Kubili House – a geothermal work of art

By Ilana Koegelenberg

The new Kubili private lodge required a seamlessly integrated, unobtrusive HVAC system that would not detract from the natural environment in its surrounds — a challenge the professional team tackled head on with great success.


The Kubili site is located 30 minutes out of Hoedspruit, near the Klaserie River in the Thornybush private nature reserve, bordering on the Kruger Park. The total area of the establishment is approximately 3 000m2 and the lodge itself lays hidden away in the untouched Lowveld savanna vegetation surrounding it as the wildlife from the Kruger Park roams around freely.

The lodge can accommodate eight people in its various villas and offers a perfect tranquil, private getaway.

The scope of work included spacial heating and cooling via an underfloor heating and cooling system; heated towel rails; spacial heating and cooling through ducted hide-away fan coil units (FCUs); heating of the three swimming pools; and the domestic hot water supply. A misting system was also installed in the covered outdoor area to cool in the summer.

The project was started in May 2016 and completion was in December 2017.

Client brief

The client required a system that “cannot be seen and cannot be heard”. The system needed to reduce the overall electrical load as compared with conventional systems. The client did not want multiple units, due to the visibility and noise of multiple units, and the system had to be low maintenance. As such, it had to be designed to run “in the bush”. This particular “bush” also came with extremely high ambient temperatures ...

The requirement was for:

  • Heating load: 175kW
  • Cooling load: 143kW.

The proposal was for a central plant that met all the demands for heating and cooling throughout the lodge. To eliminate the need for cooling towers or condensers (which would be quite visible), the suggestion was made to use ground as the energy source/dump.

Initially, the dam in front of the lodge was considered for the energy source, but was later rejected after considering that during drought, there may not be enough water available for use. Another concern was that pond loops could also possibly be damaged by the wildlife.

The decision was then made to instead install a vertical geothermal loop field comprising 30m × 100m-deep vertical ‘borehole’ loops, which would be used as the geothermal field for the plant.

System description

The system encompasses all of the lodge’s heating/cooling requirements and can be split into two sections: the central plant and the terminals.

Central plant

The heart of the plant is a heat pump-chiller, which transfers energy between the geothermal field, hot-water tanks, and cold-water tank. From these tanks, the terminals throughout the lodge are fed with hot/cold water as required to meet the demand.

The heat pump then absorbs/rejects heat to and from the geothermal field as required, or it will transfer energy directly from the cold tank to the hot tank. The heat pump has built-in circulation pumps to meet the required duty of these primary loops.

Heat is also transferred from the hot water tank, via a coil and circulation pump, to the domestic hot water as a pre-heating device. This keeps the municipal water separate to the treated, closed-circuit of the HVAC system.

Hot or cold water is delivered to various terminals through a series of pumps. Large flows required by the FCUs are delivered with Wilo Giga variable speed pumps, and Lovato mixing group-sets supply the underfloor system and deliver water that is neither too hot in winter, nor too cold in summer. This control ensures a constant temperature of water supplied to the floors according to the design parameters. Dew point is measured and in cooling mode, the temperature of the water is kept above dew point to prevent condensation.

The swimming pools are equipped with dedicated circulation pumps that supply the hot water required to heat exchangers in the pool circulation loops. Pools can be actively heated with energy from the heat pump when in heating mode, or alternatively from rejected heat in cooling mode. This ensures that in cooling mode, waste rejected heat energy is rejected to the pools before the geothermal loops-field. There are also dedicated Lovato high-temperature group-sets for the heated towel rails.

The domestic hot water is critical for the lodge. There are 2 000 litres of storage with Boiler Nova high-pressure tanks. Domestic water is heated primarily with a transfer pump on the coil of the domestic tanks from the hot water buffer tank. This achieves a temperature of around 52°C.

Twelve Alpine easy flat plate collectors are installed in two arrays of six. The solar pump transfers energy from these panels to the domestic tanks and is then diverted to the hot water buffer if the tanks are on temperature. This is topped up with high-temperature water from the solar collectors. For times when the climatic conditions do not suit the solar or the water demand in the lodge is extremely high, a 32kW Fondital Itaca condensing modulating gas boiler has been installed on the top coil of the tank. This will ensure that the lodge never runs out of hot water.


