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Heat pump owners who have previously been used to gas or oil may be tempted to set their thermostats down at night or when not at home. That can be a costly mistake. Here’s why.

Heat pumps are most efficient when the radiators they heat are slightly warmer than target room temperature, when they feel barely warm (i.e. around 30^oC).

But the amount of heating power from radiators at these low temperatures is less than from hot radiators. For this reason, heat pumps often run almost semi-continuously in winter.

The further the temperature is set back, the higher the heat pump must bring the radiators to restore a house to its target temperature within a desired period. That period is typically 2 to 6 hours and can be changed indirectly by adjusting a heat pump’s weather curve setting.

But there is another even more important reason: off-peak times are the best times for spacing heating anyway.

Altogether, several factors influence the optimum set back temperature, including:

• house thermal inertia and how well insulated it is. For example, timber frame homes warm up more quickly the brick homes and allow lower set back temperatures.
• heat pump’s energy efficiency and its maximum power output;
• electricity tariff;
• when and for how long the house is occupied and the value attached to room temperature being on target at these times.

Optimum set back temperatures are not easy to calculate by hand but are returned in an e-mail by the energy simulator automatically as an e-mail message: Optimum winter heat pump setback temperature for this configuration is xxC.

To obtain optimum set back temperature, include a heat pump in your simulation and quantify the following parameters in “location”:

• “temperature_target_celsius”: target room temperature;
• “target_hours” is an array comprising the hours in the day (e.g. 0 for 00h00m – 00h59m, 1 for 01h00m-01h59m) when the home is to be heated to target temperature;
• “temperature_half_life_days” is the time in days, when heating is switched off completely, for room temperature to fall exactly half way between target and external ambient temperature: best measured on a cold winter day;
• “intolerance_gbp_per_deg_c_per_hour” is the notional cost of inconvenience arise from the room temperature not being at its target value.

For example:

"location": {
   . . .
    "internal" : {
      "temperature_target_celsius": 21.0,
      "temperature_half_life_days": 1.5,
      "target_hours": [8,9,10,11,12,13,14,15,16,17,18,19,20,21],
      "intolerance_gbp_per_deg_c_per_hour": 0.1
    }
  },

Don’t be surprised to receive an e-mail response that the set back temperature should be the same as (or even slightly higher than) your target temperature: normal for well insulated brick/masonry homes with off-peak tariffs.

When people consider installing a heat pump, the spotlight often falls on it only: it’s brand, technology, refrigerant, and advertised efficiency. However, the major factor determining how efficient a heat pump will be is the radiators it is connected to.

This is because a heat pump’s seasonal coefficient of performance (SCOP) — a measure of how much heat energy it can deliver compared to the electricity it consumes across a whole year — is highly sensitive to the temperatures it has to work at. Radiators directly dictate those temperatures. If the radiators are undersized, inefficient, or in poor condition, the heat pump will be forced to run hotter, and its SCOP will tumble.

Radiators and Flow Temperature

Radiators don’t produce heat; they “transfer” it. The larger the radiator surface area and the more efficient its design, the more heat it can emit at a given water temperature.

• A modern double-panel, double-convector radiator might emit the same heat at 45 °C water temperature as an older single-panel radiator would at 65 °C.
• If your home has lots of small, old radiators, the only way they can deliver enough warmth in mid-winter is if the water flowing through them is hot — often 60–70 °C. That’s fine for a gas boiler, which happily produces hot water at 70–80 °C, but a heat pump’s efficiency plummets when pushed above 50–55 °C.

The simplest way to allow a heat pump to run efficiently is to have more radiators. Each additional radiator adds emitting surface area, meaning the system can deliver the same heat to the home while running at a lower water temperature.

For example:

• Suppose a room needs 2 kW of heat on a cold day.
• One old single-panel radiator might only emit 2 kW at 65 °C water.
• Replace it with two modern radiators, each sized to deliver 1 kW at 45 °C, and the heat pump can now run at 45 °C flow instead of 65 °C. That could boost SCOP by 20–40% depending on climate.

Distribution across rooms

It’s not just total number that matters, but how they’re distributed. An oversized radiator in the living room won’t help if the bedrooms upstairs are freezing because they still have tiny 600 mm singles. Heat pump efficiency depends on being able to keep the whole home comfortable without raising flow temperature for one weak link.

Floor area vs radiator count

Larger homes naturally need more radiators. But many UK homes built in the 1960s–1990s were fitted with minimal radiator counts — just enough to keep a gas-boiler system adequate. Retrofitting for a heat pump often means increasing radiator numbers by 20–50%.

Panel size and surface area

The physical size of a radiator is the single most obvious determinant of its output at a given water temperature. Tall, wide, or deep radiators simply have more surface for air to contact.

• Single-panel radiators are the least powerful per length.
• Double-panel, single-convector (P+ type) add output.
• Double-panel, double-convector (K2 type) can deliver nearly three times the heat of a slim single.
• Triple-panel designs (K3) can achieve very high outputs at low temperatures, though they are bulky.

