Solar heat collector variations, Part 1

To the untrained eye, all solar collectors may look alike. Most are big and flat, covered by glass, with a metal frame enclosing a dark interior. But, there are a surprising number of variations that exist today in the world of hydronic solar heat collectors, and the variations are mostly hidden inside, where they cannot be identified easily at first glance. 

It is important to know how a solar heat collector is configured inside, because the internal configuration must be compatible with the design intentions of any given solar heating installation. Some collectors cannot be used in certain applications because of the way they are configured internally. 

In our work with solar heated buildings, our policy is to standardize our Solar Hydronic Combisystems so that they can be deployed in a way that is largely duplicated from one building to the next. The idea here is that we feel the need to mass-produce energy efficiency and sustainability in buildings as rapidly as possible if we are going to take a bite out of our carbon footprint in a timely way. As it turns out, some types of solar heat collectors are more easily compatible than others with our New Standard design strategies.

Coolant plumbing

The typical hydronic flat-plate solar-heat collector contains of a series of pipes arranged inside the panel to provide cooling to a hot absorber plate. The absorber plate is the black surface, inside the glass cover, that heats up when exposed to solar radiation. In a hydronic solar heat collector, the absorber plate is cooled by a series of tubes attached to the hot absorber plate that allows the flow of hydronic fluid to carry the heat away in the form of hot liquid.  

The fluid enters and exits the collector through supply and return “Headers.” Attached to the Headers are smaller tubes known as “Risers,” and the Headers act as manifolds, distributing the flow of liquid coolant evenly through all the Risers to provide uniform cooling over the entire plate. Over the past 50 years or so, every conceivable configuration for the absorber plate and the coolant tubes has been tried. Today, we use the tube-plate patterns that have withstood the test of time as being most effective, reliable and serviceable for the climate and application at hand.  Following are some examples from recent installations.

The Harp configuration

The most common flat plate collector piping configuration available today is the simple pattern known as the Harp. The “Inlet” and “Outlet” Headers are mounted inside the collector, across the top and bottom horizontally. The Headers protrude out of the frame of the collector on both sides, so that there are four available pipe connections, both ends of each Header. 

Smaller riser tubes are connected so that fluid can flow from the bottom header to the top header, and multiple risers are attached side by side so that the risers resemble the strings on a Harp (the ancient musical string instrument, not the blues harmonica). The risers provide multiple parallel flow paths to cool the absorber plate which is attached to the risers for good heat transfer. The Headers act as supply and return manifolds for all the coolant passing through that collector.

In recent work on solar heated buildings, we have found Heliodyne, Radco, Sun Earth, AET, Caleffi and Solar Skies collectors that were manufactured with a classic Harp configuration. This type of collector has proven itself to be rugged, durable, reliable, economical and effective in the field. Our projects have used more Harp collectors than any other kind, and they are the “go-to” collector for most of our New Standard installations.  

Figure 87-1 shows a standard pressurized glycol installation we did in Taos, N.M. using Harp collectors to heat radiant floors, domestic hot water and a swimming pool.

Drainback applications

Most of our standard installations use pressurized glycol/water mix as the standard collector fluid. But, in some installations, plain water is preferred or specified by the owner or builder, and this is easy to accommodate without altering the New Standard plumbing or controls.  

In these cases, in freezing climates the freeze-protection for the liquid-cooled collectors must be provided by the Drainback method. This involves mounting the collectors above the mechanical room, and allowing all the collector fluid to drain by gravity back into a holding tank whenever the solar collectors are inactive. Air is allowed to fill the collectors and outdoor plumbing when the collectors are “off” using standard Drainback equipment and methods.

The most reliable solar collector that is used most often with Drainback system is (once again) the Harp configuration. The Harp pattern allows the water in the collector to fall downhill directly and unimpeded down the lower solar-cool return pipe, while air can rise up the solar-hot supply pipe to take the place of the liquid.  This, of course, depends on the supply pipes being large enough, and being installed at the correct tilt angles for proper drainage. It also depends on the collectors being installed so that their risers and headers can drain downhill fast enough to prevent freezing at sun down on a very cold day. Sometimes, this involves mounting the collectors tilted east to west so that the bottom headers drain faster.

Figure 87-2 shows a large group of tilted Drainback panels that feed downhill into a nearby mechanical room where a standard primary loop system is used in a commercial greenhouse to heat fish tanks, growing beds, radiant floors and DHW.

Caleffi has developed a “5-port” Harp style collector that modifies the tilt of the bottom Header internally, inside the collector. The modified header has a manufactured low-spot where a fifth plumbing connection is located to drain the collector. These collectors can be mounted level and still provide positive gravity drainage for each collector. 

Figure 87-3 shows Caleffi 5-port collectors being installed in a New Standard Combisystem installation in Custer, Wis. If you look closely, you can see that each collector has a drain connection in the center at the bottom that were ultimately connected for rapid drainage into a holding tank indoors.
This article will be continued with a second part in the November issue.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit for more information.