This is a guest post from Bob Ramlow, solar thermal expert and the teacher of our Solar Thermal Boot Camp. Bob’s next solar thermal training starts on September 30th. If you need to know exactly how to design, install and quote solar water heater system, this class if for you. Click here to sign up.
I asked Bob to write an article on best practices for commissioning solar thermal systems, but he quickly realized without proper understanding of installation best practices, commissioning doesn’t matter too much.
In my work with many solar water heating manufacturers, designers and installers here in the upper Midwest I have compiled a list of best installation practices for solar water heating systems. This work is collaboration and is ongoing, so if you have anything to add to this work, please send me your additions.
Here in Wisconsin where I live the climate is harsh for solar water heating systems. I have over 9,000 heating degree-days and temperatures can reach well over 1000 F during the summer and fall to -300F or colder during the winter. I have also seen the temperature drop over 400F in less than a day. This is a tough environment for a solar water heating system to survive in. If a solar water heating system can survive here in Central Wisconsin, it can survive anywhere. As is often the case, the devil is in the details, and when designing a solar water heating system it is imperative to get all the details correct if we want a quality installation.
And details are what we have in this compilation. We have included every subject matter that seemed appropriate to a best practice in solar heating system installation. Of course the installation begins with the sale, so that is where we start. My next article in this series will be about commissioning. I will be referencing this best practice manual in that next article.
Introduction to Solar Water Heater Installation Best Practices
This manual was developed as a tool to assist solar thermal designers and installers as a guideline to provide the most reliable solar hot-water systems possible. The material presented here is not intended to be used as a list of system requirements or as a type of solar code. Rather, it was assembled with the input of many parties to share lessons learned in the field. It is not inclusive and is a work in progress.
This manual was developed in Wisconsin where some parts of the state have over 10,000 heating degree days and where winter temperatures regularly fall below -30°F. In fact, the record coldest temperature recorded in Wisconsin was -55°F. During the summer, temperatures can rise above 100°F. While most climates are not this severe, the practices outlined in this manual will be helpful for system designs in all cold climates as well as in warm climates.
A properly designed solar hot-water system must not only function properly during extreme cold and hot environmental circumstances, it must also be able to safely endure sustained periods of low or no hot water draw without damage or overheating.
A best practice is defined as:
- A practice that is most appropriate under the circumstances.
- A technique or methodology that, through experience and research, has reliably led to a desired or optimum result.
A well-designed solar water heating system that is appropriate for the climate where it is located and is properly installed with appropriate solar rated components will last for many years. Being a mechanical system, some components will eventually wear out and fail. The typical wear parts in a solar water heating system include the pumps, the expansion tank, automatic valves and the solar fluid. Environmentally, lightening can damage the controller.
Reliability studies have been conducted on solar water heating systems, but they have been limited by the lack of data available. Despite the lack of data, certain conclusions have been indicated. All mechanical systems follow a common reliability path that identifies when problems typically occur. Graphically, a curve demonstrates this where the curve is shaped like a bathtub. The following table comes from a solar water heating reliability report created by Sandia Labs under a grant from the US Department of Energy.
This graph shows that the greatest probability of a failure will occur at the startup and at the end of system or component life. The failure rate early in the device’s life is characterized by startup failures due to design flaws, faulty new equipment or components, installation errors, and misuse due to ignorance (yellow area). Once these initial problems are corrected the device enters its useful operational period where failures are due to chance occurrence (green area). Later, as the device and its components age, the failures begin to increase because the system is wearing out. Failures start to slowly creep in and eventually the system fails (red area). Because most solar collectors and piping systems can last well past the average life of a pump or other shorter life components, replacing the failed component can bring a failed system back to life.
This research shows the importance of post installation inspection or monitoring to overcome the potential startup failure.
