Understanding Efficiency

Lets face it – if you are considering buying a heat pump, one of the main reasons is probably because they are very efficient. The efficiency of a heat pump is an important factor but does need to be weighed up with other factors when making your final choice.

Why do you need to consider efficiency?

The more efficient a heat pump is, the less it will cost you to run when producing the same amount of heating / cooling. Ultimately this effects your monthly electricity bill. So if you’re spending the money and investing in a heat pump you should consider its efficiency, in combination with other factors, when deciding which heat pump to buy. 

How do I work out how efficient a heat pump is? To do this accurately you need to find each units “Energy Efficiency Rating” (EER) for cooling and “Coefficient Of Performance” (COP) for heating. These figures will be published by the manufacturers. This can be quite confusing, so manufacturers are required to display star rating labels on their heat pumps – but even this can be confusing, because you don’t know how many stars you need or how much a difference of ½ or 1 star will save you in running costs. The easiest way to ensure you get a very efficient heat pump is to choose one that bears the “Energy Star” logo. Only the best of the best heat pumps (the top 25% of performers in each size) can achieve this award.

Do the running conditions effect efficiency?

Yes they do. To make comparisons fair, manufacturers are required to test their units’ capacities (and therefore efficiencies) at a given set of temperatures, when operating in a stable and continuous way. So, if the actual temperature is different when you are running a heat pump, the capacity and efficiency will also be different. A few manufacturers also publish data at alternate test conditions, but at present this is not very common. Some manufacturers “model” or “calculate” capacity and efficiency at different temperatures. This is only a guesstimate, and is unable to be verified as accurate, so should not be compared to actual tested data under any circumstances.

When inverter heat pumps slow down does the efficiency change?

Yes it does. All efficiencies quoted are when the heat pumps are running at 100% capacity (but not in “boost” mode). With inverter heat pumps, as the room gets close to the required temperature, the heat pump reduces capacity. When it does this the heat pumps efficiency will increase, sometimes dramatically. As it is very difficult to make a test which would ensure an even comparison of brands and models at reduced capacity, no data has been published for this.This increase in efficiency obtained when reducing capacity is one of the reasons why it is very important to get a correctly sized heat pump. A heat pump selected to operate mainly around the middle of its total capacity range will end up being much more efficient than one which is running at flat out or in “boost mode” the whole time.

How do I know what the payback period is for buying a more efficient heat pump?