Heat Pump Sizing in Ontario: From the Heat-Loss Calculation to the Electrical Panel

An air-source heat pump that is the wrong size costs money twice: once in comfort and efficiency, and again at the electrical panel. The right size comes from a room-by-room heat-loss and heat-gain calculation, a sizing approach that matches what the owner wants from the system, the balance point with its backup heat, and finally the electrical capacity to run it. This guide walks through each step for Ontario houses, following Natural Resources Canada's sizing and selection guide.

The Short Answer

1. Calculate the design heating and cooling loads room by room with CSA F280, at your municipality's design temperature.
2. Pick a sizing approach (NRCan options 4A to 4D) based on how much of the heating the heat pump should carry.
3. Select equipment from the manufacturer's capacity data at the temperatures that matter, not from a nominal tonnage.
4. Plot the balance point and decide the backup heat and its controls.
5. Check the electrical side: circuits from the nameplate MCA and MOP, the disconnect, the load calculation and the panel.

Why the Heat-Loss Calculation Comes First (CSA F280)

Building codes require the capacity of heating appliances to be determined in accordance with CSA F280 for new residential construction; in Ontario, the Ontario Building Code requires it for Part 9 houses. F280 is a room-by-room heat-loss and heat-gain calculation based on the house itself: its walls, windows, air leakage, orientation and the local design temperatures. It can also be used for existing houses that have undergone extensive envelope upgrades.

Never size a heat pump from the rating of the old furnace or from a rule of thumb per square foot. Neither reflects the house as it is today, especially after air sealing, insulation or new windows.

Design Temperature

Loads are calculated at the design temperatures for the site, taken from Supplementary Standard SB-1 of the Ontario Building Code. Municipalities publish them for their area. For example, the Town of Caledon lists a January 2.5 % design temperature of −21 °C and 4,450 heating degree-days, from the 2024 OBC SB-1.

The NRCan Sizing Workflow

NRCan CanmetENERGY's Air-Source Heat Pump Sizing and Selection Guide sets out seven steps. They are the backbone of a defensible selection:

1. Define the heat pump configuration: ducted, ductless or a mix.
2. Choose the indoor unit types for ductless systems.
3. Determine the design heating and cooling loads.
4. Choose the sizing approach.
5. Select equipment that meets it.
6. Define the control strategy.
7. Define the backup heating.

The Four Sizing Approaches

Step 4 is where the owner's priorities become numbers. NRCan describes four options:

Options 4A and 4B leave more of the winter to the backup heat; 4C and 4D shift it to the heat pump. Which one is right depends on the house, the backup fuel and the budget, so record the choice and its reasons with the selection.

Worked Example

The NRCan guide's F280 example is a new three-storey townhome built to ENERGY STAR for New Homes requirements, with these design loads:

The same house can therefore be served by quite different units. Option 4D, for example, needs a unit that still delivers more than 15,800 Btu/h at −20 °C. Check each candidate against the manufacturer's capacity at those temperatures, not its nominal size.

Figure 1 plots this house's load line against an illustrative cold-climate capacity curve, labelled as such because it is not a product. Where the two lines cross is the balance point; the shaded area below it is the heat the backup must supply on the coldest days.

Balance point for the worked example

Loads from the NRCan guide’s example; the capacity curve is illustrative, not a product.

Parts

1. 1

   Building heating load: a straight line from the design heating load at the design temperature (15,800 Btu/h at −20 °C) to zero load at 16 °C (60 °F). NRCan guide

2. 2

   Heat pump capacity: the manufacturer’s capacity at each outdoor temperature, maximum-capacity data where available. The curve here is illustrative, not a product.

3. 3

   Balance point: where the two lines cross. Above it the heat pump carries the load; below it backup heat is needed, and the backup cut-off is usually set here.

4. 4

   Backup heat: the hatched area between the lines below the balance point.

5. 5

   Option 4C target: 15,800 × 43 ÷ (60 − (−4)) ≈ 10,600 Btu/h at −8.3 °C (17 °F); output above the target is preferred. NRCan option 4C

• Building heating load
• Heat pump capacity (illustrative)
• Backup heat

Figure 1. Balance point for the worked example

Balance Point and Backup Heat

The thermal balance point is where the building's heating load line crosses the heat pump's capacity curve. Above it, the heat pump carries the whole load; below it, backup heat is needed. To find it, draw the load line from the design heating load at the design temperature down to zero load at 16 °C (60 °F), then plot the heat pump's capacity against outdoor temperature, using the maximum-capacity data where the manufacturer provides it.

The backup heat cut-off is usually set at the balance point. A dual-fuel system can instead use an economic cut-off, set from energy prices, the heat pump's COP and the backup's efficiency. Some heat pumps also have a low-temperature cut-off, and low-temperature faults may need a manual reset, so follow the manufacturer's operating limits.

Backup heat options include:

• Electric resistance strips in the air handler.
• The existing gas furnace, kept as backup (dual-fuel).
• Electric baseboards, often with ductless systems.

The control strategy depends on the cut-off temperature, the relative energy costs and how the heat pump's output compares with the design load.

