installation

By Sarah Cooper, Technical Reviewer, MCS Certified Heat Pump Engineer · Last reviewed

Heat Pump Flow Temperature: Why Lower Is Better & How to Opt

First published

TL;DR

  • Heat pump flow temperature (typically 35-55°C) directly impacts efficiency-lower is better for COP (Coefficient of Performance).
  • Every 1°C increase above 55°C reduces efficiency by ~2-3%, costing you £100s annually in higher electricity bills.
  • UK homes with underfloor heating can run at 35-40°C, while radiators may need upgrades to work efficiently at 45-55°C.
  • The Boiler Upgrade Scheme (BUS) rewards systems optimised for low flow temperatures-maximising your £7,500 grant.
  • Use a heat pump calculator to estimate savings before adjusting your system.

In 2023, OFGEM reported that UK households with heat pumps running at flow temperatures above 60°C saw their annual electricity bills rise by an average of £320 compared to those optimised at 45°C or below. Yet, despite the clear financial incentive, over 60% of UK heat pump installations still operate at unnecessarily high temperatures, mirroring outdated boiler settings. This isn’t just a minor tweak-it’s the difference between a system that saves you money and one that silently drains your wallet.

Heat pump flow temperature-the temperature at which hot water leaves the unit to heat your home-is the single most critical factor in determining your system’s efficiency. Unlike gas boilers, which thrive on high temperatures (60-80°C), heat pumps are designed to work most efficiently at 35-55°C. But why does this matter? And how can you adjust your system to reap the benefits without sacrificing comfort?

This guide covers everything you need to know: the science behind flow temperature, how it impacts your running costs, whether you need to upgrade your radiators, and step-by-step instructions to optimise your system. We’ll also explore how getting this right can maximise your £7,500 Boiler Upgrade Scheme grant and future-proof your home against rising energy prices.

What Is Heat Pump Flow Temperature (And Why It’s Not Like a Boiler)

The Basics: Flow vs. Return Temperature

Flow temperature is the temperature of the water as it leaves your heat pump and enters your heating system (radiators or underfloor heating). Return temperature is the temperature of the water as it comes back to the heat pump after circulating through your home. The difference between these two-known as the temperature "lift"-is what determines how hard your heat pump has to work.

For example:

  • If your flow temperature is 50°C and your return temperature is 40°C, the lift is 10°C.
  • If your flow temperature is 70°C and your return temperature is 60°C, the lift is still 10°C, but the heat pump is working much harder to achieve those higher temperatures.

Why Boilers and Heat Pumps Are Fundamentally Different

Gas boilers are designed to heat water quickly to high temperatures (60-80°C) because they rely on combustion, which is inherently inefficient at lower temperatures. In contrast, heat pumps transfer heat from the outside air (or ground) into your home using a refrigeration cycle. This process is most efficient when the temperature difference between the heat source (outside air) and the flow temperature is small.

Here’s the key difference:

FeatureHeat PumpGas Boiler
Typical Flow Temp35-55°C60-80°C
Efficiency PrincipleWorks best with small temperature liftsWorks best with high temperatures
Emitter CompatibilityUnderfloor heating (35-40°C) or oversized radiators (45-55°C)Standard radiators (60-80°C)
Running StyleContinuous, low-temperature outputIntermittent, high-temperature bursts

Source: Energy Saving Trust (2024)

The "Lift" Problem: Why Higher Flow Temperatures Kill Efficiency

The efficiency of a heat pump is measured by its Coefficient of Performance (COP)-the ratio of heat output to electricity input. For example, a COP of 4 means your heat pump produces 4 kWh of heat for every 1 kWh of electricity consumed.

The COP drops dramatically as the temperature lift increases. Here’s why:

  1. Physics of Heat Transfer: Heat pumps move heat rather than generate it. The larger the temperature difference between the outside air and your flow temperature, the harder the compressor has to work to "pump" that heat.
  2. Refrigerant Properties: The refrigerant in your heat pump (e.g., R32 or R410A) has an optimal operating range. Pushing it beyond this range reduces its ability to absorb and release heat efficiently.
  3. Defrost Cycles: In cold weather, heat pumps enter defrost mode to melt ice buildup on the outdoor unit. Higher flow temperatures increase the frequency of these cycles, further reducing efficiency.

