Planning to install a solar thermal domestic hot water system? Because Do-it-Yourself kits come with much of the pre-wiring and easy plumbing connections you can install these kits as easy as 123. Solar hot water collectors used for year-round domestic hot water should face true south and be tilted up from the horizontal at an angle equal to the latitude of the site plus 15 degrees. For example, for Denver, Colorado, 40 degrees latitude plus 15 degrees equals 55 degrees from horizontal. A south-facing surface tilted at an angle equal to latitude will actually collect maximum sunlight year-round. Where aesthetics are a factor, many people choose to mount collectors at the roof angle.
Variations 20 degrees either way will not seriously affect the total annual output (about 5%), but will create some seasonal imbalances. Tilting your solar thermal collectors up to latitude plus 15 degrees will give you fewer over heating problems in the summer and more hot water in the same pitch as the roof. A rack mount has precut or adjustable legs to tilt the collector at an angle to the roof.
Manufactured collectors often have a proprietary extruded aluminum frame incorporating a ready-made channel or other feature to attach the mounting structure with a screw, bolt, or proprietary fastener supplied by the solar thermal hot water manufacturer. If the mounts are connected to the collector with heavy, self-tapping screws, care should be taken that the screws don’t penetrate any farther than necessary, to avoid contact with collector piping or glass.
Whether the solar hot water rack is homemade or manufactured, painted angle iron can be used for mounts in areas of low humidity. Aluminum angle is preferred where steel and iron are subject to heavy rust over long periods of time. Stainless steel mounting hardware is often used in humid, rainy, or coastal climates. Be sure to choose sturdy enough sizes to support the weight, and in some communities, engineering will be required.
Many homeowners solar thermal hot water installations use treated lumber. This can provide an adequate collector mount system, but maintenance of the wood is a drawback. Although the treated wood may last for up to a few decades, screwed connections are prone to weaken over time. Through-bolts should be used for all connections to
treated wood.
An important consideration to keep in mind regarding all types of roof mounting is that the mount hardware must be fastened directly to the structural members of the roof the joists, rafters, or trusses. Screwing the solar thermal collectors to the roof sheathing will not last in a heavy wind or over time. Some local codes require that solar collectors be J-bolted to the structural members. A J-bolt wraps around the structural member and is then bolted to the mount. This requirement is not the norm, but is based on concern about lag screws weakening the structural members.
Another method of securing the solar thermal mounts is with a spanner block placed under or between two structural members in the attic. Long bolts or all-thread are run through the roof and bolted to the mounts. This works well when you have access under the roof. Lag screws, if used, should be at least 1/4 inch (6 mm) diameter.
Minimum length is 3 to 4 inches (7–10 cm) for a normal composition shingle roof with 1/2 to 3/4 inch (13–19 mm) decking. At least 2 inches (5 cm) penetration into the joist or truss is required when attaching your solar thermal stand offs. Wood shake roofs will require 4 or 5 inch (10 or 13 cm) lag screws. Care must be taken to make sure the lag screws are placed in the center of the structural members. It is often difficult to locate the exact center of 11/2 inch rafters. Cement and clay tile roofs will need to be cut and flashed, and the mounts will be right above the roofing felt under the tiles. The exact attachment details can be rather involved for each type of roof, and are not within the scope of this article.
Roof penetrations will normally need to be made for solar thermal collector piping and collector mounts. The wires needed for the collector sensor can be run alongside one or both of the insulated pipes to the collectors. Roof penetrations for piping need to be slightly larger than the diameter of the piping and its insulation. A 2 inch (5 cm) diameter hole is usually all that is required for a single pipe. A 3 to 4 inch (7–10 cm) hole may be required for two pipes.
Using one penetration for each pipe is neater, easier to seal, and exposes less piping to the elements. You should avoid contact between the pipe and roof structure, since this can cause damage to the pipes over time. Plastic pipe insulators are handy devices for running a pipe through any roof sheathing or structural member.
