15.3 Solar Water Heating Systems

Solar Water Heating Systems

Content Area G, Solar, is 5 percent of the Florida trade examination. It is a small slice, but Florida takes it seriously enough to put a dedicated reference on the Plumbing Contractor list: R145, the Florida Solar Energy Center Solar Thermal Manual: Solar Water and Pool Heating Manual, 2006, published by FSEC at Cape Canaveral — and, like the onsite sewage rule, it is one of the two references the department allows you to bring in as a bound PDF. A candidate who brings it tabbed can look up nearly every solar question.

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1. Which Code Governs

Two plumbing-code sections point the same direction, and knowing the numbers keeps you out of Section 505:

• FPC Section 601.2, Solar Energy Utilization: solar energy systems used for heating potable water, or using an independent medium for heating potable water, shall comply with the applicable requirements of the code.
• FPC Section 502.1: solar thermal water heating systems shall conform to the Florida Building Code, Mechanical.
• FPC Section 612.1, Solar Systems: "The construction, installation, alterations and repair of systems, equipment and appliances intended to utilize solar energy for space heating or cooling, domestic hot water heating, swimming pool heating or process heating shall be in accordance with the Florida Building Code, Mechanical."
• FBC-Mechanical Chapter 14, Solar Systems, then carries the substance in Sections 1401 through 1404.

[!WARNING] FPC Section 505 is Insulation, not solar. Chapter 5 of the plumbing volume runs 501 General, 502 Installation, 503 Connections, 504 Safety Devices, 505 Insulation — and 505.1 is a single sentence requiring unfired hot water storage tanks to be insulated to R-12.5. Solar lives at FPC 612 and FBC-M Chapter 14. Older prep material that cites "FPC 505" for solar is using superseded numbering.

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2. System Configurations

Florida's climate removes the hard-freeze problem that dominates solar design farther north, so the simpler configurations dominate here — but every type appears on the exam.

Every domestic system carries conventional backup — an electric element, a gas heater or a tankless unit — because a solar system is sized for an average day, not the worst one.

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3. Protecting the Potable Water

This is where the plumbing trade earns its place on the roof.

FBC-Mechanical Section 1401.2, Potable Water Supply: potable water supplies to solar systems shall be protected against contamination in accordance with the Florida Building Code, Plumbing. One exception applies: where all solar system piping is part of the potable water distribution system in accordance with the plumbing code and all components of the piping system are listed for potable water use, cross-connection protection measures are not required. A pure direct-loop system built entirely of potable-listed components does not need a backflow assembly; the moment a non-potable medium or a non-listed component enters the loop, it does.

FBC-Mechanical Section 1401.3, Heat Exchangers: heat exchangers used in domestic water-heating systems shall be approved for the intended use, and the system shall have adequate protection to ensure that the potability of the water supply and distribution system is properly safeguarded.

In practice that protection is delivered through heat exchanger construction:

• A double-wall heat exchanger places two separate walls between the transfer fluid and the potable water, with a vented space between them so that a failure of either wall leaks visibly to atmosphere rather than into the drinking water. This is the arrangement used where the transfer fluid is toxic or of unknown toxicity — ethylene glycol, or any fluid carrying corrosion inhibitors.
• A single-wall heat exchanger is used only where the transfer fluid is non-toxic — food-grade propylene glycol, for example — and where the design otherwise satisfies the AHJ.

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4. Heat Transfer Fluids (FBC-M Section 1403)

Two rules, and both are quotable:

• 1403.1 Flash Point: "The flash point of the actual heat transfer fluid utilized in a solar system shall be not less than 50 degrees F (28 degrees C) above the design maximum nonoperating (no-flow) temperature of the fluid attained in the collector."
• 1403.2 Flammable Gases and Liquids: "A flammable liquid or gas shall not be utilized as a heat transfer fluid."

The reason is stagnation. When a pump fails or power is lost on a sunny Florida afternoon, a glazed collector with no flow through it climbs far above its operating temperature — a condition the code calls the design maximum nonoperating, or no-flow, temperature. If the fluid's flash point sits near that temperature, a leak into a hot collector box becomes a fire. The 50-degree margin is the safety factor, and it explains why Florida systems either use water, use a fluid with a high flash point, or use drainback so there is no fluid in the collector to stagnate.

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5. Required Equipment and Protection (FBC-M Section 1402)

[!IMPORTANT] Section 1402.5.1 is the one candidates miss. It is not enough to put a relief valve on the storage tank. Any section of the loop that can be isolated by valves and can then develop excessive pressure needs its own relief device, positioned so a closed valve cannot separate the fluid from the relief. A solar loop isolated for service on a sunny day is a pressure vessel with a heat source attached.

Florida-specific realities layer on top:

• Wind. Collectors are roof-mounted components in a hurricane-prone state, so attachment is engineered to the wind loads of the Florida Building Code, Building, and roof-mounted equipment in the High-Velocity Hurricane Zone carries additional approval requirements.
• Freeze is rare but real. North Florida sees hard freezes. Direct systems there need recirculation freeze protection, drainback, or a closed loop.
• Scale. Florida's hard groundwater deposits scale inside a direct collector loop; softening or a closed loop with treated fluid is the answer.

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6. Solar Pool Heating

Solar pool heating is the volume application in Florida and it is deliberately simple: unglazed polymer collectors are plumbed into the existing pool filter loop downstream of the filter, with a three-way diverter valve sending flow to the roof when the controller calls for heat and bypassing it when it does not. Because a pool operates near ambient temperature, unglazed collectors with no insulation and no glazing are the efficient choice, and because pool water is already non-potable and already circulating, there is no heat exchanger and no cross-connection question.

The design points are pool-side, not potable-side:

• Collector area is conventionally sized as a percentage of pool surface area, and the FSEC manual gives the Florida guidance.
• The pool pump must overcome the added static lift to the roof plus the collector loop friction, which may require repiping or a larger pump.
• Automatic drain-down protects the panels when flow stops.
• Check valves prevent the roof array from siphoning the pool back through the collectors.
• The heater arrangement still satisfies FBC-Residential Section R4501.6.4, which sends water flow through any heater, bypass plumbing, back-siphoning protection and heat sinks to the manufacturer's recommendations.