The 'New Standard' for solar, 
hydronic combisystems

It seems that there are just as many different ways to install a solar/hydronic combisystem as there are installers. And, this may well be one major reason why solar thermal technology has not grown in leaps and bounds over the past 10 years, in the way that photovoltaic (PV) technology has. When each individual solar heating installation is a custom-made creation, the successful outcome may depend entirely on the “genius” of a single individual. And when that genius moves on, the system can no longer be controlled or maintained without his or her special knowledge of the one-of-a-kind original design.  

The PV and solar electric industries have grown enormously in recent years, and in large part, this is due to standardization. With modular components, and standard system configurations, the design of one installation becomes similar to the next. A qualified installer can successfully deploy these renewable energy systems and they can be controlled, serviced and maintained by any other qualified installer.

This is the same approach I have developed for my own use in solar thermal installations over the past 15 years or so. I have found that when I follow a certain repeatable design strategy that solar thermal installations become easier and faster to deploy, and with more dependable results. This design strategy has come to be called the “New Standard” for a solar/hydronic combisystem, and I am happy to share it with anyone who can use it. Anyone thinking of building or retrofitting a hydronic hot water boiler heating system to include renewables can benefit by following this pattern. 

An integrated approach

The diagram in Figure 71-1 shows the concept behind the New Standard. We have learned from experience that even the best solar/hydronic plumbing design by itself is useless without good controls. So, the plumbing and controls must be thought of as two sides of the same coin, and designed simultaneously to work together seamlessly. To take advantage of every opportunity for enhanced efficiency, total control of the entire combisystem must be acquired. That includes everything in the system that affects the use of thermal energy, temperature set points and energy priorities. And, to meet the needs of the owner and the occupants in the building, the controls must be primarily automatic, easy to access and adjust. 

Integrated solar-thermal applications are a large part of the mission of my company SolarLogic LLC. We have developed these principles into a packaged design method coupled with a patented control system that serves as our own New Standard system configuration. We have proven the value of this concept many times with successful installations in many diverse locations and share these design methods freely with anyone who can benefit from them.

Standardized piping configuration

In Figure 71-2, you can see a basic block diagram of the New Standard piping configuration that I have used (with minor variations) in virtually every solar/hydronic combisystem I have designed over the past decade or so.  It has evolved into a “dual primary loop” that contains closed-loop pressurized glycol in the plumbing that extends outdoors, and uses closed loop pressurized water as the boiler fluid indoors. This configuration can accommodate a wide variety of heat sources and heat loads of various temperatures, and can be easily expanded from very simple to the more complex installations in buildings of moderate size. 

The block diagram shows a relatively large system with two heat sources and four major heat loads indoors, and two intermittent heat sources and two heat loads outdoors. Notice that the heat sources and the heat loads are arranged in a specific ‘temperature-order’ around the primary loop of ascending source temperatures and descending heat load temperatures. Solar heat is not the only alternative energy that can be included in a New Standard system. Any combination of multiple heat sources can be grouped together as shown in red in Figure 71-2, including wood-fuel boilers, heat pumps, waste heat recovery or any other heat source, either "on-demand" or "intermittent."

The most important design feature of this configuration is that any major heat source or heat load can be included or excluded from the design simply by plugging or unplugging a secondary loop with two pipes (two closely spaced tees). And, any existing secondary loop can be ‘turned on’ or ‘turned off’ at any time under intelligent control simply by operating its circulator pump. Any secondary can either be included or bypassed depending on temperature and priority, including the heat-storage water-tank. This provides an adaptable hydronic plumbing configuration that can be deployed and operated in a modular way at the highest thermal efficiency. The matching control system can also be adaptable and modular to match the predictable patterns of piping and other components.

Drainback and other variations

There are a number of variations that can be implemented using this basic primary loop configuration without altering the basic principles or control strategies of the New Standard system. Figure 71-3 shows one that is often requested when using drainback solar heat collectors. In a system where there are no outdoor heating loads, and there is no other compelling reason to use pressurized glycol, a drainback collector system can be plugged in to the primary loop as shown. This variation allows plain water to be used in the solar loop using a common drainback tank with a heat exchanger. It is connected in the proper location on the primary loop in the same way as any other secondary loop with two closely-spaced tees.
The New Standard primary loop configuration tends to be adaptable, when it comes to minor variations.  We have adapted this configuration to systems that required certain hydronic heating products that already existed or had already been specified, such as variable speed pumps, mixing blocks, hydraulic flow separators, and additional heat exchangers.  These variations have been installed, sometimes with and sometimes without modifications to our standard control strategy.  We have found that the best results and the most trouble-free and profitable installations occur when we are able to stick to the most simple interpretation of the New Standard design guidelines without modifications.

Total system control

The proper control of a heating system like the one seen in Figure 71-2 requires more than just turning the solar collectors on and off at the right time. Optimization of energy efficiency must be in proper balance with the target-temperatures needed for each heating load and requires total control of all the components in the heating system. At SolarLogic, we have created a control system known at the SolarLogic Integrated Control (SLIC) that provides total control for this New Standard piping system. The comprehensive features built into the SLIC control system can be enabled or disabled to match the configuration of the final installation.   

The following features are built into this control system and are activated as needed: 

  • Room thermostat control (two stage and 'setback')
  • Domestic hot water (DHW) energy control
  • DHW instant hot water recirculator control
  • Thermal heat storage distribution (in tanks or in mass floors)
  • ‘BTU’ energy metering, data logging and data access
  • Multiple heat source management 
  • Multiple heat load management
  • Priority management (energy use/fuel type/heating loads)
  • Indoor pool and spa heating integration
  • Outdoor pool, spa and ice heating integration
  • Solar overheat control of panels & tanks
  •  NSRC — Radiant night sky cooling control

Fingertip control

While the control functions above must be intelligent and responsive, the system must be trivial to use from the occupants point of view. By turning a thermostat up or down, the occupant may be unaware of the total control functions that are at work, but may only be aware of the results: room temperature management, superior "expert system" control, and automatic, integrated and comprehensive. 

Network access

There is no way to know for sure if a multi-function combisystem is performing properly unless some kind of display screen is provided and the energy information is made visible and verifiable. Just like a hybrid car, a hybrid heating system really needs a dashboard display. Even though the occupants may never need anything more than a room thermostat, the owner and the installer will quickly learn to appreciate the level of insight that a good display provides. 

The SLIC control system uses both the local network and the internet to provide easy access to all of the following interactive functions available from any network computer that has been enabled: 

  • Dashboard-style remote monitoring (software display) domestic hot water temperature management
  • Remote control operation & adjustment (internet access) Remote thermal metering and energy display
  • Energy strategy profiles (preset performance settings)
  • Email text alertsContinuous data logging 
  • Upgradeable (network software updates)


The New Standard includes both a plumbing configuration and a control scheme that, when taken together, comes very close to being a plug-and-play solution for solar/hydronic combisystems. By using a standard Primary Loop (flow center) configuration, some complex piping can be designed very rapidly. It is then completely compatible with the SLIC packaged control system, resulting in the most reliable, verifiable and accessible renewable thermal hydronic systems we have ever installed. The SolarLogic Assisted Solar Heating Design (SLASH-D) program is available free at This program produces a New Standard piping diagram, a parts-list, and other useful analysis. 

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. 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. 

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, New Mexico, where he is involved in development of solar heating control systems and design tools for solar heating professionals.

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