Seven principles of good solar hydronic design

Solar tchermal hydronic ombisystems must be designed and installed with the highest standards of quality and reliability. There are many ways to approach solar/hydronic heating and many design details and choices to make. To sort through the choices, it helps to have some way of filtering out the best ideas. A design philosophy, if you will.

Some years ago, after reviewing the best and worst memories of all the installations, repairs, rebuilds and adjustments I have done over the years, I noticed a definite pattern emerging from the most successful installations. I organized these aspects of successful solar/hydronic design into a checklist, originally for my own use, that I called the “Six Principles of Good Solar Hydronic Design.” I found that all six principles must be included in a balanced combination for an installation to be a complete success over the long term.

Later, I realized that these principles could be summed up in one brief statement that I have come to think of as the "Prime Directive," presented below. Also, we have noticed that in our most successful combisystem installations in recent years, the issue of “verification” provides the final key to high-efficiency performance and user satisfaction. This has emerged as the Seventh Principle. Here is a brief summary of the Prime Directive and the Principles of Good Design.

The prime directive for solar heating

Always design and install solar heating equipment that is at least as reliable and trouble-free as the conventional system it replaces. Whenever possible, provide performance, longevity and controls that surpass the conventional alternatives. In other words, it must be an upgrade.

Seven principles of good design

To meet the Prime Directive, try following these seven principles, listed roughly in order of their importance:

Reliable, Effective, Compatible, Elegant, Serviceable, Efficient and Verifiable.

The success of any solar hydronic heating installation depends on the often-conflicting balance between any of these principles. Finding the balance between them defines the art of solar heating design. 

Principle 1: Reliability

Make it reliable over the long term. There is no “solar payback” and no benefit when a system stops working. If the solar heating equipment breaks down unexpectedly for any reason during its normal life expectancy, the fuel savings and all other benefits are suspended until the system can be repaired. As solar heating is more widely deployed, it is imperative that the installed systems be at least as reliable as a conventional hydronic boiler system, and carry at least the same warranty service as the best boiler heating systems. The premium associated with the solar heating components must be seen as worth the cost, and reliability over time is the key to developing this perception.

Principle 2: Effectiveness

Think about user satisfaction. “Effectiveness” includes everything that the owner needs to feel satisfied with the system. If the long-term operation of the solar heating system is effective, there will be no complaints. The initial cost must be reasonable and justifiable (cost-effective). Part of the cost may be justifiable in terms of lower carbon emissions and other environmental benefits, solar tax incentives or other intangibles unrelated to heating system performance. But above all, it must be a good heating system, providing heating comfort and hot water on demand, low fuel consumption, reasonable operating cost, low maintenance, no special attention and uncomplicated user controls. Solar and renewable heating systems will enjoy wide acceptance only if they are more effective than the more familiar conventional technologies.

Principle 3: Compatibility

Make it compatible with existing systems in every way possible. If solar/renewable hydronic heating is to be implemented on a large scale, and make a significant impact on the energy efficiency of buildings, it must be done in a way that is compatible with the way buildings are actually built. The new standard solar combisystems often described in this column are based entirely on existing methods and technologies well-proven over recent decades by the hydronics industry.

Hydronic boilers and radiant-heated floors are perfectly compatible with closed-loop hydronic solar heating collector systems. Solar heat collectors can be mounted in a variety of ways, including ground mounting, roof mounts and wall mounts. This allows for flexibility in their application, and therefore compatibility with existing and new buildings without any further technical development. Active solar heating systems are assembled, operated and maintained in much the same way as hydronic boiler systems, making them compatible with the skills of our existing installers.

Principle 4: Elegance

Do more with less equipment. Fewer parts means lower cost and fewer things to go wrong. The “keep it simple” motto applies especially well to active solar heating. I always jump at the chance to eliminate a component or substitute something less complicated if I can achieve the same function and reliability. But remember, the goal is not just to make things simple; they must be fully functional. 

