Solar water heating is solar thermal technology, not photovoltaic electricity. A solar thermal collector absorbs sunlight to heat water or a heat-transfer fluid, so the first planning question is whether your property receives enough direct sun in the right place. A careful solar water heater site assessment looks at sun access, shade, roof condition, wind exposure, tank space and pipe runs before you buy or build anything.
Start with solar access and shading
The U.S. Department of Energy’s 2003 consumer guide says the first consideration when building a solar thermal system is the site. According to that guide, a site with unshaded areas that generally faces south is a good candidate, and a professional installer can evaluate a roof as a collector location. The same guide notes that if the roof does not have enough space, the system can be installed on the ground.
The Florida Solar Energy Center’s 2006 manual, which is Florida-focused, explains that the sun’s path changes significantly with the seasons. It advises minimizing shade from buildings, trees or other collectors and planning for lengthening winter shadows. The manual describes a daily “solar window” of about six usable hours, with available solar energy at its maximum at noon.
The North Carolina Solar Center’s residential solar hot water guide says shading may come from other roof sections, chimneys, nearby buildings and trees. It warns that a location unshaded in May may be in full shade in January. That guide says some shading, particularly before 9 a.m. and after 3 p.m., is usually acceptable, and it recommends consulting a solar professional if you are unsure.
Orientation and tilt
Fixed solar collectors should ideally face true south in the northern hemisphere, according to the Florida Solar Energy Center manual. That source says facing the collector within 30 to 45 degrees east or west of due south reduces performance by only about 10 percent. It also states that the closer to the equator, the less the need to maintain orientation and direction, while still being aware of seasonal sun position.
The North Carolina Solar Center guide gives southeastern U.S. guidance. It says collectors perform most efficiently oriented toward true south, but facing collectors 45 degrees east or west of true south reduces energy output by less than 5 percent. Facing them 90 degrees off south, toward due east or west, reduces annual output by only 10 to 20 percent. That guide lists acceptable roof orientations in the southeastern U.S. as east, southeast, south, southwest and west.
For tilt, the North Carolina Solar Center says the optimum angle for annual performance is equal to the local latitude, but mounting collectors parallel to the roof is preferred for simplicity and aesthetics. It states that a 15-degree variation to suit a roof pitch makes a very small difference in output. The Florida Solar Energy Center manual says the optimum tilt angle is about the same as the site’s latitude plus or minus 15 degrees, and that collectors should generally be mounted parallel to a sloped roof unless the performance penalty is more than 30 percent.
Roof condition, age and structure
Roof-mounted collectors add weight and create roof penetrations, so the roof must be in good condition and structurally adequate. The Florida Solar Energy Center manual says the collector or its supports should be anchored to roof rafters or trusses, not the sheathing. It advises avoiding placement at the roof ridge and areas where the roof slope changes, because wind loading and heat loss are greater there.
That 2006 manual also describes roof preparation issues. Old, brittle shingles may break underfoot, and tile roofs require extensive preparation because tiles can break or slip. It says broken tiles must be replaced, and the area where tiles are removed must be sealed later to protect the underlying watertight membrane. The manual stresses following accepted roofing practices, being familiar with local building codes, and communicating with the local building inspector.
The U.S. Department of Energy’s 2003 guide specifically warns about thermosiphon systems. It says contractors must pay careful attention to roof design because the water in the storage tank is heavy. This is a key structural point for passive systems where the tank sits above the collector.
Wind loads and anchoring
The Florida Solar Energy Center manual, written with Florida’s hurricane exposure in mind, says a mounted collector must withstand wind forces as well as sunlight and ultraviolet light. It states that in parts of the world vulnerable to hurricanes or extreme wind storms, the collector and mounting structure need to withstand intermittent wind loads up to 146 miles per hour, corresponding to a pressure of about 75 pounds per square foot. It also says winds and thermal contraction and expansion may cause improperly installed bolts and roof seals to loosen over time, and it advises following local code requirements for wind loading.
The same manual says not to mount collectors near the ridge of a roof or other places where wind load may be unusually high. Mounting collectors parallel to the roof plane helps reduce wind loads and heat loss. It notes that determining the actual wind-load effect on anchors and fasteners is difficult, and most collector suppliers employ professional engineers to approve collector-mounting details, which some local code jurisdictions may require.
Space for the tank and piping runs
Site assessment is not only about the collector. You also need a place for the storage tank and a reasonable path for piping. The Florida Solar Energy Center manual says the total length of piping from collector to storage should not exceed 100 feet, because the longer the pipe run, the greater the heat loss. If a greater length is necessary, it says an increase in piping diameter or pump size may be required.
The U.S. Department of Energy’s 2003 guide describes storage sizing in general terms. It says active system storage tank size increases with collector size, typically 1.5 gallons per square foot of collector. It also gives example tank sizes: a small 66-gallon system for one to three people, a medium 80-gallon system for three or four people, and a large 120-gallon system for four to six people. These are that guide’s guidelines, not current universal standards.
