Geothermal or Air Source Heat Pumps?

What’s the best heating and cooling choice for Australian conditions?

– As designers and installers of radiant heating and cooling systems we hear some outlandish claims about the performance of geothermal heating and cooling systems. For example “By using the constant earth temperatures, a geothermal system can cut up to 75% off your energy bill”.

Geothermal, like solar electric and thermal systems, is absolutely a “green” product. The purpose of this article is not to dispute how “green” it is environmentally, rather how much financial sense it makes for homeowners. The biggest issue with a broad claim such as “geothermal is the best way to reduce your energy bills” is that it’s simply not always true. We see too many homeowners and builders being sold this message without the facts behind the claim to support it. 

A bit of background on heat pumps

Heat pumps are a fantastic way to create heating and cooling in the home; they absorb thermal energy from either the air or the ground (hence air source and ground source heat pump) and transfer it into a fluid for distribution into the home.

Electricity is traditionally considered the more expensive way to heat the home. The big advantage of heat pumps though, is that unlike a traditional electric radiator that turns that 1kWh into one unit of useful heat, a heat pump converts 1kWh into 3.5 or even more units of useful heat.

Therefore, electrically run heat pumps are actually a really efficient way to heat or cool the home. (The same concept is increasingly used in domestic hot water systems using smaller high temperature heat pumps matched to a storage tank).

Always ensure that your home is sufficiently insulated in order for the heat pump’s influence to become maximised. The next question is, which to go for – a ground source heat pump or an air source heat pump?

Air source heat pump and ground source heat pump compared

As mentioned previously, air source heat pumps absorb their heat from the air, while ground source heat pumps absorb it from the ground. Let’s compare the two technologies.

Air source heat pump units look very much like air conditioning units seen on the outside of many buildings. The size of the heat pump will be determined based on your home’s heating and cooling demand – the bigger the home, the more heating or cooling it is going to require – so the larger the air source heat pump unit.

The same principal applies to a ground source heat pump but it requires buried pipes in the garden, so the first consideration is whether or not you have space to install them. Normally the pipes are drilled in vertically, so bore drilling machinery will need to be able to access the garden. In many cases the cost of drilling can be prohibitive. Although laying the pipework horizontally takes up a larger surface area of land, it is a cheaper method.

If you have a dam or well near the home, geothermal heat pumps can also use this as a heat source and it is usually more affordable then drilled-in systems.

With ASHP, (Air Source Heat Pumps) the unit is comparatively simpler and only needs to be attached to the outside wall, with pipes connecting into the building. A typical unit might be a 1.4 metre high by a meter wide with a depth of about half a meter. This will obviously vary greatly between models, but will give you something to work with.  This option is available to the majority of the market, and therefore ASHP has the ability to be adopted by people living in densely populated areas.

Instead of spending the money on a GSHP (ground source heat pump), think about installing an ASHP and then invest the “savings” into building envelope improvements, such as more insulation, shading windows, and definitely more solar hot water and solar energy panels.

Cost of heat pumps

While the scientific basis between the two technologies is the same, the cost of having them installed is not! The cost of an air source heat pump (without considering underfloor heating) is approximately A$7,000. A ground source heat pump on the other hand is typically about A$30,000 with much of the additional cost going on bore holes that need to be drilled in your garden. 

Efficiency of Air Source heat pumps vs. Ground Source Heat Pumps

If you live in a region or country with continuous climatic extremes, the winner on efficiency would probably be ground source heat pumps. That’s why they are effective solutions in places with prolonged, very cold winters like Europe, North America and Iceland.

However in Australia the majority of the Australian population live along the seaboard in relatively mild climate zones.

With a GSHP, the heat transfer pipes are buried in the ground, where you don’t get the same temperature fluctuations as you do above ground – the ground temperature in most populated areas of Australia is fixed at about 14-19C. This means that during the winter, when the air temperature is cooler, a GSHP can provide the same heating requirements off fewer units of electricity, but you need to ask for how many hours of the day this provides a substantial benefit?

In Australia the difference in temperature between the ambient air and ground is not as extreme as in other parts of the world. That means an ASHP in most of Australia still finds it relatively easy to get to the required temperature.

Even is a much colder country like the UK, an in-depth study by Energy Saving Trust of residential GSHPs and ASHPs showed average GSHP performance to be only marginally better than that of ASHPs—and also showed a wide variation in actual performance.

Don’t forget, while modern GSHPs and ASHPs have similar performance, there’s a huge disparity in installed cost. In Australia, for example, installed GSHPs can cost four to seven times as much as ASHPs. 

Enhancing your heat pump performance

Whether you choose GSHP or ASHP, you will get even better performance if you select radiant hydronic underfloor heating or cooling rather than ducted air to deliver the energy into the home. The reason is that radiant underfloor or ceiling heating achieves the highest levels of comfort but at a lower operating temperature than most convective systems. This concept is important in understanding why underfloor heating can reduce the energy required to heat a building because it enables heat pumps to achieve their maximum rated factory efficiency, saving you even more money. 

Final Thoughts – Ground Source versus Air Source Heat pumps

In summary, heat pumps are a very worthwhile financial investment, both for comfort and energy savings particularly if you don’t have access to mains gas.

Whether you opt for a ground source or air source heat pump is dependent on your situation, but as air source heat pump technology has improved there is very little in it – it really depends on whether you can afford the additional expense of the GSHP for the marginal increased efficiency – bearing in mind that you would probably be far better off investing in more solar PV panels to supply your Air Source Heat Pump and all the other electrical equipment in your home.

If need assistance in choosing the most optimum heating system out of : Geothermal (Ground Source)  and Air Source –Contact us on 1300 790 636

Air Source Heat Pumps 

Ground Source Heat Pumps

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