Combined Water and Space Heating

Combined Water and Space Heating

Single-source water and space heating, or “combi” systems that optimize efficiency are being developed through a series of lab tests and field trials.
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Combined Water and Space Heating

Green Builder Media

Wednesday, October 14, 2015 - 7:30am

CAMPAIGN: Ethical and Sustainable Living

CONTENT: Article

Better insulation and tighter envelopes are reducing space heating loads for new and existing homes. This makes it possible for both space and domestic water heating loads to be provided with a single heating plant, saving significant amounts of energy. These systems are called combination (combi) systems and have been the focus of ongoing field and lab studies conducted by the NorthernSTAR Building America Partnership.


Combi systems can directly replace the existing forced-air furnace and water heater. They consist of a high-efficiency water heater or boiler and an optimized hydronic air handler. The air handlers are designed with large heat transfer coils to achieve highly efficient space heating.

During a space heating event, hot water leaves the heating plant, passes through the coil in the air handler and transfers heat into the airflow. The cooler water then leaves the air handler and flows back to the heating plant. Closed loop systems use a heat exchanger between the heating loop in the heating plant and the air handler.

In addition to saving energy, these systems virtually eliminate natural draft appliance spillage issues through the use of a powered or direct combustion vent.


The NorthernSTAR team created a testing laboratory to design and optimize combi systems ahead of a field trial in which such systems would be retrofitted in 300 homes participating in the State of Minnesota Low-Income Weatherization Assistance Program.

The lab phase documented the performance of currently available components and developed recommendations for optimized combi system designs. Tests were conducted on individual components, including nine heating plants and nine hydronic air handlers. Steady-state measurements of the air handlers determined output capacities that provided acceptable return water and supply air temperatures. Heating plant capacity results were used to develop algorithms to determine whether a system could meet domestic hot water (DHW) and space heating loads. Finally, multiple systems were configured and tested. Experienced contractors reviewed initial designs to provide recommendations to improve performance, reliability, ease of installation and cost.