Two-stage Geothermal Heat Pumps

What is staging, and how could it be so important?

What is Single-stage?

Not so many years ago, geothermal heat pumps, as well as other heat pumps, were only manufactured as single-stage units. When a single-stage compressor starts, it produces a constant amount of heating or cooling until the compressor shuts off. Since units are sized for the majority of the heating need (or all of the cooling need), they only run for brief periods at a time at the beginning of the heating or cooling seasons. As the season progresses, and more heating or cooling is needed, the compressor runs for longer periods of time to keep the home comfortable. At the peak of the heating season, the compressor will run very long cycles, and even continuously when outdoor temperatures are the most extreme.

When compressors operate in very long cycles, or better yet, continuously, the efficiency of the heating and cooling equipment is at its highest. It seems like running continuously would wear out a compressor faster than letting it cycle, but the facts are just the opposite. When a compressor starts up, it takes a great amount of force to get it to start rotating. For a split second, this extreme force causes the metal surfaces to rub slightly. The wear is minute, but it adds up: a million tiny wears will finally wear it out. And while a modern compressor can start up a million times, short-cycling uses up those starts in a few years.

Short-cycling was an ever-present problem for single-stage geothermal heat pumps. There was a huge need for a system that could run for long periods of time, whether it was 40°F, or 20°F outside. The solution was a geothermal heat pump that could run in different sizes, or stages.

The First Attempt at Staging

Years ago, if a customer could afford it, we would install two smaller geothermal heat pumps and operate one of them on first stage, and the other one for second stage. This type of installation was very costly and since geothermal heat pumps are not inexpensive, it was just not cost effective for most installations.

An Attempt at Making a Two-stage

Since the need was so great, engineers came up with a unit that had two complete refrigeration circuits in one geothermal heat pump furnace. These systems were designed with two compressors, and two of everything else that makes a system work, except the cabinet, and the blower motor. This unit was still costly, and even though it solved many of the cycling problems, it was just not the answer that the geothermal heat pump industry needed.

The Two-stage Compressor

Finally the two-stage Copeland Scroll Ultratech compressor was invented. This compressor was the answer that the industry had been looking for. It is able to operate at 67 percent of its capacity, and while it's running, shift to second stage and 100 percent of its capacity. It greatly reduces the short-cycling problem, as long as the equipment has been sized correctly. The Copeland Scroll two-stage compressor has been a revolution for the geothermal heat pump industry, but HVAC contractors must stop oversizing geothermal heat pumps before the short-cycling problem can be eradicated.

Variable Speed: An Even Better Compressor

The new compressor is variable speed, and has the potential to eliminate short-cycling issues. Since it will be able to operate at whatever capacity is needed, it will almost never shut off, allowing for 50 percent higher efficiency then we have now. Variable speed compressors are commonly used in mini-split air-source heat pumps, and are available in geothermal heat pumps, but are still quite a bit more expensive than equivalent units with two-stage compressors, making them have not quite as fast of a payback in most cases. We still recommend geothermal heat pumps with two-stage compressors for most installations, until the prices on the variable speed units have competitive paybacks.