By Willem Strydom and Ricky Savvides, HVAC and building services engineers at ACS Consulting Engineers

Located in the heart of Rosebank and without compromising on quality and style, The Tyrwhitt offers luxury apartments to investors and professionals in this city’s most desirable ‘work, live and play’ neighbourhood.

This development, actually pronounced “Tirrit” comprises over 200 apartments on 13 Residential floors, including luxury penthouses. The units range in size from approximately 53m2 to 135m2 and are expertly designed to make maximum use of the floor space. The penthouses offer up to 260m2 of elegant luxury, and that luxury is only topped by the magnificent views of the surrounding areas.

A street view of the building located in Rosebank, Johannesburg. Image credit: © RACA Journal | Benjamin Brits

A street view of the building located in Rosebank, Johannesburg. Image credit: © RACA Journal | Benjamin Brits

The Tyrwhitt will also offer its guests further convenience with a restaurant on the street level of the building in future, and currently includes 24-hour concierge, state of the art security and ample underground parking with three underground and two above ground basement levels giving it an international feel in a sought-after commercial and residential zone.

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The Tyrwhitt is located in the heart of Rosebank, directly adjacent to the Rosebank Mall and within 300 metres of the local Gautrain station. The suburb boasts tree-lined and pedestrian-friendly streets and a multitude of amenities for residents to enjoy.

This project was started back in late 2014 and only came to completion in 2020, owing largely to the fact that during the timeline the main building contractor as well as some other sub-contractors went out of business. Of course, several delays came about during the Covid lockdown and restriction period too.

Timing around the project in the beginning stages also played a major role as the developers wanted to wait for the opportune time to get the project started (in terms of supply and demand for this type of unit) and so although design and co-ordinations were already well underway with the architect, the project was put on ice until 2016 – whereafter it basically had to be re-launched with major changes to the design.

The building today, although handed over in 2020, is still in the “selling phase” while other units are used as short-term and long-term rentals as part of an investor-type portfolio model, others are occupied by owners themselves.

Client brief

ACS Consulting Engineers (ACS) was approached by Grapnel Property Developers, to design the entire Mechanical services of the building including HVAC, natural gas, wet services, fire smoke detection and public address systems, in addition to substantial basement ventilation systems including vehicle fume detection aspects. At the start of the design stage, ACS offered several options, one of which included a tri-generation solution for the HVAC, electrical generation and hot water, but this system was unfortunately declined by the client owing to costs.

The reason for this proposal was that this area of Johannesburg has an intricate network of existing gas infrastructure (natural gas supplied by Egoli Gas), but this solution does however include costly up-front capital investment on the equipment side, so benefits are only realised down the line.

Part of the client brief was that each unit would receive gas in the form of cooking points and therefore would require adequate ventilation to manage the standards associated with gas that would run throughout the building. As part of this project too, ACS was to incorporate different air extract systems and basement extraction (being underground), where implementation would handle the building requirements for extraction of smoke and general ventilation.

Only the penthouses would receive their own dedicated air conditioning systems as requested by the client/developer.

The VRF condensing units located at the rooftop plant area that serve the penthouses. Image credit: © RACA Journal | Benjamin Brits

The VRF condensing units located at the rooftop plant area that serve the penthouses. Image credit: © RACA Journal | Benjamin Brits

The HVAC system

This project included to a large degree a complex ventilation system. This involved the basement extract system that ACS had to design which incorporated smoke clearance and general ventilation into one system – making this less complicated, but also more cost and energy efficient. Basement areas were also fitted with CO and NO2 sensors. The basement extract supplier also included computational fluid dynamics (CFD) in their service. NO2 sensors were included because of the growing number of diesel vehicles in use where NO2 is a by-product of these combustion engine types, while CO is a by-product of both petrol and diesel engines – thus catering to both scenarios.

The basement also included the installation of smoke curtains serving four zones in this area (less than 2000m2 each). If there is a fire in one zone, the fire curtain will drop and isolate that zone to eliminate the spread of fire or smoke to other zones, which then allows for the process of evacuating occupants safely. These work in conjunction with each zone’s extract/jet fan system. The basement smoke clearance system was designed according to SANS10400 part T4.42, EN 12101-3, SANS 1238 and BS 7347-part 2, part 7 section9.

Traditionally in smoke extract conditions, if inadequate extract venting is achieved, the smoke temperature exceeds the maximum of 300°C and can reach in excess of 600°C which will then cause flashovers. If the oxygen levels are very low, backdraught will occur hence oxygen introduced suddenly at a late stage may cause an explosion, hence the inlet of fresh air via the ventilation shafts.

