Hot Water System Retrofit – GreenBuildingAdvisor – GWC Mag

Domestic hot water systems account for one fifth of household energy use, contributing to expenses and carbon emissions. To lower those numbers, I re-engineered the hot water system in our century-old home (Climate Zone 4) to take advantage of 1. hybrid water heaters, which have become common and less expensive, 2. the existing boiler and its indirect, and 3. the thermodynamics of our basement and house taken as a complete system. While this was a retrofit, the setup and the logic behind it can be used for new construction as well.

Shortcomings of standard systems

Conventional hot water systems have inherent inefficiencies and challenges; there’s the high operational costs of electric resistive water heaters and the environmental toll of natural gas. When located in the basement, as in the case at our house, they add heat to basements. This waste heat then bleeds into the ground, accounting for up to 1/3 of the heat loss in an average home.

Many green hot water solutions, from solar hot water to tankless natural gas, have drawbacks including initial cost and ongoing maintenance. Tankless water heaters give up any buffer capacity in exchange for minimizing the already-minimal heat losses from a modern, insulated tank. This necessitates all heat output be created instantaneously, and can result in cold water at low flow rates before the burner kicks in.

The most comfortable solution for homes with a boiler is often thought to be an indirect tank, but the impressive hot water production rates of these are offset by the inefficiency of running the boiler periodically all summer, which then radiates its heat to the basement or utility closet. Natural gas tank heaters have a slower but still fast recovery rate, cannot be as well insulated as other tanks, and produce emissions. Electric-resistive tank heaters have a slow recovery rate…