The purpose of this memo is to provide guidance to improve the accuracy of savings estimates from
installing cold climate air source heat pumps (ccASHP) in single and multifamily residential buildings
when using engineering calculations or energy simulation software tools such as TREAT. Most
calculation methods for savings of replacing existing heating systems with ccASHP do not account for its
varying efficiency and capacity with outdoor temperature. In cold climates, the actual average efficiency
over the entire heating season is generally lower than the manufacturer’s rated heating seasonal
performance factor (HSPF), which results in greatly overestimated savings.
This memo provides a simplified HSPF adjustment method that can be applied to the rated HSPF used in
any engineering calculation, saving estimate from a technical resource manual (TRM), or energy
simulation tool. TREAT, like most software simulation tools, uses a single input of the rated Region IV
HSPF. While TREAT does use an algorithm to adjust the rated HSPF to the climate zone of the given
weather location, it is using this same HSPF for all temperatures of the heating season and not
accounting for the varying efficiency with outside temperature. Additionally, the actual performance of
ccASHPs depends on the unit’s ducting configuration type, which TREAT handles all types the same. The
analysis for this memo found that generally ducted ccASHPs had larger reduction adjustments of the
rated HSPF than the ductless systems. To improve the accuracy of TREAT when modeling ccASHP, users
should follow the attached Modeling ccASHP in TREAT procedure.