TRURO  METHODIST CHAPEL
EXTRACT VENTILATION SYSTEM









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. 

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.

 


 

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.



 
During 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.


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