All heating and cooling terminal units, underfloor, and domestic water is reticulated in an underground insulated trench that runs in the centre of the lodge.

FCUs are the preferred method for cooling. The distance from the central plant to the furthest FCU is approximately 200m; therefore, correctly sized piping and proper insulation are required to deliver a correctly functioning system.

The underfloor system generates most of the heat required in the spaces. Underfloor cooling provides additional comfort during sweltering summer months and is carefully controlled via a hygrostat and mixing station to prevent condensation from forming on the floors if the temperature drops below dew point.

The swimming pools each have a Bowman stainless-steel shell and tube heat exchanger to provide the energy required to maintain the swimming pools at 28°C all year round, even though the pools have an overflow that acts as a ‘cooling tower’, and therefore require a larger-than-usual amount of energy.

The heated towel rails, which only operate in a heating mode, are supplied with water that does not allow the temperature to burn the skin, but only heat up the towels and space to a comfortable temperature.

Equipment selection

The true beauty of the system installed at Kubili is the integration of the various services into a cohesive, sophisticated plant that runs efficiently and meets all the client’s needs. That is why it was crucial that everyone worked closely together on the design so that everything, from the domestic hot water to the FCUs, all tie in seamlessly and no energy is wasted.

An Aermec heat pump-chiller was installed in the plant room. This is a low noise, high-efficiency unit with a dual refrigerant circuit, electronic expansion valves, and onboard high-pressure circulator pumps for the heating and cooling circuits.

The heat pump-chiller boasts the following capabilities:

  • Cooling capacity: 155kW at 7°C/12°C at 10.7ℓ/s (COP during cooling only: 5.33)
  • Heating capacity: 213kW at 55°C/50°C at 10.2ℓ/s (COP during heating only: 4.64)
  • Capacity during recovery: 126kW cooling | 165kW heating; total efficiency ratio (TER) during recovery: 6.6.

The domestic hot water system is backed up by a 32kW Fondital Itaca gas-fired direct boiler of the condensing, modulating type.

Aermec FCUs were also installed. A two-pipe configuration was selected since heating could also be achieved through the underfloor heating. Even so, electric heaters were supplied in each FCU in case there was a situation where a guest would require some instant warmth. Touch screen wall-mounted displays were supplied for each of the rooms.

In terms of operating logic and controls, a fully integrated system like this can only be controlled and managed through a central controller. In this case, a PLC was custom built for the project, and the controls integrated into a human media interface (HMI), which allows the client the control and visual operation of the entire system.

Many sleepless nights were spent to determine the logic of all the diversion valves, to ensure maximum efficiency throughout the various operating scenarios that the system would experience year-round.

After the control logic had been figured out, the software was written, and the electrical control panel was built for the site.

Valsir’s design software — Silvestro — was used to calculate the heat loads and design piping and incorporate this into the CAD drawings that the architects generated for the building. Dwg format drawings are uploaded into the software, and then various inputs are fed into the software. It then generates reports on the required heating/cooling capacity, flow rates, piping, and pressure drops.

“All of the suppliers were willing to go the extra mile and make a success of the project,” explains Dennis Holden of Reliance Eco, who designed the integrated geothermal HVAC system. “Since this was the first of its kind, there was a lot that needed to be designed and correctly selected for the application. Having the correct team was key, and the important factor was that each of these suppliers was part of the team that made it all come together.”

Going geothermal

Aqua Earth in Randburg tackled the drilling for the geothermal system. A total of 30 boreholes were drilled to a depth of 100m each — meaning that a total of 3km was drilled into this site. “The geological formations in the drilling area were predominantly granitic and mainly consist of silica and mica,” explains Ben Hefer, project manager of Aqua Earth, who was on site for this project.

Once the hole was drilled, a vertical ground heat exchanger, in the form of a single U-Tube configuration, was placed in the 100m borehole. This successfully transfers between 4.4kW and 6.2kW of thermal energy in each hole, as calculated from the site readings.

The true beauty of the system installed at Kubili is the integration of the various services into a cohesive, sophisticated plant that runs efficiently and meets all the client’s needs.

