One heating plant can handle hot water and home heating. Here’s what early field trials taught us—and what to check when choosing a system today.
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, potentially saving energy. These systems are called combination (combi) systems. The NorthernSTAR Building America Partnership studied them in lab and field trials during the early 2010s. Those results, first covered here in 2015, still offer useful lessons about sizing, installation and controls.
How They Work
During a hot water draw, the heating plant operates like a water heater: cold water comes into the storage volume as hot water leaves (for units with storage). In systems without storage, cold water enters and is heated as necessary.Open full-size graphic.
Properly sized combi systems can replace an existing forced-air furnace and water heater, with a compatible hydronic air handler and the required piping, venting and controls. 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.
Replacing natural-draft appliances with properly installed direct-vent equipment addresses the spillage risks associated with those appliances. Correct venting, combustion testing and maintenance remain essential.
In the Lab
The NorthernSTAR team created a testing laboratory to design and optimize combi systems to support a planned 300-home retrofit program in Minnesota’s Low-Income Weatherization Assistance Program. A September 2015 Building America case study reported that the work supported more than 200 installations, with detailed monitoring in 20 homes.
The lab phase documented the performance of the components available at the time 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.
In the original laboratory “maximum load” test—two showers and one space-heating event—the tankless water heaters maintained the most consistent supply temperatures. These results describe the tested equipment, not all current systems.Open full-size graphic.
Conclusions
High return water temps compromise efficiency. Lab tests showed that the heating plant steady-state efficiency decreased with increasing return water temperature; this decrease became more significant as the return temperature increased above 110°F. Optimal systems require water and air flow rates that result in low return water temperatures while still meeting loads.
The tested systems could do the job. The lab tests verified that systems were capable of meeting heating loads up to 50,000 BTU/h with acceptable return water temperatures and supply air temperatures. The tested designs provided steady-state space heating efficiencies >85 percent.
Equipment pairing limited performance in the original tests. Most hydronic air handlers in that research were not designed to produce the lower return water temperatures necessary for combi systems with a condensing heating plant. In the tested configurations, primary and secondary plumbing loops could also increase return water temperatures, reducing efficiency. Variable flow rate water pumps and fans, along with the necessary controls, could provide a greater range in heating output while ensuring low return water temperatures. Such modifications could improve system efficiency as much as 10 percent. These were improvement opportunities identified by the original research, not a promise of a 10 percent gain from current equipment.
Retrofitting Combi Systems in the Field
As a next step, the NorthernSTAR team monitored 20 homes to characterize how combi systems performed compared to existing systems. At each of the homes, the team installed a detailed monitoring system that collected data on energy use, household load and system efficiency. After a full year of monitoring, the team found that the combi systems saved 19 percent of natural gas usage for space and water heating and showed an annual average combined efficiency of 87 percent. These historical results compared the monitored systems with the older equipment they replaced; they are not a savings forecast for every home or a comparison with heat pumps. The 2015 case study confirms the 20-home sample and average gas savings.
The original research found gas consumption comparable to separate high-efficiency natural gas furnaces and water heaters, with lower first costs than some all-condensing alternatives. It also found that installation complexity could make combi systems less competitive with a condensing furnace and power-vent water heater. Those historical cost comparisons are not current bids. For a project now, compare itemized local quotes covering equipment, hydronic air handler, piping, venting, condensate disposal, controls, commissioning and any electrical or gas work.
The retrofitted combi systems typically operated in the condensing mode and provided acceptable, and in some cases, improved occupant comfort.
Lessons Learned
The water temperature returning to the heating plant must be minimized to achieve condensation. Water temperature of 105° F or lower was targeted to ensure high performance.
Custom design, equipment pairing and optimization were required to balance high performance with occupant comfort at each site.
In addition to energy savings, these systems improved the combustion safety of homes. In some cases, the flexible capacity of the combi system improved sizing and led to a more comfortable conditioned space for occupants.
Separate condensing space and water heating equipment can be installed to reach comparable energy performance and combustion safety as combi systems, but their cost relative to a combi system depends on the equipment and installation.
Advanced Controls
One of the takeaways from previous work was the need for advanced controls to further enhance the efficiency of combi systems.
The NorthernSTAR team analyzed two distinct temperature set point controls: (1) outdoor reset and/or turndown after the heating season and (2) space-heating modulation (water flow and airflow). The team investigated how these controls could increase efficiency, improve comfort and simplify sizing and design.
Lower return temperatures and longer cycles produce higher system efficiency. The air handlers discussed in that work had constant airflow and water circulation rates in heating mode. Advanced controls allow for a range of heating capacities for a single system; they improve efficiency by allowing the system to operate at lower average return water temperature and longer cycles.
