Solar thermal manufacturers usually supply the mounts along with their collectors, and factory solutions for mounting collectors definitely make a solar thermal installation easier. But if you install on a regular basis, you may find that making your own mounts is the way to go. Fabricating your own mount-sets is more time consuming but offers rewards in terms of cost and flexibility.
Even if you plan to make your own mounts eventually, it is sensible to use racks from the collector manufacturer for the first few jobs. Once you have a good idea of how the factory sets go together, you can design your own. A compromise many installers make is to stick with factory sets for most of their jobs, stock extras of the parts that will put them in a bind if they come up short, and use the materials and techniques discussed here for those custom jobs that are bound to come up.
In my 30 years in the solar thermal business, I’ve seen collector mounts fabricated from recycled bed frames, lumber, steel wall studs, and electrical metallic conduit—but these materials are not the norm. A simple, durable mount-set can be built with pipe, angle, or square stock. Angle-iron (steel angle stock) is the choice of many installers because of its low cost and ease of use in mounting systems. The common materials are steel, galvanized steel, stainless steel, and aluminum. All of these materials can make durable and strong collector mounts that will last for many decades. Here are some pros and cons of each.
Steel can be used in arid climates, since the possibility of galvanic corrosion between the steel mounts and aluminum collector frame is almost nil. In areas with higher humidity, steel stock will quickly rust and will require periodic maintenance, such as painting. The minimum dimension for 1/8-inch-thick angle iron or galvanized pipe is 1 inch. Smaller dimensions bend too much in long lengths.
Aluminum is a preferred material for its corrosion-resistance, and is the material of choice for racks and mounts in most of the solar industry. Aluminum extrusions come in a variety of different strengths and properties, and stronger aluminum is usually more brittle. Aluminum is not as strong as steel and needs to be a little larger-sized—the minimum dimension for 1/8-inch-thick extruded aluminum angle is 1 1/4 inches. While aluminum is lighter than steel, it’s more expensive, harder to weld, and, unless it is anodized or powder-coated, has a brighter color than the collector frames, so it may stand out more.
Stainless steel (SS) comes in more than 100 grades, designated by numbers that relate to different amounts of alloys. The most common grade for use in collector mounts is 304, but it should not be used in marine environments, since it is vulnerable to chloride corrosion. Instead, 316, an alloy which contains molybdenum to prevent corrosion, is recommended for coastal areas. It is hard to drill and difficult to weld but provides excellent protection from the elements—it’s the choice of boat builders for long-term saltwater exposure. If you’re planning to weld stainless mounts, 316L (low-carbon) stainless is a bit easier to weld due to the reduced carbon content. The minimum dimension for 1/8-inch-thick SS angle is 1 inch. Stainless stock is similar in cost to aluminum, which has fluctuated in the past few years.
Steel construction studs, steel Unistrut (industrial metal framing), and galvanized steel are sometimes used instead of aluminum. The material is stronger than aluminum, but can be more difficult to find. Unistrut is like a giant Erector set: it comes in a variety of angles and channels with holes. Braces, connectors, and slip-in threaded inserts (channel nuts) are available as well. It can be more expensive than other options, and is heavier and harder to cut than aluminum.
Lumber can be used to construct mounts, but it is not recommended. The ongoing maintenance (especially on roofs) is a hassle. Although using treated lumber can reduce the periodic maintenance, screwed connections are prone to weaken over time, and cracks and shrinkage can result in unreliable joints and roof penetrations. Using through-bolts will help strengthen connections to treated wood, and racks and sealed penetrations should be inspected occasionally.
Hardware. Regardless of the mount material, stainless steel hardware (nuts, washers, and bolts) is typically supplied by rack manufacturers and is recommended for most climates. In arid climates, zinc-coated hardware may be acceptable.
Mounting Feet. The “mounting foot” is the part of a mount-set that is lag-screwed or through-bolted to the rafters of the roof (or other structure). Angle stock is a good material for feet since it can be drilled for bolting to the roof and the 90-degree face can be bolted to legs or struts. One or two holes are drilled into the foot for attaching to the roof structure. A typical single-collector mount-set of this type will have four feet held to the roof with four to eight lag screws or through-bolts. My company has installed thousands of collectors using plastic roof cement (“pookie”) as the roof sealant. Care must be taken to put a bed of pookie under the foot, dip the lag bolt into the pookie prior to screwing it in, and cover the foot with a generous amount of roof cement as the last task before leaving the job. To keep roofing warranties intact, be sure that the sealant is made to work with the particular roof material.
Collector Struts. Once the feet are in place and spaced correctly, the collector strut and back leg of the mount-set can be bolted together.
There are many adaptations for constructing a basic mount-set. Mount-sets can also be strengthened using horizontal or triangulated angle stock if needed. For a row of collectors, angle stock can be used as top and bottom rails bolted to the collector struts. This configuration makes a “lay-in” mount for a row of collectors, and each collector can be secured to the mounts with self-drilling screws into the collector’s top and bottom frame extrusions.
Contributing editor Chuck Marken is a New Mexico licensed plumber, electrician, and heating and air conditioning contractor. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the University of New Mexico.
Unistrut • www.unistrut.com