7.3 Special Piping, Solar Thermal, and Ground-Source Systems

Fuel Oil Storage Tanks and Piping (Chapter 22)

Fuel oil is a common heating fuel in many residential areas, and its storage and piping are governed by Chapter 22 of the 2024 IRC. The code focuses on preventing tank leaks, fire hazards, and environmental contamination.

Tank Locations and Clearances (Section M2201)

• Indoor Tanks: Supply tanks larger than 10 gallons (38 L) must be located at least 5 feet (1,524 mm) from any burner, flame, or other source of heat or ignition. The maximum individual tank capacity is 660 gallons (2,500 L), with an aggregate capacity of 1,320 gallons (5,000 L) per building. Tanks must be installed on noncombustible floors or supported on stable masonry or concrete foundations. Tanks must be supported by steel legs on concrete pads or masonry blocks to prevent settling or tipping.
• Outdoor Aboveground Tanks: Must be located at least 5 feet (1,524 mm) from any adjoining property line.
• Underground Tanks: Must be located at least 1 foot (305 mm) from basement walls, pits, foundations, or property lines. They must be buried at least 1 foot deep (or 2 feet under driveways) and protected from corrosion and damage.

Fill and Vent Piping Requirements

• Fill Pipe: The fill pipe must be a minimum of 2 inches (51 mm) nominal diameter. It must terminate outdoors with a liquid-tight cap. It must slope continuously toward the tank. To prevent environmental contamination, fill pipe connections should be equipped with a spill containment bucket to catch drips during filling.
• Vent Pipe: The vent pipe must be a minimum of 1-1/4 inches (32 mm) nominal diameter. It must enter the top of the tank and extend no more than 1 inch into the tank. The vent must terminate outdoors, at least 2 feet (610 mm) from any building opening, and must be equipped with a weatherproof vent cap. The vent must slope back to the tank to prevent oil from pooling.
• Cross-Connections: Fill and vent pipes must be separate and must not be cross-connected with each other or with other piping.

Piping Materials and Joints

• Materials: Approved fuel oil piping materials include steel pipe, copper tubing, or brass pipe. Copper tubing (Type L or M) is the most common.
• Joints: Piping must be joined using flared fittings, threaded fittings, or welded joints. Solder joints are prohibited on fuel oil lines. In the event of a basement fire, solder would quickly melt, releasing pressurized oil and refueling the fire. Copper tubing lines run across basement floors must be protected by a steel conduit or buried in a protective trench to prevent physical damage.

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Solar Thermal Energy Systems (Chapter 23)

Solar thermal systems use solar collectors to absorb energy and heat water or a heat transfer fluid. These are regulated under Chapter 23 of the 2024 IRC.

Heat Transfer Fluids and Exchangers

• Fluid Toxicity: The toxicity of the heat transfer fluid determines the design of the heat exchanger interfacing with the home's potable water system.
• Double-Wall Heat Exchanger: Required when a toxic (non-food grade) heat transfer fluid (such as ethylene glycol) is used. The heat exchanger must have an atmospheric vent (air gap) between the two walls to detect leaks. If the internal wall fails, the fluid escapes through the vent rather than entering the potable water supply.
• Single-Wall Heat Exchanger: Allowed only when the heat transfer fluid is non-toxic and food-grade (such as USP grade propylene glycol) and the system is permanently labeled to specify the fluid type.
• Fluid Properties: The flash point of the heat transfer fluid must be at least 50°F (28°C) above the maximum system design temperature.

Solar Drainback and Safety Controls

Solar loops can reach high pressures and temperatures during periods of high sun and low heat demand (stagnation).

• Drainback Systems: Solar drainback systems are designed to drain all water or heat transfer fluid from the outdoor collectors into an indoor holding tank whenever the circulator pump shuts off, protecting the collectors from freezing in winter and boiling in summer. To ensure successful drainback, all outdoor piping must be pitched back toward the indoor storage tank at a minimum slope of 1/4 inch per foot. Inspectors must verify there are no traps, sags, or low spots in the piping that could hold water and freeze.
• Pressure Relief: Solar loops must have an ASME-rated pressure relief valve. The discharge must be piped to a safe, visible location, such as a nonpressurized container, to capture any hot fluid.
• Thermal Expansion & Vacuum Relief: An expansion tank is required to accommodate volume changes. A vacuum relief valve is required in drainback systems to prevent atmospheric pressure from collapsing piping when water drains out.

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Ground-Source Heat Pump Loop Piping (Geothermal)

Ground-source (geothermal) heat pump loops exchange heat with the earth to provide energy-efficient heating and cooling. These loops are installed horizontally in trenches or vertically in boreholes.

• Materials: High-Density Polyethylene (HDPE), PEX, or PE-RT. HDPE is the industry standard for ground loops due to its durability, flexibility, and heat transfer characteristics.
• Joining Methods: HDPE loop piping must be joined using heat fusion (butt fusion, socket fusion, or electrofusion) or listed mechanical fittings. Heat fusion creates a continuous, leak-proof plastic joint by heating both the pipe end and the fitting socket to approximately 500°F (260°C) and joining them under pressure. Mechanical joints are prohibited underground unless specifically listed for direct burial.
• Loop Pressure Testing: Before geothermal loop piping is backfilled, buried, or covered, it must be pressure tested hydrostatically. The test pressure must be at least 1.5 times the maximum design pressure, but not less than 100 psi (689 kPa), and must be held for a minimum of 15 minutes with no leaks or pressure drops.