✦ MCS & Low-Carbon Engineering

Heat Pump Size Calculator

Size Air Source (ASHP) and Ground Source (GSHP) heat pumps to exact peak thermal loads. Evaluate seasonal SCOP performance across flow temperatures (35°C–55°C), size domestic hot water cylinders, and project annual running costs.

Heat Pump Type:
Emitter Type:

🏠 1. Building Heat Loss & Winter Climate

Design Temperature Difference (ΔT): 25.0 K

🚿 2. Domestic Hot Water (DHW) & Margins

⚡ 3. Electricity Rate & Annual Heating Demand

Recommended Heat Pump Size
9.0 kW
Capacity at -4°C · 30,709 BTU/hr
Expected SCOP
3.80 (380%)
Annual Power
3,763 kWh
Est. Annual Electric Cost: $978 / yr
Flow Temperature: 45°C (ΔT25 K)
Radiator Scale Factor: 1.96× vs standard boiler
📈 Thermodynamic Carnot Principles

Interactive Flow Temperature vs. SCOP Efficiency Simulator

Observe how reducing emitter water temperature increases heat pump seasonal efficiency and lowers electricity bills.

35°C (Underfloor) 45°C (Oversized Rads) 55°C (Standard Rads) 65°C (High-Temp)

The 2.5% Rule: For every 1°C you lower the flow temperature of a heat pump, the seasonal efficiency (SCOP) increases by approximately 2.5%. Sizing radiators to run at 45°C instead of 55°C saves ~25% on annual electricity costs.

Estimated Seasonal SCOP: 3.80 (380%)
Annual Electricity (for 12,000 kWh heat): 3,158 kWh / yr
Annual Electric Bill (@ 28p/kWh): £884 / yr
Savings vs 55°C Setup: £236 / yr saved (21%)
🚿 Hot Water Storage

Domestic Hot Water (DHW) Cylinder & Coil Sizing Calculator

Match unvented cylinder volume and heat pump heat exchanger coil surface area ($m^2$) to your household occupants.

Recommended Cylinder Size 210 Litres
Min Heat Pump Coil Area 2.5 m² (High Gain)
Estimated Reheat Time (8kW HP) 38 Minutes
🔄 Technology Matrix

Air Source (ASHP) vs. Ground Source (GSHP) Comparison

Compare capital investment, operating efficiency, and installation requirements.

Metric Air Source (ASHP) Ground Source (GSHP) Hybrid Heat Pump
Seasonal SCOP 3.2 – 4.0 4.2 – 5.2 3.0 – 3.6
Sub-Zero Stability Capacity drops with ambient air temp; requires defrost. 100% stable; ground stays at ~10°C year-round. Switches automatically to gas boiler below 2°C.
Space & Ground Works Compact outdoor monobloc fan unit (1m × 1.2m). Requires boreholes (100m deep) or extensive horizontal trenches. Small outdoor ASHP unit + indoor wall-hung combi boiler.
Best Suited For 90% of suburban retrofits and urban homes. Rural detached homes with large gardens or acreage. Older homes with high heat loss and standard radiators.
⚠️ Sizing Traps

5 Fatal Heat Pump Sizing Mistakes to Avoid

Learn what causes poor efficiency and noisy compressors in poorly designed installations.

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1. Oversizing "Just to Be Safe"

Unlike gas boilers, an oversized heat pump cannot throttle down low enough during mild 8°C–12°C weather. It short-cycles on/off constantly, reducing compressor lifespan and spiking electricity bills.

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2. Retaining a Standard Gas Boiler Cylinder

Standard gas boiler hot water cylinders have a small 0.8 m² coil. Connecting a heat pump results in 2+ hour reheat times and frequent backup immersion heater usage.

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3. Designing for 60°C Flow Temperature

Running a heat pump at 60°C flow causes seasonal SCOP to crash from 3.8 down to 2.5, wiping out carbon and running cost savings.

