Hybrid Domestic Hot Water/Space Heating System

Goals of Innovation: To find the most cost effective and energy efficient solution for hot water and space heat.

Description: An 85-gallon water storage tank from a roof-mounted solar thermal collection system (evacuated tube) acts as a preheat/storage plant for domestic hot water. At times, in summer, the solar thermal system will be providing all of the house's domestic hot water. The output from the hot water tank runs through a natural gas on-demand water heater (boiler), which acts as a boost when the system needs a rise in temperature. The on-demand water heater thermostat is set to approximately 120 degrees Fahrenheit. If the water from the solar thermal system is lower, it will get a boost before traveling on. A loop off of that on-demand heater runs through a heat exchanger and returns to the tank.

The distribution system for space heating is supplied hot fluids from a heat exchanger attached to the domestic hot water system. There are two zones, each with a different distribution system. The first floor slab and second floor bathroom are heated with radiant tubing. A schematic and more details are available on the Women Working website.

Obstacles: A minor obstacle related to the building inspector being unfamiliar with this type of system. Once the heat load calculations were explained and the system was shown that it could work, the building inspector was onboard.

Cost Information: The upfront costs of the system were approximately $4000 - 5000 more than conventional, even though the system allowed for reduced equipment. The long-term costs for maintenance and operation of less equipment that is more energy efficient will be substantially lower than a conventional system.