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Which is the the best heat pump?

Posted by Graham Hazell on 30 May 2013 at 11:47 am

“Which is the best heat pump, in your view?” is the question I’m most frequently asked by callers to the Heat Pump Association's information desk. I usually turn the question back and ask them, in their view, which is the best car. Straight away they ask “what do you want it for, family, commuting, off-road, recreation, etc? What is most important to you: comfort, economy, initial cost or re-sale value, etc”?

You need to ask similar questions to choose the best heat pump: what purpose are you using it for? Heating, hot water or both? If both, which will be the most dominant? And, even after these questions have been answered, the best heat pump for you may vary depending on where you are in the country.

Usually the enquirer is really asking  “which is the most efficient?”. I point out that heat pump efficiencies vary depending on the application but most particularly to the system they are installed within. To continue the car metaphor, you may have a very efficient 1.2 litre engine but if you put it in the wrong chassis, say a heavy executive car, you will almost certainly be worse off than if you had used the engine intended for that type of car.

The efficiency in use will also depend on the aero dynamics of the car into which the engine is placed and the same is true for heat pumps. Their efficiency is dependent on the rest of the system and in particular the output flow temperature of the heat pump. The higher the supply  temperature the less efficient the heat pump will be. This is why under-floor heating systems work well with heat pumps as the flow temperature is usually relatively low. However, you can also use larger then ‘normal’ radiators or fan assisted convectors.

Then there is the source of heat; ground, air or water? When buying a car you choose between diesel- or petrol-fuelled. A diesel vehicle will be more expensive to buy but will have lower fuel costs than a petrol one, which is generally are less expensive to maintain. Similarly a ground or water source heat pump system (GSHP/WSHP) will be more expensive in initial costs but have lower fuel costs than an air source system (ASHP). In addition ASHP systems tend to have slightly higher maintenance requirements to keep the airways free of debris etc.

Most of the major brands, who are well represented in membership of the Heat Pump Association, have comparable efficiencies so look for the support and information provided as the possible differentiator, not just the unit itself. In some circumstances one product will appeal over another due to site constraints.

Above all ensure both the product and installer are registered on the Microgeneration Certification Scheme (MCS) since this opens up the prospect of financial incentives from the government. This is a minimum performance metric, whereas product on the Energy Technology List (ETL) from the Carbon Trust is best practice.

I recommend that you concentrate on the system, not just a component, with the number one rule of using heat emitters that use the lowest flow temperature possible. Ensure you have at least basic controls, time and temperature but also ask for more sophisticated controls like ambient air temperature (weather) compensation*. If you have other renewable energy sources such as solar thermal and/or solar PV you may wish to ensure the control systems are integrated. This can affect your choice of heat pump as most major brands, but not all, have weather compensation and some can coordinate with other technologies - although you may have to purchase them all from the same manufacturer for compatibility.

So, choice of the installer is probably more important than the product itself, since a good installer will give you a well described choice and should not be using shoddy products in the first place. My next blog will have questions to help you choose a good installer. Caveat Emptor!

* Ambient air temperature (or weather) compensation varies the flow temperature according to the severity of the external temperature and hence the maximum design flow temperature is only used when it is very cold outside. The rest of the year the supply temperature is reduced in proportion to the external temperature in proportion to the building heat losses.

More information about heat pumps on YouGen

An introduction to heat pumps

10 things a good installer should ask you before quoting on a heat pump

When to consider using an air source heat pump

How and where to site your air source heat pump

Picture: stiebel eltron

About the author: Graham currently works as a freelance specialist consultant and as a consultant to the Heat Pump Association.

If you have a question about anything in the above blog, please ask it in the comments section below.

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