Sizing a Solar Water Heater: Summer Sets the Collector, the Backup Covers the Rest

The store follows the collector, not the family

Once the collector area is known, the storage for an active system comes straight out of it: the DOE gives about 1.5 gallons of storage for each square foot of collector, which is about 61 litres per square metre. The family of four in the Sun Belt, on 56 square feet, needs about 84 gallons (318 litres). The same family in the north, on 64 to 68 square feet, needs 96 to 102 gallons (363 to 386 litres). The guide's own shorthand for tank sizes runs the same way: 66 gallons (250 litres) for one to three people, 80 (303) for three or four, 120 (454) for four to six.

The reason the tank is tied to the collector and not to the headcount is that the collector's output has to go somewhere. On a clear day in a house that is empty until evening, everything the collector gathers accumulates in the store; a large collector on a small tank reaches its highest temperatures on precisely the days the house draws least. Sizing the store in step with the collector gives the sun's output a place to sit until it is wanted.

The arrangement of the store is a separate choice. In a two-tank system the solar tank preheats water before it enters the conventional water heater, which stays as the backup and keeps its own storage. In a one-tank system the backup heater is combined with the solar storage in the same tank. So a sound existing heater can stay in service under a two-tank arrangement, where a one-tank arrangement replaces it with one vessel doing both jobs. A solar preheat tank can sit lukewarm through a dull week, and the temperatures stored water is expected to reach are the legionella guide's ground, not this one's.

Passive systems carry their storage differently, and the 1.5-gallon rule is not theirs. In an integral collector-storage (ICS) or batch system the collector is itself a tank, and cold water passes through it on its way to the backup heater. A thermosyphon system puts the tank above the collector so that warm water rises into it without a pump; the DOE notes that most of these hold about 40 gallons (151 litres), and that because the tank is heavy, the roof design needs careful attention.

The climate chooses the loop

Before a collector brand or a tank is discussed, the local freezing record decides which kind of system is even on the table. Water left in a collector on a clear winter night can freeze, and a system that circulates household water through the roof is only as safe as the coldest night it meets. The DOE's own placement of the four common types is the quickest way to narrow the field.

Where it freezes, the answer is usually an indirect loop: a pump circulates a non-freezing heat-transfer fluid through the collectors and a heat exchanger, and the household water never goes onto the roof. The fluid in a household system is a propylene glycol and water mix, typically half and half and adjusted to the freeze hazard. Ethylene glycol must not be used, because it is toxic. The mix degrades and normally needs changing every three to five years, and because these loops are pressurised the DOE says they should be serviced only by a qualified solar professional. A closed loop brings the fittings of any closed hydronic circuit with it — an expansion tank, a pressure gauge, a fill and drain assembly and an air vent — and its fluid volume is the collectors, the exchanger and every metre of pipe between them.

The heat exchanger is where an inspector's attention goes, and "unacceptable heat exchangers" is one of the code problems the DOE lists by name. A single-wall exchanger puts one surface between the loop fluid and the drinking water. A double-wall exchanger puts two, with a drained gap between them so that a leak shows up rather than mixing in; the DOE says double walls with drainage and leak detection are required where the fluid is toxic, and are often used even with propylene glycol. The cost is efficiency: heat crossing two walls needs a larger exchanger for the same duty. Coils inside the tank, tubes wrapped around its outside, shell-and-tube units and compact stainless plate exchangers all appear in household systems, and manufacturers rate them in Btu per hour at stated temperatures and flows.

Direct systems avoid the exchanger and the fluid and pay for it in water chemistry. Hard water leaves mineral deposits in collector tubing and pipework, and the DOE's remedy is a water softener or a mild acid solution circulated through the loop every three to five years, or as often as the water demands.

The roof comes before the collector

A thermal collector wants to face the equator — true south in the northern hemisphere, true north in the southern. The DOE's siting guidance adds the useful allowance that, depending on location and tilt, a collector can face up to 45° east or west of true south without significantly decreasing its performance. The same guidance names local weather as a reason to adjust: recurring morning fog or afternoon cloud moves the best orientation toward the clearer half of the day.