All the boreholes were filled with a mixture of silica sand and bentonite once the loop was installed.

Some of the boreholes had water (water strikes were between 30m and 40m), with a yield of 1 500ℓ/h to 2 500ℓ/h. The experimental static water level in the well was recorded at 11m below ground level.

The well was completed in a granitic aquifer, with a diameter of 165mm and a depth of 100m. The undisturbed groundwater temperature was measured at 16.5 ̊C.

The depth of the header trenches was between 1.5m and 1.8m, while the length of the trench was 200m (63mm HDPE Class 16 pipe).

When it comes to pipe sizes, the loops consist of 200m × 32mm HDPE Class 16 pipe. As there are 30 loops in total, this meant that 6km of piping was installed in the system. Together with the header’s 200m, this adds up to a total of 6 200m of piping for the geothermal installation.

After all the loops were connected to the header, the loop was flushed with water and pressure tested to ensure that there were no leaks.

A conductivity test was performed at the first borehole to determine the conductivity of the ground and to calculate the heat exchange potential in kilowatt. For the test, water is heated and pumped through the installed loop and back to the test unit. The water temperatures at the inlet and outlet of the loop are then recorded at five-minute intervals and logged in a PLC. The test was conducted over a total period of 16 hours.

Recorded data was then extracted from the PLC and sent to an engineering company to calculate the conductivity — between 4.4kW and 6.2kW, depending on the hole.

The fusion welding technique was used to connect HDPE pipes and HDPE pipe fittings to each other. A fusion welder is used to heat the inside of the pipe fitting and the outside of the pipe; once the pipe and fitting reach the required temperature, the pipe is inserted into the fitting. The heated pipe and fitting fuses together before cooling down.

The pipe and fitting are joined without using any other form of material or equipment that can leak due to deteriorating material. For example, compression fittings contain rubber seals that deteriorate over time and start to leak.

All the loops were pressure-tested to 4 bar pressure, both before and after installation, to ensure the integrity of all the pipes and joints.

Only HDPE class 16 pipes are used for fusion welding due to the wall thickness that ensures a proper weld with the fitting. Thinner pipes deform when heated and inserted into the fittings.


The biggest challenge for this project was the distance to get to site — Kubili is about five and a half hours’ drive from Johannesburg.

In terms of setting up the geothermal field, the main challenge was timing. Also, because the teams were only allowed on site between 8:00 and 17:00 and not over weekends, this extended the drilling time somewhat. Still, the team managed to drill and equip at least one hole a day.

Another issue here was the fact that some of the boreholes had substantial water yields and diverting the water from the drilling area was challenging. There was also no Eskom electricity on site, but luckily this could be overcome with the use of a generator.

“There were no serious challenges, but planning was important,” explains Holden. “The project was simplified thanks to a great deal of assistance by the main construction contractor.”

A site like no other

Some interesting things happen when working in a nature reserve, explains George Thomaides of AERSA. For example, one day the construction crew arrived on site to find a leopard sleeping in the storeroom.

“It was a great experience to work at this unique and beautiful location,” says Hefer. “Some of our crew members have seen some of the animals for the first time in their life.”

When the lodge was almost completed, a pack of wild dogs ran through the main lounge of the site, chasing an impala!

“The project was not the kind that happens every day and is a flagship that highlights many of the products, design, and installation work of those involved,” concludes Thomaides. “I believe that this is the reason all who were involved put extra effort into finding solutions to any hurdles during the project and working together as a team to see the project succeed.”

List of professionals

Project manager

John Wicks

CIR Construction


Main building

CIR Construction


Reliance Eco

Geothermal drilling, testing, and installation of ground loop system

Aqua Earth 

Product suppliers

Heat pump-chiller; FCUs, electronic thermostats, chilled water buffer tank; FCU valves; and actuators


Hot water buffer tanks, gas boiler, distribution manifolds, high-temperature and mixing group-sets for all hydronic systems, three-way diversion valves, PLC control system, and HMI

Valsir Uneeq

Alpin Easy Flat plate solar collectors

Solar Energy

VS Giga variable speed pumps for FCU supply


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