Along the ridge of the red tiled roof are 48 brick funnels topping internal stacks which pull the exhaust air out of the seven floors of offices below. Under the office floors is a mezzanine plant room behind the cross chevron screen where 32 banks of low and high volume fans draw air from the atrium through filters. This air is pushed up through the supply section of vertical ducts in the central spine core of each office wing. From the duct the air is fed through the hollow floors to low level grilles under the windows. As it is warmed by human activity it rises to the vaulted ceiling where it is sucked out via the exhaust ports at the end of each vault through a system of masonry ducts to the exhaust sections of the central vertical stacks. In the office space uplighters use the concrete vaulted ceiling to reflect light downwards and to absorb their heat.

The sandwich of the vaulted ceiling and the voided floor above acts as a heat exchanger. The cold night air passing through the void festooned with concrete teeth removes the heat of the previous day and on the following day warm external air is cooled about 3°C by the same teeth before entering the room. Normally the high volume fans run at night to give ten air changes per hour and low volume fans run during the day giving two air changes per hour. By timing the change over from low to high air velocities the optimum use of the diurnal swing of the biosphere can be utilized.

The engineers, Ove Arup & Partners, have installed a data logger which continuously records air temperature at five critical positions.

Eastgate uses 35% less total energy than the average consumption of six other conventional buildings with full HVAC in Harare. The saving on capital cost compared with full HVAC was 10% of total building cost. During the frequent shut downs of mains power, or of HVAC due to poor maintenance in the other buildings, Eastgate continues to operate within acceptable comfort levels with its system running by natural convection.

The peak internal temperatures recorded at 4:00pm on occasions during 2 to 3 weeks of the whole year have been up to 27 - 28°C on certain days within that period. Usually during the rains in November onwards cloudy nights are followed by cloudy days and the internal office peak temperatures remain below 26°C.
People and machines add 1.5°C to the internal temperature each day which if not flushed out each night tends to accumulate until the week ends when the building cools down sufficiently. However, by altering our control system we have reduced this week day build up.
The difference between low level and high level temperatures in the room is 1.5°C due to stratification in a 3m high room.
The challenge for improvement is in the following two areas.
a) Control system which maximises the use of advantages provided by the unpredictable outside air temperature swings.

b) More research into designing of the heat exchange inside the concrete floor slab, involving studies of the behaviour of air and concrete surfaces to optimise heat transfer.

Already Eastgate’s success can be measured by comparing the annual energy consumption with other conventional buildings in Harare. The bar chart shows that Eastgate consumes less than 50% of the energy used in conventionally air conditioned buildings while achieving very satisfactory comfort conditions for all but 2 weeks in 52 weeks of the year.