- Chris is considering replacing his natural gas boiler with a multi-zone ductless heat pump system. The boiler has an efficiency of 70% and the ductless mini-split has an HSPF of 11.5. Chris uses 1,000 therms per year and pays $0.66 per therm and $0.11 per kWh. How much would it cost to run each system and will Chris save money by installing the new system? Why else might he install the new system?