NABCEP SHSI cheat sheet

Collector tilt rule of thumb: tilt = latitude − 15° favors summer/domestic hot water loads; tilt = latitude + 15° favors winter space heating; tilt = latitude for year-round balanced yield.

Incident solar energy on a tilted aperture: I_tilt ≈ I_horizontal × (cos θ_i / cos θ_z); use monthly TMY data and the Liu–Jordan isotropic sky model for diffuse, plus ground-reflected term ρ(1−cos β)/2.

Useful energy delivered by a liquid flat-plate collector: Q_u = A_c · F_R · [S − U_L(T_in − T_a)]; S = absorbed solar per unit area = (τα)·G.

Hottel–Whillier–Bliss efficiency curve: η = F_R(τα) − F_R·U_L·(ΔT/G), where ΔT = (T_in − T_a); plot against ΔT/G to compare collectors.

f-chart sizing: annual solar fraction f = 1.029Y − 0.065X − 0.245Y² + 0.0018X² + 0.0215Y³, with X = collector loss reference and Y = absorbed solar reference; valid for 0.3 < f < 0.9.

DHW storage rule: 1.5–2.0 gallons of solar storage per square foot of liquid flat-plate collector (≈60–80 L per m²); draw-off volume should cover roughly one day of load.

Freeze protection selection: drainback and closed-loop glycol are the two common options; glycol concentration must be set for the site's record low ambient (typically 30–50% propylene glycol).

Expansion tank sizing must account for the full collector loop volume, the maximum temperature swing from cold fill to stagnation, and glycol expansion; undersized tanks vent fluid through the pressure relief valve.

Stagnation design: size the loop to reach stagnation safely - use a heat-dump, draindown, or covered collectors; ensure components rated to the collector stagnation temperature (often 150–200 °C) are used near the array.

Collector mounting must resist wind and snow loads per ASCE 7; roof penetrations must be flashed with a manufactured flashing and sealed with compatible sealant, never with roofing cement alone on shingles.

Row spacing to avoid inter-row shading at winter solstice: spacing ≈ collector height × cos(azimuth tilt)/tan(solar altitude at design hour) - use the noon solar altitude on December 21 for the design case.

Glycol loops require a fill/purge valve arrangement; fill from the lowest point and purge air from the highest point, then pressure-test at 1.5× operating pressure before charging.

Piping insulation must be UV-rated or jacketed outdoors; use closed-cell elastomeric or mineral wool with aluminum jacketing where exposed to weather.

Sensor placement: collector temperature sensor on the collector outlet header (hot side), tank sensor in the lower portion of the storage tank; both must be in direct contact with the pipe/tank and insulated over.

Air elimination: install an automatic air vent at the highest point and a microbubble/air separator at the pump suction; air in the loop causes cavitation and pump failure.

Pump orientation: motor shaft horizontal, strainer on the suction side, and isolation valves on both sides of the pump for service without draining the loop.

Electrical: pump and controller circuits require a disconnecting means and GFCI protection where required; low-voltage sensor wiring must not share conduit with line-voltage conductors.

Commissioning documentation at installation: as-built piping diagram, glycol concentration record, pressure test log, and sensor calibration check.

Flow rate check: liquid flat-plate loops typically run 0.02–0.05 gpm per ft² of collector (≈1–2 L/min per m²); measure with a flow meter or by timing a calibrated bucket on the drain.

Temperature differential check: a properly operating differential controller turns the pump on at ΔT ≈ 8–12 °C (15–20 °F) and off at ΔT ≈ 2–4 °C (4–8 °F); verify with a contact thermometer.

Glycol freeze point test: use a refractometer, not a hydrometer, on propylene glycol; record concentration and freeze point at commissioning and annually.

Stagnation diagnosis: high collector temperature with pump off and no flow indicates controller fault, air lock, or pump failure - check controller output voltage at the pump first.

Pressure drop diagnosis: excessive ΔP across the collector array indicates fouling, air, or a partially closed valve; compare to the design head at design flow.

Sensor calibration: compare controller readings against an independent thermometer; a 5 °C error in the tank sensor can prevent the pump from ever starting.

Air lock symptoms: pump runs but flow is low, collector temperature rises to stagnation, and the loop pressure fluctuates; purge and check the air vent.

Commissioning sequence: pressure test, flush/purge, glycol charge, flow balance, controller setpoint verification, then a 24-hour performance log.

Annual maintenance checklist: glycol concentration and pH, pressure and expansion tank pre-charge, pump amp draw, sensor calibration, controller function, and collector mounting/sealant condition.

Glycol degradation: propylene glycol oxidizes to glycolic acid over time; check pH annually (target 8–10) and replace fluid every 3–5 years or when pH drops below 7.

Expansion tank pre-charge should be checked with the system at ambient temperature and the tank isolated; pre-charge typically equals the static pressure at the tank plus 5 psi.

Record keeping: log flow, ΔT, pump hours, and glycol test results each service visit; trend data reveals gradual fouling or degradation before failure.

Seasonal shutdown for drainback systems: verify the drainback tank drains fully and the collector loop is empty when the pump is off; a partial drain causes freeze damage.

Collector cleaning: dust and bird droppings reduce transmittance; wash with water and a soft brush, avoid abrasive cleaners and cold water on hot glass.

Safety during service: isolate and depressurize the loop, allow collectors to cool, and use PPE for hot glycol; never open a loop at stagnation temperature.

SRCC OG-100 certifies collector thermal performance; OG-300 certifies complete residential solar hot water systems - specify OG-300 rated systems where required by incentive programs.

ASHRAE 93 and ISO 9806 define collector efficiency test methods; ISO 9806 is the current international standard and supersedes older test protocols.

Solar loop pressure relief valve must be rated for the system's maximum design pressure and piped to a safe discharge location, never capped or plugged.

Glycol is toxic if ingested; use propylene glycol (food-grade) for potable-water-connected systems and label all loops with antifreeze warning tags.

Backflow prevention: potable water connections to solar loops require an air gap or reduced-pressure-zone backflow preventer per local plumbing code.

Structural: roof-mounted collectors must be engineered for wind uplift and snow load per ASCE 7 and local building code; permit drawings typically require load calculations.

Electrical: PV-powered pumps and controllers must comply with NEC Article 690 where PV is present; line-voltage pump circuits follow NEC Article 430.

Stagnation temperature rating: components in contact with the collector loop must be rated for the collector's maximum stagnation temperature; standard PEX is not acceptable in the solar loop.

Labeling: all pipes, valves, and tanks in the solar loop must be labeled with fluid type, flow direction, and safety warnings per code and good practice.