All of the staircases had to also be pressurised as fire safety requires that one pressurises any escape stairs to eliminate ingress of smoke into these zones. The pressurisation of the staircases was calculated and designed according to SANS 10400 Part T and EN 12101.

Part of the Ventilation system also included the management of air using an ozone unit. This site has a main refuse yard where the dustbins get stored before removal. Obviously, this type of area produces unpleasant smells for residents as well as the community generally, so to combat this the project included extract fans and integrated an ozone treatment system to supply ozone into the refuse room. Ozone has the property of neutralising unwanted smells.

An ecology unit was installed that caters for the restaurant kitchen extract canopies so that air can discharge at low level (the kitchen will be located on the ground floor and is part of a future phase). The ecology unit cleans up extracted air associated with the cooking of food (removes smells and grease/oil/fat being extracted from the kitchen canopy). The ecology unit is a packaged unit with its own fan, grease catchers, carbon filters and UV lights. This was a good solution for this project because without low level air discharge, ducting would have to be added all the way to the roof (15 floors from the Restaurant to the roof) which also holds significant costs and design implications.

Not only does the natural gas feed to each unit of this building, it is also used to produce hot water for the building through two boilers located in the basement. In addition to the gas stove of the regular units, the penthouses are fitted with gas braais as well as gas fireplaces. Gas installations such as those found at this project need to include cross ventilation – so basically an inlet grille at low level, and an outlet grille at high level to produce natural ventilation in each apartment. The size of the natural ventilation grilles was calculated using SANS 827.

The penthouses needed air conditioning and so a VRF heat pump unit was selected and installed individually for each of these apartments. It would not have been cost-effective to select one central system for all the apartments for several reasons – mainly because this solution would be difficult to build at this site and then secondly a challenge would arise to monitor energy consumption and hence billing, so ACS opted for the individual heat pump VRF per penthouse unit solution.

This development also required significant bathroom extraction too, as almost none of the bathrooms were located at points where openable windows were possible and so this function had to be aided by means of mechanical extraction. A total of 13 extract fans are located on the roof. Each fan is connected to multiple riser ducts, each serving multiple apartments vertically aligned.

The extract ducting was internally insulated with sound absorbing material from the fan to the bottom of the 12th residential floor. This was done to reduce the mechanical noise from the fan itself and also to reduce the air noise from air moving through the ducting; especially since all the main extract fans are located near the penthouse units.

Project Challenges

During the hold period between 2014 and 2016, the owner and architect had some different ideas based on market conditions and so made significant changes to the building and also added an additional floor. This meant that ACS had to change and amend all of their drawings on Revit (which is actually not as easy as people think). 

As the scope of the building changed another challenge was the fact that one of the penthouse units wanted a custom-designed unit rather than the pre-designed specification. This then required that all of the air-conditioning be re-designed and take into account the interior designer’s layout needs to meet the clients aesthetic expectation. Wall-mounted units were changed to hideaways and changes to the sizes and locations of these units and ducting layouts all had to be updated.

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Although a tri-generation system was proposed for this project that was declined, it is still considered best practices to specify the most efficient products and to follow standard building codes and regulations.

With the basement extraction system, in reality, you won’t need it operational all of the time as the area would not have continuous traffic in and out the whole day. ACS designed the system to incorporate the CO and NO2 sensors to monitor the air quality so if there is a high CO/NO2 level in the basement, then the jet fans switch on and the main fans increase their speed, otherwise they are in an idle mode, thus saving energy.

Unique elements

The particular unique elements to note were working on a project of this magnitude and the incorporation of all professional services with one company – smoke extraction, ventilation, air conditioning, natural gas, wet services and fire elements. 

The complexity of such a ventilation system was also unique to this project. The handling of foul odours was also somewhat a unique aspect by using the ozone treatment and an ecology unit.  

List of professionals

Project name: The Tyrwhitt
Owner: Grapnel
Developer: Grapnel
Architect / Designer: GLH
Project manager: OnTrack

Consulting engineer

Electrical: CPE Consulting
Mechanical: ACS Consulting Engineers
Wet services: ACS Consulting Engineers
Civil: Knutton Consulting
Structural: Knutton Consulting


Main building: Probuild Construction Group (Started project)
Main Building: Barrow Construction (PTY)LTD (Completed project)
HVAC: Ampair HVAC Services
Wet services: Vic Ball Plumbers
Electrical: MLE

HVAC and associated product suppliers

Samsung – Air Conditioning equipment
Flakt-Woods – Basement Ventilation Fans
Colt – Smoke Curtains
General Ventilation Fans – Donkin
Ozone Generator – Azure
Pumps – Grundfos
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