All manufactured solar collectors come with a mounting system which will typically consist of some sort of rail that is anchored to the roof structure. The most common method of anchoring is with a lag screw through the mounting rail and into the rafter. Some installers will use a “J” bolt hooked to a rafter. Another method involves a long threaded rod that is bolted to a block of wood that spans at least 2 of the rafters in the attic. The key is to look for a substantial anchoring method. A lag screw in the roof decking is not considered to be substantial. If the area is not along the coast or in the mountains, where the sustained winds do not generally exceed 30-35 mph, some manufacturers allow the collector to be anchored to the roof decking without being in the rafters. If this is the case, an expansion type anchor such as a butterfly, or hollow wall anchor should be used.
General practice is to use a roof caulk to seal any roof penetrations. Every bolt should have been pre-drilled and the hole filled with caulk before the bolt is installed. The bolt head should be covered with sealant as should the mounting rail where the bolt passes through. A good quality roofing caulk should be used. A roofing caulk will remain flexible after it cures, whereas an acrylic or latex caulk will harden and crack. Silicone should not be used because it does not have good adhesive properties. Where piping penetrates the roof, a roof flashing boot should be installed. Some system manufacturers supply a special flashing boot with the collector whereas others recommend using a standard plumbing roof boot. The boot should seat on the roof so as to form a positive seal, and not allow water to pool. The boot itself should not come in direct contact with the pipe unless it is rated for high temperature applications (200 + deg. F). Wiring penetrations should have their own flashing/sealant. Wires should not come in direct contact with the pipe because the high temperatures may melt the wire jacket/insulation.
The piping run should be as short as reasonably possible. Longer pipe runs mean more heat loss in the pipe. Any horizontal runs of pipe should be supported every 6 ft. minimum. The supports should be of a material compatible with the pipe material. Most solar systems, unless they are low temperature, will use copper pipe simply because the temperatures get high enough to melt or soften any plastic pipe, even CPVC. Since the pipe will be insulated, a support around the insulation will be most common, in which case it should not come in contact with the pipe. The issue with non compatible materials is the same as with plumbing pipe in that galvanic corrosion can occur with dissimilar materials.
All of the piping between the collectors and the heat exchanger and storage tank should be insulated. Pipe insulation should be a closed cell rubber foam type such as Rubatex, Armaflex, or Armacell with no less than a ½” wall thickness. The thicker the insulation, the better insulating property it will have. There should be no openings, holes or gaps in the insulation. All seams and joints should be taped. Any insulation that is exposed to sunlight should be UV protected with an aluminum tape, paint, or an insulation jacket. Insulation that is not UV protected begins to crumble after relatively short exposure times and will quickly become non-effective.
Most manufacturer’s use a heat exchanger that is integrated with the storage tank, either inside the tank in direct contact with the water, or wrapped around the outside of the tank (but inside the jacket of the water heater.) Typically there will be a label on the tank identifying the type of heat exchanger. If it an immersed heat exchanger, it is most likely a single wall design, meaning there is only one layer of material between the heat exchange fluid and the water inside the tank. If the heat exchanger were to rupture, there could potentially be a contamination of the household and municipal water supply. When a single wall heat exchanger is used in a closed loop pressurized system, common practice is to set the operating pressure of the heat exchange loop to be less than that of the water system. With this done, if the heat exchanger were to rupture, the higher pressure of the water system will not allow the heat exchange fluid to enter the water system. With the wrap around heat exchanger, contamination of the water supply is highly unlikely.
Other manufacturers may utilize an external heat exchanger. Common external heat exchangers are the plate type and some variation of a tube in tube system. In either case they can be identified by the piping coming in to them. One side of the heat exchanger should have the piping from the collectors coming into it, and on the other side should be piping from the water storage tank. To get the maximum efficiency from the heat exchanger the heat from the solar collectors should be exchanged with the coolest water from the tank. This means that the water should come from the bottom of the storage tank (since we know from basic thermodynamics that a cooler fluid will fall while a warmer fluid will rise.) In addition to get the maximum efficiency of heat exchange, all components should be well insulated.
The fluid(s) may be actively pumped, or they may passively flow by thermal siphoning. Some manufacturer’s systems will use a combination of active and passive technologies; however, the majority of them are actively pumped. The pumps are typically small (1/25 hp or less) circulating pumps similar to those used on hot water heating systems. The pump should operate smoothly and quietly and be relatively vibration free. The pump may be hot to the touch so be careful touching it (since it is processing a heated fluid) however it should not be more than a few degrees hotter (if any) than the adjoining pipe. Typically the pump is on the return side (cooler side) in the collector loop. If an external heat exchanger is used, there will probably be two pumps. One for the collector loop and one for the water heater circulation loop. They may or may not be the same size.
The pump(s) may be AC (alternating current) or DC (direct current). The AC pump will have a controller which could be as simple as a timer, or a differential controller which senses at least two different temperatures that it uses to determine when to turn on and off. The pump is typically wired to or plugs in to the controller. The temperature sensors are most commonly located at the bottom of the tank (coldest point) and the collector out (hottest point). A temperature difference between these two points of 15 deg. F is enough for efficient heat exchange and is a common set point for turning the pump on. After the pump runs for a while the temperature of the collector will drop and the temperature at the bottom of the tank will increase. When this temperature difference reaches the 3-4 deg. F range the controller signals the pump to turn off. A differential controlled system will go through many on/off cycles on a sunny day.
A DC pump may or may not have a controller. Most DC pump systems are directly coupled (wired) to a PV (photo-voltaic) panel that is mounted with the collectors (same orientation and tilt). When there is sufficient solar exposure for the thermal collectors to provide heat, there is enough solar radiation for the PV panel to generate electricity to power the pump. The PV panel acts as a controller for the system. A DC system may also use a controller such as the one described for the AC system.
The next most important component is the heat exchange fluid. Most areas of the US require a freeze protected system, and most freeze protected systems use propylene glycol as the heat exchange fluid. Propylene glycol is a food grade anti-freeze. The glycol should be checked for ph and concentration. These are the same checks that you will do for the anti-freeze in your car. A 50/50 mixture of glycol and water is typically recommended to prevent freezing. Usually a basic visual inspection will be enough to determine the condition of the glycol. The system should have a drain valve at or near the heat exchanger. The valve should be slowly and carefully opened draining a small amount of the fluid in a cup. A white colored cup is best so the color of the fluid can be evaluated. Glycol that has begun to oxidize will change colors (begin to darken). The original color may be green, yellow, or red. If any discoloration or burnt smell is noticed it is probably time to change the fluid. A ph test paper will confirm this if there is any doubt.
Most manufacturers have their systems certified by an independent organization such as the Solar Rating Certification Center (SRCC) or Florida Solar Energy Center (FSEC). If the system is certifed by these organizations, then a label indicating the certification and rating will be found on the collector body. Both of these organizations require that the system be accompanied by an operations manual which is typically attached to the storage tank. This manual is important for any new homeowner to have so that they understand how to operate and maintain the system in their new home. If the current owner does not have the manual, then the manufacturer of the system should be contacted and a new manual acquired.
Complete Resources is a green building construction company in Athens, GA that specializes in renewable energy products such as solar water heating systems. More information can be found on their website at www.CompleteResources.net
Tags: Athens Georgia, green, home inspector, inspection, solar
