The Buena Vista St home is a small, 800 sq. ft. house built in 1946.
While the energy efficiency improvements to this home may appear modest—adding wall insulation, air sealing rim joists, and upgrading windows—the heating system solution is truly noteworthy. A 45,000 BTU/h, 96% efficient single-stage Armstrong furnace is paired with an efficient variable speed Breeze33 1.5-ton heat pump (SEER2 16.1 and HSPF2(region V) 7.8). Calculations show that this relatively low cost (price comparable to traditional furnace + AC replacement) dual system could potentially reduce heating energy use by 49%!
The previous furnace was an 80% efficient, 80,000 BTU/h unit. The new duct work in the basement features a 4” Aprilaire filter bay. Larger filters provide a lower pressure drop, making it easier for air to pass through and reducing the fan’s energy consumption.
Rounded-corner ducts help minimize air friction, improving overall efficiency. Relocation of some of the ducts into the interior walls reduces undesirable heat losses. Additionally, downsizing the equipment minimizes low-load cycling, which helps extend the lifespan of the system by preventing excessive wear.
The Breeze33 heat pump is capable of meeting 70% of the annual heating load and achieves 335% efficiency at 47°F. This means that for every unit of energy the heat pump consumes, it generates 3.35 units of heat for the home. In comparison, the furnace provides only 0.96 units of heat per unit of energy.
E3’s calculations, using CoolCalc version of Manual J, incorporate:
- Measured air leakage of 1270 CFM@50 (or 8 ACH@50)
- Insulation levels de-rated for imperfect installation
- An assumed average COP of 3.1 for the heat pump over its likely operating temperature range
Using T6 Pro thermostat, the system is configured so that the heat pump “leads the way,” with the furnace activating only if the indoor temperature deviates from the setpoint by more than 2–3°F. At outdoor temperatures of 40°F and above, the furnace does not turn on at all. Special thanks to Home Mechanix for working with E3 to fine-tune these settings. This home achieved a Home Energy Score of 9—up from 3 when we started!
It remains uncertain how these settings will translate to actual energy savings, particularly if the homeowner frequently uses the “Emergency Heat” option, which activates the furnace for faster heating. Heat pumps are best at maintaining steady temperatures but warm the home more gradually.
Compared to running an 80% efficient furnace, the operating cost of this well-designed and optimized dual system are similar to prior cost, despite much higher electricity rates relative to gas. Paired with insulation and windows improvements, the homeowner’s utility bills are expected to be reduced. Moreover, the energy savings are significant, and we believe we are making an impact!
