Enter System Assumptions
Use metric inputs. Change defaults to match the planned system.
Example Data Table
| Input | Example value | Purpose |
|---|---|---|
| Heated floor area | 140 m² | Defines the radiant area served. |
| Heat-loss rate | 55 W/m² | Represents building heat demand at design conditions. |
| Supply water temperature | 35°C | Supports low-temperature radiant heating. |
| Rated COP | 4.00 | Starting performance value before temperature adjustment. |
| Loop temperature difference | 5°C | Used to estimate water flow through circuits. |
| Electricity tariff | 0.30 per kWh | Converts estimated energy into seasonal cost. |
Formula Used
The calculator uses simplified engineering relationships. They support early comparisons. They are not a substitute for verified equipment curves.
Climate factor = (Indoor temperature − Outdoor temperature) ÷ (Indoor temperature − Design outdoor temperature)
Room heat demand (kW) = Floor area × Heat-loss rate × Climate factor ÷ 1000
Heat pump thermal output (kW) = Room heat demand ÷ Distribution efficiency
Adjusted COP = Rated COP + 0.075(Source temperature shift) − 0.045(Supply temperature shift)
Effective COP = Adjusted COP × Part-load efficiency
Electrical input (kW) = Thermal output ÷ Effective COP + Auxiliary electrical load
Loop flow (L/min) = Thermal output × 60 ÷ (4.186 × Loop temperature difference)
Seasonal cost = Daily electricity × Heating season days × Electricity tariff
The COP correction is an estimate. Use published performance maps for final equipment sizing.
How to Use This Calculator
- Choose the heat source type for the proposed unit.
- Enter the heated area and realistic heat-loss rate.
- Add indoor, outdoor, and design outdoor temperatures.
- Enter source and supply water temperatures from your system plan.
- Use the manufacturer's rated COP and matching rating temperatures.
- Set distribution, part-load, operating, and tariff assumptions.
- Select Calculate Heat Pump Performance.
- Review the result panel above the form and compare scenarios.
Radiant Heat Pump Performance
Radiant heating moves warm water through floor, wall, or ceiling circuits. Broad surfaces release gentle heat. Rooms can feel comfortable at lower air temperatures. A heat pump produces this water efficiently when temperatures stay low. This calculator estimates design demand, water flow, electrical input, and seasonal operating cost. It uses practical assumptions for early planning. Results help compare scenarios. They do not replace a room-by-room heat-loss calculation, hydraulic design, manufacturer performance data, or professional installation advice for a finished system.
Heat Demand and Temperature
Heat demand starts with conditioned floor area and an estimated loss rate. Better insulation lowers the required watts per square metre. Large windows, drafts, and exposed walls often increase it. Outdoor temperature also changes the current demand. The calculator compares outdoor temperature with a chosen design temperature. Colder weather increases the climate factor. Milder weather lowers it. This avoids applying one extreme design value throughout the season. Realistic assumptions about insulation, ventilation, windows, and air leakage improve the estimate substantially.
Why Lower Water Temperatures Help
Radiant circuits usually need lower supply temperatures than conventional radiators. Lower supply temperature reduces the heat pump temperature lift. That can improve the coefficient of performance, called COP. The calculator starts with a rated COP. It then adjusts COP for source temperature and supply temperature differences. This is a linear planning method. Actual performance follows manufacturer maps. Defrost operation, humidity, compressor speed, water flow, controls, and installation quality also matter. Confirm ratings at the exact design conditions before selecting equipment.
Flow Rate and Distribution
The loop must carry enough water to move delivered heat. Required flow depends on thermal output and loop temperature difference. A smaller difference needs more flow. More flow can raise circulator power and pipe resistance. A larger difference reduces flow but may cause uneven circuit temperatures. The distribution efficiency represents piping losses, control losses, and imperfect floor transfer. Insulation below the circuit is important. It directs heat toward the room. Manifold balancing and pipe lengths help each zone receive heat.
Electricity Use and Cost
Electrical input equals delivered heat divided by effective COP. The calculator also includes auxiliary power for circulation pumps and controls. Daily energy follows operating hours. Seasonal energy multiplies daily use by heating days. Electricity price converts energy into cost. Actual bills will change with weather, solar gain, occupancy, and thermostat settings. Air-source units may lose output during cold damp weather. Ground-source and water-source systems may see steadier source temperatures. Compare several realistic operating scenarios instead of relying on one result.
Practical Design Checks
Keep supply water temperature as low as comfort permits. Confirm that floor coverings allow the needed surface output. Protect wood and sensitive finishes from excessive temperature. Create separate zones for rooms with different schedules or solar exposure. Check pump selection, manifold balance, pipe spacing, and circuit length. Review electrical capacity, condensate routing, safety controls, and local requirements. A qualified designer should verify all final values. Careful commissioning supports even warmth, efficient operation, and dependable service life across changing winter conditions.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates room heat demand, required heat pump output, loop flow, COP, electrical use, seasonal energy, and operating cost. It is intended for planning and comparisons.
2. Is the displayed COP guaranteed?
No. It is a temperature-adjusted planning estimate. Final COP depends on the selected unit, controls, source conditions, defrost behavior, water flow, and manufacturer test data.
3. Why does lower supply temperature improve results?
Lower supply temperature reduces temperature lift. The compressor usually needs less electrical work. Radiant systems can often use lower temperatures because their emitting surface is large.
4. What heat-loss rate should I enter?
Use a room-by-room heat-loss study whenever possible. For early planning, use a cautious rate that reflects insulation, windows, air leakage, climate, and exposed surfaces.
5. Why is distribution efficiency included?
Some heat is lost or delivered imperfectly through pipes, controls, and floor construction. Distribution efficiency increases required heat pump output to allow for those effects.
6. What does loop temperature difference mean?
It is the supply water temperature minus the return water temperature. It affects required water flow. Smaller differences require higher flow for the same thermal output.
7. Can this calculator size every radiant circuit?
No. Circuit sizing needs pipe diameter, spacing, length, pressure loss, manifold details, flooring, and zone output. Use a dedicated hydraulic design for final layouts.
8. Should I include pump power?
Yes. Enter expected circulation and control power as auxiliary electrical load. This gives a more complete electricity estimate than compressor input alone.
9. Does the source type change the calculation?
The selection identifies the system in the results. Source temperature still drives the COP adjustment. Use realistic entering temperatures for air, ground, or water sources.
10. Why can actual seasonal cost differ?
Weather, occupancy, solar gains, thermostat settings, electricity tariffs, defrost cycles, and maintenance all affect actual energy use. Run several cases to understand the likely range.
11. When should a professional review the design?
Use a qualified designer before equipment purchase or installation. They can verify heat loss, electrical capacity, hydraulic balance, controls, safety requirements, and local rules.