direct_design

Key Considerations

Sizing
To get the best performance from your heat pump it is important to consider the impact the geological and hydrogeological ground conditions has on the performance to avoid oversizing (spending too much) or undersizing (system failing). There are costs implications of both.

In a German study for “Thermal Response Test, A Routine Method to Determine Thermal Ground Properties for GSHP Design” the costs of under-sizing and over-sizing GSHP systems was shown for a 12 borehole array system – note the optimal thermal conductivity is 2.2:

Thermal Conductivity [W/m/K] SPF (Seasonal Performance Factor) Annual Power Consumption [MWh/Year] Annual Electricity Cost [Euros/Year] Incremental Cost [Euros/Year]
2.2 4.0 26.3 €3,945 -
2.0 3.5 30.0 €4,500 €555
1.8 3.1 33.9 €5,085 €1,140
1.6 2.8 37.5 €5.625 €1,680

Table 1 – Cost of Undersizing

Thermal Conductivity [W/m/K] Necessary Length for 12 Borehole System [m] Total Borehole System length [m] First cost of Borehole System [Euros] Incremental Cost [Euros]
2.2 102.2 1,226.4 €91,980 -
2.4 96.7 1,160.4 €87,030 €4,950
2.6 91.5 1,098.0 €82,350 €9,630
2.8 86.7 1040.4 €78,030 €13,950

Table 2 – Cost of Oversizing
In some cases there has been a practice of oversizing heat pumps to guarantee that you will meet the heating requirement, but this does come at a price as the heat pump will short cycle. Short cycling is where the heat pump turns off and on within a short period of time (multiple times per hour). Each time a heat pump starts it will draw a large starting current, which in turn will impact the efficiency of your heat pump.

Building Insulation

The insulation of the building – installation in a poorly insulated property or building will result in a poorly performing system and you may have to install a larger heat pump than you need to. It is good practice to retrofit insulation prior to installing a GSHP.

Heating Requirements

For space heating, a general rule of thumb for peak heating requirements is that 25 square metres of living space, in a well insulated building, requires 1kW of thermal heating. However, this is only a generalisation, and a more precise sizing of system can be determined from a properties SAP report. For hot water heating there are numerous other considerations to take into account, including business requirements/lifestyle habits, although a similar rule of thumb can be used.

Replacing your existing heating source

Heat pumps can save you more on your heating bills if you’re replacing an electric, LPG or coal system – but, currently, might not lead to great savings if you are on the gas network. What they will provide is security from future energy price rises.

Your current radiator system

The heat produced by a ground source heat pump comes at a lower temperature than other forms of heating, making it best suited to underfloor heating, which requires lower temperatures, rather than radiators (or, if radiators are used, they should be properly-sized or of the low temperature variety).

Supplementary Electric Heating

Running costs can be higher if you’re also using the system for your hot water supply, and you may require a supplementary electric immersion heater to keep up with your heating needs and for the majority of heat pumps, heat the water to above 60° C once a week (or every 24 hours in commercial installations) to prevent legionella. If the system is required to provide hot water, a thermal store will be required also. This is similar to a hot water tank, although tends to be larger, with a larger heat exchanger coil within the cylinder.

Solar PV and GSHP

The payback period will be shorter if your current heating system is electric, LPG or oil. This can be further shortened by generating the electricity required to run the GSHP through renewable energy sources such as Solar PV.

Solar PV and Ground Source Heat Pump

Solar PV and Ground Source Heat Pump

A product provider and installer should do a site survey before you decide to purchase one of these products, and this should help you choose which system best suits your needs. Following installation there should be very little maintenance and Ground Source Heat Pumps have a lifespan of over 25 years. Tailoring a solution that properly takes account of ground conditions and site constraints are crucial.

Our aim is an optimised, high performance system that prevents unnecessary expense and delivers you the best value for money. We believe that our clients should not pay additional cost for poor design, but get a designed solution that delivers the best performance and therefore the best rate of return.