Feb 1, 2014

Retrofitting Forced Air Combi Systems: A Cold Climate Field Assessment


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.

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 DHWonly operation.