What Equipment Is Required for Solar Thermal Piping and Storage Tanks? (IRC 2018)

Quick Answer

IRC 2018 Section M2301.2 requires solar thermal systems to include a circulation pump, differential temperature controller, expansion tank, pressure-relief valve, and a listed storage tank. Storage tanks must comply with the applicable standard for the fluid being stored — potable or non-potable — and must be sized to store the daily solar energy production efficiently. All piping and components must be compatible with the heat transfer fluid and operating temperature range of the system.

What M2301.2 Actually Requires

IRC 2018 Section M2301.2 establishes equipment requirements for solar thermal systems through a combination of explicit requirements and cross-references to manufacturer's installation instructions and applicable equipment standards. The section requires that the solar thermal system include all components necessary for safe and efficient operation as documented in the system design submitted with the permit application.

The circulation pump must be listed and rated for the heat transfer fluid (glycol or water) at the maximum operating temperature. Variable-speed pumps are increasingly common because they allow the controller to optimize flow rate based on solar insolation; fixed-speed pumps are simpler and acceptable under IRC 2018. The pump must be located in the cool section of the collector loop — on the return side where fluid temperature is lowest — to extend pump life and avoid vapor lock.

The storage tank must be constructed of materials compatible with the fluid it contains. For potable solar water heating systems, the tank must comply with the NSF/ANSI 61 standard for contact with drinking water. Glass-lined steel tanks are the most common choice; stainless steel tanks are used in high-quality systems. The tank must be sized to store one to two days of solar energy production, which for a typical two-person household with a two-collector system is approximately 80 to 120 gallons. An undersized tank saturates early in the day, reducing collector efficiency; an oversized tank delivers tepid water due to excessive dilution.

The heat exchanger — which transfers heat from the glycol collector loop to the potable water in the storage tank — must be double-wall construction if the heat transfer fluid is toxic or if there is any risk of cross-contamination with the potable water supply. Propylene glycol systems often use double-wall heat exchangers as a best practice even though propylene glycol has low toxicity. Drainback systems using water in the collector loop may use single-wall heat exchangers in jurisdictions that permit them.

All piping must be compatible with the heat transfer fluid and temperature range. Copper is the standard choice; CPVC and some plastics are not suitable for the operating temperatures involved. Solder used on copper fittings must be rated for the operating temperature — use lead-free solder with a high silver content for collector loop joints exposed to elevated temperatures.

IRC 2018 Section M2301.2 requires that all components of a solar thermal system including collectors, storage tanks, pumps, heat exchangers, controls, expansion tanks, and pressure relief valves be listed or approved for their intended service. Storage tanks for solar thermal systems must be rated for the temperature and pressure range of the system. Standard water heater tanks rated at 150 degrees F maximum storage temperature are not appropriate for solar thermal storage, where temperatures may reach 180 to 200 degrees F under high solar input conditions. Solar storage tanks must be rated for at least 200 degrees F service temperature and must have an appropriate temperature and pressure relief valve set to relieve below the tank's rated temperature and pressure. The relief valve discharge pipe must terminate at a visible, safe location where it will not create a scalding hazard if the valve activates during stagnation or other high-temperature conditions in the system.

Why This Rule Exists

A solar thermal system without proper equipment — undersized storage tank, missing expansion tank, incompatible heat exchanger — will either fail prematurely or fail to deliver the promised energy savings. More critically, missing safety equipment (pressure relief, high-temperature limit) creates hazards. The equipment requirements in M2301.2 define the minimum complete system that will perform safely and reliably over the intended 20-plus-year service life.

What the Inspector Checks at Rough and Final

The inspector will review the permit application for a system schematic identifying all components. At rough inspection, all major equipment should be on-site and positioned for verification. At final inspection, the inspector will check: pump installed on return side, storage tank listed and sized per design, heat exchanger type (single or double wall) appropriate for the fluid, expansion tank installed and pre-charged, pressure-relief valve present and properly routed, differential temperature controller installed and wired, all piping insulated, and system pressure-tested prior to inspection. The inspector may also check glycol concentration and verify that the heat transfer fluid matches the type specified in the permit.

