Heat Pump Running Costs
How much does a heat pump cost to run?
We can break running costs down into 3 variables.
Total energy required to heat your house.
Heat pump COP.
Price of electricity.
To estimate how much a heat pump will cost to run you need to know how much energy is required to heat your house every year. An EPC certificate estimates the annual energy consumption for heating and domestic hot water; this is a good start.
Alternatively, you could use the total gas consumption per year to estimate running costs, however, this is only accurate if you you only use gas for heating and not for cooking.
To calculate a heat pump running cost estimate, take your annual energy usage (in kWh) and divide that by 3 (our assumed SCOP). This gives the estimated heat pump electricity consumption.
Then multiply your estimated electricity consumption by your current electricity unit price to get your annual heat pump running cost.
For example, if your annual energy usage is 20,000kWh, and your electricity unit price is 30p, then your heat pump running cost will be £2000 per year.
How to reduce my heat pump energy bills.
We can look at each aspect of your heat pump running costs. Firstly is consumption. This can be reduced by either reducing the heat loss of your house by improving insulation. Or by reducing your usage - basically, having a colder house. Doing both of these will reduce your bills no matter what heating system you have.
Finding the cheapest electricity rate available is obvious. However if you are on a variable tariff then there are ways to lower your bills. For example, if you get a cheaper rate at night then you can program your system to heat the DHW cylinder during those times to store the heat energy in your tank. Increase the storage temperature of your DHW cylinder to store more energy at that cheap rate and avoid having to reheat it during the day.
Finally we have your heat pump efficiency or COP. Coefficient of performance is the ratio of heat output from electricity input. The main consumer of electricity in the heat pump system is the compressor. The compressor increases the pressure of the refrigerant. So we can reduce it's consumption by reducing the difference in pressure that is has to create.
We reduce compressor ∆P by reducing the flow temperature, or increasing the outside air temperature (ground temperature for GSHP).
What is flow temperature?
Flow temperature is the temperature of the water leaving your heat pump. It is controlled by the heat pump and is programmed when the system is first commissioned.
You want your flow temperature to be as low as practicable, for your system to be as efficient as possible.
The heat loss of your house determines the power requirement for each room. Your heat emitters have to meet the loss of each room. You should have your flow temperature at the point where the the system has to be on continuously to just meet the heat loss of the room. However, most systems are designed by making vast assumptions. Flow temperature is often set higher than necessary.
Increasing the size of your radiators will allow you to meet the heat loss of the room with a lower flow temperature. If you are limited on space then you can fit fan convectors which have a much greater heat output, or underfloor heating, which makes your whole floor area a radiator.
Because the heat loss of the room is dependent on the outside temperature we use weather compensation. This means that the heat pump automatically increases flow temperature as the outside air gets colder.
Most systems allow you to offset flow temperature in the user menu. You hay see 0°C on the controller. This means that the flow temperature is set to what the heat pump's weather compensation is programmed to. By changing this to -5 you can decrease the flow temperature by 5°C.
How to increase outside air temperature
Obviously, you can control the weather, however you can use system design and settings to consume when the outside air is warmest.
Heating your DHW tank can be set to mid afternoon when the air is warmest.
Additionally you can situate your heat pump facing south and out of shade so that the sun heats up the air going into your evaporator. Note that if the heat pump is in direct sunlight the temperature sensor may misread. Most heat pumps come with an external temperature sensor option that can be situated in the shade.
Heat pump COP drops with outdoor temperature. Having supplementary heating during very cold ambient temperatures will drastically reduce your energy bills. Wood burners are a fantastic way of heating your house when below freezing outside. Alternatively, you can design your system to a higher ambient temperature (5°C for example) and have secondary heat source such as an oil boiler which only kicks in when it's cheaper to use oil than electricity with a heat pump.
This is called bivalent operation and most heat pumps are able to be setup this way.