Water heating designs have been pretty solid for decades, but it’s always possible to make improvements to the systems. Andrew made some modifications that work for his situation—here are a few others to consider.
Heat loss from a backup water heater can cause unnecessary fossil-fuel usage if the solar storage tank is hot enough to supply the hot water but there is no demand to move the solar-heated water to the water heater. With the tanks normally piped in series, this can happen with intermittent hot water usage. Andrew chose to equalize the two tanks and combine the equalization with a hot water recirculation system (more on that below). Another possibility would be to circulate between the two tanks, rather than throughout the household’s water delivery system.
Extra insulation on the water heater can help. Many older water heaters have poor insulation, and wrapping the tank with extra can cut standby heat loss considerably. In areas where hard water isn’t a problem, a tankless water heater that will modulate its output with the incoming solar-heated water solves the standby loss.
Gas, propane, and fuel-oil water heaters have an uninsulated flue pipe running up the center of the tank and through a home’s roof to dispose of the combustion products, which contributes to heat loss. If your backup water heater is fossil-fuel fired with a conventional flue pipe, consider a highly insulated electric tank as an alternative. At this time, the price of oil is low, but a few months ago the price of propane and fuel oil made electric water heaters attractive. Electric tanks are always a better backup type of tank since they don’t have a flue to add to standby heat loss.
Hot water recirculation systems are becoming more popular in upscale residential construction. They’ve been standard in larger buildings for decades. They provide instant hot water no matter how far the tap is from the water heater, which can reduce the amount of wasted water flowing down the drain while waiting for the water to heat. However, this convenience and water savings has an energy cost. The power that the pump uses is one cost and the heat lost through the pipes as it endlessly circulates through the system is the other cost. Timers and devices like Andrew designed mitigate both of these losses considerably, but it’s always a trade-off. Where water is abundant and energy is precious, these systems are of questionable value, except for convenience. In areas where energy is more abundant and water is scarce, the recirculation systems can be beneficial as well as convenient.
Recovering heat from a drainback tank is a novel idea and could be beneficial. However, it would be best not to route the water recovery system through the collectors at night. To keep pumps running as cool as possible, locate them on the cool side of the heat exchanger—where the heat has been pulled out already.
—Chuck Marken, Home Power Solar Thermal Editor