Just what is a batch heater,
Thousands more have been assembled
by handymen using only their common
sense to guide them.
On the other hand, with a little more
attention to detail you can build a batch
heater that will rank among the very
best, and rival the performance of more
complex systems costing two or three
times as much. Surprisingly, only a
handful of factors separate the winners from the also-rans.
The Water Tank
The water tank is the heart of any batch heater. Its size, shape, and positioning within the heater's enclosure determines how well it does its job. A useful rule of thumb for sizing batch heaters suggests that the tank should hold from one to two-and-one-quarter gallons of water for every square foot of glazing on the batch heater enclosure.
This insures that the tank is large enough to provide a reasonable amount of hot water, but not so large that it requires many hours of solar heating before reaching the desired temperature of 110 -- 120 Degrees F. Our own batch design uses a 40-gallon tank with 28 square feet of collection area -- about 1.4 gallons per square foot. This seems to be nearly ideal for providing both adequate storage and high delivery temperatures.
Regardless of the gallonage, long, narrow tanks are best because they have a large surface area relative to their volume, and thus effectively get the sun's heat into the water, where it belongs. Our batch heater utilizes a tank five feet tall but just 14 inches in diameter. This, too, seems nearly ideal.
Some batch heaters use one tank; others use two, three, or even more. (See Illustrations A through D.) Single-tank systems are usually cost-effective for average families (and our design is a single-tank model), while the multiple- tank systems' greater storage capacity works well for larger families.
In multiple-tank designs, there are two very different ways of plumbing the tanks. In the first, the tanks are connected to a shared inlet and outlet (parallel flow). In the second, the outlet of one tank is connected to the inlet of the next (series flow). The series flow produces slightly higher outlet temperatures, and usually is preferred for that reason.
Regardless of the number of tanks,
you have a choice of how they're
mounted. Illustrations A and B show
horizontal mounts, with the tanks on
their sides; C and D show vertical units,
with the tanks on end. From a performance standpoint, the vertical mount
seems better because it encourages
"stratification"; that is, the hottest
water tends to rise to the top of the tank
where it easily can be drawn off. Stratification is much less pronounced in
horizontal tanks, and lower outlet temperatures can result from the mixing of
the cold incoming water with the tank's
stored hot water. Because of this, the
design of a horizontal tank's inlet and
outlet pipes is crucial. Illustration A's
plumbing schematic shows one way to
arrange the plumbing to minimize the
harmful effects of mixing the tank's
water in a horizontal single-tank design;
Illustration B's schematic shows the
correct technique for multi-tank units.
Preventing Heat Loss
If you used hot water only while the sun was shining, then you simply could insulate the walls of the batch heater's enclosure, and that would be that. But most families use large amounts of hot water twice a day: first around breakfast time, and again after supper. So a batch heater must be constructed to hold the day's solar heat through the evening and into the following morning.
At night (assuming the walls of the heater's enclosure are thoroughly insulated), the glazing will be the principal cause of heat loss. Because of this, a batch heater should be double-glazed to minimize this loss. In cool climates, it's also a good idea to add some form of movable insulation (see Illustration A) that can be opened in the morning and closed at night. Movable insulation is highly effective, but it has a drawback because the owner must schedule twice-daily trips to the heater in order to operate it. If you forget to open the insulation, you'll get no heat for the day. Also, though the work involved in opening or closing insulation doors is hardly major, it's not really in keeping with the purely passive concept of hatch heating. A more elegant solution is to use triple glazing on the enclosure to minimize convective heat loss, and a "selective surface" on the tank to minimize radiant heat losses. (A selective surface is a special product that absorbs large amounts of solar energy, but reradiates very little, keeping the heat inside where
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23 RODALE'S NEW SHELTER JULY/AUGUST 1981