According to MCS-certified data, a heat pump running at a 55°C flow temperature in 5°C outdoor conditions might achieve a COP of 2.8, while the same system at 35°C could reach a COP of 4.5. That’s a 60% improvement in efficiency-and a corresponding drop in your electricity bill.

The Golden Rule: How Lower Flow Temperatures Boost Your Heat Pump’s Efficiency

COP and SCOP: The Metrics That Matter

  • COP (Coefficient of Performance): A snapshot of efficiency at a specific outdoor temperature and flow temperature. For example, a COP of 4 at 7°C outdoor temperature and 45°C flow temperature means your heat pump is 400% efficient at that moment.
  • SCOP (Seasonal Coefficient of Performance): The average COP over an entire heating season, accounting for varying outdoor temperatures. This is the metric used to compare heat pumps in the UK and is critical for assessing long-term savings.

The Inverse Relationship Between Flow Temperature and COP

The relationship between flow temperature and COP is non-linear. As flow temperature increases, COP drops-but not at a steady rate. Here’s a real-world example based on data from Vaillant and Mitsubishi Electric:

Flow Temperature (°C)Outdoor Temperature (°C)COP (Approx.)Annual Electricity Cost (£)*
3574.5£650
4573.8£770
5573.0£980
6572.2£1,330

*Assumptions: 3-bed semi-detached house, 15,000 kWh annual heat demand, electricity price of £0.24/kWh (Octopus Energy Cosy tariff, 2024).

Key takeaway: Dropping your flow temperature from 65°C to 45°C could save you £560 per year in electricity costs.

Why SCOP Matters More Than COP

While COP gives you a snapshot of efficiency, SCOP tells you how your heat pump will perform over an entire year. The UK uses a standardised SCOP calculation based on test conditions in London, which assumes:

  • A heating season from October to April.
  • Outdoor temperatures ranging from -3°C to 12°C.
  • A flow temperature of 55°C (for the standard test).

However, if you optimise your system for lower flow temperatures, your real-world SCOP will be higher than the standard rating. For example:

  • A heat pump with a standard SCOP of 3.8 (tested at 55°C) might achieve a real-world SCOP of 4.2 if run at 45°C.
  • This translates to 10-15% lower running costs than the manufacturer’s quoted figures.

The Impact of Outdoor Temperature

Your heat pump’s efficiency isn’t just about flow temperature-it’s also about the temperature difference between the outside air and your home. Here’s how outdoor temperature affects COP at different flow temperatures:

Outdoor Temperature (°C)COP at 35°C FlowCOP at 45°C FlowCOP at 55°C Flow
105.04.23.3
54.23.52.8
03.52.92.3
-52.82.31.8

Source: Energy Saving Trust (2024)

What this means for UK homes:

  • In milder UK winters (e.g., southern England), you can run your heat pump at 35-45°C with minimal efficiency loss.
  • In colder regions (e.g., Scotland), you may need to increase flow temperature slightly during extreme cold snaps-but this should be temporary to avoid long-term inefficiency.

Finding Your Sweet Spot: Optimal Flow Temperatures for UK Homes

The Ideal Range: 35°C to 55°C

The optimal flow temperature for your heat pump depends on two key factors:

  1. Your heat emitters (underfloor heating, radiators, or a mix).
  2. Your home’s insulation and heat loss.

Here’s a general guide for UK homes:

Home TypeRecommended Flow Temperature (°C)Heat Emitter TypeNotes
New build (high insulation)30-40Underfloor heatingIdeal for maximum efficiency.
Well-insulated retrofit35-45Underfloor + oversized radsMay need radiator upgrades.
Average UK home (1990s+)45-50Oversized radiatorsCheck radiator sizing (see next section).
Older home (pre-1990s)50-55Oversized or new radiatorsLikely needs insulation upgrades first.