A quote widely attributed to Albert Einstein says, "Everything should be made as simple as possible, but not simpler." The new standard combisystems use component parts arranged in a repeatable pattern that is intended to epitomize this very idea. It is as simple as possible, without eliminating or limiting the intelligent control functions required by a wide range of interconnected heat sources and heat loads.

Principle 5: Serviceability

Make it easier to install, repair and adjust. Isolation valves can be a bargain. When things go wrong, a quick and economical repair is essential to keep costs down and to maintain the confidence of the owner. When purchasing a solar heating system, a buyer will often apply pressure to get a lower price. But, it is false economy to eliminate critical ball valves, drains, air vents, check valves, unions and other equipment that allows for rapid filling, draining, air elimination or component replacement. It can take hours to do something that should only take a few minutes if the serviceability is not properly designed. The use of pre-fabricated plumbing modules may offer a good solution here. These modules typically contain a pump, isolation valves, drains, gauges, check valves, relief valves, air vents and other components already pre-assembled. The use of pre-fabricated plumbing modules can significantly reduce the labor cost of an installation, while providing service features already built in. Of course, the controls and electrical components must also be serviceable, easy to adjust and easily replaced.

Principle 6: Efficiency

Do more with the energy used, both thermal and electrical. The whole idea behind the solar hydronic combisystem is to reduce conventional energy consumption, so let’s not forget about all the ways we can do that. Using good solar heat collectors will get you started in the right direction by delivering good solar thermal performance. Check out the web pages of the Solar Rating and Certification Corporation for the current list of test ratings for all the solar heat collectors available in the U.S. market. If you install a high-efficiency backup system and control it properly, giving priority to the solar and renewable heat, then your job providing thermal efficiency may be done.

But don’t forget about the electrical consumption of the system components. If you use a 200 watt circulator pump to do a job that requires only 20 watts, then you may have missed the boat. It is often possible to cut the electric consumption of the heating system in half (and up to 80%), by properly sizing and controlling pumps, eliminating transformers, choosing zone valves carefully and taking other steps to curtail electrical use, also known as parasitic energy. The solar equipment, the electrical components and the controls must all work together to provide high energy performance with some way to verify it.

Principle 7: Verification

Demonstrable performance, monitoring, data-logging and remote control. One of the chronic problems that has plagued the solar heating industry in the past is the difficulty in verifying that a complex solar combisystem is working properly day-to-day and season-to-season. With old school control systems, hours could be spent in the mechanical room pondering a slew of manual control settings and watching for proper system response. Today, data-loggers are used to record temperatures and system status so that a long-term record of performance may be used to make a better-informed diagnosis. But the most common kinds of data-loggers still require a trip to the site to gather the data. 

If the primary purpose of the system is to save conventional fuel (and reduce carbon emissions), there must be a way to demonstrate this, to prove these savings have been accomplished and to allow proper adjustments if needed. Here at SolarLogic, we have developed the SolarLogic Integrated Control (SLIC) system specifically to provide such a solution for our own projects. This is an integrated control system that includes continuous data logging that is accessible over the Internet. It also provides real-time remote monitoring and remote control over the Internet.

Energy metering is built in, with a dashboard display as well as all the intelligent control functions and capabilities needed to control a combisystem in a single control box that requires no programming.

The conflict 

You may have noticed that the Seven Principles can sometimes be in direct conflict with one another. If you focus too much on one goal, you may fail to satisfy one of the others. For example, an Elegant strategy that is excessively simple may not be Effective at meeting consumer expectations. A very Efficient design may not be Compatible with common construction practices. The most Energy-Efficient design strategy may not be the most Reliable over the long term. The objective is to strike a reasonable balance between conflicting goals, and still end up with a system that is satisfactory to the owner in price, features, performance and longevity. This essentially defines the art of solar and renewable energy combisystem design. Our most successful installations in recent years have been modeled after the new standard piping configuration using SLIC Internet-enabled control.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 sq. ft. 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. l

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.

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