With an ever increasing focus in the Northeast on Heat Pump technology, strategic electrification, reducing consumer carbon footprint and utility bills, all while increasing the comfort of the home, the system design process is paramount. I can say that some residential HVAC Contractors & Building Inspectors have been well educated on the load calculation report, but that is only the first part of the design process! Once the amount of heat being lost and/or gained by a residence is established (but/hr), there is specific guidelines in equipment selection that must be followed to avoid uncomfortable seasons, high energy bills, and even premature equipment failures.

In the latest version of ACCA Manual S (Second Edition, 2014), the limits set for maximum oversizing of a Heat Pump depends on your technology (Single Stage/Multi/Variable), climate, and sometimes even the results of your Load Calculation.

Based on the information outlined in Manual S, I created the chart to be a simple reference for you. Which design criteria should you use? There are a few factors:

• Which type of Heat Pump (or A/C Only) System are you specifying (Single Stage, Multi, or Variable)?
• Know the ratio of Heating Degree Days vs. Cooling Degree Days for your climate
• Very tight homes in a Cold Climate may find that regardless of the above, the latent gains are significant compared to the sensible cooling gains.
• Important Update: Clarification for using Condition B for Heat Pump Selection also requires a SHR of .95 (or higher), or installation of a whole-home dehumidifier (properly sized)!

If the system is designed properly with the above criteria, comfort issues will least likely be affected. This is particularly important with single-stage systems. Multi-Stage and Variable capacity systems have the luxury of operating a part capacity, for part load conditions. This is not a silver bullet! I have seen many over-sized variable capacity heat pumps that act like single-stage compressors, because they are so oversized you cannot operate low enough to meet the low demand of low load days!

ACCA Manual S addresses the sizing criteria around the load calculation, or the design day(s). If you do not have enough heat at this point (within the maximum oversizing guidelines), or below your design temperature, you should install supplemental heat that is sized for the difference. I am personally less concerned with heat pump performance these days as the technology has proven more than impressive in the harshest of climates. My concern is the rest of the year, the 98% or so of the run hours that are higher than your design temperature in heating.

Why doesn’t Manual S include part load design criteria?

My recommendation: Make sure your Variable Capacity system can operate low enough (minimum capacity) to meet the heat loss at 47F (not just maximum capacity at your design temperature of 5F for example)!