Adjusting the thermostat

What you can do

Heating

Set the thermostat to a maximum of 20oC while the house is occupied, as low as 15oC overnight or when the house is not occupied.

If you have a programmable thermostat you might raise it about 30 minutes before you expect to be home or wake up (although some thermostats automatically adjust the start time). Set it to the lower temperature 30 minutes or more before you go to bed.

If possible, wear sweaters and set the thermostat even lower.

Cooling

If you are using an air conditioner, set the thermostat to 25oC (higher if possible) when the house is occupied. 

Background

There is a common misconception that it takes more energy to heat (or cool) a house if the temperature is set back overnight or when when the house is not occupied.

However, the rate of heat loss depends upon the difference in temperatures. For example, if the surrounding (outdoor) temperature is 10oC and the indoor temperature is 15oC the temperature difference is 5oC. But if the indoor temperature is 20oC, the temperature difference (10oC) is twice as much and hence the rate of heat loss is twice as much. (The rate of heat loss also represents the amount of heat that must be provided to maintain that temperature.) In these diagrams, the total heat loss is proportional to the area between these two lines.

Constant indoor and outdoor temperatures

Scenario 1

Indoor and outdoor temperatures remain constant throughout the day

Set back indoor temperature

Scenario 2

Indoor temperature is low overnight, increases to the daytime temperature and then decreases to the nighttime temperature. In this example the outdoor temperature is considered to be constant (which is not usual.)

Constant indoor temperature with varying outdoor temperature

Scenario 3

A more realistic case where the outdoor temperature decreases at night and the indoor temperature is maintained at a constant (daytime) temperature.

Setback indoor temperature, varying outdoor temperature

Scenario 4

The preferred solution where the indoor temperature is lowered overnight.

Energy savings

Since Scenario 4 has a smaller area than Scenario 3 it requires less energy. Energy saved by setting thermostat down overnight This explains how setting back the thermostat at night reduces energy.

For air conditioning, the reverse applies. Since the indoor temperature is lower than outdoors, energy flows from outdoor to indoor. This forces the air conditioner to remove more heat when the temperature difference is greater. This requires the air conditioner to work longer and harder.

Open coolers/freezers at grocery stores, wine stores etc are a major concern, since cold air is heavier than warm air, the cold air will flow out of the cooler and be replaced by warm air, forcing the compressor to work harder. For chest type freezers that isn't the case since the cold air can't flow out. There is still some heat lost where the top is open, but not a lot.

A more serious problem is the amount of energy wasted by soft drink vending machines. According to a study by the California Center for Sustainable Energy "Vending machines present an excellent opportunity for energy conservation. They operate 24/7 and consume six times as much energy as a household refrigerator. By installing vending machine power controllers, energy costs can be cut in half."