Cold-Climate Heat Pumps

Cold-climate heat pumps are designed to keep their capacity at low outdoor temperatures, which is what options 4C and 4D rely on. When comparing units, look at the rating data at cold temperatures: the NEEP cold-climate list gives data down to −15 °C (5 °F), while CEE/AHRI data goes to −8.3 °C (17 °F). For Ontario design temperatures, ask the manufacturer for extended capacity data at the site's design temperature.

The Electrical Side

This is where mechanical sizing meets the panel.

The outdoor unit's circuit

The wire and the breaker are sized from the nameplate MCA and MOP, not from the tonnage. The wire must have an ampacity equal to or greater than the MCA on the nameplate, and the overcurrent device (the breaker) must not exceed the MOP. The MCA and MOP guide explains both values.

The disconnect

For air-conditioning and refrigeration equipment, Rule 28-604 5) requires the disconnecting means to be within sight of and within 3 m of the equipment; the HVAC electrical coordination guide explains the disconnect rules for mechanical equipment.

The load calculation (Rule 8-200)

For a single dwelling, the heat pump and its backup are added to the service calculation under Rule 8-200, which takes space heating per Section 62 and air conditioning at 100 % (Rule 8-200 1) a) iii)):

• Interlocked heating and cooling: where heating and air conditioning cannot run at the same time, the larger of the two loads is used (Rules 8-200 1) a) iii) and 8-106 3)).
• Electric furnace or duct heater (including the air-handler strips): counted at 100 % of the rating (Rule 62-118 4)).
• Baseboard heat with room thermostats: 100 % of the first 10 kW and 75 % of the balance (Rule 62-118 3), via Rule 8-200 1) a) iii)).

The panel

The panel upgrade guide lists heat pumps at 30–60 A in its load table, so check the service capacity before adding one. An energy management system can sometimes avoid a service upgrade by managing loads, the same approach the EV charging guide describes for chargers. In Ontario, the OESC amends Rule 8-500 to cover energy management systems in general, and Rule 8-106 10) sets the demand load of loads controlled by an energy management system at the maximum load the system allows (OESC Rules 8-106 10) and 8-500, Ontario amendments).

Electrical points of an air-source heat pump

A typical house installation, not a specific project. Not to scale.

Electrical points

1. 1

   Outdoor unit nameplate: the wire ampacity is at least the MCA, and the overcurrent device (the breaker) does not exceed the MOP; not sized from the tonnage. MCA / MOP

2. 2

   Disconnect: within sight of and within 3 m of the air-conditioning and refrigeration equipment it serves. Rule 28-604 5)

3. 3

   Branch circuit to the outdoor unit: from the panel through the disconnect.

4. 4

   Air handler backup strips: on their own circuit; an electric furnace or duct heater counts at 100 % of its rating in the load calculation. Rule 62-118 4)

5. 5

   Controls and interlock: where heating and cooling cannot run at the same time, the larger of the two loads is used. Rules 8-200 1) a) iii), 8-106 3)

6. 6

   Panel and service: add the heat pump and backup to the load calculation; an energy management system can sometimes avoid a service upgrade; its controlled loads count at the maximum the system allows. OESC Rules 8-106 10), 8-500 (Ontario amendments)

• Power circuit
• Controls and interlock

Figure 2. Electrical points of an air-source heat pump

Rebates and Paperwork

Ontario programs, such as the Home Renovation Savings Program, often require a heat-loss calculation and eligible equipment. Programs and their rules change, so check the current program rules before buying equipment, and keep the calculation and the equipment data with the project file.

When You Need a P.Eng.

The steps above are written for Part 9 houses. Larger buildings are different: multi-unit residential, commercial and institutional projects need a heat pump design by a professional engineer, coordinated with the electrical design that supplies it. The P.Eng. stamp guide explains when Ontario requires stamped drawings.

Common Mistakes

• Oversizing for heating with a single-stage unit: it short-cycles in summer.
• Sizing from the old furnace instead of a CSA F280 calculation.
• Ignoring the balance point and backup: the house is cold on the coldest days, or the backup runs far more than expected.
• Forgetting the electrical capacity: the circuit, the disconnect and the service load calculation.
• Sizing a single-stage unit to the heating load in a cooling-dominated design.

Frequently Asked Questions

Sources

1. Ontario Electrical Safety Code, 29th edition (2024), CSA C22.1:24 with Ontario amendmentsRules 8-106 3) and 10), 8-200 1) a) iii), 8-500, 28-604 5) and 62-118 3) and 4).
2. NRCan CanmetENERGY — Air-Source Heat Pump Sizing and Selection Guide, Version 1.0 (2020-12-21)CSA F280, the seven-step workflow, sizing options 4A–4D, the worked-example loads, balance point, cut-off temperatures, cold-climate data and backup heat.
3. Town of Caledon — Climatic Design DataJanuary 2.5 % design temperature and heating degree-days from the 2024 OBC SB-1.
4. Guide to the Canadian Electrical Code, 26th Edition — A Road Map: Section 8Rule 8-200 and interlocked heating and cooling loads.
5. IAEI Magazine — Residential Load CalculationsBackground on electric heating loads in the residential load calculation.