Variable Speed Scroll Compressors: Why They Dominate Modern Heat Pumps

How Much Compression do you Need?

The amount of Compression accomplished in a Refrigeration or Air Conditioning System is directly related to the amount of Capacity the system delivers. Capacity is the amount of usable cooling effect your system is able to deliver to the Refrigeration/Conditioned Space. Variable Speed Compression allows great control over the Capacity at which the system maintains.

This is the 3rd of 3 articles in the series on Scroll Compressors – the 1st was on Single-Speed Scrolls, and the 2nd addressed Two-Stage Scrolls.

Capacity Control Explained

Capacity Control is simply maintaining System Suction Pressure near design conditions. This keeps the Compressor(s) running longer to avoid Short Cycling. This is a benefit in that optimal or longer running cycles allow better oil circulation through the system, and also avoid more power consumption if the Compressor(s) were to turn on and off rapidly. Longer cycles will also mitigate Compressor Motor or Start Capacitor overheating.

There are different ways to accomplish Capacity Control, including Compressor Staging, Cylinder Unloading, and Hot Gas Bypass. Variable Speed Compression is the most precise and sophisticated, and can also be used in conjunction with the other mentioned types, depending on the Compressor and System application.

Variable Speed Compression

For comparison, we will use an example of two Compressors piped in parallel for Compressor Staging Capacity Control. We have two 1-ton Compressors (12,000 BTUs/hour in 1 ton), which serve full load when both operate, and part load when one operates. Again, think of Suction Pressure as the metric that controls the staging of Compressors.

Under “full load”, both Compressors run for 2 tons of capacity. Under “part load”, one Compressor runs for a 1-ton capacity. Between “full load” and “part load”, there is a load that this arrangement cannot accurately match. Below the “part load”, there is another load that a single operating Compressor will provide more than the required capacity for before turning off. This leaves something to be desired for both energy consumption and product/space comfort and consistency.

With Variable Speed Compression, you can select a 2-ton Compressor, which can operate anywhere from 15% to 120% capacity (not 1-100% as sometimes stated). This Compressor will start up at a low speed for minimum starting Motor Amperage Draw, and ramp up gradually to the current required capacity. If this is 100%, the VSD (Variable Speed Drive) will gradually ramp the Compressor Motor speed to 100% to match the capacity requirement (example of 100% capacity could be 3600 rpm).

Note: a Variable Speed Drive is an accessory that modulates the Motor speed that is driving the Compressor. The VSD can be affixed to the Compressor/Motor assembly or externally mounted. The VSD varies electrical frequency to the motor—higher frequency means higher speed. Proper grounding is critical to prevent electrical issues including bearing damage from shaft currents.

For VFD fundamentals, listen to the HVAC Know It All podcast on Variable Frequency Drives Basics with Anish Suthesh Arul.

Oil Return at Low Speed

When compressors run at low speed, refrigerant velocity drops and oil can accumulate in evaporators rather than returning to the compressor. Manufacturers address this through periodic “flush cycles” that temporarily increase speed, internal oil pumps, or suction line accumulators. This is why minimum capacity is typically 15% (not lower).

Watch to Glen Schwarzman discuss compressor protection and failure prevention on the HVAC Know It All podcast.

Vapor Injection for Cold Climate Performance

Variable Speed Scrolls in heat pump applications often include vapor injection technology. A small brazed plate heat exchanger acts as an “economizer,” injecting refrigerant vapor into an intermediate compression stage. This can increase heating capacity by 10-30% or more at low ambient temperatures, depending on system design and operating conditions, enabling heat pumps to work effectively below 0°F without backup heat.

Discussing a Danfoss variable speed compressor with vapor injection technology at AHR 2025.

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Charging Variable Speed Systems

Variable speed equipment demands precise, weighed-in charges. Unlike single-speed systems that tolerate some variation, inverter-driven compressors operate across wide capacity ranges where incorrect charge significantly impacts efficiency and reliability. Always follow manufacturer specifications and use accurate scales. Reference proper refrigerant charging procedures for detailed guidance.

Precise charging requires knowing what you’re walking into. Property.com’s “Know Before You Go” tool lets you research the property before your service call. Arrive prepared with the right equipment and realistic time estimates for variable speed system work.

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Electrical Protection Requirements

Inverter equipment requires protection that single-speed systems don’t need:

• Surge protection: VFDs are sensitive to voltage spikes
• Voltage monitoring: Prevents operation outside acceptable range
• Buck-boost transformers: Correct voltage when utility supply is high or low

Many warranty claims are denied due to improper electrical protection. Install surge protection on every inverter system.

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Application of Variable Speed Scrolls

The Variable Speed Scroll’s ability to accurately match loads while saving energy, combined with the Scroll’s already energy-efficient and low-maintenance design, makes it an extremely popular Compressor. This Compressor is often selected in modern HVAC/R systems due to its versatility and great load-matching ability.

Variable Speed Scrolls have the benefit of not having to be accessed very often for maintenance, checks, or oil changes. The VSD also allows great control and accuracy over the operation of the Compressor(s) in the system, no matter the control strategy utilized.

The VSD Scroll is utilized in all parts of the HVAC/R market: domestic, commercial, and industrial applications. They are selected for equipment that provides comfort cooling for Air Conditioning + Heating, or to cool down or heat up a manufacturing process.

Note: the heating mentioned above would utilize the Variable Speed Scroll in a Heat Pump Application – a common application of VSD Scrolls. Variable speed is particularly dominant in heat pumps because continuous modulation enables better defrost recovery, maintains comfort during heating mode, and pairs well with vapor injection for cold climate performance.

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Comfort Benefits

Beyond efficiency, variable speed delivers superior comfort:

• Temperature consistency: No cycling means no temperature swings
• Better dehumidification: Extended run times at reduced capacity remove more moisture
• Quieter operation: Lower speeds mean lower noise levels

For customers complaining about humidity despite adequate cooling, variable speed is often the solution.

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Summary

Variable Speed Scroll Compressors are a valuable Compressor type in the HVAC/R Industry. Their enhanced efficiency and control by the addition of a Variable Speed Drive to the Scroll Compressor creates an impressive machine.

Applications for VSD Scrolls are wide in the industry, from Domestic to Industrial, for both cooling and Heat Pump applications.

The Variable Speed Scroll offers great control, efficiency, and a low maintenance requirement. These Compressors are seen all over the HVAC/R field and are not going anywhere.

For related topics, see The Inverter Compressor and Do You Need To Design a Mini Split System?