The freeze-protection approach for single-tank systems can be either drainback or closed-loop glycol, so there’s a lot of design flexibility and component options. The best approach will depend on your local climate and the ease of installation of one system type or the other at your home.
If your household is four people or fewer, and your water use is somewhat routine, consider a single-tank system. If space is an issue, the single-tank system may be your only choice. But perhaps the better question is, “Is there anyone who shouldn’t consider a single-tank system?”
If you have five or more in your household, or use large amounts of hot water, the extra capacity of a two-tank system certainly has value. If you have the problem of frequently running out of hot water (teenagers, perhaps?), then you may need all the hot water you can get. A single-tank system is a good fit for conservationists—but if you’ve tried everything you can and still don’t have enough hot water, then stick with two tanks. Something as simple as installing 1.5 gpm, low-flow showerheads instead of conventional 2.5 gpm showerheads can be enough to solve the problem of not having enough hot water from a single-tank system.
If you currently have a gas-fired tank-style water heater and there’s no room for an additional 240 VAC breaker in your electrical panel, then putting in new electrical service just to add the hot water circuit might not be cost-effective. If this is the case, you can still install a solar hot water system—just keep the existing tank heater or use a gas-fired tankless backup water heater. However, be aware that tankless heaters, especially gas models with venting requirements, can be expensive, and their savings are not as great as most people anticipate.
One disadvantage of a single-tank system is occasionally not having enough hot water when use is unusually high and sunshine is unusually low. Single-tank system owner Geri Bogart from Baker City, Oregon, reported that she ran out of hot water for the first time over Christmas when there were four extra houseguests and no sun for days. “I figured out I just had to space out hot water use, allowing a half hour or so between showers. Then everything was fine.”
Single tanks can also be expensive to replace if they fail ($1,800, plus a half-day’s labor at a plumber’s rates). But tanks can last 20 to 30 years if the anode rod is routinely replaced. (See “New Life for Your Old Water Heater” in HP106.)