Making The Case For Oversizing Variable-Capacity Heat Pumps

Report Number




Heat Pumps; Buildings; HVAC; Humidity; Cooling


Oversizing of fixed-capacity (FC) heat pumps and air conditioning (A/C) systems is understood to reduce space conditioning energy efficiency, and is not permitted or at least severely restricted by various standards, state codes, and programs. Central ducted variable capacity (VC) systems capable of delivering ~40%-120% of nominal capacity are available, promising high cooling and heating efficiency. Lab house experiments found that Nordyne iQ Drive 2 and 3-ton VC heat pumps are 43% more efficient at lowest capacity versus full nominal capacity (which is ~86% of maximum capacity) and 64% more efficient at lowest capacity versus maximum capacity. Analysis of two years of lab house data found that the 3-ton VC system consumed less cooling and heating energy, and had reduced peak demand, compared to the right-sized 2-ton system. By contrast, oversizing the FC heat pump generally yielded higher seasonal and peak energy consumption. Experimental data also suggest that concerns about the ability of oversized cooling systems to control indoor relative humidity (RH) are overblown.

This paper presents results that support right-sizing for FC but oversizing for VC systems. Oversizing VC heat pumps substantially decreases cooling and heating season energy consumption and peak demand. Oversizing heat pumps (both VC and FC) would also reduce operation of electric resistance back-up elements, yielding both seasonal and peak demand heating energy savings. Codes, standards, and programs that limit oversizing of heat pumps should be evaluated and likely modified to encourage or at least permit oversizing of VC systems.

Date Published



This paper published in the Proceedings of the 2014 ACEEE Summer Study on Energy Efficiency in Buildings,

© 2014 American Council for an Energy Efficient Economy

Local Subjects

Buildings - Cooling; Heat Pumps; Humidity; Buildings - HVAC


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