Low-temperature efficiency

Because heat pumps thrive at lower water temperatures, radiators need to be oversized relative to what a gas boiler required. A room that previously had one P+ might need a K2 or even two radiators at 45 °C design flow.

Vertical radiators

These have become fashionable, but many have less surface area than horizontal radiators of the same height. Unless carefully specified, they can be a liability for heat pump SCOP.

Radiator Efficiency Beyond Size

It’s not only surface area that matters. Design tweaks influence how effectively a radiator converts hot water into room heat.

• Convection fins: Modern convector plates welded between panels greatly boost airflow and heat transfer.
• Airflow patterns: Radiators placed under windows use rising warm air to counteract downdraughts; when moved to less effective positions, their practical efficiency drops.
• Active flow: Adding very low power fans to radiators increases airflow over a radiator’s surface at a negligible running cost. This enhances heat transfer by boosting the convection effect, helping distribute heat more evenly and quickly, even with lower water temperatures.
• Radiant vs convective balance: Most radiators are largely convective (heating air), but designs like cast iron or aluminium can emit more radiant heat, making rooms feel warmer at lower air temperatures.

High-efficiency radiators allow the heat pump to run cooler and keep SCOP higher.

The Condition of Radiators

Even the best-sized radiator can underperform if it’s in poor condition.

Sludge and corrosion

• Over years, central heating systems accumulate sludge (magnetite particles) and rust.
• This reduces water flow, creates cold spots, and slashes radiator output.
• A sludged-up radiator might deliver 20–40% less heat than its rating, forcing the heat pump to raise flow temperature to compensate.

Air trapped in radiators

Trapped air pockets reduce effective surface area. Regular bleeding is essential.

Fouling of fins

Dust clogging the convector fins under radiators impedes airflow and reduces output. A quick vacuum can measurably improve performance.

Valve performance

Old radiator valves may not open fully, restricting flow. Poor hydraulic balancing leads to some radiators running too cool while others hog flow, again pushing up system temperatures unnecessarily.

Keeping radiators clean, flushed, and balanced is a surprisingly powerful lever on SCOP.

Interaction with Heat Pump Control Strategy

Radiators and controls work together.

• Weather compensation: A heat pump can automatically vary flow temperature based on outdoor temperature. With adequately sized radiators, weather compensation keeps flow very low in spring/autumn, giving spectacular COP figures.
• Constant low-flow operation: Radiators designed for continuous low-temperature heat work best with heat pumps. Stop-start control or undersized emitters undermine this.
• Zoning pitfalls: Shutting down too many radiators (e.g. only heating one room) can reduce flow volume through the system, making it harder for the heat pump to modulate efficiently.

Practical Examples of Radiator Impact on SCOP

Let’s take a worked example to see how radiator capacity alters SCOP.

• A typical 3-bed UK semi requires ~8 kW peak heat load at −3 °C outdoor temperature.

Case A: Old boiler radiators

• Existing system: 7 single-panel radiators sized for 70 °C flow.
• At 45 °C flow they can only emit 4 kW.
• Heat pump must run at 60 °C+ to keep house warm.
• Average SCOP over winter: ~2.7.

Case B: Upgraded radiators

• Replaced with 11 double-panel convectors sized for 45 °C design.
• Heat pump runs at 45 °C most of winter.
• Average SCOP: ~3.8–4.0.

Case C: Oversized radiators

• 14 radiators, all K2 or K3 types.
• Design temperature 35 °C.
• Heat pump SCOP: 4.5–4.7, with some days reaching instantaneous COP above 5.

A difference in running cost between Case A and Case C could be £400–£600 per year for the same house, plus lower CO₂ emissions.

If you’re adding solar PV, it’s important to get the best possible financial return on your investment by maximising the amount of power generated across the year:

• use the maximum solar pv area¹ possible: your installer should advise. South facing is best and, in order of suitability, this is usually:
  • pitch roofs get most sun and make best use of space because panels can be butted together;
  • flat roofs make less efficient use of area because they require spacings between panel arrays and may require planning permissions, then;
  • ground installation: if you (and your neighbours) can live with them.
• if you have sufficient panel area to generate more than the 3.68kW export limit, get your installer to make a G99 application to your DNO to get the limit raised. The application is often free and will allow your installers to fit a bigger pv system;
• oversize your solar pv generation capacity to the maximum “oversize” limit permitted for the inverter you are using: this is usually between 130 and 150 percent of its maximum power output. This allow your inverters to run safely at their maximum output for a greater proportion of the time.

1. Subject to your inverter capacity and G98 or G99 export limit. ↩︎

Photovoltaic (pv) panels convert solar energy to electrical energy for consumption, export or storage.