They hold the pipe to avoid movement, which may cause wear and tear, or stress the weatherproof seal. A roof jack is required for all pipe penetrations. A roof jack is a formed, sheet metal component with a flat bottom and an attached metal or rubber cone-shaped projection that has an opening for pipe, duct, or conduit. The flat portion can be slid under shingles and nailed or screwed to the roof. The cone projection prevents rain and snow from entering the attic or roof space.
The hole in the roof should be placed so that the flat part of the roof jack will slide under an existing shingle above, and over the existing shingles below. Coat the top of the fastener for the solar thermal roof jack with a generous dollop of roof sealant. If you are penetrating a metal roof, you should use the roof jacks provided by the manufacturer of the roofing material.
Sealing the mount screws or bolts and the part of the mounts that are directly in contact with the roof surface can be done with roof sealant. Contractor’s silicone caulking is good for metal or other nonporous surfaces. All of these products, and roof jacks of various sizes and types are available at home centers and plumbing supply houses.
The solar thermal collector supply pipe is always connected from the pump to the cold inlet at one end of the bottom header pipe of the collector. The return pipe runs from the hot outlet of the collector(s) and runs to the heat exchanger next to the storage tank. The hot collector outlet is always at the end of the top header that is diagonally opposite and farthest away from the cold inlet at the end of the bottom header. This piping arrangement is called “reverse return,” and will give an even flow through the solar collector(s).
All solar thermal pipe insulation exposed to ultraviolet (UV) rays of the sun needs protection for long-term durability. A good UV-resistant paint will last from five to ten years, and manufacturers of high-temperature, closed-cell insulation have recommended products.
If you want a maintenance-free covering for the insulation that will last a lifetime, flat, architectural-grade aluminum used for camper shells and gutters is a good solution. It is easily bent around the insulation and can be fastened with very short screws (using proper care), or bent to form a self-fastening clip.
Almost all hot water solar tanks heaters use the same type of electronic control, a differential control. The differential control is used to control the system if AC pumps are used. The Goldline GL-30 has two temperature sensors. One is located at the outlet (top) of the collector piping. The other is located at the cold DHW piping on the solar thermal storage tank.
You should keep in mind that solar thermal hot water systems tend to overproduce in the summer, and any tilt angle less than the recommended optimum will produce even more in the summer. The loss with a lower tilt angle will be in the winter months when the systems tend to produce the least.
Ideally, your solar thermal collector orientation should be exactly true south if you have an unobstructed solar window. Fortunately, solar hot water systems are surprisingly forgiving as far as orientation. Orientations 15 degrees
off true south still capture 90 percent of total daily sunshine. Orientations up to 30 degrees off true south are acceptable, but may lose as much as 20 percent of optimum sunshine. You can increase your collector size to compensate for a less than ideal orientation.
If you have a choice of facing the solar collectors more easterly or westerly because the home’s orientation prevents a due south solar hot water installation, choose the west for slightly increased performance. The afternoon has higher ambient temperatures. Prevailing cloudiness that exists in some locations may also have a bearing on the orientation of your collectors. Locations with morning clouds will have better performance if collectors are faced in a more westerly direction, and easterly works better for prevailing afternoon clouds.
Your compass lies. It points to magnetic south. In some parts of the U.S., true south can be as much as 22 degrees east or west of magnetic south. To find true south, you need to adjust for the magnetic declination of your site. In Denver, Colorado, the magnetic declination is 14 degrees east. This means that true south is really 14 degrees east of magnetic south or a compass reading of 166 degrees. Refer to the accompanying map for magnetic declination for the U.S. See Access for additional info on magnetic declination.
Solar collectors don’t work well in the shade. Solar collectors should be totally unshaded from 9 AM to 3 PM standard
time, year-round. Avoid shading earlier and later in the day if you can.
Solar collectors used for heating domestic hot water are usually mounted on roofs, where there is often plenty of unused space. Shading from trees and buildings is usually less of a problem on roofs. Mounting hardware products can be supplied by the solar collector manufacturer or you can build it yourself.
Authors; Chuck Marken Ken Olson Printed With Permission Home Power Magazine