Do solar water heaters work in winter in the Northeast?

One of the most common (and important) questions asked about any solar-related technology in the Northeast is this: How well will it work in the middle of winter when it’s below freezing outside and the sunlight is weak?

The first thing to remember is that solar water heaters are not meant to completely replace your water heater. A typical solar water heater will be able to heat 60-80% of the water you use over the course of that year. From April through September, nearly all of your hot water will be solar heated. In winter, the percentage of your hot water heated by the sun drops to as low as 10-20%—as you might expect with short days and weak sun in December. That’s why practically every solar water installed in the US will be connected to a backup conventional water heater to ensure that your hot water needs continue to be met even in January.

As for risk of freezing, any active solar water heating system (see here for an explanation of the different types of water heaters) should be designed to not freeze in the winter. There are a couple ways in cold climates to prevent freezing: the first is by having the solar water heater circulate a non-toxic antifreeze mixture through the collector and heat the water in your tank indirectly (through a heat exchanger) rather than circulating water, which could freeze and damage the collector. The other is to use what is referred to as a drainback system, which circulates either water or an antifreeze mixture, using a temperature sensor combined with a pump to drain the fluid out of the system when it is too cold or when the sun stops heating the water. Any solar water heater system installed by a reputable installer in New England will be protected from freezing.

Despite the long winters, Massachusetts actually ranks among the top states in the United States in solar water heater installations--only behind the usual, sunny suspects (California, Hawaii, Florida, and Arizona).