Solar Domestic Hot Water and On-Demand Domestic Hot Water
For DHW the Strawbale House uses active solar and an on demand (tankless) system. Hot glycol from the flat plate collectors enters the house and passes through the 120 gallon DHW storage tank. The storage tank is a well insulated stainless steel tank with a built in internal heat exchanger.
This system generally supplies us (a family of three) with about 80 percent of our DHW needs from mid-May through mid-October and about 50 percent the rest of the year. For the balance of our DHW needs we use an on-demand or tankless heater fueled by propane. The on-demand system uses stainless steel tubing coiled around a small but powerful combustion box. When you turn on the hot water faucet the on-demand unit detects a pressure drop in the hot water line and automatically fires the combustion box, water moving through the coil is heated instantaneously. When the hot water faucet is turned off the combustion process ends this system eliminates the need to store water heated by fossil fuel.
Americans love their hot water. But what folks do not realize is that hot water heaters are the largest energy gobbler in the home, after heating and cooling units. According to Jay Burch, a solar specialist at the National Renewable Energy Laboratory “Running the water heater of a single family home for one year creates more emissions than driving an automobile 12000 miles”. The average American household spends 20 percent or more of its energy bill on hot water, and much of what is paid for is heat lost through the thin walls of the storage tank in the basement or utility room. This waste of energy can hit the pocketbook and environment hard. “The greatest inefficiency of a hot water heater lies in the heat the tank loses during the time it sits around”, said Yen Chin of Seattle City Light.
A conventional system runs 24 hours a day in order to heat and store many gallons of water. Home owners pay for all this energy though no hot water is used while they are at work, a sleep, or on vacation. Even after water heaters have ceased to work, they continue to harm the environment. Because a life span of a tank heater averages 10 years, homeowners must replace them frequently. According Get Tankless, a water heater manufacturer, more than 7.3 million hot water tanks are land filled in the U.S. each year. (Bluefish.org.)
Pressurized Glycol Systems
In this active, closed-loop system, incoming potable water is routed to the solar storage tank, but never into the collectors. A water and antifreeze mixture circulates from the collectors through a coil of pipe in the solar tank, and then is pumped back through the collectors. (In most climates, a 50/50 propylene glycol and water mixture will keep collectors from freezing.) The potable water is warmed by heat transfer through contact with the pipe.
These systems require an expansion tank and a few other auxiliary components for filling, venting, and maintaining the system. A definite advantage to antifreeze systems is that the collectors can be mounted anywhere. These systems are pretty much the only choice in very cold climates.
Solar Domestic Hot Water DHW and Tankless On demand Domestic Hot Water in the Strawbale House Tour