In a December 2014 Plumbing & Mechanical Hydronics Workshop column, we discussed the benefits of dual-fuel systems that combine a geothermal water-to-water heat pump with a mod/con boiler. The boiler can be configured to provide supplemental heat when the heat pump can’t quite keep up with the load. This allows the heat pump to be sized for 50 percent to 75 percent of design load while still providing the majority of the total seasonal heating energy requirement. In mission-critical applications, the boiler could be sized to provide full design load output if the heat pump was not operating. The latter also is helpful if you need to supply high domestic hot water (DHW) loads.
In areas where natural gas is not available, the mod/con boiler in a dual-fuel system will likely be supplied by propane. The cost of a heat produced by burning propane in that boiler may be significantly higher than that of heat delivered by the geothermal heat pump. For example, as of mid-December 2014, the price of propane in the Albany, New York, area was about $3.21 per gallon. If this propane fueled a mod/con boiler with an average thermal efficiency of 92 percent, the heat produced would have a unit cost of $38.03 per MMBtu (1 MMBtu =1,000,000 Btu).