In-tank heat exchangers

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

Nearly every solar heating installation I have done in recent years has included a solar-heated domestic hot water (SDHW) tank. The smallest systems sometimes include no other solar heating load. While the larger solar combisystems may include radiant heated floors, hot water baseboards, swimming pools and other heating jobs, the SDHW tank is virtually always included.

I tend to shy away from external solar heat exchangers when potable water is involved. This is because, in my region of northern New Mexico, hard water is a fact of life. I have lost count of the number of times that I have pulled apart an old solar water heater and found the potable water-side of the original external heat exchanger completely clogged with minerals, and the water-side circulator ruined for the same reason. The heat exchanger looks like someone filled it with concrete. In some cases, this has happened as quickly as only two years or so after installation.

External heat exchangers have been (and still are) marketed as a cost-saving way to convert a conventional DHW tank into a “solar” tank. But when this equipment clogs up and dies prematurely, where is the savings in that? So, whenever the water quality might be questionable, the solution I have found to be both Reliable and Elegant is the in-tank heat exchanger. It continues to produce solar hot water year after year, even when the internal surfaces become coated with minerals, and there is no need for a water-side circulator, since the heat exchanger is immersed in the potable water. My goal, as a solar designer, has always been to make the solar equipment last as long as conventional equipment or longer. The in-tank heat exchanger has proven equal to the task.

Single wall or double wall

Many manufacturers of in-tank heat exchangers offer the choice of either single wall or double wall construction. Single wall heat exchangers have become more popular in recent years, since non-toxic heat transfer fluid has become a common standard in solar hydronic systems. Single wall construction is typically lower in cost and higher in thermal performance. Single wall heat exchangers are usually permitted if they satisfy all of the following requirements:

1. The heat transfer medium is potable water or contains only substances which are recognized as safe by the U.S. Food and Drug Administration (e.g. propylene glycol).

2. The pressure of the heat transfer medium is maintained less than the normal minimum operating pressure of the potable water system.

3. The equipment is permanently labeled to indicate that only additives recognized as safe by the FDA shall be used in the heat transfer medium.

A leak in a single wall heat exchanger will cause the higher pressure water to mix with the lower pressure hydronic fluid, typically causing the hydronic pressure relief valve to discharge and diluting the hydronic fluid with water. When the toxicity of the hydronic fluid is in question, or when the administrative authority will not allow single wall for any reason, a double wall heat exchanger will usually pass inspection. It is best to confirm this detail with the relevant authority before ordering the tank.

Water heaters with double wall heat exchangers meet the Uniform Plumbing Code for installation in all potable water systems. The double wall construction provides protection in the event that either the potable or hydronic heat exchanger barrier is penetrated. The fluid will move along in integrated leak-path between the walls of the heat exchanger, leaving the exchanger through a weep hole located in the fittings on the outside surface of the tank. In this way, any leak becomes visible in the mechanical room on the plumbing connections on the tank.

Fin-tube coils

When most people think of a heat exchanger, they commonly think of tubes with fins. Fin coils are well known, since they have been in use in side boiler “side arm” tanks (a.k.a. boiler indirect water heaters) for many decades. Fins are added to small diameter tubing to increase its outside surface area. The heat transfer of an immersed tube is driven only by the natural convection of the water surrounding the coil. Adding fins allows more liquid to make contact with the tube, increasing the amount of convection at any given time. The fin tubes are coiled and folded into a compact shape that will fit through a small access hole.

Figure 38-1 shows two examples of fin-tube tanks that I have used successfully on solar heating installations in the past. The Amtrol examples on the left are “boiler side-arm” tanks that have made an easy transition to the SDHW world. The Vaughn example on the right allows slightly easier removal of the coils through the side of the tank, rather than through the top and bottom.

When exposed to hard water, mineral deposits build up on the fins. The fins can become completely covered by minerals, and the heat transfer will slow down considerably, but the system will still make hot water, heated by lumps of hot minerals. In many cases, maintenance has been overlooked for a decade or more, even with significant mineral buildup. It is possible, however, to remove the coils and dip them in cleaning solution to remove the mineral deposits. This procedure can be less frequent and less costly than repairing or replacing the clogged tubing and failed pump from an external SDHW heat exchanger.

Smooth-tube coils

Another way to increase the surface area of an immersed coil is to use a smooth tube (no fins) with a larger diameter. An example is shown in Figure 38-2. There has been a large increase in the number of tanks that offer this choice in recent years, especially in a variety of stainless steel products. These tanks show great promise in both longevity and thermal performance. This seems to be an instance where we have rediscovered that “simpler is better.” As the larger diameter tubing heats and cools during its daily routine, the metal surface expands and contracts. The smooth surface tends to break away from the mineral deposits, which can actually fall away from the tubing. So, the smooth tubing is more likely to be self-cleaning than the fin tubing during thermal cycling. Even coated with minerals, the large smooth surfaces will continue to provide heat to the surrounding water.

Another version of this can be seen in Figure 38-3, in which a stainless steel water tank is contained inside a second tank. The surface of the water tank itself becomes the “smooth tube” single wall heat exchanger. The water tank is essentially immersed in a bath of hot hydronic fluid. If minerals build up, they do so on the entire inside surface of the water tank; again, this will not stop the production of hot water in the tank. We have installed a number of tanks like those seen in Figures 38-2 and 38-3 in recent years, and so far they have lived up to our expectations.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 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 only to provide examples for illustration and discussion and do not constitute recommendation or endorsement.

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 www.solarlogicllc.com for more information.