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Consequently, to get the most from any heat pump, one needs to maximise the temperature at which it takes in heat and minimise that at which it goes out. This makes the way a heat pump is used very important. Consequently, Sikora says that it is very much better to use the hot water from a pump for an underfloor heating system rather than in conventional radiators because an underfloor system uses water at 35°C while radiators might operate at 60°C. Similarly, a heat pump should only be used to pre-heat the water used for baths and washing up, not to raise it to its final temperature.
Ground source heat pumps score over air-source ones because they have higher intake temperatures on exactly the days when heat is needed most. In Ireland, the ground is a constant 11 – 13°C several metres below the surface whatever the time of year, so the temperature of the mixture of water and antifreeze flowing into the heat exchanger on the heat pump can be quite high – perhaps 10°C if a long enough collecting pipe is used. However, if the outside air is the source of heat, a pump's performance in midwinter when temperatures fall below freezing on a cloudless, windless night can be very poor.
The line shows how the heat output of an air-source pump drops off sharply at around 6 deg. C
Derek Phelan of G.T. Phelan Ltd, which sells Toshiba air-to-air heat pumps, says that all air-source pumps are rated on the basis of a 7°C external air temperature and that there is a sharp fall off in efficiency when the external temperature falls below 6°C. As a result, by the time the outside temperature is down to zero, the pump will only give about 75% of its output. If the outside air drops as low as -15°C, the output is down to 45% .
Part of the reason for loss of efficiency is that, because of the high humidity in Ireland, ice forms on the cold metal surface of an air source pump's heat exchanger and prevents the energy that can still be found in the cold outer air passing through to warm the refrigerant within. To deal with this, air-source pumps have to have a defrost cycle to melt the ice and allow the heat exchanger to work properly. Naturally, providing this heat means that there is less available to warm the house. “At external temperatures of +5°C or below, the average defrost time is about 7 minutes per hour,” Phelan says, “but as the external temperature drops, its moisture content also falls and so the defrost, interestingly enough, becomes less of an issue.” So, while an air source pump can match the COP of a ground-source one if the external air temperature is 7°C or more (some Toshiba pumps have a COP of over 4) they have lower COPs than ground-source ones during periods of cold weather.
Since only 5% of Irish electricity comes from renewable sources such as the Arklow banks wind farm, the vast majority of heat pumps are currently reliant on fossil fuel generated electricity to operate
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