Heat Pumps

Air source

This is the most common design of heatpump in use in the UK as it is relatively easy to find a suitable location for the external unit (which looks a bit like an airconditioning unit) and it has the least expensive up-front cost. It uses a fan to blow external air across a heat exchanger that extracts heat from the air as it passes through.

Over the last few years, their efficiency has improved dramatically and modern units are capable of seasonal coefficients of production in excess of 4.3.

All heatpumps ware only capable of producing Domestic Hot water with the use of a hot water storage cylinder.

They come in 3 main layouts and 2 main temperature ranges:

Layouts:

Split
These systems use refrigerant pipes to link an internal and external unit. The refrigerant pipes move heat from outside to inside and transfer it into the water pipes used for central heating or Hot water cylinder via a “hydrobox” which is visually similar to a wall hung fossil fuel gas boiler. These systems are typically the most efficient design and have the smallest external units.

Monoblock
These combine the functions of the external unit and hydrobox into a single external unit. The heat they extract from the air is transferred to water inside the monoblock itself and is sent into the property via water pipes. There are still some components required inside the property but they generally require less internal space than a split system. The external unit is usually larger than a split system unit and the efficiencies are usually slightly lower. As there is water in the external unit, the entire heating system needs to be treated with glycol antifreeze which adds cost and actually reduces the heat carrying capacity of the heating system. The main reason for their popularity is that the installer does not have to be certified to use refrigerant.

Hydro-split
These combine the benefits of a split system with the ability to use larger quantities of newer generation refrigerants. There is both an external unit and a hydrobox but the heat is transferred by a water circuit. They are typically used for system designs above 8kW that require larger quantities of refrigerant.

Temperatures

“Low temperature heatpumps”
Low temperature heatpumps are typically capable of producing warm water at upto around 50 to 55 degrees C. They are capable of running central heating systems upto around 50 degrees Flow but are significantly more efficient if running at 35 to 40 degrees Flow. They are capable of producing Domestic Hot water at 48 degrees C in the cylinder which is acceptable for normal daily use. They do require the assistance of a direct electric immersion heater for a weekly 60 degree C legionnaires purge. They are perfectly matched with underfloor heating but can also work well with suitably sized radiators or fan convectors.

“High temperature heatpumps”
There are several methods of achieving higher temperatures with heatpump technologies:

• Different refrigerants can be used in a single compressor that tend to be less efficient overall but allow higher temperatures to be achieved,
• Heatpumps using 2 compressors with different refrigerants can be used, one optimised to generate lower temperatures and one optimised to lift those lower temperatures to higher temperatures. Tese use 2 compressors so are less efficient than single stage units.
• Very modern units like the Daikin HT hydro-split that uses a new design of compressor to allow very efficient production of heat at lower temperatures but are still capable of producing higher temperatures relatively efficiently. These units can produce legionnaires purge hot water cycles without the use of an immersion heater yet are as efficient as most low temperature units when running at lower temperatures.