﷯Solar Water Heating

A lesson in how the “old school” ways can change to become solar water-efficient. 

In the past year or so, we have visited a number of public schools that have existing solar domestic hot water (SDHW) heaters. The Albuquerque school system is committed to energy efficiency for its buildings, old and new. Our job was to document and evaluate what was already in use so that it could be modified and improved upon wherever possible, and then upgrade the design for use in the new schools planned for construction in the near future.  

In our investigation of “old school” solar water heating (pun intended), we have uncovered a common piping configuration that can significantly limit the performance of a solar water heater. We find that this plumbing detail has been duplicated in many solar water heaters, not only in the public schools we visited, but in residential and small commercial installations. This is a common plumbing pitfall that is easy to avoid, and so, here is a brief description of the issue.
What is wrong with this picture?

Figure 99-1 shows a simplified schematic diagram of the typical SDHW system found in the schools we visited. This is a “series tank” configuration, where the solar water heater is added in series with a conventional domestic hot water (DHW) heater, and the solar heat storage tank is connected between the cold water supply and the boiler-heated DHW tank. Solar heat collectors deliver heat to the solar storage tank where solar heat accumulates and the water temperature rises during the day. When any hot faucet in the building is opened and hot water pours out, cold make-up water enters the solar tank and solar-heated water moves from the solar tank to the boiler heated tank. In this way, solar heated water provides pre-heat to the boiler, which boosts the water temperature if needed before moving on to the open faucet.  

An anti-scald thermal mixing valve is provided to make sure that the hot water delivered to the fixtures is always a safe temperature, even when the solar-heated water exceeds the normal boiler temperature. The proper anti-scald system must be chosen to be compatible with solar temperature fluctuations, maximum hot water flow rates and DHW re-circulator flow rates. (These details are very important but are not covered here.)

There is also a DHW re-circulator pump that moves hot water from the DHW tank around the building to all the fixtures and then back into the DHW tank. This ensures that hot water is instantly available at all hot faucets in the building. In New Mexico, this is an important water-saving feature in our desert climate to prevent lukewarm water stranded in the pipes from going down the drain. Of course, the instant hot water is also a time-saver and provides enhanced satisfaction for the hot water user.

The main purpose behind the solar heating system is to provide a (free) renewable heat source every sunny day at a high enough temperature to prevent the boiler from firing for a significant number of hours each day. But, this common piping configuration limits the boiler savings in a way that is unnecessary and unintentional.

Rethinking the popular series-tank piping

The system in Figure 99-1 will work adequately (as described above) as a functional solar water heater, but it is not optimal. This is not whole-system design thinking, but rather, treats the solar water heater as an afterthought added onto a conventional boiler-heated DHW system. The boiler DHW operates on its thermostat setting independently of the SDHW system, and the only time the boiler “knows” what the solar temperature is, is when water is running out a hot faucet. It is only when hot water is running, that the solar-heated water moves into the DHW tank and has a chance to shut off the boiler thermostat and provide that solar savings that everyone is expecting.

So, when the hot faucets are not open (which is most of the time), the solar heat will build up in the solar storage tank with nowhere to go and even cause occasional overheating in the solar collectors. Even though the solar collectors are typically capable of eliminating the boiler standby operation, they are disabled from doing so in this plumbing configuration. This is a serious disadvantage especially in a school building, since there are long periods of standby with little hot water flow such as on weekends, weeklong vacation periods and of course the low-occupancy period all summer long. A properly-sized SDHW system like this is capable of virtually eliminating boiler run-time during periods of low occupancy as well as providing a healthy solar heating fraction during normal school hours with only a minor modification to the plumbing.

A small improvement with big rewards

After we identified this widespread opportunity for improvement, we developed a plan to modify this type of SDHW system to improve the solar heating performance. We accomplished this without replacing any of the heating system components and using the simplest possible plumbing changes.

Figure 99-2 shows a simplified schematic diagram of our modified piping strategy. You may not notice it at first, but the only plumbing modification is to disconnect the (purple) cool return pipe from the DHW re-circulator from the DHW tank, and reconnect it to the solar storage tank. This may seem like a trivial change, but it has a number of real benefits. The DHW re-circulator can now be controlled to run all the time or turn on during high or low occupancy or whenever the solar storage tank is hot. Whenever it runs, the solar-heated water moves from the solar tank into the boiler tank, even when there aren't any  open hot faucets. In this way, the solar temperature can be delivered to the boiler tank so that the boiler thermostat can be shut off whenever the solar is hot enough. As an added bonus, we have now increased (e.g. doubled) the size of the solar heat storage water tank to include the full volume of the boiler DHW tank. So we can store more useful heat over weekends and holidays and even add more solar heat collectors to match the size of the larger heat storage tank capacity.

This retrofit to the re-circulator plumbing makes much better use of solar heat to offset boiler run time. It does this by using the re-circulator as a solar heat transfer pump, which can be controlled intelligently and efficiently. The SDHW system is no longer largely disabled during times of low (or no) hot water usage. In a public school with typical intermittent use patterns, this virtually doubles the number of days per year that the solar heat is allowed to make its full contribution to fuel savings.

Other improvements and upgrades

In our investigation into existing SDHW systems we have also found other obvious targets for upgrade and improvement. The next SDHW system to be installed in a new Albuquerque public school will likely have a number of enhanced features to boost reliability and effective performance. The plumbing plans include an improved series tank design, improved use of heat exchangers, more compatible thermal mixing valves, more solar collectors with better heat storage and fail-safe thermosiphon self-cooling. Updates to the control system will include better boiler control, higher efficiency circulators, energy metering, data logging and internet monitoring with remote control.

Final notes

Thanks to Dr. Fred Milder for the schematic diagrams seen here. 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 in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. Back issues of this column can be found in the archives at TMB Publishing and SolarLogic LLC websites. 

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 CTO for AMEnergy/SolarLogic LLC in Santa Fe, New Mexico, where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit solarlogicllc.com for more information.

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