Solar water heating systems have a unique situation where it is difficult to notice a system failure because there is always a full-size backup water heating system in place. In a water heating system using a conventional single water heater, if the system fails there is no hot water and the owner knows it immediately and can arrange for a service call. In the case of a solar water heating system that has a backup water heater, the owner may not know if the solar water heater is not functioning because the backup water heater will provide hot water. This situation shows how critically important it is that the solar water heating system be checked periodically. Owner involvement is mandatory and the system owner must be aware of this responsibility before the installation is started. If the owner is not willing to check the system at least monthly, then the sale should not take place unless a service contract is in place or unless some type of alarm is in place that would alert the owner of a system failure. It also shows that the installer should conduct a follow-up inspection within a reasonably short period of time after the system is commissioned to identify any startup failures.
Site Assessment For All System Types:
Harnessing the sun’s energy requires proper orientation and location of the solar collectors to maximize system performance, efficiency and ease of installation. A site analysis should be performed before purchasing equipment to ensure there is access to the southern sky without excessive shading and available space for the installation of the solar collectors, solar storage and drainback tanks, pumps or integrated pump stations and associated piping. Steps for an effective site analysis:
- Try to have all decision makers present during the site assessment or the sales call.
- Make sure the client understands what solar hot-water systems can and cannot do. Many potential system owners are enthusiastic about the prospect of owning a solar hot-water system but may not really understand the characteristics or limitations of this type of investment.
- A south facing location for the collectors is ideal. A north facing location will not provide adequate access to the sun’s energy and are not suitable for locating the solar collectors. East and west facing roof locations may be used but will require tilt kits to orient the collectors towards the southern sky. Web sites with satellite imagery (such as Google Maps) can often be used to survey the orientation of the roof before a site visit.
- The best horizontal orientation is achieved when the collectors are facing due south plus or minus 30°, this is often referred to as the azimuth angle.
- The best vertical orientation for year-round applications is achieved when the collectors are tilted at an angle equal to the geographic latitude of the location. Tilt kits are available to achieve the optimal vertical angle. NOTE: Customers often prefer to have the solar collectors flush mounted to the roof for aesthetic reasons. Modern solar collectors are efficient enough that flush mounting to pitched roofs will still provide reasonable performance for domestic water heating. Therefore customer’s preferences should always be considered.
- Placing the collectors as close as possible to the peak, less 3 feet provide clearance for maintenance, on pitched roofs will make installation easier by providing increased attic access. Placing the collectors near the edge of the roof will make installation difficult since attic access is more restricted at this point. The attic space must be examined during the site analysis to confirm adequate space is available for installing the solar collectors in the proposed location. Be aware that the top 6 feet on the South side of the peak is known as the snow surcharge area (drifting).
- The solar collectors should be located as close to the solar storage tank as possible to minimize heat loss in the piping runs, pump power and reduce installation cost.
- The proposed location must have access to the southern sky with a minimum amount of shading between 9:00 AM and 3:00 PM each day throughout the year.
- Determine the load:
- Most residential clients have no idea how much hot water they actually use; where feasible, meter the hot water load for a month. Otherwise, do a load profile based on the ANSI/ASHRAE 90.2-2007 formula:
- AGPD = [CW + SPA + B](NP)
- AGPD = average gallons per day of hot water consumption
- CW = 2.0 gal/day per person if a clothes washer is present in living unit, otherwise zero
- SPA = 1.25 gal/person per day additional hot water use if a ‘spa-tub is present in living unit, otherwise zero
- B = 13.2 gal/person
- NP = number of people in living unit; if exact information is unknown, estimate as follows, where NSR = number of sleeping rooms:
- (1.0)(NSR) for single-family detached and manufactured (mobile) homes with one to four sleeping rooms, plus (0.5)(NSR) for each sleeping room beyond four, or
- (1.25)(NSR) for multifamily buildings with one to four sleeping rooms per dwelling unit, plus (0.5)(NSR) for each sleeping room beyond four
- Inquire whether the household may bear any behaviors or activities that will consistently exceed or reduce the estimate based on the ASHRAE guidance.
- Encourage the replacement of old appliances.
- Document whether loads are consistent or intermittent by inquiring about vacation patterns or other absences in occupancy throughout the year.
- On both residential and commercial systems, look for multiple loads that a single system can satisfy. If possible, try to find both winter and summer loads to satisfy so the system can provide heat all year round.
- Do not install collectors on a bad roof.