What Contractors Need to Know

Match the storage tank volume to the collector area using the rule of thumb of 1.5 to 2 gallons of storage per square foot of collector area. For a two-collector system with 64 square feet of absorber area, this yields 96 to 128 gallons of storage — a 120-gallon solar storage tank is the typical choice. Size the heat exchanger area to achieve a heat exchanger effectiveness of at least 0.7 — undersized heat exchangers reduce system performance. Use a high-limit aquastat on the storage tank set at 180°F to prevent overheating.

For the expansion tank, calculate the expansion volume for the temperature range from cold fill (40°F or local design minimum) to stagnation temperature (from the collector data sheet, typically 300°F to 400°F). Use the expansion tank manufacturer's sizing tool or consult the glycol manufacturer's volume expansion tables. Add 10 to 15% margin to the calculated volume.

When selecting a solar thermal storage tank, verify that the tank is rated for the maximum inlet temperature from the solar collectors and that the tank's internal coating or lining material is compatible with the heat transfer fluid. For systems using propylene glycol heat transfer fluid with an indirect heat exchanger, the storage tank is a standard domestic water tank on the potable water side. For direct-circulation systems where pool water or potable water passes directly through the collector, the tank must be compatible with the water chemistry of the connected system. Consult the tank manufacturer's specification sheet for the maximum operating temperature, pressure, and compatible heat transfer fluids before specifying the storage tank. Installing an incompatible tank such as a glass-lined water heater tank that is not rated for solar service temperatures creates a warranty violation and a safety risk if the tank fails under high-temperature operating conditions.

What Homeowners Get Wrong

Homeowners frequently assume that a larger storage tank is always better. In practice, an oversized tank stores more cold water in the morning, which the collector must heat before the tank temperature rises — reducing the useful solar fraction. The other common error is neglecting the heat exchanger condition over the system life. Heat exchangers can develop scale deposits that reduce effectiveness by 20 to 30% over 10 years. Annual inspection and periodic descaling are necessary to maintain performance.

State and Local Amendments

IRC 2018 states — TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO — follow M2301.2 equipment requirements without significant modification. Georgia and North Carolina have state energy codes and utility programs that specify minimum solar storage volume and collector area for systems claiming energy credits or rebates. Florida, while not on IRC 2018, has the most specific solar thermal equipment requirements in the country; systems installed in Florida must comply with Florida Building Code Chapter 13, which is more detailed than the IRC.

IRC 2021 added specific language about pump placement (return side requirement made explicit) and expanded the cross-references to NSF/ANSI 61 for potable storage tanks. These requirements were implied in IRC 2018 but not as directly stated.

When to Hire a Licensed Solar Thermal Contractor

Solar thermal system design — sizing the storage tank, heat exchanger, expansion tank, and pump — requires training in solar thermal engineering. An NABCEP-certified solar thermal installer or a licensed mechanical engineer experienced in solar thermal design is appropriate for residential systems. The 20-plus-year service life of a properly designed solar thermal system makes getting the equipment specifications right at installation critically important. Undersized or mis-specified components are expensive to replace once installed.

Common Violations Found at Inspection

• Storage tank not listed to NSF/ANSI 61 for potable water service — uses a non-potable tank on the domestic hot water side
• Single-wall heat exchanger used with a toxic heat transfer fluid (ethylene glycol) — cross-contamination risk
• Pump installed on the supply side (hot side) of the collector loop instead of the return side — reduced pump life
• Expansion tank missing or sized only for normal operating temperature, not stagnation temperature
• No high-temperature limit on storage tank controller — tank overheats during extended sunny periods with low hot water demand
• Undersized storage tank saturates before noon on clear days — collector efficiency reduced by overtemperature cutout
• Piping joints made with standard 50/50 solder — inadequate for operating temperatures on collector loop
• No provision for draining and purging the system for maintenance or freeze emergency