Does a Solar Thermal System Need Pressure Relief or Temperature Controls? (IRC 2018)

Quick Answer

Yes. IRC 2018 Section M2301.2 requires solar thermal systems to include pressure-relief protection and temperature controls. Every pressurized solar thermal system must have a pressure-relief valve rated for the maximum operating pressure of the collector and piping. Temperature controls are required to prevent the stored water from scalding and to protect system components from stagnation damage during periods of high solar gain and no load.

What M2301.2 Actually Requires

IRC 2018 Section M2301.2 requires that solar thermal systems include the following safety controls and devices: a pressure-relief valve on the collector circuit set at or below the maximum working pressure of the weakest component in that circuit, an expansion tank sized to accommodate the thermal expansion of the heat transfer fluid over the operating temperature range, high-temperature protection for the storage tank such that water delivered to fixtures does not exceed 120°F (or as required by P2903.7 for scald protection), and a differential temperature controller that manages pump operation to prevent overheating.

The pressure-relief valve must discharge to an approved location — a floor drain, indirect drain, or exterior terminus — not into an occupied space or near electrical equipment. The discharge pipe must be as short and straight as possible so that back-pressure does not prevent the valve from fully opening. The relief valve must be listed for the operating temperature and pressure of the system.

For stagnation protection — the condition that occurs when the pump stops (power failure, controller shutdown) and the collectors continue to absorb solar energy — M2301.2 requires that the collector and piping be rated for stagnation temperatures. Flat-plate collectors can reach 300°F to 400°F at stagnation; evacuated tube collectors can reach even higher temperatures. The heat transfer fluid must be rated for stagnation temperatures, and the expansion tank must be sized to accommodate the volume change if the fluid boils partially to steam (a condition called steam dumping).

The differential temperature controller that runs the circulation pump must include high-temperature limit logic that shuts off the pump when the storage tank reaches a maximum set temperature (typically 180°F). This prevents overheating of the storage tank and protects the heat exchanger and stratification layers in the tank.

The pressure relief valve required by IRC 2018 Section M2301.2 for solar thermal collectors must be sized to relieve the maximum stagnation pressure that can develop when the collectors are exposed to full sun with the pump off. Stagnation conditions occur during power outages, pump failures, or when the system is shut down for maintenance in summer. Under stagnation, the collector temperature can reach 300 to 400 degrees F depending on the collector type. If the heat transfer fluid vaporizes under stagnation, system pressure can spike rapidly. The PRV must be set at a pressure below the working pressure rating of the weakest component in the system, typically the collector glazing seals or the storage tank inlet fittings. SRCC collector ratings specify maximum operating pressure and stagnation temperature for each listed collector model; the PRV setting must be appropriate for the specific collector's stagnation characteristics as documented in the SRCC rating sheets provided with the permit application.

Why This Rule Exists

Solar collectors are uniquely capable of generating extremely high temperatures — far higher than any other residential water heating device. Without pressure relief, a stagnating collector can pressurize a closed circuit to levels that rupture piping or blow fittings. Without temperature controls, a storage tank can reach scalding temperatures that cause severe burns at fixtures, or the tank itself can be damaged by prolonged exposure to extreme heat. The required safety devices provide automatic protection against both overpressure and overtemperature without requiring any action from the homeowner.

This requirement reflects the fundamental principle of the IRC that electrical and mechanical systems must be installed in a manner that protects occupants over the life of the building, not just at the moment of installation. Proper installation documented at inspection provides future owners and service technicians with confidence that the system was built to code, reducing liability and preventing disputes about pre-existing conditions.

What the Inspector Checks at Rough and Final

The inspector will verify at final inspection: a pressure-relief valve is installed on the collector loop, rated for the maximum working pressure of the system; the discharge pipe is routed to an approved location and is not blocked; an expansion tank is present, properly sized, and pre-charged to the correct pressure; the differential temperature controller is installed and wired with high-temperature limit logic; the storage tank has a temperature-and-pressure relief valve (if it is a pressurized tank); and the system has been pressure-tested prior to inspection. The inspector may ask for the manufacturer's documentation showing the expansion tank sizing calculation.