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4. Omitting a Volumetric Buffer or Bypass

During winter defrost cycles, the heat pump needs rapid thermal energy from the system water to defrost the outdoor coil. Without sufficient buffer volume, it trips on error codes.

🧰 Dedicated Calculation Engines

Explore the Heat Loss Calculator Suite

Access our complete family of specialized thermal engineering tools for rooms, whole houses, radiators, heat pumps, and insulation assemblies.

🏠
Single Room

Room Heat Loss Calculator

Calculate precise heat loss for individual rooms to size replacement radiators.

Open Dedicated Tool →
🏘️
Whole Building

Whole House Calculator

Aggregate whole-building heat demand to size central heating boilers and heat pumps.

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📋
Multi-Zone + CSV

Room-by-Room Calculator

Comprehensive multi-room inventory with cumulative heat loss schedules.

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📐
ISO 6946

U-Value Calculator

Calculate composite U-values for multi-layer wall, roof, and floor assemblies.

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4-Way Converter

R-Value to U-Value Converter

Instantly convert between imperial R-values (hr·ft²·°F/BTU) and metric U-values.

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🔥
BS EN 442

Radiator Sizing Calculator

Size panel radiators for ΔT50, ΔT40, and low-temperature ΔT30 heat pumps.

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⚙️
Combi & System

Boiler Sizing Calculator

Size condensing gas, oil, or LPG boilers with domestic hot water allowances.

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♻️
MCS & SCOP

Heat Pump Sizing Calculator

Determine design kW capacity and seasonal coefficient of performance (SCOP).

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🌡️
80+ Cities Database

Design Temperature Lookup

Search regional 99.6% winter design outdoor temperatures across the globe.

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💨
Blower Door & MVHR

Air Infiltration / ACH Calculator

Estimate ventilation and air leakage heat loss based on air changes per hour.

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Frequently Asked Questions

Find clear, expert answers to common questions about heat loss calculations, heating system sizing, U-values, and building thermal efficiency.

Gas boilers have high instantaneous modulation ratios (e.g. 1:10) and can heat a house quickly in bursts. Heat pumps operate most efficiently during steady, continuous low-temperature running. Oversizing a heat pump leads to severe compressor short-cycling, rapid starts/stops per hour, premature component failure, and a sharp collapse in seasonal COP efficiency.
COP (Coefficient of Performance) measures instantaneous efficiency at a single laboratory test point (e.g. 7°C outdoor / 35°C water). SCOP (Seasonal Coefficient of Performance) calculates the true average efficiency weighted across the entire heating season, factoring in sub-zero winter frosts, defrost cycles, and mild spring days.
Heat pumps supply lower water temperatures (50°C–55°C) than gas boilers (70°C–80°C). Because the temperature differential between the heating water and the cold domestic water is smaller, the heat transfer rate per square metre of coil is reduced. A dedicated heat pump cylinder requires a large surface area heat exchanger coil (typically 2.0 to 3.2 m² vs 0.8 m² for a boiler cylinder) to reheat water in 35–45 minutes.
A bivalent system pairs a heat pump with an auxiliary backup heater (such as an internal electric immersion element or a secondary gas/oil boiler). The heat pump provides 100% of the heating needs down to a calculated 'bivalent temperature' (e.g. -2°C), above which 95%+ of annual heating occurs. Below this point, the auxiliary heater kicks in to meet peak sub-zero deficits.
A buffer tank or low loss header provides hydraulic separation between the heat pump primary circuit and the radiator heating circuits. It guarantees minimum water volume for defrost cycles (preventing the heat pump from extracting heat from cold radiators) and prevents short-cycling when individual room TRVs close down.
Between 0°C and 5°C with high relative humidity (80%–95%), moisture in outdoor air freezes directly onto the evaporator coil. The heat pump must periodically reverse its refrigerant cycle into 'defrost mode' (every 45–90 minutes) to melt the ice, temporarily reducing delivered heating output by 5%–10%.