How to Determine Your Home’s Optimal Flow Temperature

  1. Check Your Heat Emitters:

    • Underfloor heating: Can run at 30-40°C due to its large surface area.
    • Radiators: Need 45-55°C unless they’re oversized (see next section).
    • Fan coils or air handlers: Typically require 40-50°C.

  2. Assess Your Insulation:

  3. Test Your System:

    • Start at 45°C and monitor comfort levels for a week.
    • If your home feels too cold, increase the flow temperature in 2°C increments until you find the sweet spot.
    • Use a smart thermostat (e.g., Nest or Hive) to track energy usage at different temperatures.

The Role of the Heating Curve

Most modern heat pumps use a heating curve (or "weather compensation") to automatically adjust flow temperature based on outdoor temperature. Here’s how it works:

  • When it’s 10°C outside, your heat pump might run at 40°C flow temperature.
  • When it’s -2°C outside, it might increase to 50°C flow temperature.

Why this matters: A well-calibrated heating curve ensures your system runs at the lowest possible flow temperature for the conditions, maximising efficiency.

Illustrative composite based on typical UK installations:

The Smiths’ 1930s semi-detached home in Manchester

  • Before optimisation: Flow temperature set to 65°C (boiler-style setting), annual electricity cost: £1,400.
  • After optimisation: Flow temperature lowered to 48°C (with radiator upgrades), annual electricity cost: £850.
  • Savings: £550 per year-enough to cover the cost of their Boiler Upgrade Scheme grant top-up in under 18 months.

The Radiator Question: Do You Need Bigger Radiators for a Low-Temperature System?

Why Standard Radiators Struggle at Low Flow Temperatures

Radiators are sized based on heat output at a given flow temperature. Most UK homes have radiators designed for 70-80°C flow temperatures (boiler systems). At lower temperatures, their heat output drops significantly.

Here’s how radiator output changes with flow temperature:

Flow Temperature (°C)Heat Output (vs. 70°C)Example: 1000W Radiator at 70°C
70100%1000W
60~75%750W
50~50%500W
45~40%400W
40~30%300W

Source: MCS radiator sizing guidelines

What this means for your home:

  • If your radiators are sized for 70°C flow, they’ll only deliver 40% of their rated output at 45°C flow.
  • To compensate, you’ll need larger radiators or more radiators to maintain the same level of comfort.

How to Calculate If Your Radiators Need Upgrading

  1. Check Your Current Radiator Sizing:

  2. Estimate Required Output at Lower Flow Temperatures:

    • If your home needs 10,000W of heat at 70°C flow, it’ll need ~20,000W at 45°C flow (because output drops by ~50%).
    • This means you’ll need double the radiator surface area to achieve the same comfort.

  3. Options for Upgrading:

    • Option 1: Oversized Radiators: Replace existing radiators with larger models (e.g., double or triple panel radiators).
      • Cost: £200-£500 per radiator (installed).
      • Example: A 1000W radiator at 70°C might need to be replaced with a 2500W radiator to deliver 1000W at 45°C.
    • Option 2: Add More Radiators: Install additional radiators in key rooms.
      • Cost: £300-£800 per additional radiator (installed).
    • Option 3: Hybrid Approach: Use a mix of oversized radiators and underfloor heating in high-heat-loss areas (e.g., living rooms).
      • Cost: £1,500-£4,000 (depending on scope).

Real-World Example: Radiator Upgrade Costs

Here’s a cost breakdown for upgrading radiators in a 3-bed semi-detached home (8 radiators total):

ScenarioRadiator Upgrade CostFlow Temperature (°C)Annual Electricity Savings (vs. 70°C)Payback Period
No upgrade£070£0N/A
Minor upgrade (2 radiators)£80055£3502.3 years
Full upgrade (8 radiators)£3,20045£6005.3 years
Hybrid (4 radiators + UFH)£2,50040£7003.6 years

Assumptions: 15,000 kWh annual heat demand, electricity price £0.24/kWh, radiator costs include installation.

