Monitoring Heat Pump Efficiency in Cold Climates

In collaboration with RDH Building Science and Fortis BC, SMT provided sensors and instrumentation assistance for a unique study of Heat Pump performance in cold climates. It was a fascinating study that we were thrilled to be a part of, and demonstrates the value of using real-world monitoring to verify the performance of components compared to their data sheets.


What is a Heat Pump?

Heat Pumps are gaining popularity worldwide as a solution for keeping indoor temperatures at a comfortable level. Building Science professionals and enthusiasts laud Heat Pumps for their efficiency, effectiveness, and reduced environmental impact when compared to furnaces.


Heat Pumps are great for both heating and cooling indoor spaces. Put simply, they use electricity to move a refrigerant between an indoor and outdoor unit; this refrigerant absorbs heat which is pumped inside in the winter, outside in the summer.


Although they are known to work best in mild climates, Heat Pumps have the potential to bring energy savings to those living in much colder parts of the world - however they are generally tested in a laboratory under controlled conditions. In order to deepen our understanding of Heat Pump performance, SMT assisted RDH Building Sciences with a unique study to provide “a better indication of the real-world efficiency and the resulting energy reduction and potential cost savings of air-source heat pump retrofits in existing homes in cold and moderate regions of British Columbia”.


Methodology

In order to test Heat Pumps in real-world conditions, twenty-six heat pump systems were monitored at twenty-four distinct single-family residential sites on Vancouver Island and in the interior of British Columbia. All the test sites were in Climate Zones 4 and 5 (i.e the most extreme inhabited areas of the globe that require the most energy to heat). The Heat Pumps were instrumented with sensors that collected performance data, while outdoor environment conditions were also monitored. Data was captured at five-minute intervals for each site over a one-year monitoring period.





Temperature Sensor

Pictured: SMT Temperature Sensor







Findings


Note: “Coefficient of Performance” (COP) is an efficiency ratio that represents the units of energy output per unit of energy consumed by the system. In general, the Heat Pumps were found to be operating at an efficiency of greater than 1 COP, meaning that even in cold climates they were providing a greater efficiency than 1-1. Overall, ductless mini split Heat Pumps were found to be 10-20% more efficient than central systems.


One interesting finding was that many occupants were using heat pumps to cool the interior when outdoor temperatures were already below typical interior temperatures. In some cases this led to short-cycling, where the Heat Pumps were unable to operate efficiently. The study concluded that “Although this phenomenon was not exhibited for all units, educating homeowners on the strategies and benefits of passive cooling strategies (i.e., natural ventilation) could reduce hours of heat pump operation during mild outdoor temperatures”.


The average COP during heating season was found to be lower than in cooling season, in some cases by as much as 50%. Most of the units would actually stop normal operation between 3°C and -5°C, relying on a backup heating system to supplement the heating load at lower outdoor temperatures. The COP was almost always greater than 1, even down to -14°C, except in a couple of cases for which faulty equipment (leaking refrigerant) was deemed to be the culprit.


Having evaluated the efficiency of Heat Pumps in extreme conditions, the researchers gathered utility data from 18 participants - with 67% reporting annual energy savings. The benefits were most consistent in cases where electricity was the primary heating source, with average savings of 5650 kWh and $810 for the year-long monitoring period. Cases that used non-electric primary heating fuel sources varied significantly, however the average savings for these homes was still found to be 1520 kWh and $231 for the year.

Relative Humidity Sensor

Pictured: SMT Relative Humidity Sensor



SMT engineers have years of experience adapting our sensors to unique challenges, and through this instrumentation we are able to learn valuable lessons about the factors impacting the real-world performance of building components. Lessons learned based on some site monitoring intricacies discovered during this study were outlined to improve future in-situ heating pump instrumentation techniques, including suggested return and supply air temperature sensor placement. Some key factors affecting heat pump performance were also explored, including volumetric flow rate, equipment sizing and short-cycling, defrost control, backup heating, and some installation considerations, with the following findings:


  • The Volumetric flow rate of all the Heat Pumps monitored was measured at an average rate of 64% when compared to the manufacturer’s data sheet.

  • Central System Heat Pumps performed closer to manufacturer ratings (72%)

  • Mini Split Systems performed lower at 62%

  • Mini Split Systems were installed with limited ceiling clearance, which could account for the reduced performance when compared to manufacturer data sheets

  • Manufacturer’s specified minimum ceiling clearance of between 3.9” (100mm) and 1.2” (30mm) was mostly complied with by installers, however the researchers suggest greater study of the impact of low clearance is needed

  • In typical heating periods, 33% of units showed evidence of short-cycling (cycles of fewer than 8-5 minutes), which suggests some units are oversized for their location

  • In general, the participants in the study seemed to have reviewed or had access to little documentation on the most efficient pump for their space

  • Some Central System Heat Pumps are designed to operate on electric resistance backup heating coils, which accounted for an average of 22% of total space conditioning (and as high as 63% for one unit)

  • It was estimated that more than half of the usage of backup coils was for defrosting the system, rather than for supplementary heating

  • For Mini Split pumps, less than 1% of total energy consumption was used for defrost, however they are not typically equipped with coils for backup heat

Conclusions


As with any real-world study, there were limitations; small sample size, variations in home size and construction, different primary and backup heating sources, occupant behaviour, and instrumentation limitations. Being aware of these limitations allows us to temper our response to our findings, however the study still produced encouraging results. The final report found that there is potential for “widespread adoption of heat pumps in British Columbia, particularly for retrofit applications in homes that rely primarily on electricity as their primary heating source”.


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