If you’re aiming to reduce your carbon footprint, you’ve probably thought about installing a solar hot water system. Here’s the good news: if you have an unshaded south-facing roof, you can install a solar hot water system that will meet about half your annual hot water needs.
The bad news: the typical solar hot water system costs between $6,000 and $10,000.
ICS, Drainback, or Antifreeze?
There are three main types of solar hot water systems (also known as solar thermal systems):
Integrated collector-storage (ICS) systems, also known as “batch” systems, store hot water in a roof-mounted tank above the collector. Such systems are common in Israel, Hawaii, and other warm climates where freeze damage is unlikely.
Those of us who live in frosty climates need a freeze-resistant system. To protect against freeze damage, a solar hot water system either drains all the fluid from the collector when it’s cold — the drainback approach — or circulates an antifreeze solution through the collectors.
The collectors in a drainback system are usually dry. When temperature sensors indicate that the sun is shining, a control activates a pump which circulates water through the collector pipes. Later, when the control senses that the sun has gone away, all of the water in the collectors is automatically drained to a “drainback tank” — a special tank, separate from the home’s hot-water storage tank, that holds the collector water. The next day, when the sun comes out again, the water from the drainback tank is again circulated through the collectors. Whenever the pump is operating, hot water from the collectors circulates through a heat exchanger to raise the temperature of the water in the main storage tank.
Propylene Glycol Antifreeze
The second way to make a solar hot water system freeze-resistant is to use an antifreeze…