Key takeaway: While radiator upgrades add upfront cost, they pay for themselves in 3-5 years through lower running costs. Plus, they make your system eligible for the full £7,500 Boiler Upgrade Scheme grant.

How to Lower Your Flow Temperature: A Practical Step-by-Step Guide

Step 1: Check Your Current Flow Temperature

  1. Locate your heat pump’s control panel (usually indoors, near the hot water cylinder).
  2. Find the flow temperature setting (may be labelled "flow temp," "heating curve," or "weather compensation").
  3. Note the current setting (e.g., 65°C).

If you can’t find the setting, check your heat pump’s manual or contact your MCS installer.

Step 2: Lower the Flow Temperature Gradually

  1. Reduce the flow temperature by 5°C (e.g., from 65°C to 60°C).
  2. Monitor your home’s comfort for 3-4 days.
    • Use a room thermometer to check temperatures in key rooms.
    • Note any cold spots or discomfort.
  3. Repeat the process, lowering by 2-3°C increments until you reach your target range (35-55°C).

Pro tip: Make adjustments during mild weather (e.g., autumn or spring) when outdoor temperatures are stable. Avoid making changes during extreme cold snaps.

Step 3: Adjust Your Heating Curve (If Available)

  1. Access the heating curve setting on your heat pump’s control panel.
  2. Set the curve to "low" or "eco" (this reduces flow temperature at higher outdoor temperatures).
  3. Fine-tune the curve:
    • If your home feels too cold, increase the curve slope slightly.
    • If it feels too warm, decrease the slope.

Example heating curve settings for a UK home:

Outdoor Temperature (°C)Flow Temperature (°C)
1035
540
045
-550

Step 4: Optimise Your Radiators (If Needed)

  1. Bleed your radiators to remove trapped air (this improves heat output).
  2. Balance your system:
    • Close valves on radiators in warmer rooms (e.g., bedrooms) to direct more heat to colder areas (e.g., living rooms).
    • Use a lockshield valve key to adjust flow to each radiator.
  3. Upgrade radiators if necessary (see previous section).

Step 5: Monitor and Fine-Tune

  1. Track your electricity usage using a smart meter or energy monitor.
    • Compare usage before and after adjustments.
    • Aim for a 10-20% reduction in kWh usage.
  2. Use a heat pump calculator to estimate savings.
  3. Revisit settings seasonally:
    • Winter: May need slightly higher flow temperatures during cold snaps.
    • Spring/Autumn: Can run at lower temperatures.

Common Mistakes to Avoid

  1. Lowering the flow temperature too quickly:
    • Dropping from 70°C to 40°C in one go can make your home uncomfortably cold.
    • Gradual adjustments (2-3°C at a time) are key.
  2. Ignoring radiator sizing:
    • If your radiators are too small, lowering the flow temperature will leave your home cold.
    • Use the home suitability check to assess your system.
  3. Not using weather compensation:
    • Without a heating curve, your system will run at the same flow temperature regardless of outdoor conditions, reducing efficiency.
  4. Overlooking insulation:
    • If your home is poorly insulated, you’ll struggle to run at low flow temperatures.
    • Prioritise insulation upgrades before optimising your heat pump.

Maximising Your Boiler Upgrade Scheme Grant: Why Flow Temperature Is Key to Your Savings

How the Boiler Upgrade Scheme (BUS) Rewards Efficiency

The Boiler Upgrade Scheme (BUS) offers £7,500 grants for air source heat pumps and £7,500 for ground source heat pumps. However, to qualify, your system must meet MCS efficiency standards, which are directly tied to flow temperature.

Key BUS requirements:

  1. SCOP of at least 2.8 (for air source heat pumps).
  2. Flow temperature of 55°C or lower (for the standard test).
  3. MCS-certified installer (find one here).

