Air duct and ventilation system inspection

Investigation of existing air ducts and ventilation system within a 1910, grade A listed, Old Town Hall with a large extension. Survey for Rare Architects on behalf of their client.


We have carried out a number of air duct and ventilation system inspections. One such inspection was conducted was at a large grade A listed building.

The Old Town Hall was built in 1910, with a large extension added in 1937. In its history the property served as a bank, a town hall, a council office and conference centre and is currently used as a film set for various film and media companies. The scope of this inspection was to identify the location and dimensions of existing ductwork at three locations within the building. The council chambers in the old section and the meeting rooms and conference hall in the new section. The goal was to identify the ducts and follow their route through the building to determine their relationship identify approximate dimensions and any changes noted within the ducts.


We carried out an investigation of the existing air ducts within the property as requested and specified by Rare Architects on behalf of the client. The investigation was carried out using an Olympus IV6C635 Video Probe and Everest CA Zoom Camera and monitor station in accordance with in house company procedures. No break-in technique were used to provide openings for the camera systems. Grills screwed onto the walls were removed and replaced without damage to any surfaces. The findings of the inspection were as follows: The Former Council Chamber

The point of entry at each location was an existing metal grill, fixed onto the existing wall surface. These grills were located just above floor level at the bottom of what appears to be a column in the wall. There was a total of seven locations in the hall, situated around three sides of the hall, the fourth side being an elevated stage area. After removing the grills and looking to the left and right, there was no evidence of any duct or covered flue system that could be used to transfer air around the hall. The grills appear to provide ventilation to the underside of the suspended floor and do not go above a line at the approximate height of the skirting board. The void area has various items of debris such as bits of wood, piping and general rubbish at various points around the areas inspected. The Conference Room

The existing ducts in this room were against the wall on the North and South sides of the room. They were located under grills below the window line, one at each side of each of three windows. This resulted in six ducts along each wall. The ducts were 175 x 360mm in section. Each duct ran past the conference room floor level, straight to the basement where it collected into the main system and was fed direct from the plant in the basement area. There were no other openings or breaks at other floor levels, the condition of the duct material was very good with no obvious sign of any splits, deformed or stressed sections. The grills at floor level along the seating arena appeared to be ventilation for the podium level seating. When you looked through the grills all you could see were the dwarf walls supporting the seating levels at different heights. There was no evidence of duct systems or of any duct leading away from the void area through the wall. The Committee Rooms

There was a central corridor that split the building on an East West axis. The corridor was interrupted by a main stairway up to the first floor level. There was a false ceiling construction in the corridor that is split into an East section and a West section. Each of these sections was fed from plant rooms on the roof level. The only section of duct was from the plant room in each case to an opening in the roof level. It did not extend any further down the building. The duct was feeding air into a masonry shaft, one in the East side and one in the West side. The shaft in the East side went down to the first floor and stopped, the shaft in the West side went down to the basement.

There was other plant room on the roof that fed down into three rooms on the second floor and nowhere else. These rooms were just to the left of the South East corner of the building, on the second floor only. In some areas of the ceiling void it was possible to see where an active corrosion cell had been established and the reinforcement in the concrete slab had corroded. On grid line C3 – D4 there was a redundant “Dumb Waiter” shaft that was also being used to provide ventilation into the false ceiling void. It appeared to run from the ground floor to the soffit of the second floor where it had been sealed off with a wooden partition.

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