Heat Pump Flow Temperature and Radiator Output
Flow temperature is arguably the single most important number in any heat pump installation. It determines how efficiently your heat pump runs and how much heat your radiators can deliver. Get it right and you have a warm, efficient home. Get it wrong and you will either shiver in cold rooms or pay far more in electricity than necessary.
This guide explains why flow temperature matters so much, how it affects radiator output, and what you can do to ensure your system is optimised.
What Is Flow Temperature?
Flow temperature is the temperature of the water leaving your heat pump and entering your heating distribution system — whether that is radiators, underfloor heating, or a combination of both.
With a gas boiler, the flow temperature is typically set at 70 to 80°C. This is far higher than needed for most situations, but boilers are cheap to run at high temperatures because gas is inexpensive relative to electricity. With a heat pump, the economics are reversed: every degree you lower the flow temperature improves efficiency and reduces running costs.
The Return Temperature
Water flows out of the heat pump (the flow), passes through your radiators where it gives up some of its heat, and returns cooler to the heat pump (the return). The difference between flow and return temperatures is called the delta T (ΔT). For heat pump radiator systems, a typical ΔT is 5 to 10°C, compared to 10 to 20°C for gas boilers.
Why 35 to 45°C Is the Sweet Spot
The efficiency of a heat pump is measured by its coefficient of performance (COP) — the ratio of heat delivered to electricity consumed. COP varies dramatically with flow temperature:
| Flow Temperature | Typical COP (Air Source, 7°C Outside) | Electricity for 10 kWh of Heat | Cost at 24.5p/kWh |
|---|---|---|---|
| 35°C | 3.8 | 2.63 kWh | 64p |
| 40°C | 3.3 | 3.03 kWh | 74p |
| 45°C | 2.9 | 3.45 kWh | 85p |
| 50°C | 2.5 | 4.00 kWh | 98p |
| 55°C | 2.2 | 4.55 kWh | £1.11 |
The difference is stark. Running at 35°C instead of 55°C costs roughly 42% less in electricity for the same amount of heat. Over a full heating season, that translates to hundreds of pounds in savings.
The range of 35 to 45°C represents the practical sweet spot for most radiator-based heat pump systems. Below 35°C, standard radiators struggle to deliver enough heat. Above 45°C, efficiency drops noticeably and running costs climb.
How Flow Temperature Affects Radiator Output
Radiators are rated at a standard condition called Delta T 50 (ΔT50), which assumes a flow temperature of 75°C, a return of 65°C, and a room temperature of 20°C. This is how manufacturers publish their outputs — a radiator rated at 1,000 watts delivers 1,000 watts under those specific conditions.
When you reduce the flow temperature, the radiator delivers significantly less heat. The relationship is not linear — it follows a curve that drops off sharply:
| Flow Temperature | ΔT (approx.) | Output as % of Rated | A "1,000W" Radiator Delivers |
|---|---|---|---|
| 75°C | 50 | 100% | 1,000W |
| 55°C | 30 | 51% | 510W |
| 50°C | 25 | 40% | 400W |
| 45°C | 20 | 30% | 300W |
| 40°C | 15 | 22% | 220W |
| 35°C | 10 | 14% | 140W |
This table reveals a critical point: a radiator rated at 1,000 watts at standard boiler conditions delivers only 300 watts at a flow temperature of 45°C. That means you need roughly three times the radiator surface area to deliver the same amount of heat to the room.
Sizing Radiators for Heat Pump Flow Temperatures
The process for sizing radiators for a heat pump is straightforward but different from how a gas boiler system is designed:
Step 1: Calculate Room Heat Loss
Every room in your home loses heat through walls, windows, the roof, and the floor. A proper heat loss calculation — which your installer should perform — determines exactly how many watts each room needs to maintain a comfortable temperature on the coldest days. For an average UK home, a living room might need 1,500 to 2,500 watts, while a bedroom might need 800 to 1,500 watts.
Step 2: Choose Your Design Flow Temperature
Your installer will recommend a flow temperature based on balancing efficiency against practicality. For most existing homes with upgraded radiators, 45°C is a common design point. For very well-insulated homes, 40°C may be achievable. For homes with limited options for radiator upgrades, 50°C might be necessary.
Step 3: Select Radiators with Sufficient Output
Using the correction factors above, select radiators whose output at your design flow temperature matches or exceeds each room's heat loss. A radiator sizing calculator simplifies this process.
For example, if a bedroom needs 1,200 watts and your design flow temperature is 45°C (30% of rated output), you need a radiator rated at 1,200 ÷ 0.30 = 4,000 watts at ΔT50. That is a substantially larger radiator than a gas boiler system would require — typically a Type 22 double-panel convector measuring 600mm high by 1,400mm or more in length.
Weather Compensation: Automatic Flow Temperature Adjustment
Modern heat pumps include weather compensation as standard. An outdoor temperature sensor tells the heat pump how cold it is outside, and the system automatically adjusts the flow temperature according to a preset heating curve.
