Field Trial Shows Heat Pumps are Underperforming

by Jamie on September 13, 2010

Heat pumps are  a clever piece of microgeneration technology that have the potential to play a big part in cutting CO2 in our homes.

They work in much the same way as a fridge with energy being extracted from one place and dumped into another. In the case of a fridge, heat is extracted from the compartment and dumped out of the coils into the air in your kitchen. In the case of a heat pump, heat is extracted from the ground, air or even water (in a few cases) and then dumped into your home via oversized radiators or underfloor heating.

Exactly like a fridge, heat pumps need electricity to operate so CO2 emissions are generated at the power station, but the clever bit is that for each unit of electricity they consume, in excess of 3 units of heat can be delivered to your home.

In theory.

Unfortunately in the real world it appears that performance can be a long way from the optimum: the Energy Saving Trust has just published the results of the first year of heat pump trials and it doesn’t look very good.

The trials monitored 83 installations of ground and air source heat pumps around the UK over a period  of 12 months.  The trials looked at the system coefficient of performance, system efficiency, installation practices, user behaviour and heating patterns and the economics of the technology.

The coefficient of performance represents the number of units of heat that you get out to the number of units of electricity you put in over the course of a year. So the higher the number, the better the performance and the bigger the reduction in carbon emissions and operating costs. The study also looked at the system efficiency which considers the energy required to deliver all of the heat and hot water that you need and this is the important figure for the householder.

What the study found found was that typical system efficiencies for ground source heat pumps were around 2.3 to 2.5, with a maximum of over 3; for air source heat pumps it was slightly lower at 2.2 on average, again with a maximum efficiency in excess of 3.

The problem was that there was a big range in efficiencies and this was mostly due to poor system design and/or a poor understanding on the part of the householder of how to use the system most efficiently.

Heat pumps put out a fairly low temperature heat and therefore need large radiators or ideally underfloor heating. They’re also quite complex and work very differently to conventional heating systems so the home owner needs to learn how to set it correctly (something that should be clearly explained by the manufacturer or installer).

In other countries, especially in Scandinavia, heat pumps are very popular and perform well. In Scandinavia they have more low carbon electricity than they can shake a stick at so electric heat pumps make a lot of sense. Their homes are also very efficient and so have a low heat demand (even though it’s so darned cold for a lot of the year).

As the industry matures in this country the performance of heat pumps should improve, but if you are thinking about opting for one, all I can say is be careful and read up on the subject before taking the plunge.

As it is, heat pumps should only really be considered for homes that are off the gas network at this time as you get a bigger CO2 and financial saving by switching from oil or electricity, so they’re not currently suitable for homes in Stoke Newington. As our electricity supplies decarbonise, then we can start looking at homes that are on the gas network.

You can read the report here. It’s a good little report with a clear explanation of how the technology works and an overview of the monitoring study and results.

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