Here’s a sample description of a pv installation:

"solar_pv": {
  "include": true,
  "cost": {
     "install_gbp": {
        "install": 19000.0,
        "plant_room": 2000.0
        },
        "maintenance_pa_gbp": 200.0
        },
   "area": {
      "shading_factor": 0.85,
      "border_m": 0.2
    },
    "panels": [{
       "panel": "AIKO 455W",
       "width_m": 1.134,
       "height_m": 1.722,
       "cost": {
          "per_panel_gbp": 0.0,
          "maintenance_per_panel_pa_gbp": 5.0
       },
       "efficiency": {
          "percent": 23.1,
          "loss_percent_pa": 0.3,
          "loss_percent_per_celsius": 0.29,
          "temperature_reference_celsius": 25
       },
       "power_max_w": 455.0,
       "lifetime_years": 25,                     "thermal_inertia_m2_second_per_w_celsius": 1000
       }],
       "inverter": {
          "cost_install_gbp": 0,
          "power_threshold_kw": 11.5,
          "power_efficiency": 0.96
       },
        "collectors": {
           "south": {
                "include": true,
                "area": {
                    "dimensions_footprint_axis": {
                        "tilt_m": 11.5,
                        "other_m": 3.6
                    },
                    "orientation": {
                        "type": "tilted",
                        "tilt_degrees": 35,
                        "azimuth_degrees": 185
                    }
                },
                "panel": "AIKO 455W",
                "panels_number": 16,
                "cost": {
                    "install_gbp": 0.0,
                    "maintenance_pa_gbp": 0.0
                }
            },
            "west": {
                "include": true,
                "area": {
                    "dimensions_footprint_axis": {
                        "tilt_m": 5.9,
                        "other_m": 3.9
                    },
                    "orientation": {
                        "type": "tilted",
                        "tilt_degrees": 10,
                        "azimuth_degrees": 184
                    }
                },
                "panel": "AIKO 455W",
                "panels_number": 10,
                "cost": {
                    "install_gbp": 0.0,
                    "maintenance_pa_gbp": 0.0
                }
            }
        }
    },

To describe your pv panels:

• use the “include” flag to include a collector;

• insert your install and annual maintainance costs, “install_gbp” and “maintenance_pa_gbp” respectively, in “costs”. These will be summed with any costs you declare in “collectors”;
• describe each of your pv collector areas using “collectors”;
• include a “panel” or “panels” description. In either case, for each panel, give:
  • “width_m” and “height_m”: panel dimensions;
  • “cost”:
    • “per_panel_gbp”: cost per panel to be summed by number of panels;
    • “maintenance_per_panel_pa_gbp”: maintenance per panel per annum to be summed;
  • “efficiency”:
    • “percent”: power efficiency when panel is new at the reference temperature (see below);
    • “temperature_reference_celsius”: nominal reference temperature in celsius;
    • loss_percent_pa”: loss of power efficiency in percent per annum;
    • “loss_percent_per_celsius”: loss of power efficiency in percent per degree celsius above the reference temperature (see above);
  • “power_max_w”: maximum output power in watts;
  • “lifetime_years”: maximum operating lifetime in calendar years;
  • “thermal_inertia_m2_second_per_w_celsius”: panel thermal inertial, or how long it takes to heat up when illuminated.
• “inverter”: the collectors are all assumed to feed their generated power through an inverter, so you must include its properties:
  • “cost_install_gbp”: total installation cost;
  • “power_threshold_kw”: maximum power in kilowatts;
  • “power_efficiency”: overall power efficiency.

Thermal panels use solar energy to heat water in a buffer tank to satisfy hot water and space heating energy demands. Here’s an example:

"solar_thermal": {
    "include": true,
    "cost": {
      "install_gbp": 2000.0,
      "maintenance_pa_gbp": 0.0      
    },
    "panel": {
      "width_m": 1,
      "height_m": 1.6,
      "efficiency": {
        "percent": 70
      }
    },
    "collectors": {
      "primary": {
        "include": true,
        "cost": {
          "install_gbp": 1500.0,
          "maintenance_pa_gbp": 0.0      
        },
        "area": {
          "orientation": {
            "type": "tilted",
            "tilt_degrees": 45,
            "azimuth_degrees": 185
          },
          "dimensions_footprint_axis": {
            "tilt_m": 6.0,
            "other_m": 1.9
          },
          "shading_factor": 0.9          
        }
      },
      "secondary": {
        "include": true,
        "cost": {
          "install_gbp": 1500.0,
          "maintenance_pa_gbp": 0.0      
        },
        "orientation": {
          "type": "tilted",
          "tilt_degrees": 45,
          "azimuth_degrees": 270
        },        
        "shading_factor": 0.7,
        "panels_number": 5
        }
      }
    }
  },

To describe your thermal panels:

• use the “include” flag to include a collector;

• put your install and annual maintainance costs, “install_gbp” and “maintenance_pa_gbp” respectively, in “costs”;
• describe each of your thermal collector areas using “collectors”;
• state your panel dimensions and their energy efficiency in “panel”, as in the above example;
• include a “panel” or “panels” description. In either case, for each panel, give the panel dimensions “width_m” and “height_m”, and efficiency in “percent”.