The SLIC controller is programmed to prevent solar overheating and to maintain safe high limits and comfortable temperatures. Keeping the collectors below 230°F prevents the propylene glycol from breaking down and becoming acidic, corroding the pipes over time.
When the solar heat is not needed and the collector temperature approaches 200°F, several mass floor zones are opened automatically to cool the collectors by 5°F or so. The cooling cycle only takes a few minutes and does not typically contribute any noticeable heat to the floor. (See the graph on day 2 when the cooling cycle occurs five times.) It is most common to use a garage floor, outdoor ice-melt zone, or swimming pool as heat sinks.
When heat in the house is not wanted, the flat-plate solar collectors are used to radiate heat to the night sky. The DHW tank is used as a heat accumulator by day, and can be cooled through the solar collectors by night. This can be very useful when the house is unoccupied and hot water is accumulating in the storage tank. The floors in the warmest rooms in summer can be cooled by night circulation through the collectors as well.
The homeowner carefully recorded heating fuel consumption, both before and after the solar heating retrofit. Between 2004 and 2006, some fuel savings came from using thermostat setbacks with the old boiler. But, because some of the rooms became uncomfortably cold, the thermostats were raised to around 65°F between 2006 and 2009.
The owner’s analysis of this data includes some interesting highlights. Propane use has been reduced from about 2 to 3 heating degree-days (HDD) per gallon before the retrofit to about 5 to 7 HDD per gallon after the retrofit. For HDD determinations, an outdoor baseline temperature is established (65°F) where it is assumed that no space heating is used. Whenever the outdoor temperature drops below this baseline, it is assumed that the house will need some heat. If the average outdoor temperature drops 1°F (to 64°F) for 24 hours, that condition is defined as “1 HDD.” If you know how cold it is in HDDs over a given period of time, and you know how much fuel you used (e.g., in gallons), then you can calculate gallons per HDD, or the inverse: HDD/gallon. This is a good way to compare the fuel efficiency of your house over any period of time, much the same way automobiles are compared using mpg.
For domestic hot water, propane use is down to an average of 0.6 gallons a day versus 1.5 gallons per day previously. This past winter, the house netted 273 to 375 kBtu per day of solar heat (80 to 110 kWh per day). Annual propane consumption has dropped by about two-thirds, saving about 1,300 gallons a year. At current local prices, this translates into saving $3,000 per year. The total cost of the retrofit was $57,315. After state and federal tax credits totalling $21,459, the net system cost was $35,856, resulting in a simple return on investment of 10 years.
Bristol Stickney has been designing, manufacturing, repairing, and installing solar hydronic heating systems for more than 30 years. He holds a B.S. in mechanical engineering and is a licensed mechanical contractor in New Mexico. He holds several patents related to solar/hydronic heating and control and is the Chief Technical Officer for SolarLogic, where he develops solar heating control systems and design tools.