Over the last month, Efficiency Vermont has received complaints from a small number of customers about high energy consumption of their cold climate air source heat pumps (CCHPs). We have also become aware of some recent investigations that have found high energy use among multi-split heat pumps that do not modulate well at partial load.1
There are indications that single-zone (or “one-to-one”) CCHPs, terminating in either a single cassette or a "compact-ducted" (also referred to as “mini-ducted”) unit on the interior, will perform better in many situations. The Northeast Energy Efficiency Partnership (NEEP) has compiled a Guide to Sizing & Selecting Air-Source Heat Pumps in Cold Climates, which is a valuable resource on this issue. They now recommend that contractors consider using separate single-zone systems, rather than installing multi-zone systems in order to optimize performance.
Here are some other factors to be mindful of to avoid performance issues and high energy consumption, according to NEEP:
When installing any CCHP, it is always important to look for manufacturer extended performance tables that cover a wide range of indoor and outdoor temperatures. In addition, you should be sure that a unit can modulate effectively at milder temperatures, when the load of a building may be just a fraction of the design load. Select equipment that can turn down to meet part load without short cycling. For example, on some projects we have observed great performance in units with a turndown of 8:1 or better, and poor performance with a turndown in the 2:1 range.
If you think this issue is impacting any of your customers, please contact us. We are gathering information to assess if this is a widespread issue being experienced by our Vermont customers and partners as well as those applications that present the highest risks.
1 Monitoring Mechanicals; Rosenbaum; slides 13-26; Elm Place: Post-Occupancy; Lescaze, Bushey, and Simmons; slides 9-11 and 37-52.