Advanced controls can also reduce the need for custom engineering of combi systems. “Smart” control of airflow and water-flow rates could allow a single air handler to provide more efficient performance over a wider range of heating loads and should eliminate time-consuming manual adjustments to the airflow and water-flow rates. This way, the installer can focus on assembling functional systems, while the controller optimizes system operation.
A 2016 follow-up report found that temperature-reset controls produced modeled and laboratory-supported savings of 2.1–4.3 percent for storage and hybrid water-heater combi systems in moderate-load homes. The team’s March 2017 installation guideline then set out a four-step process: assess loads, select compatible equipment, install it and optimize its operation. Neither finding turns a single appliance rating into a guarantee of whole-system performance.
Choosing a System Today
Two current gas-fired options are Navien’s NCB-240/110H, rated at 95 percent AFUE for space heating, and Bosch’s Greenstar combi models, also rated at 95 percent AFUE. Both use condensing technology to recover heat that would otherwise leave through the exhaust; modulation helps match output to demand. These are equipment examples, not verified matches for the original test homes. The linked manufacturer pages provide product and contractor information; obtain a local installed quote rather than treating the boiler price alone as the system cost.
Ask the designer to check both the home’s heat loss and simultaneous hot-water demand, along with the minimum heating output that matters in a low-load house. Natural Resources Canada recommends evaluating combined-system performance rather than relying on a water-heater-only rating. Gas combi systems still burn fuel; also compare an electric heat-pump approach when reducing fossil-fuel use is a project goal.
For an existing installation, check the exact model and serial number against safety notices before recommending reuse. A 2018 Navien recall covered certain older NCB-180E boilers converted to propane—not the NCB-H model identified above. Bosch also publishes Greenstar service bulletins for installers. Have a qualified contractor check the applicable instructions and bulletins for the actual equipment.
The original lab tested nine heating plants: four condensing combi boilers (two with internal storage), three condensing storage-tank water heaters, one condensing tankless water heater and one condensing hybrid water heater. Historical research equipment.
Testing, Testing
The NorthernSTAR combi lab evaluated nine heating plants individually using three different tests:
Idle tests. These were conducted by recording gas and electricity consumption when the heating plant had no DHW use or space heating load. Infrared thermography was used to help identify sections of the heating plant with significant heat loss.
Steady-state space heating efficiency. These tests were conducted using a “work horse” heating plant that supplied hot water at a set flow rate and temperature, simulating the water conditions returning from a hydronic air handler. Efficiencies were computed by measuring the energy into the heating plant, in natural gas and electricity, and the energy output in hot water. Return water temperatures (ranging from 80°F to 120°F) were monitored in order to track the efficiency reduction with higher return temperatures. This helped determine an optimized flow rate and heat output sizing of the hydronic air handler.
Transient performance. Conducted to examine delivery capabilities, these tests tracked water temperatures during a sudden increase in DHW flow in order to determine hot water delivery times. The tests also tracked transient supply water temperature under three simulated conditions: (1) a space heating event interrupted by a DHW event (e.g., a shower); (2) a shower interrupted by a second shower; and (3) a shower interrupted by a space heating event.
Combined Water and Space Heating
One heating plant can handle hot water and home heating. Here’s what early field trials taught us—and what to check when choosing a system today.
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, potentially saving energy. These systems are called combination (combi) systems. The NorthernSTAR Building America Partnership studied them in lab and field trials during the early 2010s. Those results, first covered here in 2015, still offer useful lessons about sizing, installation and controls.
How They Work
During a hot water draw, the heating plant operates like a water heater: cold water comes into the storage volume as hot water leaves (for units with storage). In systems without storage, cold water enters and is heated as necessary. Open full-size graphic.
Properly sized combi systems can replace an existing forced-air furnace and water heater, with a compatible hydronic air handler and the required piping, venting and controls. 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.
Replacing natural-draft appliances with properly installed direct-vent equipment addresses the spillage risks associated with those appliances. Correct venting, combustion testing and maintenance remain essential.
In the Lab
The NorthernSTAR team created a testing laboratory to design and optimize combi systems to support a planned 300-home retrofit program in Minnesota’s Low-Income Weatherization Assistance Program. A September 2015 Building America case study reported that the work supported more than 200 installations, with detailed monitoring in 20 homes.
The lab phase documented the performance of the components available at the time 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.
In the original laboratory “maximum load” test—two showers and one space-heating event—the tankless water heaters maintained the most consistent supply temperatures. These results describe the tested equipment, not all current systems. Open full-size graphic.
Conclusions
High return water temps compromise efficiency. Lab tests showed that the heating plant steady-state efficiency decreased with increasing return water temperature; this decrease became more significant as the return temperature increased above 110°F. Optimal systems require water and air flow rates that result in low return water temperatures while still meeting loads.
