TRURO
METHODIST CHAPEL
EXTRACT VENTILATION
SYSTEM
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The Chapel was built in
1830 and designed by Mr Sambell the Architect who
remarkably was deaf and dumb.
In its early years it was heated by a warm air stove
installation. During the recent refurbishment works the underfloor
builders work ducts which led from the warm air heating
stove were discovered whilst the flooring was being
removed. A later restoration in 1885 was
by Sylvanus Trevail a local Architect who left a legacy of many fine
buildings in Cornwall. These alteration works appear to have included
the installation of gas lighting, raising the roof and the erection of
a new ceiling. As
the congregation at this time was in the region of 2000 this must have
led
to the need for a ventilation system to be installed.
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For the Chapel a new innovative type of
ventilation system was installed which used the venturi effect to
assist in extracting air. This passive ventilation system was installed
within the roof space, and used centrally sited rising air ducts to
induce air from the
perimeter extract ducts.
Circular grilles in the ceiling sited
along the centre line of the Chapel allowed the warm air
rising from the congregation and from the gas lighting to pass through
and be collected by the circular hoods in the roof space sited above
the ceiling grilles.
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This extracted air is then accelerated by the
effect of a venturi whereby the ducting is decreased in cross-sectional
area to
increase the
velocity of the air. This air increased velocity was then used to
induce
and draw in extracted air from the perimeter ducting in the roof space.
The perimeter ducting is continuous and its air inlet is connected to
the
decorative
mouldings at the cornice edge of the building.
Fixed
lengths of rectangular perimeter ducting in the roof space are each
connected by circular ducting which then connects into the main riser
ducts
above the venturi reductions. The vitiated air then
escapes to
atmosphere through circular turret vents sited along the ridge line of
the roof.
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large congregations the high internal height of the Chapel
helps to increase the stack effect within the building, and thus
increase the
quantity of air drawn into the roof ducting. |
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To compensate for
and balance the amount of
extracted air, fresh air inlet boxes (each complete with a flap control
damper)
are installed in the majority of the window recesses at both
floor
levels. Additional boxes are fitted flush within the
thickness
of the external walls where access is restricted. These air inlet boxes
are similar to the Tobin
Tubes
as illustrated in the Walter Jones book ‘Heating by Hot Water’ third
edition
published in 1904.
The
ventilation system was effectively made
redundant
during the 1960’s when the circular ceiling grilles were frosted over
and
lighting installed in their positions,
Architects The Jerry Kent
Consultancy
of
Barnstaple during their refurbishment works carried out in the year
2000
changed the
type and method of lighting in the Chapel which then enabled the
central
ceiling ventilation grilles to be reinstated and reused for their
original purpose.
This then allowed the extract ventilation system to be brought
back
fully into use.
Early
indications of the effectiveness of this
passive ventilation system, when the Chapel has held around a capacity
of 1000
people, have showed the performance of the system to be acceptable.
Prior
to the reinstatement of the system the internal atmosphere was
oppressive with surface condensation occurring on external walls
indicating minimal
air movement which produced a stagnant internal environment.
Our ancestral
Victorian engineers
without
the benefit of electricity devised and installed an effective extract
ventilation system for the Chapel which after a brief period of
disuse has now been brought back again into operation 115 years after
its initial installation.
CIBSE HERITAGE GROUP
APRIL 2002