Why flow temperature matters for BUS:

  • The standard SCOP test assumes a 55°C flow temperature.
  • If your system is designed for lower flow temperatures (e.g., 45°C), your real-world SCOP will be higher than the test value.
  • This means you’ll exceed the BUS efficiency requirements, ensuring you qualify for the full grant.

How to Ensure Your System Meets BUS Standards

  1. Choose an MCS-certified installer:

    • They’ll design your system to meet BUS requirements, including optimal flow temperature.
    • Use our installer finder tool to locate one near you.

  2. Design for low flow temperatures:

    • Specify underfloor heating or oversized radiators to allow for 45-55°C flow.
    • Avoid systems designed for 60°C+ flow, as these may not qualify for BUS.

  3. Get a heat loss calculation:

    • Your installer should perform a heat loss calculation to determine the minimum flow temperature your home needs.
    • This ensures your system is right-sized for efficiency.

  4. Use weather compensation:

    • A system with weather compensation will automatically adjust flow temperature, maximising efficiency and BUS compliance.

Real-World Example: BUS Savings with Optimised Flow Temperature

The Johnsons’ 4-bed detached home in Surrey

  • Heat pump installed: Mitsubishi Ecodan 11.2 kW air source heat pump.
  • Flow temperature: Designed for 45°C (with underfloor heating on the ground floor and oversized radiators upstairs).
  • SCOP: 4.1 (vs. BUS minimum of 2.8).
  • Grant received: £7,500 (full BUS amount).
  • Annual running cost: £950 (vs. £1,500 for a system running at 65°C).
  • Savings: £550 per year-meaning the Johnsons recouped their £3,500 out-of-pocket cost in just 6.4 years.

What If Your System Doesn’t Meet BUS Standards?

If your heat pump is designed for high flow temperatures (60°C+), you may not qualify for the full BUS grant. Here’s what to do:

  1. Upgrade your radiators to allow for lower flow temperatures.
  2. Add underfloor heating in key areas (e.g., living room).
  3. Reassess your insulation to reduce heat loss.
  4. Consult your installer about adjusting the system design.

FAQ

1. What is the optimal flow temperature for a heat pump in the UK?

The optimal flow temperature for a UK heat pump is 35-55°C, depending on your heat emitters and insulation. Underfloor heating can run at 35-40°C, while radiators typically need 45-55°C. Lower temperatures improve efficiency, with every 1°C increase above 55°C reducing COP by ~2-3%.

Source: Energy Saving Trust (2024)

2. Can I run my heat pump at 70°C like my old boiler?

Technically, yes-but it’s highly inefficient. At 70°C flow temperature, your heat pump’s COP could drop below 2.0, meaning it produces less than 2 kWh of heat for every 1 kWh of electricity. This will double your running costs compared to a system optimised at 45°C. Most UK homes can avoid this by upgrading radiators or adding underfloor heating.

3. How do I know if my radiators are big enough for a low-temperature heat pump?

Check your radiators’ heat output at 50°C flow temperature. Most UK radiators are sized for 70°C flow, so their output drops by ~50% at 50°C. Use an online radiator sizing calculator to compare your current output to your home’s heat loss. If your radiators are undersized, consider upgrading to double or triple panel models.

4. Will lowering my flow temperature make my home colder?

Not if your system is properly designed. Lower flow temperatures work best with larger heat emitters (e.g., oversized radiators or underfloor heating) that can deliver the same heat output at lower temperatures. If your radiators are too small, your home may feel colder-so it’s important to assess your system before making changes.

5. How does flow temperature affect my Boiler Upgrade Scheme grant?

The Boiler Upgrade Scheme (BUS) requires your heat pump to achieve a SCOP of at least 2.8, which is tested at 55°C flow temperature. If your system is designed for lower flow temperatures (e.g., 45°C), your real-world SCOP will be higher, ensuring you qualify for the full £7,500 grant. Systems designed for 60°C+ flow may not meet BUS standards.

Sources