The solution to my first efficiency problem was to install an instant hot water circulator (IHWC). This eliminates the issue where the boiler turns on to reheat the existing water heater tank despite having plenty of hot water available in the solar storage tank. This device also eliminates the annoyance of waiting for hot water to arrive at the faucet.
An IHWC is installed under the sink that is farthest from the water heater and monitors the temperature of the hot water at the sink. When the temperature drops below an adjustable set point (100°F, in this case), it turns on its circulator pump, which draws hot water from the water heater, keeping hot water at the faucet all the time. If not used in the sink, the water returns to the tank via the cold water line. When the hot water line reaches the required temperature, the IHWC pump shuts off. In this way, hot water is kept very close to all the faucets along the same hot water supply line.
While having on-demand hot water is nice, I wanted to save oil, not use extra by drawing water from the water heater tank during times when solar-heated water of sufficient temperature was not available. I accomplished this by plugging the IHWC into an X-10 appliance module and connecting an X-10 PowerFlash transmitter module to a small thermostatic switch installed at the top of the solar tank.
When the solar tank is at 125°F (5°F higher than the water heater tank’s set point), the thermostatic switch closes and causes the X-10 transmitter to send a signal through my home’s electrical wiring to the appliance module at the IHWC to power the IHWC’s automatic operation. When the temperature in the solar tank drops below 125°F, the thermostatic switch opens, and the transmitter signals the appliance module to turn off the IHWC. This means the IHWC runs only when the tank temperature is more than 125°F and the IHWC temperature at the sink is less than 100°F. With the system properly adjusted, the boiler should stay off as long as sufficiently heated water is available from the solar storage tank. (For homes with 240 V single-phase service split into two 120 V legs, both X-10 modules must be on the same leg to communicate.)
The second efficiency issue—where heat becomes “trapped” in the drainback tank—is more complex. Provided a good amount of hot water has been used (cooling the bottom of the storage tank), and it’s not too cold outside, the SunEarth controller can be manually set to run the circulator. But if it’s too cold outside and glycol is circulating, the cold solar collectors will absorb usable heat from the glycol and begin to cool the solar hot water tank. If it’s warm out and the circulator is running, it will quickly heat up the collector until it is hot enough to fool the controller into thinking the sun is shining, keeping the system running. At this point, the controller should be set back to automatic mode. When the solar controller finally shuts off the circulator, the storage tank will be much warmer, having absorbed much of the drainback tank’s residual heat. Nineteen gallons of 140°F glycol can raise the bottom third of the 40-gallon tank from 75°F to 105°F. This is more than enough heat to take a hot bath and saves about 0.1 gallons of oil.
Unfortunately, no differential solar controller I’m aware of, including the dual-output models, will allow this process to be automated. What’s needed is a special, additional controller that will turn on the circulator for a few minutes when the drainback tank’s temperature is a few degrees hotter than the solar tank’s bottom (and the collector is not too cold), until the automatic function of the main differential controller takes over. Since no commercial solution appears to be available, I hope to design one myself. (Contact me via e-mail for more information.) For now, the only way to extract the trapped drainback heat is to manually set the controller.
Historically, my family used about 550 gallons of oil for heating water, split almost evenly between the nonheating and heating seasons. The solar hot water system should offset 80% to 90% of this during the nonheating season, and about 25%, or 56 gallons of oil, during the heating season. Annually, the system offsets about 250 gallons of oil. At $4 per gallon, that’s a savings of about $1,000 per year, which covers most of my loan payments for the system.
Calculating the savings from the IHWC is more complex. During the nonheating season, I estimate my boiler (which burns 3 gallons of oil per hour) fires six times per day to compensate for water heater tank cooling. It fires for about 3 minutes each time, for a total of 18 minutes, burning about 0.9 gallons of oil per day. From June to September, the solar hot water system will supply all our hot water on clear days. According to meteorological statistics, there are likely to be about 32 completely cloudless days in my area for that time period. This means the IHWC and associated components are likely to save a minimum of 28.8 gallons (32 days x 0.9 gallons) of oil per year, recouping costs in about three years.
To calculate the potential savings from my system that extracts heat from the drainback tank, it’s necessary to estimate the number of times a cloudy day follows a clear day from June to September. I figure that a cloudy day comes after a clear day for a total of 32 days during that period. Since the drainback tank stores the equivalent of about one-tenth of a gallon of oil (as discussed above), an automatic system would save 3.2 gallons per year. This estimate does not include the reduced losses or increased efficiency savings discussed previously.
Andrew Goldbaum has an MBA, and is an electrical engineer, programmer, and director of software development for a small New York-based defense contractor.