For a thermosiphon system, the collector must be installed below the storage tank so warm water will rise into the tank, according to the same DOE guide. That placement requirement affects both roof structure and the space available inside or on the roof.
Ground mount option
If the roof is shaded, weak, badly oriented or otherwise unsuitable, a ground-mounted collector can be an alternative. The Florida Solar Energy Center manual says the lower edge of the collector should be at least one foot above the ground so it will not be obstructed by vegetation or soaked by standing water.
That manual lists several ground-mounting points: keep threads protected with masking tape when setting anchor bolts in concrete, avoid dissimilar metals or isolate them with neoprene pads or silicone, coat threads with durable elastic material before tightening nuts to prevent rusting, and use galvanized anchors when possible. It also says to check the threads, because hot dip galvanizing sometimes puddles in the grooves.
Site assessment checklist
Use this checklist as a starting point. It is a decision aid, not a substitute for a professional structural or code review.
- Find an unshaded area that faces south, southeast or southwest, or within about 45 degrees of true south.
- Check for shade from trees, chimneys, other roof sections and nearby buildings, and think about winter shadows.
- Check the roof pitch and compare it with the latitude-based tilt guidance.
- Inspect the roof surface for age, brittle shingles, cracked tiles or signs of water damage.
- Confirm that the roof structure can carry the collector and, for thermosiphon systems, a heavy storage tank.
- Plan to anchor to rafters or trusses, not sheathing, and keep collectors away from the roof ridge.
- Consider wind exposure and local wind-load code requirements.
- Measure the pipe run from collector to tank and keep it as short as practical.
- Identify a location for the storage tank and backup water heater.
- If the roof is not suitable, check whether a ground mount area is unshaded and well drained.
Factor, what to look for, and source
| Factor | What to look for | Source |
|---|---|---|
| Solar access | Unshaded area facing south, southeast or southwest; account for winter shadows | DOE 2003 guide; FSEC 2006 manual |
| Orientation | True south is ideal; up to 45 degrees off south has a modest penalty | NC Solar Center; FSEC 2006 manual |
| Tilt | About latitude plus or minus 15 degrees; parallel to roof is usually acceptable | FSEC 2006 manual; NC Solar Center |
| Roof structure | Anchor to rafters or trusses; avoid ridge and slope changes | FSEC 2006 manual |
| Roof condition | Check for brittle shingles, cracked tiles, existing leaks or rot | FSEC 2006 manual |
| Wind load | Follow local code; keep collectors down from the ridge | FSEC 2006 manual |
| Tank space | Room for storage tank; thermosiphon tank must sit above collector | DOE 2003 guide |
| Pipe run | Keep collector-to-storage piping under 100 feet if practical | FSEC 2006 manual |
| Ground mount | Lower edge at least one foot above ground; avoid vegetation and standing water | FSEC 2006 manual |
Codes, permits and professional review
Anything connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes and may need a permit and a licensed plumber. The U.S. Department of Energy’s 2003 guide says some cities and counties have ordinances or require permits for home improvement construction, including solar system installation. It also advises checking homeowners association rules and neighborhood bylaws, known as codes, covenants and restrictions.
The Florida Solar Energy Center manual says standards and codes are sometimes ambiguous about what can and cannot be done to a roof, and it advises always following accepted roofing practices, being familiar with local building codes, and communicating with the local building inspector. For roof penetrations, it says to seal and flash pipe and sensor penetrations using permanent sealants such as silicone, urethane or butyl rubber.
Safety details matter beyond the roof. Solar water heating can produce very hot water, so scalding protection such as a tempering valve is relevant, and pressure and temperature relief is part of safe storage. Freeze damage is a risk in cold climates, and any materials that touch drinking water must be potable-water-safe. These topics are covered in more detail in the DIY Solar Water Heater Safety section and the roof and structural safety article.
Frequently asked questions
Does the collector have to face exactly south?
No. The North Carolina Solar Center guide says facing collectors 45 degrees east or west of true south reduces energy output by less than 5 percent in the southeastern U.S. The Florida Solar Energy Center manual says facing within 30 to 45 degrees of due south reduces performance by only about 10 percent.
Can I mount a solar water heater on the ground?
Yes. The U.S. Department of Energy’s 2003 guide says a ground installation is an option if the roof does not have enough space. The Florida Solar Energy Center manual says the lower edge of a ground-mounted collector should be at least one foot above the ground to avoid vegetation and standing water.
How long can the pipe run be between the collector and the tank?
The Florida Solar Energy Center manual says the total length of piping from collector to storage should not exceed 100 feet, because longer runs lose more heat. If a longer run is necessary, it says an increase in piping diameter or pump size may be required.
For the broader planning picture, see the site assessment category page, the DIY project planning section, and the pillar article Planning a DIY Solar Water Heater: Needs, Site and Materials.
