Local Geology

Geology and ground conditions must be considered for both types of collector. Generally, the best areas for horizontal collectors are saturated or wetter areas of the site; to give the best long-term heat recovery and ensure lasting efficiency for years to come. Saturation is key to the horizontal collectors performing at their best. In our experience, the temperature within the collectors buried at 1.5m would fluctuate between 8ºC (cold winter) 14ºC (hot summer) at 1.5m depth, depending on seasonal fluctuations. As you go deeper into the ground, the temperature of the ground becomes much more stable and this is where boreholes come into their own.

Depending on the geology found on site, the temperature at 100m deep would remain at approximately 12ºC all year round, unaffected by seasonal changes. At these depths, the collector is well into the permanent water table level and would remain saturated all year round increasing conductivity too.

System Lifespan

Regarding the physical installation, i.e. pipework, provided that the correct materials are used for the installation and fusion welded, both ground loop and borehole systems should have a similar expected lifespan. It is anticipated that both systems would be in excess of 50 years, theoretically longer as there should be very little degradation to the plastics used as they are not exposed to sunlight.  Both systems would require the glycol (heat transfer fluid) to be changed periodically.

It is imperative that there are no mechanical connections underground which cannot be mapped and ultimately be reached if necessary. Best practice would minimise joints in the system as far as is possible.

The collector system should always be checked annually when the heat pump is serviced to ensure that all of the collector loops are working correctly.

System Efficiency

Even in a perfectly designed ground collector system, their relative proximity to the surface means that there is potential for seasonal changes in ground temperature, and slightly increased heat pump inefficiency during winter.  And the same effect can happen in summer if you are cooling a building; when using the warm ground to cool water to be used in the building for active cooling.

Boreholes conversely benefit from far greater depths and the water table and are not affected by the same fluctuations that horizontal loops are. As a result, it enables your heat pump system in a cold winter to be running at the same efficiency as in warmer weather.  When cooling, this is particularly beneficial and, as a result, boreholes are often used in office complex installations because of this superior cooling property.

Each collector type has its pros and cons. The best solution for your build will depend on cost, local geology, and bespoke technical requirements. Either way, you will be pleased to hear that the expertise for design and installation in this complex area is close at hand.

If you still have any questions about ground source heat pumps, or anything else to do with your low-energy home,
please don't hesitate to contact the Mesh team today.