Bristol’s six principles for good solar hydronic design

The evolution of ‘Direct’ active solar heating

BY BRISTOL STICKNEY,

contributing writer

So far in this series of articles we have been discussing the key ingredients for solar/hydronic design and installation. They can be divided into six categories, listed below, roughly in order of their importance:

Reliability; effectiveness; compat­ibility; elegance; serviceability; efficiency.

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

A fundamental concept that we employ in every heating system we install is the idea of “Direct” solar heating. This concept is not widely used yet in the hydronic heating industry, but we have been using it with good results for many years in our local region. You might say we are the pioneers of Direct Active Solar Home Heating. Let’s take a closer look at it now.

What is Direct Active Solar Heating?

First let’s get a few definitions out of the way. Passive is the opposite of active. Passive solar heating is when solar heat is delivered only by natural means, such as natural convection, radiation, thermal siphoning and is most often Direct. When a house is designed with many windows facing the sun (allowing it to warm up in much the same way a greenhouse does) it is known as “Direct Gain” solar heat. The heat is collected into the house in a single step, as it passes through the window into the heated space.

Active solar heating is when another energy source (besides the Sun) is used to transfer the solar heat from one place to another that is not provided by nature. It is typical for active components like circulator pumps, fans or motorized valves to be used that are powered by electricity and consume “parasitic” energy. In many Active solar designs, the heating is “Indirect” since it is often delivered through external heat exchangers and holding tanks before it goes to its final destination.

How can an Active Solar heat collector be a Direct system? By delivering the solar heat from the collector to the final heating job in one step. A good example of this is a solar water heater with an immersed heat exchanger in the water tank. A glycol pump delivers solar heat directly from the collector into the potable water. The existence of the pump and its power source and controller make this an Active system, but the solar heat delivery into the potable water is Direct.

From water heating to Direct Active Solar Floor Heating

It is also possible to apply this same solar heating system to a warm concrete mass floor. In fact, one of the earliest Direct Active Solar Heated floors I know of was constructed around 1957 here in Santa Fe New Mexico by a local solar pioneer, Peter van Dresser. The idea was to pump heat directly from solar heat collectors into the heat storage mass of a masonry floor, which has about 1/2 to 1/3 of the heat storage capacity of the same volume of water.

The floor warms up slowly and stays warm well into the evening on cold winter days. When the collectors are sized and tilted properly, the floor is provided with the right amount of heat that does not cause uncomfortable temperatures at any time of the year. When you control the heat in the floor within the comfort range, you realize that maybe you don’t need those enormous solar heat storage water tanks that everyone else is using. The floor acts as a “solar accumulator,” to use a phrase that’s come into favor lately.

Back in ’57, van Dresser was using solar hot air collectors and air ducts in his mass floor. But the idea is even more practical when using hydronic tubing in warm mass floors. Throughout the past 20 years, we have been building Direct solar heated floors. In our climate, a well-insulated mass floor can be heated with about 10 to15 % of the floor area in collectors, and the collectors work quite well when mounted vertically on a south facing wall. The vertical tilt keeps them from overheating in summer, because the high summer sun angle prevents the collectors from gaining heat. The low winter sun angle provides maximum olar heat to a vertical collector during the cold season.

Most people want it all

In recent years we have been seeing less demand for individual solar heated floors or single water heaters and began getting more demand for combination systems with an integrated backup system as well. In an attempt to simplify the installation and control of these combinations systems, I developed a standard plumbing approach that was inspired by a class I attended in the mid 1990s, presented by Dan Holohan, who was teaching about primary/secondary piping.

Primary loop piping offers a comprehensive alternative to the “Spaghetti Diagrams” so prevalent in current solar manufacturer’s installation literature today.

Most of the components for residential heating systems can be treated as modules and plugged together with two pipes. Modular design is already popular in Europe and manufacturers like paw, Caleffi, Precision Hydronics, Watts Radiant, and others are offering more modular components in the U.S. market all the time. These modular designs, using flow separators, can be used in place of primary/secondary site-built piping.

If the plumbing modules can plug in and out with two pipes, the controls can be designed to do the same thing. It is really the controls that make Direct Solar floor heating possible. I have found that controlling the solar heat storage in the mass floors is easier and less expensive than installing big solar heat storage tanks nearly every time. At my company, Cedar Mountain Solar, we build our own control systems that do this job. We use 2-stage room thermostats to allow the solar heat priority over boiler heat. And we use the primary loop system to send the solar heat to where ever it is needed most. The collector is tilted more toward vertical if little heat is needed in summer, and tilted back more if there is a big water heater load or a swimming pool.

In this way we can eliminate big solar water storage tanks in virtually any house that has well insulated hydronic heated mass floors. We have done hundreds of houses like this over the years and it seems like a natural for the hydronic heating industry as a whole to adopt this approach on a wider scale.

Regional conclusions may vary

Final collector to floor area ratios and collector tilts are very climate-dependent and also dictated by the number and type of heating loads attached to the solar heating system.n

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.