Kelly monitors heat pump performance through a Google Nest Hub. This particular graph is showing heat delivered in watts (purple) and power consumed in watts (blue) over the last 24 hours. The ratio between those two numbers is the COP.By David McMichael and Dan Vastyan
Few homeowners have interest in collecting detailed energy use and heating system performance data at their residence. Even fewer are qualified to do so.
However, as a research and development engineer with Efficiency Nova Scotia, Ryan Kelly was more than interested in collecting data on his home’s new heat pump. He manages a team that runs pilot projects impacting efficiency programs in the province. More specifically, he helps determine how best to retrofit heat pump technology to existing ducted systems. More often than not, the ductwork is a remnant from an oil-fired furnace installation.
Kelly owns a 1,400 sq. ft., 90-year-old house in Halifax, Nova Scotia. The home was originally heated with an oil-fired furnace, but in the 1990s, the previous owner retrofitted the furnace in favour of electric resistance baseboard upstairs and an electric thermal storage (ETS) unit downstairs. The five-inch round ductwork was still in use because a unitary air conditioner had been added.
Kelly hired Sunshine Renewable Energy to install a two-ton, 19-SEER (Region IV HSPF 10.7) ducted mini-split heat pump to replace the existing heating and cooling systems.
Richard Ross, owner of Sunshine Renewable Energy, immediately saw the potential to repurpose the existing ductwork by installing a single-zone Airstage H-Series heat pump and ducted air handler. The 60-person solar and HVAC company completed the retrofit in a single day. “We replace a lot of oil furnaces and electric baseboard with mini splits,” said Ross. “Our focus is energy efficiency, electrification, helping customers with rebates, and maximizing their return on investment. This job, like many others, qualified for Canada’s Greener Homes grant.”
Volumetric flow rate is calculated using measurements of fan motor current and a fan curve created using readings of fan current and the results of a duct traverse with a hot wire anemometer at each fan speed.Data collection
Kelly’s home is typical for the average house in Nova Scotia. It has a modestly higher performance compared to similar 90-year-old homes complete with R-24 ceilings, R-13 exterior walls, R-30 basement walls, vinyl double-pane windows, and an air leakage rate of 6.9 air changes per hour (ACH) at 50 Pa. He wanted to see how the single-zone Fujitsu H-Series heat pump performed in an average home. Homesol Building Solutions’ heat load calculation using a -16 C (3 F) outdoor temperature was an estimated 26,860 BTU/h.
“I installed monitoring equipment at the panel to measure power and energy at the breakers for the heating equipment,” said Kelly. “Continuous, direct measurements of airflow are not practical. To estimate airflow indirectly, I built a curve from spot measurements of fan motor current as well as airflow using a hot-wire anemometer. Fan current is then measured continuously, and that curve is used to estimate instantaneous airflow. Temperature and humidity sensors were installed on both the supply and return sides of the air handler mounted in a basement utility room. Outdoor temperature and humidity sensors were also installed.”
Data from half of the 2022/2023 heating season was archived, but the most telling numbers were collected during a mid-February record-breaking cold snap.
As a maritime province, Nova Scotia’s winters are milder (albeit wetter) than most of Canada. The average February high temperature in Halifax is 0 C, with an average low temperature of -8 C.
“When I saw the forecast calling for -24 C, I knew I had a unique opportunity to capture data that would test the low-temperature performance of the heat pump,” said Kelly. “I double checked the monitoring equipment and left the thermostat at 21 C.”
A Fujitsu air handler was installed to retrofit the heat pump to the home’s existing ductwork.Real-world performance
The expectation was that the heat pump would turn off and the nine-kW backup electric coil in the air handler would run for a considerable portion of the cold snap. What actually happened was much different.
When temperatures plummeted, reaching a low of -21 C, the heat pump stayed on. It operated at maximum capacity during the coldest periods. Both the coefficient of performance and total capacity reached similar levels to what is outlined in Fujitsu’s engineering manual, even at the lowest temperatures: 1.65 and 5.3 kW, respectively. Considering that the home is an uncontrolled environment, these numbers were remarkable.
“The electric coil came on briefly during the coldest period, but on a full-year basis, this is a rare occurrence,” said Steve Shellnutt, outside sales at Master Group, who learned the results of Kelly’s data collection shortly after the severe cold had passed.
“It’s possible that a 2.5-ton unit would have kept up without having to rely on backup, but it’s likely not worth the additional cost for the larger unit, considering the overall fraction of time that backup was needed,” continued Shellnutt.
Interestingly, the fan stayed on “low” nearly the entire time. When retrofitting a heat pump to ductwork designed to operate at high supply air temperatures – such as an oil-fired furnace – there’s potential for velocity noise from the fan having to operate at a higher speed. This didn’t happen. The heat pump capacity varied accordingly, increasing supply air temperatures as needed, with return air temperatures remaining consistent.
Conclusive testimony
“The energy results were super impressive,” said Ross. “Over the years, we’ve come across some equipment that doesn’t operate as well in the real world as it does during manufacturer lab testing. It was encouraging to verify that this heat pump performed as advertised.”
Coefficient of performance (green) is the ratio of heat delivered to power consumed. COP is estimated using measured power readings and estimated heat delivered. Outdoor temperature (green) is shown to demonstrate the relationship between COP and outdoor temperature.“I wasn’t shocked by the results,” said Kelly. “It was consistent with the engineering manual described for COP and available capacity. We’ve seen that before, in mini-split field trials conducted on Prince Edward Island, which has a similar climate. Those tests yielded similar results.”
The ambient temperatures captured in this application are record-setting for Nova Scotia, reports Shellnutt. “That provides conclusive testimony that these heat pumps are applicable to even colder climates. When sized correctly, these systems are fully capable of providing efficient heating capacity where very low temperatures occur more frequently.”
Since the energy consumption of the old, electric resistance heating system was monitored prior to the retrofit, a regression analysis on energy consumption data collected pre- and post-installation allowed for a weather-normalized estimate of energy savings. Using a baseline temperature of 18 C and historical temperatures from the weather station nearest Kelly’s home, a typical year sees about 3,600 HDD (heating degree days). Based on the observed performance of the heat pump, a 3,600 HDD year would result in energy savings of 8,700 kWh or $1,400, assuming $0.16/kWh, the current electricity rate in Nova Scotia.