ThermalPoint Case Study 1
Prepared by The Sustainable Technologies Evaluation Program (STEP)
March 2025
Contact: step@trca.ca
Background
The thermal balance point temperature (TBPT) is a key metric for sizing heat pumps. It represents the outdoor temperature at which a heat pump can no longer meet a home’s heating demand without supplemental heating. Graphically, the TBPT is the outdoor temperature where the heat pump capacity curve intersects the heating load line of the home.
In practice, a home does not have a single, fixed thermal balance point temperature, as heat loss, internal gains, solar gains, thermostat setpoints, and other factors can vary throughout the day—even if the outdoor temperature remains constant. However, TBPT is nonetheless a useful metric for heat pump sizing and thermostat programming.
Using historical natural gas consumption data, the new ThermalPoint web application (thermalpoint.ca) estimates the TBPT and the percentage of the annual heating load that can be met solely by a heat pump.
Introduction
This case study series presents the results of applying ThermalPoint to real homes. In previous work [1], STEP piloted hybrid heating systems in Peel Region, collecting utility consumption data before and after heat pump installations in several homes. Smart thermostat data was also gathered. This data was used to assess the accuracy of the ThermalPoint tool in predicting TBPT.
Home Profile
Home 1 from [1] is a 1,637 ft² (above grade) detached, two-storey home in Mississauga, built in 1985. An EnerGuide audit, completed just prior to the installation and shared with the analysis team, estimated the home’s design heat loss (DHL) at 11.87 kW (40.5 kBTU/h). This is shown in Figure 1. On November 22, 2023, an air-source heat pump (Outdoor unit: Bosch BOVA-36-HDN1-M20G; Indoor Coil: BMA*2430BNTD; AHRI #203025008) was installed (Figure 2). The outdoor unit may be installed with different indoor units to achieve a different rated capacity. For the configuration in this home, the rated nominal capacity of the system at 47 °F was 2-Tons. It was integrated with an existing Carrier 59SC2D060 (1-stage) furnace, with a rated AFUE of 92.1% and a heating output of 56,000 BTU/h.
Figure 1. The DHL was estimated by an EnerGuide audit. Note that the furnace specifications assumed in the EnerGuide audit differ slightly from the actual specifications listed above, but this would not have impacted the DHL calculation.
Figure 2. The heat pump was installed at the home in November 2023.
Data Collection
Natural gas consumption data was totaled from utility bills for one year before the heat pump installation. From December 22, 2021, to December 20, 2022, the home consumed 1,729 m³. Gas consumption data from the summer months suggested a fixed base load for domestic hot water and other uses of approximately 25 m³ per month.
Methodology
TBPT was estimated using three approaches:
- Standard Approach: A heating load line was plotted using the EnerGuide audit DHL and the design outdoor temperature of -18.3°C assumed for the location [2], and also assuming zero heat load at 16°C. The heat pump maximum capacity curve (sourced from the [3]) was overlaid to estimate the TBPT.
- ThermalPoint: Inputs including the annual gas consumption for 2022 (1,729 m³), base monthly gas load (25 m³), heat pump maximum capacity curve (31,000 BTU/hr at 8.3°C; 20,000 BTU/h at -8.3°C; 17,400 BTU/h at -15°C), and furnace AFUE (92.1%) were entered into the ThermalPoint tool algorithm to estimate the TBPT. Note that this case study considered annual gas consumption for 2022, while ThermalPoint currently requires the most recent data from 2024 because that is what is most readily available via the Enbridge “My Account” portal.
- Smart Thermostat Data: Ecobee thermostat data provided 5-minute interval runtime for the heat pump and furnace, along with outdoor temperature. A curve was developed showing the proportion of heat pump runtime relative to total heating runtime, including both furnace and heat pump, as a function of outdoor temperature. TBPT was inferred as the temperature at which the furnace took over for the heat pump and began to operate more significantly. This data reflects actual system performance but may not represent a control strategy that optimized heat pump utilization to the greatest extent. Thermostat data from January 25th 2024 to May 31st 2024 were used – this time period omitted an initial time period where the thermostat control parameters were different from their final settings.
Results
- Figure 3 shows that the standard approach estimated a TBPT of -3.4°C while ThermalPoint estimated a TBPT of -12.4°C with a 0% safety margin. With a 25% safety margin, the TBPT estimated by ThermalPoint was -8.6°C. The safety margin is user-defined and simply applies a multiplicative factor to the estimated heat loss from the tool, which then predicts TBPT to be at a warmer value.
Figure 3. The thermal balance point is the intersection of the heating load line and the heat pump capacity curve. The standard approach estimated TBPT to be -3.4°C while ThermalPoint estimated it to be -12.4°C (assuming 0% safety margin).
- Smart thermostat data indicated that the heat pump operated independently down to an outdoor temperature of -13°C, and below that the furnace turned on (Figure 2). The thermostat data also showed that the home used a setback schedule and was most often set at 22°C during the day, and 19oC to 21oC overnight.
Figure 4. The bars plot the total runtime for the heating system including both the heat pump and furnace. The gold line plots the fraction of the total heating runtime that was from the heat pump. For example, at -10°C there was 20 hours of heating system runtime and it was 100% from the heat pump. Below -13°C, the furnace engaged substantially.
Conclusion
The standard approach estimated a TBPT that was approximately 10°C warmer than the real-world value, whereas ThermalPoint closely matched observed performance when a 0% safety margin was applied. The overestimation in the standard approach is partly because the DHL is a conservative estimate designed to prevent insufficient heating in extreme conditions. While this ensures the home remains warm in worst-case scenarios, it may not accurately reflect typical heating needs, making it less reliable for predicting when backup heating is required in practice. Overall, this case study demonstrated that ThermalPoint provided accurate TBPT predictions that aligned well with the home’s real-world heat pump performance.
[1] STEP. Replacement of Air-Conditioners with Cold-Climate Air-Source Heat Pumps: Pilot Project Findings in The Region of Peel. July 2024. https://sustainabletechnologies.ca/reports/hybrid-heat-pump-pilot-in-peel-region/
[2] NRCan. HOT2000 Climate Map. https://open.canada.ca/data/en/dataset/4672733b-bbb6-4299-a57f-f19ab475ac11
[3] NEEP. ASHP Product List. https://ashp.neep.org/#!/product_list/
Supporting data and analysis available as a Python Jupyter Notebook on request to step@trca.ca.
Funding support for ThermalPoint provided by The Atmospheric Fund (TAF).
