Bristol’s six principles for good solar hydronic design

BY BRISTOL STICKNEY,

contributing writer

Over the past two months I have introduced the six principles of good solar heating design (reliability, effectiveness, compatibility, elegance, serviceability, & efficiency). The key to using these principles as guidelines is to accept that the principles are in conflict with one another, and incorporate ideas and technology that achieve a balance between different principles. For example, if you only design a system for efficiency, you may lose so much reliability that the system will not provide its promised benefits.

Last month I wrote about how the balance between reliability and efficiency affects a choice basic to all solar hot water systems -- the use of a PV circulator for the solar collectors rather than a differential controller with ac circulators. In New Mexico, where my company Cedar Mountain Solar installs, the climate is unforgiving and the power grid susceptible to failures and voltage spikes, making PV pumps a great choice. Since PV pumps circulate only when it’s sunny, using them is also a very elegant solution because even without temperature controls they work well in almost all of our applications. We’ve used all kinds of PV pumps, from Ivan Labs’ El Sid 5w pumps to Dankoff SunCentric 150w pumps, for different sizes of collector arrays. These days we use the Laing D5 Strong pump almost exclusively because it seems like the most reliable and flexible solution, working with different PV wattages depending on flow requirement. For larger arrays we use several pumps in series or in parallel rather than going to a larger pump with a brush-type motor that will require maintenance over time.

As I did last month, I’m going to give you an example of how the principles affect our choice about a major system design decision, which again, impacts almost all of our solar hot water system installations at Cedar Mountain Solar. Reliability is so important to long-term customer satisfaction, and therefore to the growth of the solar industry, so I am including it in this month’s example as I did in last month’s.

Principle #1: Reliability -- Make it reliable over the long term.
As we’ve said, for solar heating to be widely adopted, it is imperative that installed solar heating systems be at least as reliable as a conventional Hydronic boiler system and carry at least the same warranty service as the best boiler heating systems. Reliability is the key to developing the perception that the extra investment in solar heating is worth it for the client.

Principle #2: Effectiveness -- Think about user satisfaction.
If we provide solar heating systems that are Effective, customers will have little to complain about. “Effectiveness” includes everything that the owner needs to feel satisfied with the system. The initial cost must be reasonable and justifiable, (cost-Effective) usually in terms of reduced fuel use and environmental benefits. The heating system has to be effective in other ways as well, providing heating comfort (or hot water) on demand, reasonable operating cost, low maintenance, no special attention and uncomplicated user controls. The systems we install at Cedar Mountain Solar require the customer only to set their room thermostats as they would with a non-solar system. There are no valves to adjust, collector covers to install in summer, or atmospheric tanks to “top off.” (More on atmospheric tanks in future articles.)

Let me share an example with you of reliability coming into conflict with effectiveness in system design:

The exchanger experience
Have you noticed the solar equipment ads that show how easy it is to hang an external heat exchanger on an existing hot water tank to transform it into a solar heated tank? The main advantage of doing this is you can save the cost of a new solar storage tank. It is presumably cost-effective. Some heat exchange units are modular, with two pumps and a st control pre-assembled. Some have only one pump (for the solar fluid), and the water is supposed to thermo-siphon by gravity flow into the water tank. These modules conform to the laws of physics, and when first installed, usually work quite well.

But then something insidious can happen. It may take a few months, or a few years, but the water side of the heat exchanger can clog up with minerals, causing the water-side pump to burn out. In my region of New Mexico this happens with such regularity that I no longer use external heat exchangers on open domestic water unless there is a proven source of soft water or a maintenance contract in place. Even if this type of failure happens to only a small number of solar equipment buyers, those people will not get their expected solar benefits, and will tell everyone they know about it. The last thing the solar heating industry needs is another black eye.

At Cedar Mountain Solar, we religiously use in-tank heat exchanger coils for solar domestic hot water. This is because even when the coil becomes coated with minerals, it still continues to make hot water and there is no water-side pump to burn out. The solar thermal efficiency may drop over time, but the solar hot water maker does not simply quit, causing the solar collectors to overheat. For us, this is a clear case of long-term Reliability outweighing short-term cost-Effective (false) economy. For lower-cost tanks with internal heat exchangers, check out the Vaughn sepco or Heat Transfer Products’ Solar Contender tanks. Higher-end tanks, usually stainless steel, are made by many manufacturers including Oventrop, Crown, and Amtrol. (More on choosing a tank in future columns.)
Again, regional conclusions may vary.

I encourage you to use the Six Principles based on your own experience and local requirements in mind. Principles can never be great design tools if applied in a vacuum -- it’s applying them to your unique situations that will make your system designs and installations most successful.

In coming months I’ll examine different system types, such as closed-loop and drain-back systems, as well as different component types, such as different collector technology, through the prism of the Principles.