Air Source Heat Pumps
The term Air Source Heat Pump is used to designate a heat pump which extracts heat from air and heats water. Typically, these would be used to deliver space heating and hot water, but there is no reason why a suitable system cannot also run cold rooms, or produce chilled water or ice, either on a standalone basis, or whilst simultaneously heating water for both sanitary and hydronic heating systems.
Essentially air source heat pumps are very similar to air conditioning units but for the fact that they have water cooled condenser coils. Some cheaper air source heat pumps are modified air conditioning units, however in climate zones where winter temperatures get close to or drop below freezing, it is best to use units designed from the ground up for the specific purpose of heating water for both sanitary and space heating purposes. (Using an air source heat pump of the converted air conditioner type to heat a swimming pool in Southern Spain might be a different story).
What's Different about a Purpose Built Air Source Heat Pump?
The biggest difference between an air conditioning unit and an air source heat pump is in the temperature difference between the temperature of the air source verses the temperature of the service delivery.
On a hot day, an air conditioning unit might dump heat to 35 centigrade outside air and deliver chilled air at 15 centigrade to the room a temperature difference ΔT = 20. In winter when heating the temperature difference is greater so that on a very cold day in the UK you might have air at -5 centigrade delivering warm air at 25 centigrade ΔT = 30.
For an air source heat pump on that same -5 centigrade day, the heat pump would need to deliver hot water at 55 centigrade (to assure of Legionella safety), and water for space heating at maybe 35 to 40 centigrade in an optimised system ΔT = 40 to 60 centigrade. This much bigger ΔT combined with the fact that typically an air source heat pump will be asked to deliver all the heat requirements of a building for both space heating and hot water – not just space heating in a few rooms at most as would be the case with a multi-split air conditioning unit makes the technical demands on an air source heat pump far greater.
In order to have an efficient system, you need all parts to integrate well with each other and to be appropriately specified – not just an efficient heat pump. Add a large poorly insulated hot water tank, or poorly configured controls to an otherwise good system, and the system efficiency suffers.
A heat pump's efficiency depends on the size and quality of the compressor, expansion valves, heat exchangers, evaporators etc. Typically, air conditioning units converted for air to water application have undersized components compared to purpose built air source (air to water) heat pumps – which under-sizing impacts operational efficiency, reliability and operational life. Typically, a converted air conditioning unit will be designed for average rather than worst case conditions, so will not be able to operate efficiently in the coldest conditions or when cool damp conditions apply. Such units have a very narrow spacing between fins on the evaporator compared to a system designed from the ground up as an air source (air to water) heat pump. The fins being so close together quickly fill up with ice and need frequent de-icing which has a parasitic energy requirement and worse still results in the evaporator being exposed to many more heating / cooling cycles which can compromise the life of the evaporator. This is just one example of a design compromise.
In short, whilst modified air conditioners are cheap – due to high volume production, small products and a well-developed supply chain, they perform badly, have a compromised life expectancy and are a poor investment compared to buying the correct system for the job. The modified air conditioning unit is in addition, only a heat source, not a heating system, and once all the other necessary components are added in, the cost advantage of a cheap air conditioner can largely evaporate.
It is quite possible that if you look up the COP and nominal output of the two systems and compare them, that on paper based on standard test conditions that nominal performance may be very similar, however in the real world, and taking into account damp or very cold days, the purpose built unit will deliver significantly better performance than the AC derived unit. For example, at 4 centigrade on a 100% humid windy rainy day, the purpose built unit will deliver significantly more heat with less energy in per unit of heat out than the AC derived unit.
An AC heat pump can expect a life of around 5 to 7 years on this application compared to 15 – 20 years on a properly designed air source heat pump. Note:- A shorter life can be expected if used continuously such as to heat a year-round swimming pool.
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Having specified a purpose built optimally sized air source heat pump with appropriate controls and heat storage, the most important thing in achieving efficiency is to minimise ΔT.
Typically, air source heat pumps are designed to be able to produce sanitary hot water at >55 centigrade, and water in the space heating system to between 35 and 40 centigrade. In some cases, such as with heat pumps using CO2 as the refrigerant gas, 90 centigrade can be achieved.
The efficiency of any heat pump has a fundamental physical limit set by the Carnot equation.
In practice no heat pump in operation comes close to delivering the theoretical level of energy efficiency, however the less temperature difference between the hot and cold sides of the system, the greater the efficiency, and the higher the COP of the heat pump.
To achieve efficiency in practice – use of underfloor heating or oversized radiators or fan assisted radiators allows heating to be delivered by water at 35 to 40 centigrade rather than the 65 to 70 centigrade more typically used in systems with gas boilers.
In systems with hot water tanks where the water to be delivered hot is stored in bulk, (your standard hot water tank), it is necessary to heat the water to 55 centigrade to have confidence that Legionella bacteria are suppressed, so typically this is the storage temperature for sanitary hot water.
In an alternative configuration, a “closed heat storage tank” is employed, and the same water or alternative thermal storage medium is held in the tank at all times. In this system, there is a large heat exchange coil, and cold water enters at the beginning and hot water exits at the other end of the coil. In this configuration, there is no bulk amount of hot water potentially sitting stagnant, Legionella risk is not an issue, and the heat store can be held at a lower temperature e.g. 40 – 45 centigrade which is hot enough for a shower or bath.
Typically, heating water to 40 centigrade in the UK will involve around half the ΔT of heating to 70 centigrade (generally only achievable with a CO2 heat pump), and all other things being equal, the efficiency and heat output capacity of a heat pump delivering 40 centigrade will be about twice that of one delivering 70 centigrade.
Enhancing Energy Efficiency
Two further actions which can enhance energy efficiency are
- Wherever possible, site the air source heat pump on the South side of the building. On a sunny still spring day, the apparent temperature in such a location can easily be 10 – 20 centigrade higher than in the shade on the North of the same building substantially reducing ΔT and boosting efficiency.
- Capture heat discharged from elsewhere – e.g. If you have an uncomfortably hot industrial kitchen, and are using an air source heat pump for heating adjacent offices and providing hot water, then you may be able to site the heat pump in the industrial kitchen, or have a duct system which allows you to source the feed air for the heat pump either from the kitchen, or from outside to the heat pump, so controllably cooling the kitchen with negligible additional energy consumption.
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