Check the valves. The package I purchased came with a spring-controlled check valve, which has the advantage of working either vertically or horizontally. However, this particular one required more opening pressure than the circulation pump could provide. After diagnosing the problem, I cut the pipes, redesigned some of the routing, and switched to a swing-type check valve. Though swing valves won’t work vertically, they open with less pressure and are more reliable over time. Spending the extra $6 for this check valve at the start could have saved me both time and money.
Avoid overheating. During low- or no-use periods, the glycol solution in closed-loop systems can overheat if hot water in the storage tank is not being drawn out for household use. Our oversized system will sit idle occasionally when we are gone on vacation. To avoid overheating, I added a manually operated “vacation bypass” on the closed loop. In vacation mode, this ball valve routes the antifreeze mix around the check valve. This allows heat from the storage tank to thermosiphon upward and radiate through the collectors at night, cooling the water in the tank so it won’t overheat the next day.
Check your thermostat. Before installing our SHW system, we had to keep our electric tank-type heaters’ thermostats set relatively high (145°F) to produce enough hot water to fill the bathtub. Now that the electric tank is fed from the SHW storage tank, we found that its thermostat could be turned down to 120°F.
Temper your hot water. Domestic water from SHW systems can reach temperatures of 160°F or more, which is dangerously hot for human use. A tempering valve mixes enough cold water into the hot water line to prevent scalding water. Make sure to place one where the hot water exits your final hot water tank, and that will cover all the taps in the household.
Let the air out. Air bubbles in the closed loop can hamper the flow of the antifreeze solution. Though I was careful about filling the closed loop, some air bubbles lingered in the system. A manual ball valve (to preclude leaks) topped with a Schrader valve (to purge trapped air) allows you to manually release trapped air without losing antifreeze. Place the two valves at the highest point in your closed-loop system, where the antifreeze solution exits the collector and air bubbles will collect. You’ll only need to use the valve once or twice after filling the closed loop.
Calculate your optimum mix. The percentage of glycol in your heat-transfer fluid depends on your local temperatures. Too much glycol makes the mix more expensive and less efficient; too little glycol leaves your system vulnerable to freezing and breakage. For our local temperatures, the ideal mix is a 40% glycol solution, which I determined based on recommendations in Bob Ramlow’s book, Solar Water Heating.