Optimum setback temperatures for heat pumps

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.