Single-source water and space heating, or “combi” systems that optimize efficiency are being developed through a series of lab tests and field trials.
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.
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.
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.
In the Lab
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.
The graph shows the change in supply temperatures from start to finish of a”maximum load” test, which consisted of two showers and one space heating event. The tankless water heaters maintained the most consistent temperatures.
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.
Today’s combi technology can do the job. Tests verified that systems are capable of meeting heating loads up to 50,000 BTU/h with acceptable return water temperatures and supply air temperatures. These designs provide steady-state space heating efficiencies >85 percent.
The performance of combi systems is limited by currently available equipment. Most hydronic air handlers were not designed to produce the lower return water temperatures necessary for combi systems with a condensing heating plant. Also, the manufacturer requirement that plumbing for combi systems be configured with a primary and secondary loop significantly increases return water temperatures, thus reducing system 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. Several manufacturers, at least one in direct reaction to the findings from this project, have begun to improve combi 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.
The combi systems were shown to have the same gas consumption as separate high-efficiency natural gas furnaces and water heaters, at lower first costs. However, combi systems are tricky to install, and many installers are unfamiliar with them. This makes it difficult for combi systems to economically compete with a common installation of a condensing furnace and power vent water heater.
The retrofitted combi systems typically operated in the condensing mode and provided acceptable, and in some cases, improved occupant comfort.
- 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 separate systems are more expensive.
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). Each increases system efficiency, improves occupant comfort, saves energy and simplifies the sizing and design process.
Lower return temperatures and longer cycles produce higher system efficiency. Yet all air handlers have a constant airflow rate and constant water circulation flow rate for the 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.
The nine heating plants tested included four condensing combi boilers with heating loops for space and domestic water heating, two with internal storage for hot water, three condensing storage tank type water heaters, one condensing tankless water heater and one condensing hybrid water heater.
The NorthernSTAR combi lab evaluated nine heating plants individually using three different tests:
1. 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.
2. 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.
3. 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.