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Ochsner Ground & Air Source Heat Pumps from Barres

Choosing a Heat Pump

Each type of heat pump, whether ground source or air source, has its advantages and disadvantages and the type of heat pump used will vary from project to project.

Ground source heat pumps generally offer the best efficiencies and multiple types of heat collector can be used. The most common method is ground loops - horizontal pipework within the ground. Other methods can include boreholes, open well boreholes, river collectors and more.

Air source heat pumps can offer efficiencies that come very close to ground source heat pumps, and are generally easier and quicker to install than most ground source options. They are also more suitable where space is limited and horizontal ground loops or boreholes are not an option.

The Ochsner split air source units are offering class leading COPs - taking into account the higher summer COP of the air source, then the real life seasonal performance factor (SPF) will not be that much less than the ground source heat pump.

Ground source heat pump - closed loop options

Ground Source Heat Pumps

Ground Source with Ground Loops

Ground loops are usually the best option if a suitable area of ground is available. Horizontal runs of pipe are installed at a depth of around 1.2 meters under the ground, connected back to a manhole. Sizing the ground loops is very important as it can make the difference between an excellent system and a very badly performing system.

The following table shows the maximum sustainable heat that can be extracted from various soil types over 1800 and 2400 hours per year. This information is in accordance with the internationally recognised standard VID4640.

Underground Specific extraction output for Horizontal ground loops
Soil type 1800 hours per annum 2400 Hours per annum
Dry, non-cohesive soils 10 W/m 8 W/m
Cohesive soils, damp 2030 W/m 1624 W/m
Water saturated sand/gravel 40 W/m 32 W/m

The figures of 1800 hours and 2400 hours are average figures per year based on how the heat pump is being used. 1800 hours is the average for heating only, while 2400 hours is for both heating and hot water.

Using the figures from the table, it can be seen that with a 10kW heat pump on a site that has cohesive, damp soil (giving an average of 20W/m) would require 500m of ground. For dry soil that has a lot of sand in it, the same heat pump would require 1250m.

At Barres we always use linear runs of pipe, we never use slinkies or other space saving devices. As shown above its the area of ground that is the key factor.

Ground Source with Boreholes

Where ground loops are impractical, the use of boreholes is possible. There are two types of borehole that can be used:

Closed Loop Boreholes
Similar to ground loops but the pipe installed vertically instead of horizontally, six inch boreholes are drilled to a depth of 90m, and two lengths of pipe joined using a special connector are inserted into each borehole. The boreholes are then back filled with a special grout called bentonite. The pipes are connected to a manifold and which runs back to the heat pump.

On average, you can extract 3-4.5kW from a closed loop borehole, depending on the rock type. A 10kW heat pump would require 2-3 boreholes to operate efficiently. For domestic projects, the cost of drilling usually makes boreholes unviable, however for commercial projects as the scale of the project becomes larger, ground loops start to become impractical and therefore boreholes will be the more viable option.

Open Well Boreholes
Extracting and using the ground water directly, two boreholes are drilled a set distance apart, with the intention of hitting a good source of water. A deep borehole pump is placed in one of the boreholes and water is pumped from the borehole, through the heat pump and discharged through the second borehole.

This is by far the most efficient way to run a ground source heat pump but is potentially the most problematic. With open well boreholes, the water has to be tested as certain minerals or elements can damage the heat exchanger of the heat pump over time. There also has to be high level of confidence the water will be available long term.

Air Source Heat Pumps

Air source heat pumps are often seen as a cheaper alternative to ground source and many models on the market are derived from mass produced air conditioning units which are not specifically designed for the job. All too often they are undersized, and this is one of the reasons that air source heat pumps have had a poor reputation in the UK. These units, while they may be cheap, may end up costing more in the long run due to poor efficiencies.

Air source heat pumps should be designed with the same considerations as a ground source, based on the project in question and sized appropriately.

The Ochsner air source heat pump delivers class leading efficiency. It's a split unit, meaning that the evaporator is external to the property, and the heat pump unit is internal, with a refrigerant circuit in-between. This type of installation provides the highest efficiencies and lowest external noise.

The horizontal evaporator design of the Ochsner external units, also give the best performance in terms of defrosting as the vertical fins allow water to fall out of the heat exchanger rather than being trapped inside.

With the advent of these high efficiency and high quality Ochsner heat pumps, air source is no longer the second best choice.

Air source heat pump - options

Barres is part of the GA Barnie Group
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