- AGPD = [CW + SPA + B](NP)
- If shingles are nearing the end of their useful life (curling, breaking, or significant loss of aggregate), the building should be reroofed before the collectors are installed.
- When the site analysis is complete and it has been confirmed that the proposed location will provide adequate access to the sun’s energy and room to install the equipment sizing and equipment selection can be made.
- Ensure the local codes regarding all mechanical components, particularly single wall or double wall heat exchanger requirements are understood before equipment is purchased. Order double wall heat exchanger systems if required by local codes.Typically propylene glycol systems don’t require double-wall heat exchanger (verify with local code official). Ethylene glycol systems always require a double-wall heat exchanger to potable water.
- Use a site assessment tool to help determine the best place for the collectors:
- Document the solar window by taking a digital photo of the site assessment tool. Provide a copy to the owner and keep a copy in your files.
- Collectors can be oriented within 30 degrees of South with little difference in output.
- Model system performance:
- When using computer modeling tools, use the following parameters:
- When shading occurs within the solar window, it is typically the case that the site’s shading occurs in the winter months. Do not recommend a space-heating component if that is the case. When shading is a concern, note that while nearly all heat is collected during the hours of 9 AM to 3 PM (solar time), a majority of the heat is actually collected between 10 AM to 2 PM. If this window is less than 10% shaded, it is considered a good site for a solar water-heating system.
- Count branches of a deciduous tree at 50% shaded during the hours impacted if the shading occurs from October to March.
- Pay attention to future tree growth horizons. Recommend to the owner that most types of trees should not be planted within 50 feet of the site.
- When using computer modeling tools, use the following parameters:
- If options are available, involve the client in deciding which sites are acceptable for collector placement. This will prevent misunderstandings about placement and last-minute changes to the pump size. If the site has very limited solar access, document the reasons for exact collector placement.
- Don’t recommend a system if the site is more than 35% shaded. While most of the energy collected from any solar thermal system will be in the spring, summer and fall months, you want customers to be satisfied with their investment year-round. In case of summer uses (i.e. cabin, pool), winter shading can be ignored.
- If walls will be opened, document repair/carpentry costs.
- Record measurements of stairs or door openings and determine whether they are large enough to allow tank placement.
Typical system design
- Undersize rather than oversize:
- Size the system to provide a maximum of 100% on best solar day. This sizing scheme results in systems that do not overheat as well as systems that have the highest possible return on investment (ROI).
- Specify appropriate system type:
- Consider drainback systems for intermittent loads or seasonal load types, if practical.
- Consider pressurized glycol systems for systems that have pipe runs that cannot maintain a ¼” per foot slope back to the drainback tank and for ground mounted systems.
- Typically, the area available for the collector array will determine the size of system, especially in commercial applications. Another space limitation, particularly for commercial installations, is the available room for the solar storage tanks and the balance of system components in the mechanical room.
- If collector arrays will be in a saw tooth configuration, make sure the southern array will not shade the northern array. Note: A little shading when the sun is at its lowest angle will not seriously impact the performance of the system.
- Systems that serve multiple loads typically have a better return on investment than single load applications.
- Plan installation carefully so you have all components on site.
Residential system design
- System sizing: In order to qualify for the current federal tax credit, a residential system must be sized to cover half of the household’s domestic hot water load. This is the ideal maximum for solar hot-water systems without space or pool heating.
- Space Heat: This option is very popular in cold climates. The collectors should be tilted to maximize the winter sun (location latitude plus 150). To minimize potential summer overheating, consider including a heat diversion circuit to dissipate unwanted heat when necessary, or recommend a drainback system.
- Aesthetics: Many potential solar hot-water system owners would prefer that the collectors be flush mounted (parallel to the roof). While this practice will have only a small impact on the performance of the solar hot-water system in most climates, it is important that the prospective owner be aware that in a climate that experiences both a significant amount of annual snowfall plus experiences prolonged below freezing temperatures, there will be a reduction in overall system performance if the collectors are not tilted to an angle of at least 450. Production will be lost during the winter when daily production is at it’s lowest.