Tilt is where a water heater parts company with a photovoltaic array. The tilt that maximises the year's delivery, with the best months in spring and autumn, is about equal to the site's latitude. Because a water heater needs the most heat in winter, when the water arrives coldest, the DOE advises tilting water-heating collectors steeper than that, in contrast to PV arrays that often sit flat or at a shallow angle. In practice most collectors are mounted flush and simply take the roof's pitch, so the pitch belongs in the sizing conversation rather than being discovered on the day of installation.

Shade matters most in the middle of the day and in winter. The DOE's guidance is to avoid shading above all between 10 am and 2 pm, and in winter when the sun is lowest; a tree that is harmless in June can cover the collector in December. It also recommends checking for new shade every year, at mid-morning, noon and mid-afternoon, because trees grow and neighbours build.

The roof itself has to outlast the system. The DOE puts a solar water heater's life at about 25 years and advises installing only on a new or sound roof that will not need replacing in that time. Composite shingle is the easier and cheaper surface; wood shingle and tile are harder. Blocking between rafters is often needed, stanchions and their flashing are often a roofer's job, and ground mounting on piles is the alternative when the roof is wrong. "Exceeding roof load" heads the DOE's list of common code problems, and a thermosyphon's tank sits full of water on the roof itself. The same list names improper wiring and unlawful tampering with the potable water supply, and the zoning list adds side yards, roof protrusions and setbacks from streets and boundaries — all of which a permit application will ask about before the scaffold goes up.

Proving it works on the first clear day

The heart of an active system is a differential controller with two temperature sensors: one on the collector outlet, one at the bottom of the solar store. The controller starts the pump when the collector is hotter than the tank and stops it when it is not. Its faults show up as a pump running at the wrong time, which makes the first sunny day after installation the real commissioning test.

The DOE's troubleshooting reads straight off the controller. A pump running at night points to a short-circuited collector sensor or an open-circuited tank sensor; a pump idle on a sunny day, to the reverse, and each sensor's resistance compared against its reference value shows which has failed. A common and embarrassing cause is simpler: the sensor has fallen off the pipe it was meant to measure, which is why it should be fixed with a lug or a stainless steel clamp rather than tape.

1. Before the sun is up, check that every valve is in its operating position and, on an indirect loop, that the loop's pressure gauge reads what the installer set.
2. From mid-morning on a clear day, listen for the pump. If it is silent, the controller or the pump has failed — and the DOE notes the culprit is often the pump's starting capacitor, which can be replaced without replacing the pump.
3. After the system has run through a clear, warm day, feel the pipes going into the storage tank. Hot pipes mean the system is working.
4. After dark, confirm the pump has stopped. A pump still running points at the sensors.
5. Lift the lever on the pressure relief valve to prove it is not stuck open or closed.
6. Walk the insulation on the pipe runs, and cover it with a protective plastic or aluminium wrap wherever it is outdoors.
7. Leave the owner with the maintenance schedule: the loop fluid's pH and freeze point measured and the fluid replaced when out of specification, the steel tank's sacrificial anode replaced at the supplier's interval, and the tank flushed of sediment.

Pools: the water is the store, and there is usually no backup

Solar pool heating skips most of the above. There is no storage tank because the pool is the store, and usually no separate pump, because the pool's filtration pump sends the water through the collectors. The collectors are usually unglazed — a dark absorber of metal or polymer with no cover or enclosure — and the DOE notes that most people do not use a backup heater with them at all, spas being the exception.

The size is a share of the pool's surface area, 50% to 100% depending on location and season. The DOE's example is a 15 by 30 foot pool in Florida, used all year, which takes collectors equal to 100% of its surface: 450 square feet (42 m²). Pools in northern California, used six to eight months a year, are typically sized at 60% to 70%. More collector lengthens the season, and a pool cover holds the heat longer. The pump, filter and turnover the collectors depend on are the pool circulation guide's subject.