December Sizing for a Shaded Drainback DHW Retrofit
A four-person household at 44° latitude in a cold climate (design outdoor −20 °C) keeps an existing 60-gal electric tank at 60 °C. The owner wants solar to carry "most of our hot water" and has budgeted for three flat-plate collectors (4 × 8 ft, 32 ft² aperture each), a 120-gal drainback tank, and a 1/2 hp pump. Site data: annual plane-of-array irradiance 4.3 kWh/m²/day at latitude tilt; December 1.5 kWh/m²/day; mains water 8 °C in winter and 15 °C in summer. The 8/12 roof faces 20° east of true south, and a chimney shades the array from 09:30 to 11:00 in December. Daily hot-water use is estimated at 20 gal per person. The contractor proposes a 60 °C tank setpoint with a thermostatic tempering valve and asks you whether the array, tank, and tilt are adequate before ordering equipment.
1. Estimate the daily hot-water load and the collector area needed for a 60% solar fraction in December. Does the proposed three-collector array meet it?
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Load: 4 × 20 = 80 gal/day = 303 L. At 60 °C storage and 8 °C mains, ΔT = 52 K.
Q = 303 kg × 4.186 kJ/kg·K × 52 K = 65,900 kJ ≈ 18.3 kWh/day.
60% of that = 11.0 kWh/day.
December output: 1.5 kWh/m²/day ÷ 10.76 = 0.139 kWh/ft²/day. At a realistic cold-weather collector efficiency of 50%, usable gain ≈ 0.070 kWh/ft²/day.
Area needed = 11.0 ÷ 0.070 ≈ 157 ft², i.e. five 32 ft² collectors.
Three collectors (96 ft²) yield about 6.7 kWh/day, roughly 37% of load; the 09:30–11:00 chimney shade likely removes another 10–15%, so the true December fraction is near 31–33%.
The array is annual-adequate (4.3 kWh/m²/day × 0.55 × 96 ft² ≈ 21 kWh/day on an average day, more than the load) but winter-short. The honest answer: three collectors with electric backup will cover maybe 50–60% annually and about a third in December - not "most" in winter. Either reset expectations or add two collectors and matching storage.
2. Recommend tilt, azimuth, shading, and storage changes, and explain the trade-offs.
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Tilt: latitude tilt is an annual-energy compromise. For a winter-limited DHW load, tilt at latitude +10–15° (54–59°) to improve December incidence and shed snow. The cost is some summer overproduction, which in a drainback system simply stagnates and drains - no fluid damage, but wasted potential.
Azimuth: 20° east of south costs roughly 3–5% annually, acceptable, but it worsens the morning shade overlap. If the array can shift west of the chimney, do it; otherwise model the shade and add collectors rather than guess.
Shading: a December morning shadow over the array's lower-left corner hits the coldest, lowest-sun part of the day. Trimming the chimney, moving the array, or accepting a lower December fraction are the only options. Do not sell the system on unshaded performance.
Storage: DHW-only sizing is 1.5–2 gal/ft². For 96 ft² that is 145–190 gal; the proposed 120 gal is marginal (1.25 gal/ft²) and will cause frequent high-limit cycling. If you go to five collectors, plan 240–320 gal of storage. Confirm the drainback tank sits below the collectors with continuous 1/4 in./ft fall, or the system will not drain.
3. What freeze protection and stagnation strategy should be specified, and what installation details make or break it?
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Drainback is the right call for a cold climate with a winter-short array: no glycol to degrade, no freeze risk during outages, and stagnation is managed by simply draining the collectors.
Critical details: • Continuous slope of at least 1/4 in./ft from every collector and every pipe run back to the drainback tank; no local low spots, no flexible hose sags, no lifted insulation. • A vacuum breaker or automatic air inlet at the high point so the loop can drain; without it, the collectors hold water and freeze-split. • A tank sized for the entire loop volume plus headspace, vented, with the pump inlet above the tank bottom and protected from vortexing. • Pump selected for the static lift from tank to collector, not just friction; a 1/2 hp pump may be oversized for flow but undersized for lift if the mechanical room is low. • Controller with collector-high-limit and freeze protection disabled or set below drainback logic - drainback does not need a freeze-recirculation setpoint. • Insulation UV-jacketed outdoors and sealed at fittings; wet insulation defeats drainback.
If the contractor insists on glycol, then specify propylene glycol, an expansion tank, annual pH/freeze-point testing, and a double-wall heat exchanger - a materially different and higher-maintenance system.