What Contractors Need to Know

Size the expansion tank for the maximum temperature differential the system will experience — from cold fill temperature (typically 40°F in northern climates) to stagnation temperature (which can exceed 300°F). Use the collector manufacturer's data sheet to determine the stagnation temperature rating. Over-sizing the expansion tank slightly is preferable to under-sizing; an undersized tank causes the relief valve to open frequently, wasting fluid and causing pressure cycling fatigue on fittings.

Set the high-temperature controller limit on the storage tank at 160°F to 180°F to prevent scalding and tank damage. Install a thermostatic mixing valve downstream of the storage tank to deliver water at the fixture-safe temperature (typically 120°F) regardless of storage tank temperature. This separation of storage temperature from delivery temperature allows the tank to be kept at a higher temperature for Legionella control (above 140°F) while still protecting against scalding.

For drain-back solar thermal systems, stagnation protection is inherent because there is no fluid in the collectors to overpressure the system when the pump stops. Drain-back systems are popular in cold climates because they also provide freeze protection without antifreeze fluid. However, drain-back systems require careful system design to ensure complete drainage. Any low points in the collector loop that retain fluid can freeze or flash to steam under stagnation. For pressurized closed-loop antifreeze systems common in freeze-prone climates, the expansion tank must be sized for both the normal thermal expansion of the fluid and for potential steam formation under stagnation. An undersized expansion tank allows pressure to exceed the PRV setpoint repeatedly, causing fluid loss and system degradation over time. Size the expansion tank to maintain adequate pre-charge pressure at the highest anticipated operating temperature per the manufacturer's published sizing charts.

What Homeowners Get Wrong

Homeowners sometimes assume that because they have a pressure-relief valve, they do not need an expansion tank — or vice versa. These two devices serve different functions: the relief valve is a last-resort safety device that opens when pressure exceeds the set point; the expansion tank is the normal operating device that accommodates fluid volume changes and prevents the relief valve from opening during routine thermal cycles. If the expansion tank fails (waterlogged or air charge depleted), the relief valve will open daily, wasting glycol and creating a maintenance problem. Check the expansion tank pre-charge annually.

State and Local Amendments

IRC 2018 states — TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO — follow M2301.2 pressure and temperature control requirements without significant modification. Southern states with high solar insolation may see more stagnation events due to the frequency of high-heat, low-demand periods (summer vacations, for example). Contractors in these regions should size expansion tanks conservatively and specify heat transfer fluids with high stagnation temperature ratings.

IRC 2021 added more explicit requirements in M2301.2 for stagnation management, including cross-references to collector listing data for stagnation temperature ratings. The 2018 edition addressed these concerns through the general requirement for listed equipment and manufacturer's instructions, but 2021 made the stagnation provisions more explicit.

When to Hire a Licensed Solar Thermal Contractor

Sizing pressure-relief valves and expansion tanks for solar thermal systems requires knowledge of thermodynamic principles and fluid expansion calculations that go beyond basic plumbing skills. An NABCEP-certified solar thermal installer or a licensed mechanical contractor with solar thermal training should perform this work. The consequences of an undersized relief valve or an oversized expansion tank can range from nuisance (frequent valve opening) to catastrophic (ruptured piping or scalding). Always permit and inspect solar thermal systems.

Common Violations Found at Inspection

• No pressure-relief valve on the collector loop — system lacks overpressure protection
• Relief valve discharge pipe routed into an occupied space or near electrical equipment
• Expansion tank missing or undersized — relief valve opens during every thermal cycle
• Expansion tank pre-charge not set to system design pressure at cold fill
• No high-temperature limit on the storage tank controller — tank can reach scalding temperatures
• No thermostatic mixing valve at the storage tank outlet — hot water delivered at storage temperature directly to fixtures
• Relief valve rated for a pressure higher than the maximum working pressure of the collector — not protective of the weakest component
• Stagnation temperature of the collector not verified against the heat transfer fluid rating — fluid degrades or boils unexpectedly