The tested systems could do the job. The lab tests verified that systems were capable of meeting heating loads up to 50,000 BTU/h with acceptable return water temperatures and supply air temperatures. The tested designs provided steady-state space heating efficiencies >85 percent.
Equipment pairing limited performance in the original tests. Most hydronic air handlers in that research were not designed to produce the lower return water temperatures necessary for combi systems with a condensing heating plant. In the tested configurations, primary and secondary plumbing loops could also increase return water temperatures, reducing efficiency. Variable flow rate water pumps and fans, along with the necessary controls, could provide a greater range in heating output while ensuring low return water temperatures. Such modifications could improve system efficiency as much as 10 percent. These were improvement opportunities identified by the original research, not a promise of a 10 percent gain from current equipment.
Retrofitting Combi Systems in the Field
As a next step, the NorthernSTAR team monitored 20 homes to characterize how combi systems performed compared to existing systems. At each of the homes, the team installed a detailed monitoring system that collected data on energy use, household load and system efficiency. After a full year of monitoring, the team found that the combi systems saved 19 percent of natural gas usage for space and water heating and showed an annual average combined efficiency of 87 percent. These historical results compared the monitored systems with the older equipment they replaced; they are not a savings forecast for every home or a comparison with heat pumps. The 2015 case study confirms the 20-home sample and average gas savings.
The original research found gas consumption comparable to separate high-efficiency natural gas furnaces and water heaters, with lower first costs than some all-condensing alternatives. It also found that installation complexity could make combi systems less competitive with a condensing furnace and power-vent water heater. Those historical cost comparisons are not current bids. For a project now, compare itemized local quotes covering equipment, hydronic air handler, piping, venting, condensate disposal, controls, commissioning and any electrical or gas work.
The retrofitted combi systems typically operated in the condensing mode and provided acceptable, and in some cases, improved occupant comfort.
Lessons Learned
Advanced Controls
One of the takeaways from previous work was the need for advanced controls to further enhance the efficiency of combi systems.
The NorthernSTAR team analyzed two distinct temperature set point controls: (1) outdoor reset and/or turndown after the heating season and (2) space-heating modulation (water flow and airflow). The team investigated how these controls could increase efficiency, improve comfort and simplify sizing and design.
Lower return temperatures and longer cycles produce higher system efficiency. The air handlers discussed in that work had constant airflow and water circulation rates in heating mode. Advanced controls allow for a range of heating capacities for a single system; they improve efficiency by allowing the system to operate at lower average return water temperature and longer cycles.
Advanced controls can also reduce the need for custom engineering of combi systems. “Smart” control of airflow and water-flow rates could allow a single air handler to provide more efficient performance over a wider range of heating loads and should eliminate time-consuming manual adjustments to the airflow and water-flow rates. This way, the installer can focus on assembling functional systems, while the controller optimizes system operation.
A 2016 follow-up report found that temperature-reset controls produced modeled and laboratory-supported savings of 2.1–4.3 percent for storage and hybrid water-heater combi systems in moderate-load homes. The team’s March 2017 installation guideline then set out a four-step process: assess loads, select compatible equipment, install it and optimize its operation. Neither finding turns a single appliance rating into a guarantee of whole-system performance.
Choosing a System Today
Two current gas-fired options are Navien’s NCB-240/110H, rated at 95 percent AFUE for space heating, and Bosch’s Greenstar combi models, also rated at 95 percent AFUE. Both use condensing technology to recover heat that would otherwise leave through the exhaust; modulation helps match output to demand. These are equipment examples, not verified matches for the original test homes. The linked manufacturer pages provide product and contractor information; obtain a local installed quote rather than treating the boiler price alone as the system cost.
Ask the designer to check both the home’s heat loss and simultaneous hot-water demand, along with the minimum heating output that matters in a low-load house. Natural Resources Canada recommends evaluating combined-system performance rather than relying on a water-heater-only rating. Gas combi systems still burn fuel; also compare an electric heat-pump approach when reducing fossil-fuel use is a project goal.
For an existing installation, check the exact model and serial number against safety notices before recommending reuse. A 2018 Navien recall covered certain older NCB-180E boilers converted to propane—not the NCB-H model identified above. Bosch also publishes Greenstar service bulletins for installers. Have a qualified contractor check the applicable instructions and bulletins for the actual equipment.
The original lab tested nine heating plants: four condensing combi boilers (two with internal storage), three condensing storage-tank water heaters, one condensing tankless water heater and one condensing hybrid water heater. Historical research equipment.
Testing, Testing
The NorthernSTAR combi lab evaluated nine heating plants individually using three different tests:
Updated content · November 3, 2026
By Green Builder Staff