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.