If you’re considering converting your fossil-fueled forced-air furnace to a geothermal (aka ground source) heat pump system, one of the most important decisions you’ll need to make is whether to go with a heat pump with a two-stage compressor or a variable-speed compressor. In the Geostar product line, the Aston series and its de-featured cousin, the Magnolia Plus series, are equipped with a two-stage compressor; the Sycamore series features a variable-speed compressor. Other than the compressor, the Aston and the Sycamore can be outfitted to be fairly comparable (e.g. variable-speed blowers, communicating thermostats, and remote monitoring & control), so let’s look at how they’re different – the compressor.

To fully appreciate the impact of the compressor difference between the Aston and the Sycamore heat pumps, it’s essential to understand a little about the process of designing a geothermal system and sizing of the heat pump. An analysis is performed on the building to be heated to estimate its heat loss at “design temperature”. Design temperature is based on historical outside temperatures at or near the location and is defined as the temperature at which 99% of the time the outside temperature is at or above it. In the Finger Lakes Region, that’s typically between 2 and 8oF. Generally, the total system heat capacity for a residential setting needs to be at least 130% of design heat loss (aka design heat load or, for heating-dominant climates, simply design load) to ensure inside comfort throughout the winter.

Another key design factor is that a heat pump’s durability and efficiency are negatively impacted when the heat pump is regularly short cycled. By that, we mean that the heat pump is energized (by a signal, aka “call”, from the thermostat) and, in short order (i.e. 5 minutes or less, as a rough rule of thumb), satisfies the call and is turned off. Combined with the requirement of heating capacity of 130% of design load, the designer must balance these opposing criteria in selecting the size – heating capacity – of the heat pump. In the case of the Aston (or Magnolia Plus), this becomes a compromise: enough system heat capacity to handle the rare occasions when outside temperatures dip to single digits or lower, yet not so much capacity at the heat pump’s lowest output that it frequently short-cycles during the Spring and Fall, when outside temperatures typically range from 30 to 60oF and the load is substantially less than design load.

For the heat pumps with two-stage compressors, the output can only be throttled to ~65% of capacity – first stage, whereas the Sycamore compressor can operate at 12 different speeds and as low as ~20% of maximum capacity, better enabling it to satisfy both criteria. In practice, when a heat pump with a two-stage compressor is specified, the designer will typically choose a size that has a maximum heat pump capacity less than design load (to reduce the propensity to short-cycle during the shoulder seasons) and incorporate an optional electric resistance heating element which is automatically activated by the thermostat and supplements the heat pump capacity to ensure combined capacity of at least 130% of design load.

Under partial load conditions, typical of Spring, Fall, and mild Winter days and nights, the Sycamore can throttle its capacity to better match the building heat loss to the outside, enabling the heat pump to run for longer and fewer cycles. This results in higher efficiencies (and thus lower annual operating costs), less wear on the heat pump, and generally more even temperatures over time and space.

In addition, if the designer is able to slightly oversize a Sycamore, this will enable the heat pump to more quickly recover from overnight thermostat setback and possibly avoid activating the electric resistance heat  (if present).

Considering the government incentives as of October 2021, expect to pay a net premium of $1,700-2,600 to upgrade from a comparably featured Aston to a Sycamore. It may well be worth it over the life of the heat pump, so give the compressor type serious consideration while you’re considering geothermal.