Heat Pump Balance Point Calculator

Building heat loss
Find it on ASHRAE's site
  1. Open ASHRAE Climatic Design Conditions.
  2. Select the edition year you want, usually the newest, or the one your energy code calls for.
  3. Click IP for °F.
  4. Right click your location on the map, or use the magnifying glass to search for it.
  5. Select the nearest weather station from the list.
  6. Scroll down to the Annual Heating table and read Heating DB at 99% (residential) or 99.6% (commercial). That is the outdoor design temperature.
Heat pump capacity, from the manufacturer

How to use it
  1. Enter the No-heat temperature, the outdoor temperature above which the building needs no heat, because internal and solar gains cover the losses. 65 °F is the traditional degree-day base and the conservative default; many buildings, especially commercial ones with high internal gains, stop needing heat at a lower temperature, which would lower the balance point and the strip heat.
  2. Enter the outdoor design temperature and the building's design heat loss (from a load calculation).
  3. From the manufacturer's data, enter the heat pump's heating capacity at two outdoor temperatures. The AHRI rating points, 47 °F and 17 °F, are the usual pair.
  4. The results update as you type. They show the balance point, the shortfall at the design temperature, and the strip heat, in kW, that covers it. The chart shows both lines: point at it to read the heat loss and capacity at any outdoor temperature, and click or tap it to make that temperature the outdoor design, with the design heat loss read off the building line at it.

Use Copy link to these results to save or share the calculation.

Balance point and supplemental heat

Enter the design heat loss.

The strip heat is the calculated need, not rounded up. Choose a strip heater at least that size from those the manufacturer offers for your air handler.

Methodology, equation and example

A heat pump's heating capacity falls as the outdoor temperature drops, while the building's heat loss rises. The balance point is the outdoor temperature at which the two are equal. Above it, the heat pump alone carries the building. Below it, supplemental heat (usually electric strip heat in the air handler) has to make up the difference.

The calculator treats both as straight lines:

Heat loss(T) = Design heat loss × (T₀ − T) / (T₀ − T_design)

Capacity(T) = Q_low + (Q_high − Q_low) / (T_high − T_low) × (T − T_low)

The balance point is where they cross:

T_bal = (m_L × T₀ − Q_low + m_C × T_low) / (m_C + m_L)

Supplemental heat = Design heat loss − Capacity(T_design), and zero if the heat pump covers the design load. Converting to kW: 1 kW = 3,412.14 Btu/h.

Where:

  • T₀: outdoor temperature at which the building stops needing heat, °F
  • T_design: outdoor design temperature, °F
  • m_L: slope of the heat loss line, Design heat loss / (T₀ − T_design), Btu/h per °F
  • m_C: slope of the capacity line, (Q_high − Q_low) / (T_high − T_low), Btu/h per °F
  • Q_low, Q_high: heat pump capacity at the low and high rating temperatures T_low and T_high, Btu/h

The supplemental heat is given as calculated, in kW, and is not rounded up to a strip heater size, because the sizes offered differ by manufacturer and air handler. The calculator also gives the emergency heat: the strip heat covering the full design load, for when the heat pump is off or locked out.

Example: A house loses 15,000 Btu/h at a 17 °F design temperature and needs no heat above 65 °F. The heat pump delivers 17,800 Btu/h at 47 °F and 10,700 Btu/h at 17 °F.

  • m_L = 15,000 / (65 − 17) = 312.5 Btu/h per °F
  • m_C = (17,800 − 10,700) / (47 − 17) = 236.67 Btu/h per °F
  • T_bal = (312.5 × 65 − 10,700 + 236.67 × 17) / (236.67 + 312.5) = 24.8 °F
  • Capacity at 17 °F = 10,700 Btu/h, so the supplemental heat is 15,000 − 10,700 = 4,300 Btu/h = 1.26 kW of supplemental strip heat.

Notes

  • Use the capacities the manufacturer publishes for the matched indoor and outdoor units. The calculator uses them as entered.
  • Below the lower rating temperature, the calculator extends the straight line through the two rating points. If the manufacturer publishes capacity at the design temperature itself, enter that as the low rating point.
  • Strip heater sizes vary by manufacturer and air handler. Choose one at least as large as the calculated need from the sizes available for the unit you are specifying.
Top