After we had designed a tight shell, preliminary calculations with Energy 10 predicted an energy load for heating of 8,482 kilowatt-hours per year (kWh/yr). Andy’s model accounted for HRV effectiveness and assumed we would be able to get the shell to a reasonably low blower-door number of 600 cfm at 50 Pascals — tight but achievable as long as we paid attention to air sealing.
The low heating load narrowed the number of practical options for the mechanical system. We looked briefly at the possibility of a hybrid system using solar thermal panels for hot water and space heating, but this would have required either a fossil-fuel appliance for backup or electric backup, which would have increased overall energy use. Given the low design loads, the simplest non-fossil-fuel option for the heating plant was a ground-source heat pump. (At the time we were building, the available air-to-air mini-split heat pumps were not as efficient in cold climates as they are today, or we might have considered those.) We chose an Econar model (econar.com) that also produces domestic hot water fairly efficiently.
Because we had to drill a well for our drinking water, it made sense to use the same well for heating — a type of open loop setup, common in New England, in which domestic water and water for heating are drawn from one well and the return water from the heat pump is delivered back to the same well near the top of the water column. A bleed control activates if the well water gets too cold. In our system, the bleed water will dump into an existing shallow well, but so far the control has never had to be activated.
Ground-source heat pumps produce more output heat energy than the energy consumed in operation, as measured by the COP, or coefficient of performance. Heat-pump manufacturers typically advertise the AHRI (Air-Conditioning, Heating and Refrigeration Institute) rating for the heat pump, which is tested under conditions much more favorable than we see in the North. These ratings are also for the heat only and don’t include the other pumps and controls in the system.
Because we would be providing power on site, we had to account for the total energy needed to run the system, not just the heat pump. So Andy calculated what he calls the “effective COP,” which includes pumps and controls. To do this accurately, you have to dive into the unit’s engineering data, like performance across a range of possible ground-water and heated-water temperatures. By looking at the conditions we expected for each month of the heating season and for domestic hot water all year, Andy estimated the annual effective COP for the system for both heating and hot water.
He also found that using a Gould variable-speed well pump with a VFD (variable frequency drive) controller saved about 50 percent of the pumping energy and increased the effective COP by around 13 percent compared with using a conventional single-speed well pump