Combi Heating Systems
The efficiency of furnaces and boilers for space heating have significantly improved in recent years. For instance, 95% annual fuel utilization efficiency (AFUE) two-stage gas furnaces and condensing wall-hung boilers are almost common-place. Space heating loads are also much lower as houses have become better insulated. In some very energy-efficient Syracuse houses, we have measured water heating loads that were larger than space heating loads on an annual basis. In contrast, water heaters still have a relatively low efficiency, and high-efficiency alternatives can be expensive.
Therefore a combined appliance or “combi” unit that uses the same gas burner to meet both space and water heating loads can be a cost effective and energy efficient solution. Combi units can include:
Internal Heat Exchanger (HX). A wall-hung boiler with an internal heat exchanger to heat potable domestic hot water (i.e., a four-pipe unit). These units combine two appliances in one: a space heating boiler and a tankless water heater.
Indirect Tank. A conventional boiler with an indirect tank. This option has been used for years. A separate circuit on the boiler heats a coil inside the tank.
Potable Water Systems. A water heater tank or tankless unit that uses potable hot water (i.e., hot water from your faucet) in a heating coil or air handler. Some fan coil products are now available with special controls to ensure potable water does not remain isolated in the piping system for prolonged periods and become a health risk.
The “From the Ground Up (FTGU) Houses that were built as part of an architectural design competition sponsored by Syracuse University all used combi units for better efficiency and reduced installation cost. The FTGU houses (Livework, TED House, and R-House) all used variations of the combi system concept. Detailed monitoring by the Syracuse COE and CDH Energy is summarized in a final report. This field testing demonstrated that combi systems do not always realize the expected efficiency gains from combining space and water heating.
Concepts that use potable water for space heating offer the potential for lower installed costs by eliminating heat exchangers and other components. Extensive development and testing in Minnesota (Schoenbauer) demonstrated that these systems could be cost effectively implemented as a retrofit solution.
Research at the Gas Technology Institute (GTI) is helping to further refine components and system arrangements to ensure the efficiency and cost reduction benefits of Combi systems are fully realized.
Combi Boiler Unit with Internal HX for DHW
Wall-hung Boiler with Indirect Tank for Water Heating