Heat Pump Balance Point Calculator
ANA›Life Services Authority›National Calculator Authority›Heat Pump Balance Point Calculator
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Heat Pump Balance Point Calculator
Determine the balance point temperature — the outdoor temperature at which your heat pump's heating capacity exactly matches your building's heat loss rate. Below this temperature, supplemental heat is required.
### Building Heat Loss
Design Heating Load (BTU/hr) ?
Indoor Design Temperature (°F) ?
Outdoor Design Temperature (°F) ?
### Heat Pump Capacity
Rated Heating Capacity at 47°F (BTU/hr) ?
Heating Capacity at Low Temp (BTU/hr) ?
Low Temperature Rating Point (°F) ?
Calculate Balance Point
### Results
Balance Point Temperature —
Heat Loss Rate (UA) —
HP Capacity at Balance Point —
Supplemental Heat Needed at Design Temp —
HP Capacity at Design Outdoor Temp —
System Status —
function heaCalc() { const errEl = document.getElementById('hea-error'); const resEl = document.getElementById('hea-result'); errEl.style.display = 'none'; resEl.style.display = 'none';
// --- Parse inputs --- const designLoad = parseFloat(document.getElementById('hea-design-load').value); const indoorTemp = parseFloat(document.getElementById('hea-indoor-temp').value); const outdoorDesign = parseFloat(document.getElementById('hea-outdoor-design').value); const ratedCapacity = parseFloat(document.getElementById('hea-rated-capacity').value); const lowTempCapacity = parseFloat(document.getElementById('hea-low-temp-capacity').value); const lowTempRating = parseFloat(document.getElementById('hea-low-temp-rating').value);
// --- Validation --- const errors = []; if (isNaN(designLoad) || designLoad 80) errors.push("Indoor design temperature must be between 60°F and 80°F."); if (isNaN(outdoorDesign)) errors.push("Outdoor design temperature is required."); if (isNaN(ratedCapacity) || ratedCapacity = indoorTemp) errors.push("Outdoor design temperature must be lower than indoor design temperature."); if (!isNaN(lowTempRating) && !isNaN(indoorTemp) && lowTempRating >= indoorTemp) errors.push("Low temperature rating point must be below indoor temperature."); if (!isNaN(ratedCapacity) && !isNaN(lowTempCapacity) && lowTempCapacity > ratedCapacity) errors.push("Low-temperature capacity should not exceed rated capacity at 47°F (capacity decreases as temperature drops)."); if (!isNaN(lowTempRating) && lowTempRating >= 47) errors.push("Low temperature rating point must be below 47°F.");
if (errors.length > 0) { errEl.innerHTML = "" + errors.map(e => "").join("") + ""; errEl.style.display = 'block'; return; }
// --------------------------------------------------------------- // FORMULAS // --------------------------------------------------------------- // // 1. Building UA (heat loss coefficient, BTU/hr·°F): // UA = Q_design / (T_indoor - T_outdoor_design) // // 2. Heat pump capacity model (linear interpolation between two // manufacturer rating points — 47°F and low-temp rating point): // Cap(T) = Cap_47 + (Cap_lowT - Cap_47) / (T_lowRating - 47) * (T - 47) // This is a linear fit: Cap(T) = m * T + b // where m = (Cap_lowT - Cap_47) / (T_lowRating - 47) // b = Cap_47 - m * 47 // // 3. Building heat load at outdoor temp T: // Load(T) = UA * (T_indoor - T) // // 4. Balance point: Cap(T_bp) = Load(T_bp) // m * T_bp + b = UA * (T_indoor - T_bp) // m * T_bp + b = UA * T_indoor - UA * T_bp // T_bp * (m + UA) = UA * T_indoor - b // T_bp = (UA * T_indoor - b) / (m + UA) // // ---------------------------------------------------------------
// Step 1: UA const deltaT_design = indoorTemp - outdoorDesign; const UA = designLoad / deltaT_design; // BTU/hr·°F
// Step 2: Linear capacity model const m = (lowTempCapacity - ratedCapacity) / (lowTempRating - 47); // BTU/hr per °F const b = ratedCapacity - m * 47; // BTU/hr intercept
// Step 3: Balance point // T_bp = (UA * T_indoor - b) / (m + UA) const denominator = m + UA; if (Math.abs(denominator)
#### Formulas Used
1. Building Heat Loss Coefficient (UA):
UA = Q_design / (T_indoor − T_outdoor_design) Where UA is in BTU/hr·°F, Q_design is the design heating load (BTU/hr), and temperatures are in °F.
2. Building Heat Load at Any Outdoor Temperature T:
Load(T) = UA × (T_indoor − T)
3. Heat Pump Capacity Model (Linear Interpolation):
m = (Cap_lowT − Cap_47) / (T_lowRating − 47) b = Cap_47 − m × 47 Cap(T) = m × T + b Linear interpolation between the two AHRI manufacturer rating points: 47°F and the low-temperature rating point (typically 17°F).
4. Balance Point Temperature:
Cap(T_bp) = Load(T_bp) m × T_bp + b = UA × (T_indoor − T_bp) T_bp = (UA × T_indoor − b) / (m + UA)
5. Supplemental Heat Required at Design Conditions:
Q_supp = max(0, Load(T_design) − Cap(T_design))
#### Assumptions & References
- Heat pump capacity is modeled as linear between the 47°F and low-temperature AHRI rating points. Real performance curves may be slightly non-linear; consult manufacturer expanded performance data for higher accuracy.
- Building heat loss is assumed to be linearly proportional to the indoor-outdoor temperature difference (standard steady-state conduction/infiltration model per ACCA Manual J).
- The design heating load should be calculated per ACCA Manual J or equivalent energy audit methodology — not simply based on equipment size.
- The outdoor design temperature is the 99% heating design dry-bulb temperature from ASHRAE Fundamentals or local climate data (ACCA Manual J Table 1).
- This calculator does not account for defrost cycles, which temporarily reduce heat pump output (typically 5–15% reduction in average capacity in cold, humid climates).
- Heat pump minimum operating temperature (lockout point) is not modeled; verify with manufacturer specifications.
- Supplemental heat sizing should follow ACCA Manual S equipment selection guidelines.
- References: ASHRAE Handbook — Fundamentals (2021); ACCA Manual J (8th Ed.); ACCA Manual S; AHRI Standard 210/240 (Unitary Air-Conditioning and Air-Source Heat Pump Equipment).
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