Update on a Potential CCHP performance Issue

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:

  • Proper sizing is important (oversized equipment can lead to "excessive cycling, low efficiency, and ineffective summer dehumidification")
  • Avoid "padding" load calculations with additional safety factors (calculations already have safety factors built in)
  • Pay special attention to calculations for individual, small block loads (e.g. for a single room)
  • The minimum capacity of the system may be just as important as the maximum ("A good equipment selection for comfort and efficiency has adequate turn-down to perform well during mild weather, low-load conditions…and is particularly important for multi-zone equipment.")
  • "Do not choose outdoor multi-zone equipment sized at more than 115% of the heating load at design conditions, just to provide individual room zone control."

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.