Bristol's six principles of good solar hydronic design

Part 4: Effectiveness and Serviceability

By Bristol Stickney

Technical Expert

In this article, I continue our ongoing discussion of the broadest principles of good solar heating design. Over the past few decades, I have found that the best solar heating installations do not depend upon any single idea or product but rather on the careful integration of all of these principles into every installation. The key ingredients for solar/hydronic design and installation can be divided into the following six categories, listed roughly in order of their importance: reliability, effectiveness, compatibility, elegance, serviceability and efficiency.

The success of any solar hydronic home heating installation depends on the often-conflicting balance between any of these six principles. In past columns, I described the conflict between many of these principles to illustrate the need for a balance in the overall design. This month I would like to focus on Effectiveness and Serviceability. These concepts are not always in conflict. In fact, sometimes they reinforce each other nicely. Let's take a closer look at these two principles.

Principle 2: Effectiveness -- Think about user satisfaction.
Don't you hate it when customers complain? Well, if we offer solar heating systems that are effective, they 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). Part of the cost may be justifiable in terms of lower carbon emissions and other environmental benefits, solar tax incentives or other intangibles unrelated to heating system performance. But it must be a good heating system as well, providing heating comfort (or hot water) upon demand, low fuel consumption, reasonable operating cost, low maintenance, no special attention and uncomplicated user controls.

An effective solar heating system has all the same requirements as, say, a hybrid car. A well-designed hybrid car will meet the driver's expectations in every way, when compared to the driver's old car. A solar heating system in today's market must do everything that the old heating system did but with the added benefits of comparatively low operating cost and low emissions. Solar heating will enjoy the same acceptance as hybrid cars are gaining only if they are more effective than the old home heating system was.

Solar hydronic heating systems, whether used for domestic hot water or for larger heating needs, apply the "hybrid car" approach to home heating. In the car, electrical energy is stored in batteries to be delivered to the drive wheels later, at times when the battery power can offset the use of gasoline. This allows the gasoline engine to run less often and at a rate that is more fuel-efficient.

Active solar heating systems have developed over time to work the same way. The solar heat is stored in thermal mass, allowing it to be used later to offset conventional heating fuel. This reduces the use of conventional fuel and can also be designed to improve the efficiency of the fuel consumption. The thermal mass can be installed in several different forms, such as domestic hot water tanks, other insulated water tanks, a swimming pool or hot tub, or the thermal mass of the masonry radiant heated floors themselves.

Just as in the hybrid car, the mechanical system and controls must be in careful balance. The car batteries must be regulated to prevent overcharging them. When in danger of overcharging, there must be a safe way to deal with the excess electrical energy. When the batteries are low, the control system must decide when to start the gasoline engine, and when to send energy-regeneration from the brakes to the batteries. When the battery or regulator malfunctions, you still want the gasoline engine to get you home reliably. The designers of hybrid cars had to anticipate every possible contingency and mode of failure. Then they had to build in enough intelligence in the control system to make it both fail-safe and reliable. If they hadn't done this, few people would want to own one.

This is the same approach that we are taking with solar heating these days. We don't take the solar collectors too big, because that would require too much heat storage. The controls have enough intelligence to prevent solar overheating, while providing seamless operation of the conventional backup heat in a prudent yet efficient way. All types of possible malfunctions are anticipated and compensated for by the controls, without the sacrifice of reliability. The mechanical configuration and the control system go hand in hand to allow every practical heating need to be supplied by solar heat automatically, whenever possible.

Principle 5: Serviceability -- Make it easier to install, repair and adjust.
Isolation valves can be a bargain. When things go wrong, a quick and economical repair is essential to keep warranty costs down and to maintain the confidence of the owner. When purchasing a solar heating system, a buyer will often apply pressure to get a lower price. If we give in to this pressure, we may do things that we will soon regret. It is false economy to eliminate critical ball valves, drains, air vents, check valves and other equipment that allows for rapid filling, draining, air elimination or component replacement. I hate to see a repair crew struggle for hours to do something that should only take a few minutes if properly designed.

Pre-fabricated plumbing modules are being offered by more manufacturers (such as Caleffi and P.A.W.) these days, for a variety of hydronic heating applications. These modules typically contain a pump, isolation valves, gauges, check valves, relief valves, air vents and other components already pre-assembled. The use of pre-fabricated plumbing modules promises to reduce the labor cost of an installation, allowing completion of more installations each month. Also, since it is not possible to remove the critical components from a pre-fab module, that temptation is eliminated.

The following example illustrates how serviceability and effectiveness play out through the use of internal versus external heat exchangers in closed-loop solar domestic hot water systems. When implementing a design based on any of our principles, keep in mind that the best offense is a good defense. Avoiding a life-long maintenance problem is a great way to make a system more serviceable.

The exchanger experience
Have you noticed the 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 saving the cost of a new solar storage tank. It is, presumably, cost effective. Some such exchangers are modular with two pumps and a delta-T 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 and the water-side pump will then burn out. In my region of New Mexico this happens with such regularity that I no longer use external heat exchangers for domestic water unless there is a proven source of soft water or a maintenance contract, under which the heat exchanger is regularly flushed with acid solution. Even if chronic scaling 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. Installing tanks with internal coils avoids a life-long need for system maintenance, making the system more serviceable, and, as a result, more effective. We mostly use tanks made by Heat Transfer Products and Oventrop. Recently, Caleffi announced its packaging of a new solar domestic hot water system using a tank with internal coil. In my opinion, this system will prove more reliable, effective and serviceable than its competitors.

 

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