Retrofitting Forced Air Combi Systems

Executive Summary

This project analyzed combined condensing water heaters or boilers and hydronic air coils to provide high efficiency domestic hot water (DHW) and forced air space heating. Called “combi” systems, they provided similar space and water heating performance less expensively than installing two condensing appliances. The system’s installed costs were cheaper than installing a condensing furnace and either a condensing tankless or condensing storage water heater. However, combi costs must mature and be reduced before they are competitive with a condensing furnace and power vented water heater (energy factor of 0.60).

Better insulation and tighter envelopes are reducing space heating loads for new and existing homes. For many homes, decreased space heating loads make it possible for both space and domestic water heating loads to be provided with a single heating plant. These systems can also eliminate safety issues associated with natural draft appliances through the use of one common sealed combustion vent.

The combined space and water heating approach was not a new idea. Past systems have used non-condensing heating plants, which limited their usefulness in climates with high heating loads. Previous laboratory work (Schoenbauer et al. 2012a) showed that proper installation was necessary to achieve condensing with high efficiency appliances. Careful consideration was paid to proper system sizing and minimizing the water temperature returning from the air handling unit to facilitate condensing operation.


Twenty sites were selected for a detailed monitoring project to characterize how combi systems perform when installed in real homes. The homes were monitored for 1–2 months to evaluate the performance of the existing systems. A combi system was installed in each home after the completion of the existing system monitoring. At each of the 20 sites a detailed monitoring system was installed, collecting data on energy usage, household load, and system efficiency. This report describes the monitoring protocol, installed combi systems, energy savings and installed performance of combi system. Combi system monitoring was conducted for a full year in order to obtain information on both combined space/DHW operation and DHW-only operation.


Monitored field data were used to characterize combination systems. Collected data
were analyzed to determine:

  • The annual energy consumption of both the existing furnace and water heater and the combi system
  • The installed efficiency of the combi systems
  • The delivery capabilities for space heating and DHW.

The output measurement data were also analyzed to access the delivery capabilities of these systems. Capabilities were assessed both in terms of occupant comfort and sizing capacity. Both delivered air and water temperatures were analyzed to access occupant comfort.

Fifteen sites have been monitored and analyzed for both the existing and combi systems (instrumentation and occupancy issues prevent complete analysis at five sites). Combi systems, on average, saved 19% of natural gas usage for space and water heating. The measured annual combined efficiency was 81%–92% with an average 87%.

System installation was very important to achieve these savings. Careful equipment pairing and operation parameter selection guidelines were required to achieve high efficiencies and good savings. Deviation from these installation parameters would have reduced performance considerably.

Full report (PDF):
Retrofitting Forced Air Combi Systems