Best ideas in solar combisystems

In this column, I will presented a wide variety of issues related to the design and installation of solar heating systems in buildings, A.K.A. solar combisystems. Each article represents a few pieces of a larger picture which includes the design, installation, control, maintenance and operation of a hydronic combisystem that will be successful and reliable over the long term. 

In our projects, we consistently use a modular piping configuration combined with an integrated control system that has come to be called the “New Standard” solar/hydronic combisystem. This approach incorporates the best ideas, proven in the field that can now provide the same reliability, automatic operation, and intelligent energy control in solar/renewable systems to equal or surpass any other conventional hydronic equipment. 

Following is a brief review of some of the most successful “Best Ideas” that we typically build into these systems today.

Over the past 30 years, I have dismantled, repaired and remodeled many solar heating systems. As a result, many of our best practices now included in the “New Standard” approach are a direct response to the incomplete or unreliable installation details found in older solar heating installations from the past.  

Following is the short list of ideas that appear in past columns and have withstood the test of time.  In recent years they have proven their worth in hundreds of solar installations, both retrofit and new. I now include virtually all of these measures in every combisystem I design these days.


Make a complete piping, wiring and control diagram. Solar plumbing design and the controls are two sides of the same coin. The plumbing system will not work without compatible controls and vice versa. This does not happen by accident, and must be planned carefully before construction starts.  

The time spent documenting a complete piping and wiring plan is rewarded many times over in the time saved during installation, startup and operation. It is also invaluable to the future users and service people throughout the considerable decades in the life of the system. In our internet-enabled computer control systems, we include a system diagram that can be displayed on the user’s computer screen on demand. This way, as long as the control system is functional, the system diagram can never be lost or misplaced.

Primary loop configuration 

Standardize the solar/hydronic piping. A Primary Loop “Flow Center” piping configuration allows multiple heat-sources to be connected to multiple heat-loads and provide heat (1) directly, or to (2) bypass any source or any load, or (3) allow simultaneous operation of any source or load. 

In past articles, a simple primary loop combisystem called ‘Combi 101’ has been presented to illustrate these features on a system that includes a bank of solar heat panels, a boiler, a domestic hot water tank and warm floor space heating. Larger primary loop systems may also include swimming pools, baseboard zones, wood boilers, heat pumps and heat storage water tanks attached together on the same loop (just to mention some examples from recent installations).  

Our company, Solarlogic, has developed a software design tool that speeds the design process of the standard primary loop system.  We call it the ‘Slash-D’ (SolarLogic Assisted Solar Heating Design) and it is available for free through our website. 

Figure 92-1 shows a schematic system diagram using the “New Standard” piping configuration. 

Direct, in-floor solar heat storage

Use concrete floors Instead of water tanks. By using 2-stage thermostats for room heating integrated into the solar control system, the considerable thermal storage capacity of concrete radiant heated floors can be used directly as solar heat storage. 

In many cases, this will down-size or even eliminate the need for large heat-storage water tanks. The “Slash-D” design software will help make that determination. The same principal has been used successfully to heat swimming pools and hot tubs when radiant heat tubing is embedded in the concrete shell of a pool or spa.

Controlled overheat dissipation

Prevent collector overheat by strategic circulation. Solar heat collectors can cause a lot of trouble if the liquid inside them is allowed to overheat. We now employ control systems that can dissipate extra heat safely into an existing hot water tanks, garage floor, ice melt sidewalk or other normal masonry heating zone to cool the collectors in a controlled way.  

When controlled properly, human comfort is not compromised and steam is prevented in the collectors, using existing in-floor or in-ground zone loops. The use of existing heat distribution equipment for overheat control can eliminate the need for more complex cooling system add-ons. This approach can extend the life of the solar heating equipment by keeping it within a more moderate temperature range during normal operation. It will not, however, offer any temperature protection during a power failure on a sunny day.

Figure 92-2 shows the Heat Dissipation indicators on the display of one of our control system installations. 

Passive self-cooling

Some self-cooling methods work during a power failure. Thermosyphon Self-Cooling Fins can be added to any bank of flat plate solar heat collectors, as long as the piping inside and outside the collectors meets some simple requirements.   Most of our recent Solar Combisystem installations include this cooling option to reduce the need for service over the long term.

Drain-back solar heating systems will also survive power failures just fine, because they empty themselves when the solar pump loses power. Another commonly used passive measure is the “steam back” expansion tank system. This does not prevent high temperature steam in the solar heat collectors during a power failure, but rather allows it to happen without the loss of any collector fluid.

Figure 92-3 shows a photo of a passive self-cooling fin-tube system installed on the back of a bank of eight solar heat collectors.

Night sky radiant cooling

Flat plate panels can be used at night for cooling. NSRC cooling can be accomplished using glazed flat-plate solar heat panels or (even better) using unglazed flat panels (often used to heat swimming pools).  

In many recent installations, we have included control settings that allow the warm floors to be cooled at night in summer by running the solar collectors backwards at night.  Similar control functions can be programmed to dissipate heat at night from overheating water tanks when the stored heat is not being consumed.

Performanze verification

Data-loggers, remote display and remote control. One of the chronic problems that has plagued the solar heating professional in the past is the difficulty in verifying that a complex solar control system is working properly from season to season. This used to require a site visit where hours were spent in the mechanical room pondering over a slew of manual control settings.  

Many hours can be spent on-site trying to observe proper system response which varies with the weather conditions. Some installers have adding data-loggers to record temperatures and system status so that a long-term record of performance can be used to make better informed adjustments. Some conventional solar controls now come with a data card built-in that can be removed and downloaded to your computer. These kinds of data-loggers still commonly require a trip to the site to gather the data.  

At SolarLogic we have overcome this problem for our own projects by developing the SLIC system (SolarLogic Integrated Control). This is a central control system for solar home heating that includes data logging that can be downloaded over the internet. The SLIC also provides heat-energy metering, remote monitoring and remote adjustment and control over the internet. And as an added bonus, it provides all the control functions and capabilities mentioned above in a single control box.

Figure 92-4 shows a sample energy graph using the remote energy heat-metering display function from one of our combisystem installations. 

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand 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.  Back issues of this column can be found in the archives at the Plumbing Engineer and SolarLogic LLC websites. 


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

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