Once the hallmark of high-efficiency ductless equipment, more manufacturers are incorporating inverter-driven compressors into their ducted air-source heat pump (ASHP) offerings. The reasons for this are simple: Inverter technology improves energy efficiency, provides better comfort, and offers quieter operation.

While consumers have responded positively to these newer energy-efficient products, inverter technology is more expensive and, therefore, mainly offered on higher-end heat pumps (and air conditioners). Even though these systems can cost significantly more than non-inverter ASHPs, manufacturers note their higher price tags are more than offset by significantly higher efficiencies.

Operation and Benefits

In a standard heat pump or air conditioner, a fixed-speed compressor comes on at full capacity to meet the indoor temperature requirement and then turns off once the target temperature is met. This results in peaks and valleys in both power consumption as well as inside temperature, said Farooq Mohammad, director, product management, air conditioning division, Rheem Mfg. Co.

By contrast, inverter-driven heat pumps and air conditioners have a variable frequency drive (VFD) that incorporates an adjustable electrical inverter to control the speed of the motor, compressor, and cooling and heating output, explained John Mix, residential split system platform manager, Carrier Corp. “The inverter, or VFD, uses a rectifier to convert the incoming alternating current (ac) to direct current (dc), then uses pulse-width modulation in an electrical inverter to produce ac of a desired frequency. The variable-frequency ac drives a brushless motor, or an induction motor, as the speed of an induction motor is proportional to the frequency of the ac. The compressor can then run at different speeds.”

The ability to modulate the speed means the system can adjust its output in very small increments to maintain a constant temperature, said Matt Lattanzi, director of product management, Nortek Global HVAC. “Because the system makes nearly constant small adjustments, it offers longer, quieter run times and greater indoor comfort. You can liken the technology to cruise control on a car, which maintains a near constant speed by making very small adjustments. As a result, the ride is smooth; there is no loud, quick acceleration; and miles per gallon are maximized.”

Basically, inverter technology allows the heat pump to more closely match the changing load of the home, which greatly improves comfort, said Mix. “A traditional one-stage system cycles on and off based on the space temperature climbing above the thermostat set point in cooling mode or dropping below the set point in heating mode. Consequently, the temperature in the home cycles over a wider range as the one-stage system switches on and off. Inverter technology can minimize these temperature swings by operating the compressor at a lower speed for a longer period of time. And, by adjusting the speed and capacity output of the compressor, it is possible to better control humidity in the space during cooling mode.”

In addition to more precise temperature control, inverter technology results in higher efficiencies, said Mohammad. “Inverter technology allows the compressor to slow down or speed up based on the cooling/heating needs of the homeowner, which results in energy savings. Customers will also appreciate that, under normal operating circumstances, inverter-driven heat pumps perform very quietly.”

While there are numerous benefits homeowners, contractors can also benefit from offering inverter-driven heat pumps, said Lattanzi. “Inverter-driven products help contractors distinguish themselves in the home. While the competition may offer a two-stage heat pump as their best offering, a contractor with access to inverter-driven heat pumps can offer a multifaceted solution that addresses the majority of homeowner concerns — extreme energy savings, quiet operation, and precise temperature control. Some inverter heat pumps also offer advanced diagnostics, which can help contractors service the systems more effectively.”

Complexity, Cost

As with any newer technology, there is a learning curve associated with inverter-driven heat pumps. “Inverter systems do have slightly more sophisticated controls compared to standard systems, which means contractors need to be properly trained before they install or service them,” said Mohammad. “Before we launch our line of 17- and 20-SEER inverter-driven heat pump models in the second half of 2015, we will offer a robust inverter training program to ensure contractors and dealers are prepared to work on Rheem inverter systems.”

When Nortek Global launched its first inverter-driven systems in 2006, its initial iQ Drive platform had several installation requirements that made it more complex than a traditional heat pump. But, the company’s newest inverter-driven heat pump — the iQ Drive FT4BG — is much easier to install, and no factory training is required, said Lattanzi. “Because it does not have any special wiring requirements, it works with most two-stage thermostats. In general, just like any technology curve, as manufacturers get more adept with inverter technology, the heat pump designs will become friendlier.”

In addition to greater complexity, inverter-driven ASHPs are premium systems that come at a higher price, but that increased cost is offset by the system’s improved efficiency, said Mix. “When properly designed, inverter-driven heat pumps can be very comparable to the cost of non-inverter heat pumps of the same efficiency (SEER and HSPF) ratings. If customers are buying a high-end heat pump, wouldn’t they want the best performance they can get? That’s why we offer the Infinity^® 20 variable-speed heat pump with Greenspeed® intelligence, which was designed to provide the best comfort, temperature, humidity control, heating performance, and variable-capacity performance in a ducted heat pump system. The inverter technology is key to this extraordinary performance.”

Focusing on extraordinary performance and comfort rather than the higher initial cost is what contractors should do when selling these higher-end heat pumps. “Contractors who do well with these systems are the ones who know how to sell on overall value and comfort benefits,” said Lattanzi. “Energy efficiency is a key feature, but to be successful with these high-end heat pumps, the contractor needs to go beyond efficiency and sell all of the other comfort benefits equally.”

Publication date: 5/25/2015

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