 psi or less if the line regulator fails wide open.

Downstream of all that sits the appliance regulator, usually built into the combination gas valve, which sets the manifold pressure at the burners. Typical settings are about 3.5 inches water column for natural gas and 10 to 11 inches water column for propane, but the rating plate is the authority -- two-stage equipment carries a separate low-fire figure. Startup is not finished until you verify the actual input rating by clocking the gas meter and comparing the result with the nameplate Btu/h.

Exam Trap: Candidates who memorize "6 inches" for the relief discharge often answer that the line must terminate within 6 inches of the floor, and then pick an option showing the pipe stubbed 1/2 inch off the slab. IPC 504.6 item 10 is a window, not a ceiling: not more than 6 inches above the floor and not less than two discharge-pipe diameters above it. A 3/4-inch line stopped 1/2 inch off the floor violates the lower limit and can be blocked by debris or standing water.