Historically, the most robust solar heating markets have been in warm-weather states such as Florida and California. Consequently, the solar heating industry in the United States has developed an emphasis primarily on solar pool heating and solar domestic water heating systems.

Trends show the market for solar combisystems — systems that provide both domestic water and space heating — has potential to expand. An estimated 42,000 of these systems are installed in the United States, and the market is growing slowly.

Many of the solar combisystems utilized in the United States to-date are custom systems. Much of this is due to the limited availability of combisystem-specific products and the lack of thorough industry standards related to these systems. As a result, the success of this segment of the industry has depended heavily upon the expertise of the designer and installer, who are often the same person.

Without a significant depth of knowledge about the design considerations unique to solar heating systems, even highly competent hydronics professionals make key oversights that significantly affect system performance and durability.

Solar heating systems have two somewhat unique characteristics: the potential for freezing and the inability to shut off the fuel source (the sun). As a result, we commonly find issues such as:

• Check valves in the wrong location;

• Autofill valves piped into a freeze-protected loop containing propylene glycol;

• Undersized expansion tanks;

• Critical components installed in locations where they will be exposed to steam;

• Controls so complex they are difficult to service; and

• The solar supply integrated into the heating and distribution system at locations that are extremely inefficient.

Given these common errors and the lack of solar literacy across the conventional heating industry, there is significant need for solar combisystem standards. Unfortunately, the North American market has not been large enough to support the development of detailed solar combisystem standards.

Fortunately for us, the implementation of solar combisystems in Europe can serve as a guide. Germany alone has nearly 15 times as many solar combisystems as the entire United States, and these systems account for nearly 20% of Europe’s solar heating market.

To support the growth of solar combisystems in European markets, several industry representatives participate in the CombiSol project. As a result, several key documents related to best practices in design and installation are available at www.combisol.eu.

Building combisystem success

The CombiSol project and lessons from the U.S. market provide some guidance for those who seek to expand their work with solar combisystems. Below are nine recommendations that will help support your long-term success.

1. Know when to say “no.” While there is often an aversion to saying “no” to a potential client, it is important to recognize the applications where solar combisystems are not favorable. Inefficient homes, houses with high temperature distribution such as standard hydronic baseboard and applications with low demand for domestic hot water are unlikely to perform well.

Installing systems in these applications may ultimately reflect poorly on your business when the client is dissatisfied with the system’s contributions.

2. Use a standard design. A number of standard designs can be used for a solar combisystem. Many of them are designed around specific buffer tanks that simplify integration of multiple heat sources and heating loads. When utilized effectively, these products increase the ease of installation, simplify the control strategies for each subsystem, and minimize the auxiliary fuel required for domestic water and space heating.

Additionally, some unique products are on the market that allow for a standardized design through the use of a single, advanced control or inclusion of the auxiliary boiler in the storage tank.

3. Understand stagnation. Stagnation occurs when the system is unable to remove heat from the collectors. This commonly occurs in combisystems when the maximum storage tank temperature has been met or when a power outage makes the circulator inoperable. In antifreeze-type systems, there is an art to managing stagnation through appropriate collector selection, expansion tank positioning and sizing, and the selection of particular components.

Stagnation also can be avoided by using a drainback system that empties liquid from the collectors when the circulator is off.

4. Target low return temperatures. The production from a solar heating system is heavily dependent upon delivery temperatures — the higher the temperature of the fluid circulating through the solar collectors, the lower the solar production will be. This relationship favors the use of radiant distribution, where return temperatures can be kept to a minimum. Optimizing the delivery temperatures to radiant distribution through an outdoor reset control further increases the solar contribution.

5. Know the auxiliary heat source. Solar combisystems integrate well with low-mass condensing boilers that also benefit from low return temperatures. Care must be taken when integrating solar with high-mass boilers, since the boiler return temperatures often must be maintained above 140° F to protect the boiler from condensation. As a result, the solar integration should focus on the return temperatures from the radiant system, not the return piping to the boiler when integrating with high-mass boilers.

With condensing boilers, there is more versatility since the optimal operating range for the solar supply and boiler return temperatures are complementary to one another.

6. Consider total system efficiency rather than simply solar efficiency. Solar heating systems are energy-efficiency devices: Their primary goal is to reduce the demand for auxiliary heat. It is very easy to get caught up in efforts to maximize the heat production of the solar heating portion of the system, but this does not always equate to maximum energy savings.

For instance, producing high-grade heat for domestic water heating often results in less heat production from the solar array. There may be benefits in maintaining the high-grade temperatures at a loss to solar heat production since these high-grade temperatures are what trigger the aquastat to fire the boiler.

7. Pay attention to domestic hot water comfort. In many solar combisystems, the high-grade temperatures required for domestic hot water drive the setpoint temperature for the auxiliary boiler. Limiting the domestic water heater setpoint by utilizing insulation on the distribution piping and minimizing heat loss from fittings at the top of the tank through the use of thermosiphon heat traps to reduce standby losses are critical details.

When considering the domestic hot water setpoint, it is important to consider the effects that lower temperatures may have on Legionella contamination.

8. Consider the economics of a solar fraction.The solar fraction is the portion of the heating demand that is satisfied by the solar heating system. While it may be tempting to try to reach the highest solar fraction possible, this approach is often least economical and involves the most complexity.

Many of the combisystems in Europe use storage tanks with capacities of roughly 200 gal. These residential systems may only achieve an overall solar fraction of 20% to 30%, but the cost/benefit is often much more attractive than a system utilizing much larger storage volumes. They are also much simpler to design, install and maintain.

9. Isolate controls. Using standard controls whenever possible helps to make the system serviceable. Strong standardized designs often use a standard solar differential controller to run the solar circulator, use a switching relay or zone valve control with an outdoor reset control for distribution, and a single aquastat with the boiler control.

This type of approach greatly reduces the complexity of servicing by future heating professionals and ensures that a malfunction of the solar control will not affect the comfort in the home.

There is a great deal of benefit in keeping things simple and learning from past successes and failures. Systems that are easy to install, easy to maintain and align with industry best practices will make your efforts much more enjoyable and help grow your business.


Vaughan Woodruff is the owner of Insource Renewables in Pittsfield, Maine. In addition to solar contracting and consulting, he provides training for Solar Energy International and HeatSpring. His course with HeatSpring, “Solar Approaches to Radiant Heating,” is one of the courses offered in its RPA University series.