Bristol’s six principles for good solar hydronic design

By BRISTOL STICKNEY,

contributing writer

In the past few months I have been discussing the principles of good solar heating design. In my experience, the best solar heating installations do not depend upon any single idea or product, but rather the careful combination of all of these principles (reliability, effectiveness, compatibility, elegance, serviceability, and efficiency, listed roughly in order of their importance) into every installation.

In past columns I described how the principles of reliability, efficiency and effectiveness need to be balanced with one another. Now let’s consider the constant battle between the need for Compatibility and the desire for Elegance. A very elegant (simple) system may not be compatible with industry standards, existing construction, or with consumer tastes. A system that is highly compatible may also become too complicated, unreliable, or impractical in other ways. Let’s take a closer look at these principles.

Principle 3: Compatibility.

Make solar heating compatible with existing systems.

Remember the Roof Pond? It was a passive solar heating and cooling system that resembled a very large atmospheric “water bed” covering the entire roof of a specially-constructed flat-roof house. The water acted as passive solar heat storage in winter and cool storage in summer, resulting in very little conventional energy required for heating or cooling. Despite its obvious Efficiency, Elegance, Reliability and other potential benefits, it never caught on because it was not compatible in so many ways.

If solar heating is to be implemented on a large scale, it must be done in a way that is compatible with the way buildings are built now, and have been built in recent years. Let’s start with the buildings that employ hydronic heat distribution, since hydronic boilers and radiant-heated mass floors are perfectly compatible with closed loop hydronic solar heating collector systems. Closed loop active solar heating systems are assembled, operated and maintained in much the same way hydronic boiler systems are, making them compatible with the skills of our existing installers. This is a timely opportunity for the hydronic heating industry to step in and become the leaders in solar heating.

Principle 4: Elegance.

Do more with less equipment.

Fewer parts mean lower cost and fewer things to go wrong. The “Keep It Simple” approach applies especially 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. There are always many opportunities to simplify every installation.

Example: Drain-back debacle

I am often asked why Cedar Mountain Solar does not install drain-back solar heating systems. After all, aren’t they more Elegant than closed-loop glycol systems? They use plain water in the collectors. The water drains by gravity into a reservoir at night so the collectors don’t freeze. There is no pressure gauge, no diaphragm expansion tank and no “charge pump” is required to fill the system when first installed. Why not use this seemingly simpler approach?

I agree that the drain back system looks seductively elegant. But my answer is that in our climate and market, the drain-back system is less compatible than the closed-loop glycol system in many of the important applications that we encounter every day.

This lack of compatibility can prevent the installation of solar collectors in many common applications, and even worse, cause catastrophic failure to the collectors and drain-back pipes when they are installed, which would never happen with glycol systems when things go wrong.

Just last week we visited a large drain back heating system with steam puffing and water dripping from multiple broken collectors, damaged by freezing and thermal shock. The owner had gotten used to pouring 50 gallons of water into the reservoir every day to keep it working. This is what I mean by “debacle.”

As I mentioned last month, I am inclined to make decisions based on the “three good reasons” theory. I have encountered more than three good reasons to choose glycol over drain-back so many times, that our company now installs pv pumped glycol systems almost exclusively. Fig 2 outlines some of the reasons drain-back solar collectors have lost favor with us in our service area.

Most of these drain-back disadvantages are Compatibility issues. Aesthetic and architectural as well as technical Compatibility have proven to be important for the long term acceptance and satisfaction of our clients. Most importantly, our field experience has proven that for us the closed loop glycol system is more Com-? patible with principles 1 and 2: Reliability and Effectiveness.

Regional conclusions may vary

Keep in mind that we are working in a climate (around Santa Fe, N.M.) where it is brilliantly sunny by day with bitter cold, hard freezes occurring all winter. Your choices in other climates may be different than ours. Also, we are targeting the wider home construction and retrofit market for solar hydronic heating installations which require large collector arrays and repeatable designs to be profitable. A narrower market, such as smaller rooftop solar water heater installations, for example, might favor another approach.

I encourage everyone to think through the Six Principles with their own experience and local requirements in mind. The regional differences in building materials, climate, weather patterns, local industry, and consumer preferences may justifiably influence your conclusions to be different from mine. Remember that if you can think of at least “three good reasons” to justify your choice then you are probably on the right track.                   

Bristol Stickney, partner and technical director at Cedar Mountain Solar Systems in Santa Fe, N.M., has been designing, manufacturing, engineering, repairing and installing solar hydronic heating systems for more than 30 years.