- If the owner of a large house wants a solar hot-water system, but currently there are only 1-2 occupants, system sizing will depend on the future intentions of the owner. If the plan is to have children or to sell the home in the next few years, size the system slightly large and consider the following:
- 1) Tilt the collectors to the winter angle.
- 2) Oversize the storage tank.
- Two-tank systems outperform one-tank systems in climates that experience extended cloudy periods.
- All systems require a listed Thermostatic Mixing Valve (TMV) at the exit hot water outlet of the back-up heater.
- If the back-up heater is on-demand, the TMV may be installed between the solar storage tank and the on-demand heater. Check with the water heater manufacturer to determine the maximum incoming water temperature allowed; and if necessary install the TMV between the storage tank and the on-demand heater set the TMV at or below this temperature.
- If the back-up water heater is an on-demand type, be sure that the on-demand heater will modulate to the “off” position if the incoming preheated water is already up to temperature.
Non-residential system design
- Never install an automatic water fill valve on pressurized glycol systems.
- It is acceptable to use an a glycol fill system (injection pump) that injects a pre-mix of glycol into the solar loop if the pressure drops in that loop (sometimes called a glycol makeup system)..
- Size the Heat Exchanger (HX) for a worst-case scenario with maximum possible water temperature and solar fluid temperature). To accommodate this worst case, the HX cannot be too big.
- Install Pressure Relief Valve (PRV) in the mechanical room:
- 1) Pipe the PRV to within 6” of the floor.
- 2) Locate the PRV between the collectors and any isolation valves in the system.
- 3) Size the PRV appropriately in relation to the maximum BTU output of the system.
- Maximum flow rates for copper tubing:
- Size the piping to maintain 5 feet of water column (head) per 100 feet of pipe. The following graph also shows the amount of heat that can be pushed through a pipe size at the identified flow rates and temperature rise.
|Pipe Size(in)||Flow(gpm)||Energy Delivered(BTUH @ 20°F temp rise)|
- Another method of pipe sizing is based on fluid velocity (between 2 and 5 feet/second) and head loss. The table below summarizes this method.
|Pipe Size(in)||Flow Rate(gpm)|
|1/2||1 – 3|
|¾||3 – 7|
|1||5 – 12|
|1 ¼||8 – 19|
|1 ½||11 – 28|
|2||20 – 49|
|2 ½||31 – 76|
|3||44 – 110|
|4||78 – 296|
- Sizing with a flow rate greater than 5 feet Per second (undersizing the pipe) results in pipe erosion and requires excessive pumping energy. This is important because it differs from the plumbing code. Closed-loop piping with pumps and glycol is different than open-loop piping with water.
- Sizing less than 2 feet per second (oversizing the pipe) results in excessive costs, the inability to move air through the piping (which is especially critical in drainback systems), and potentially a significant amount of heat loss through the pipe because its residence time is so high.
- Sizing for head loss is also important because it determines the amount of pumping energy that will be required. In space heating systems with radiant floor/sandbed loops, or in large commercial systems, going up one pipe size can, in some cases, save enough pumping energy to overcome the extra installation costs in just a few years. Oversizing in the case of planning for system expansion is justifiable. In every other case, oversizing has to be done carefully. The extra costs may often be overlooked. It is not just additional cost in pipe, but it is also more costly labor, fittings, and hangers. It carries over to larger insulation and jacketing, more solar fluid, larger expansion tanks, etc. In commercial systems, the difference is many thousands of dollars. And this is the cost that must be offset by the benefits: savings in pumping energy and flexibility for future expansion.
- Add parallel lines together.
- Pipes can be oversized but will increase the cost.
- 8 gpm for a 1” header means the max number of panels linked together should be 8 to ensure 1 gpm per collector. The 8 limitation of maximum collectors linked together is also a function of manifold expansion and contraction. This applies to harp style absorber plate collectors. Connecting more than 8 four foot wide collectors can result in more expansion than the collectors can withstand without harming the absorber plates and possibly the collector frame as well. Refer to the collector manufacturer for specific information about this point.
- Max of 4 collectors for ¾” header.
- Long pipe runs may require expansion loops, L-bends, Z-bends or U-bends per 2008 ASHRAE HVAC systems and Equipment 45.11