How the Heating Curve Works
The heating curve is a simple relationship: when it is colder outside, the heat pump sends hotter water to the radiators. When it is milder, it reduces the flow temperature. A typical curve might look like this:
- -3°C outside: 45°C flow temperature
- 5°C outside: 38°C flow temperature
- 10°C outside: 32°C flow temperature
- 15°C outside: System off or minimal flow
The beauty of this system is that the heat pump operates at its highest efficiency most of the time. The UK has relatively few extremely cold days, so for most of the heating season, the flow temperature is well below the design maximum. This means your seasonal average efficiency is considerably better than the worst-case COP at design temperature.
Getting the Curve Right
Your installer should set the heating curve during commissioning and adjust it based on your feedback. If your home is too warm on mild days, the curve is too steep and should be flattened. If rooms are cold on the coldest days, the curve may need shifting upward or the maximum flow temperature may need increasing.
What If Your Radiators Are Not Big Enough?
If a heat loss calculation reveals that your current radiators cannot deliver enough heat at your target flow temperature, you have several options, broadly ordered from least to most disruptive:
1. Accept a Higher Flow Temperature
Running at 50°C instead of 45°C reduces efficiency but avoids replacing any radiators. The COP difference is roughly 0.4 points — noticeable in running costs but not dramatic. This is sometimes the pragmatic choice for one or two marginal rooms.
2. Upgrade Individual Radiators
Replace undersized radiators with larger double-panel convectors or aluminium radiators. This is the most common approach. A plumber can typically swap a radiator in two to three hours, using existing pipework if the valve positions are compatible.
3. Add Extra Radiators
In some rooms, there is not enough wall space for a single large radiator. Adding a second radiator on another wall increases total output without needing one enormous panel. This requires additional pipework.
4. Install Fan-Assisted Radiators
Fan convectors (like Jaga or Biddle units) use a small electric fan to push air over the radiator fins, boosting output at low flow temperatures by 50 to 100%. They are more expensive but solve space constraints where a standard large radiator will not fit.
5. Add Underfloor Heating
In rooms where you are already planning floor works, adding UFH provides ample output at very low flow temperatures. This is particularly effective in kitchens and living areas.
Common Mistakes with Flow Temperature
Setting the Flow Temperature Too High
Some installers set the flow temperature to 55°C or higher "to be safe." This makes radiators feel hotter but slashes heat pump efficiency. If your radiators are properly sized, there is no need for flow temperatures above 45 to 50°C, and weather compensation should reduce it further most of the time.
Turning Off Weather Compensation
Some homeowners disable weather compensation because they do not understand what it does, or because they want "hotter radiators." This forces the heat pump to run at a fixed, higher flow temperature all the time, wasting electricity.
Ignoring the Return Temperature
If the gap between flow and return temperature is too small (less than 3 to 4°C), the water is flowing too fast through the radiators and not giving up enough heat. If the gap is too large (more than 10°C), flow rates may be too low. Your installer should check and adjust flow rates at each radiator during commissioning.
Monitoring and Optimising Your System
After installation, you can monitor flow temperature through your heat pump's display or app. Many modern heat pumps (such as those from Vaillant, Samsung, and Daikin) show real-time flow and return temperatures, COP, and energy consumption.
Key things to check:
- Average flow temperature through the season: Aim for a seasonal average of 35 to 40°C with weather compensation enabled
- COP readings: A seasonal COP (SCOP) of 3.0 or above indicates the system is running efficiently
- Room temperatures: If rooms consistently reach their target temperatures, the system is working correctly
If your seasonal average flow temperature is above 45°C or your SCOP is below 2.5, it is worth having your installer review the system settings and radiator sizing.
Frequently Asked Questions
What flow temperature should my heat pump be set to?
Most heat pump radiator systems in the UK are designed for a maximum flow temperature of 45 to 50°C on the coldest days, with weather compensation reducing this to 30 to 40°C for most of the heating season. Your installer should set this based on a proper heat loss calculation.
Why are my radiators only lukewarm?
This is normal with a heat pump. At 40 to 45°C flow temperature, radiators feel warm to the touch rather than hot. If the room is reaching its target temperature, the system is working correctly even if the radiators do not feel as hot as they did with a boiler.
Can I turn up the flow temperature to make rooms warmer?
You can, but it reduces efficiency and increases running costs. If rooms are not reaching their target temperature, the better solution is to check radiator sizing, insulation, and system settings rather than simply increasing the flow temperature.
Does flow temperature affect hot water production?
Yes. Heat pumps typically raise the flow temperature to 50 to 55°C when heating the hot water cylinder, which reduces efficiency temporarily. Some systems use a separate hot water cycle to minimise the impact on space heating efficiency.
What is the minimum flow temperature for radiators?
Radiators can theoretically work at any flow temperature above room temperature, but below about 35°C, the output from standard radiators is so low that they struggle to heat most rooms adequately. For radiator-based systems, 35°C is generally the practical minimum; underfloor heating can operate lower.
Should I leave the heat pump running all the time?
Yes. Heat pumps are designed to run continuously at a low output rather than cycling on and off. With weather compensation, the system automatically adjusts the flow temperature throughout the day and season, maintaining steady room temperatures efficiently.