Washington Mews: From office to residential

Washington Mews: From office to residential

By Rory Macnamara, with technical information by Steve Franklin and David Wills of Wills Franklin Pretorius, and John Taylor of Rehoboth Projects

Washington Mews, previously a four-storey office building situated in Randburg has been developed into a residential complex by the owners, Netwater Properties.

The owners added an additional floor in keeping with the original design of cascading floors. Architecturally, the building has new cladding, facades, roofs over external parking and cladding over external plumbing ducts. The result is a completely transformed building in the most modern architectural design.

The building has 80 one bedroom, one bathroom apartments, and 20 two bedroom, two bathroom apartments, each with fitted kitchens. There are two levels of undercover basement parking, with hot and cold-water plantroom situated in the lower basement.

The design team faced a major challenge in that the existing building had post-tensioned concrete floor slabs. As the position of the structural cables in the slabs were unknown, and the fact that many core drillings had to be done to accommodate the internal plumbing to fit the architectural design, the risk of damaging the cables was high.

As the cost to repair post-tensioned structural cables would have been extremely costly and time wasting, the client instructed that the vertical plumbing ducts were to be installed on the external façade of the building.
Of primary importance was water saving and the owner invested heavily in providing for this.

Measures included a separate wastewater drainage reticulation system serving wash basins and showers only. This wastewater will be connected to a future basement wastewater treatment plant where filtered and sterilised water will be pumped up to a roof level gravity tank where it can be used for WC flushing and potentially irrigation. The soil and kitchen waste system are separate. Initially both drainage systems connect at the western perimeter of the building before connecting to the municipal sewer. Clearly, significant savings of municipal water can be made.

Electronic water meters are provided with the relevant data collectors for transmitting the readings for billing and statistics. These are:

  • Hot water usage per apartment
  • Cold water for WCs has multiple water meters that measure each vertical toilet shaft of five flats below it. This allows the owner to gather comparative figures per stack of five flats.
  • Total cold water to the building (municipal check meter)
  • Total hot water usage
  • Borehole water usage

Smart Meters – electricity and water

Washington Mews uses smart meters for all electricity and water measurements and billings. Readings are taken every 30 minutes and consumption graphs are available on the internet.

Early warning leak detection

The building has its own bulk smart meters to compare consumption with the council meters record for electricity
and water.
The building also has multiple sub-meters to record internal consumption for cold water, hot water, toilets, borehole, and irrigation.

The owner’s operational experience with utilities has shown that toilets are the biggest risk when it comes to water leaks (for example, mechanism gets stuck, toilets run all day and waste water – can be up to 10 000 litres per day).

Each stack of five apartments (1 per floor) has its own toilet shaft with a smart water meter on the toilet feed pipe in the top of shaft. Consumption data is checked regularly and compared to the other shafts. Excessive water consumption is flagged and investigated, generally within 48-hours. Without our early leak detection methodology, leaks can waste water for several weeks.

Historically, and without smart meters, we only realised a building had a leak when we received an inflated utility bill from council a month later. The council does not read meters every month and often use estimated readings, which may allow leaks to continue for several months.

The plumbing installation consists of:

  • Soil waste and ventilation drainage pipework
  • Vertical soil stacks, kitchen waste stacks and waste stacks installed in vertical extended ducts
  • Horizontal waste pipes and soil pipes run in dry-wall cavities, raised floors to the external ducts
  • Ventilation pipes installed as required
  • Horizontal drainage pipes are collected on the soffit of the upper basement parking level and under the steel parking level roof. Rainwater pipes follow a similar route.

Cold water installation

Municipal cold water supplies polyethylene tanks from constant pressure variable speed type booster pumps in the basement plant room. These provide cold water to the building and to the hot water plant.

Hot water plant

The plant provided by Elemental Energy Supply Company (EESCO) is very innovative. The hot water generation is provided by their own EESCO brand heat pump that uses two heat exchangers for the refrigeration cycle and water heating. These have very small footprints and are highly efficient in terms of heat exchange.

The hot water is heated via a brazed plate heat exchanger (BPHE) which heats the water with the high temperature refrigerant. The second half of the refrigeration cycle traditionally uses an air to refrigerant fan coil that extracts heat from the ambient air and transfers to the liquid refrigerant, evaporating it back to a gas to complete the cycle.
However, in this case with the plant located in the basement, re-cycling cold air would occur if this technology were used. As a solution the company have developed a heat pump that uses a water to refrigerant heat exchanger (Shell & Tube) to evaporate the liquid refrigerant.

The water used in this cycle is heated by a standard air – water fan coil heat exchanger which is located 50m away on top of the guard house roof. The fact that this unit can be located a distance from the plant and the flats means there is little chance of their being any noise pollution from the fan coil unit. The water that is heated by the fan coil is stored in 2 x 5 000ℓ plastic tanks located in the basement plant room.

“The result is a completely transformed building in the most modern architectural design.”

This further assists with heat pump efficiency, especially in the winter when water can be heated during the warmer daylight hours and used at night when ambient temperatures drop.

This is a hot water storage vessel – condenser water tanks, pipework, and pumps.  An outside air fan unit on top of the gate house, some 50m away, assists in noise reduction.
 The noise and space requirements of traditional ‘all-in-one’ air-to-water heat pumps were completely re-thought at Washington Mews and the previous heat pump and storage vessel plant room was made available for commercial space.

The hot water reticulation rises in the extended ducts to the apartments. Each apartment has a hot water meter. The copper insulated circulation system is provided with thermostatic balancing valves connecting to the basement hot water plant room where the circulation pumps are positioned.

A borehole pumps water to a roof tank where irrigation water serves the garden area through gravity tanks. The future treated wastewater could provide irrigation water.

The separate water reticulation serving WCs is initially supplied with municipal water via WC gravity tanks. This reticulation will be served by treated and sterilised wastewater in the future. After testing and treatment the WC system could also be supplied with borehole water.

Other aspects of this complex plumbing installation include the fire installation serving hose reels and hydrants with a basement tank and a hose reel booster pump and basement booster pumps. 

Owner/Developer: Netwater Properties (Pty) Ltd
Main Contractor and QS: Netwater Properties (Pty) Ltd
Architect: Agora Consulting (Pty) Ltd
Wet Services Consulting Engineers: Wills Franklin Pretorius (Pty) Ltd
Plumbing Contractor: Rehoboth Projects (Pty) Ltd
Hot Water plant and pumps: Elemental Energy Supply Company
Electronic water meters: Kamstrup
Taps and mixers: Viking
Pipes, fittings, sanware: Plumbgo

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