Best solar DHW pre-heat systems

IN LAST MONTH'S column ("Active and passive solar pre-heat systems," pg. 36 ), the final system we addressed was the closed-loop pressurized system. One of this system's major Achilles heels is the potential for exposure to stagnation conditions. This generally happens during the summer when the system has maxed out the solar storage tank at its top recommended temperature of 180F, and there is still

IN LAST MONTH'S column ("Active and passive solar pre-heat systems," pg. 36 ), the final system we addressed was the closed-loop pressurized system. One of this system's major Achilles heels is the potential for exposure to stagnation conditions. This generally happens during the summer when the system has maxed out the solar storage tank at its top recommended temperature of 180F, and there is still a lot of solar radiation shining on the solar panels.

A simple means of overcoming this problem is to install a temperature sensor on the return line from the solar collector. If the fluid temperature comes back at temperatures higher than 180F, the solar collector pump and the storage tank pumps are kept running until the fluid cools down to below 175. This keeps the fluid from breaking down due to thermal degradation, the loop pressures within defined limits and the system from melting down.

Although problematic by nature, the pressurized closed-loop system may be your only alternative if the solar array is lower than the storage tank and mechanical package. Don't discount its use. Just make sure you've covered all the potentials it might see, including long-term stagnation. With these controls added to the features, the only thing your system won't be capable of handling is a major power failure during peak solar gain, which rarely happens.


It is possible to shut off your backup water heater during the summer months.


Now on to the ultimate system. A drain-back closed-loop solar system is similar in design to the closed-loop pressurized system. It has a heat exchanger, a small drain-back reservoir and an additional pump to move water between the heat exchanger and the DHW preheat storage tank. With this system, it is generally required that the solar collectors be mounted higher than the mechanical package. It is also required that the piping between the solar collectors and the drain-back reservoir be pitched so that when the solar pump turns off, the fluid drains back from the collectors to the storage reservoir. Hence, the name, "drain back."

Its advantages? During freezing weather, the fluid (water or water/glycol mix) is drained back into the heat of the house and the collectors are not subjected to freeze-burst potential. If the solar system should happen to max out the storage tank during the summer months, the collector loop pump turns off and the fluid drains back to the reservoir. The collectors are made to handle the stagnation temperatures ( 350  ) that will be seen during peak insolation and non-flow conditions.

The disadvantages? The system requires that the solar collectors be mounted higher than the mechanical package, and that all attached piping, including the solar collector array, be pitched at 1/4 in. per ft. back to the mechanical package. The pump required to move the solar fluid is generally a high-watt consumption pump because it has to lift the water from the basement to the top of the solar collectors and retain adequate velocity to establish-a siphon on the down-comer. The collectors can be quite noisy if the collector pump is started back up during a peak stagnation condition. Collectors sitting at 350 have a tendency to generate steam when they are hit with much cooler water.

In general terms, these systems are sized with one 32-sq.-ft. panel per person in the family. In addition to the collectors/heat exchanger/drain-back package, you will need a good solar preheat storage tank. The storage tank is sized based on array size, and you will want to keep the ratio of gallons per square foot at around 1 to 2 gal. per sq. ft. A system of this configuration will be capable of delivering about 20 gal. per person per day, or about 70% of the annual DHW energy needs for residential consumption. This means that it will provide 110% of the hot water needs during the summer months, and about 50% of the winter DHW needs. It is actually possible to shut off your backup water heater during the summer months.

For commercial use, you would still only want to size the system based on its capacity to provide 70% of the annual DHW demand. Sizing the system for smaller solar load fractions ensures that the "system" as a whole will operate at peak efficiency, and that there will be very few times of the year that the solar falling on the face of the panels is not being fully utilized. The most expensive solar system is one that is under-utilized because it's been oversized. Keep your system design as simple as possible. Remind the customer that the system must have an annual maintenance inspection to ensure reliability and high performance.

Tune in next month as we continue to delve into the hydronic heating systems of the future. Until then, happy solarized hydronicing!

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.

Joulè merger takes firm private
EDISON, N.J. — Joulè Inc. has completed its merger with JAC Acquisition Co., a Delaware corporation owned by Emanuel N. Logothetis, the founder, chairman of the board and chief executive officer of Joulè; members of his family; and John G. Wellman Jr., president and chief operating officer of Joulè.

As a result of the merger, most shares of Joulè common stock were converted into the right to receive the merger consideration of $1.70 per share in cash. The certificate of merger was filed Aug. 13 with the Delaware secretary of state.

At the effective time, Joulè common stock ceased to be traded on the American Stock Exchange and became eligible for deregistration under the Securities Exchange Act. JoulÈ said it intends to deregister its common stock soon.