Ringing Room Air Extractor

by Mike Redfern & Alan Rowe

Requirement

Fan

Board Layout

Requirement

When ringing was allowed to start again in May 2021, we tested the effectiveness of the ventilation in the ringing room using a carbon dioxide monitor. This showed that the ventilation was adequate with all three windows open, but it was obvious that this would not be viable during colder weather.

As an alternative, we set out to design a means of extracting air close to the ceiling, reducing the build up of aerosols and allowing the other windows to be kept close or only partially open. We decided that the best way to do this was to mount an extractor fan to a board that could be mounted in place of the largest window.

Design considerations were:

It should be possible to easily remove the board - the window is also an emergency exit.
It should be possible to fit the board without needing to remove the window.
The noise level should be low enough to not disturb ringing.
It should be possible to leave the bord in place for extended periods exposed to the elements.
The extractor fans can be rotated to allow the units to blow air in if needed
The unit should be powered by connected to the mains supply via a suitably fused three-pin plug.
It should be possible to easily modify the extractor to blow fresh air in to the ringing room.

Fan

We initially looked at fans intended for window mounting, but these either had relatively high noise levels or could only extract low volumes of air. Alec Humphrey was undertaking a similar project at Sandiacre, and decided to use a Manrose MF100T. This is designed to be mounted above a ceiling but can still be mounted on the board. It can achieve an airflow of 245 cubic metres per hour. Our ringing room is approximately 67cubic metres, so this delivers about 3.5 air changes per hour. The unit runs with a noise level of just 24dB, although this can be as low as 20dB with a reduced airflow of 180 cubic metres per hour.

The size of the fan meant that it was possible (just) to mount two on the board. We initially chose not to do this, but ensured that the design allowed a second fan to be easily fitted at a later date. As well as allowing us to double the volume of air extracted, it would also allow the option to have one fan extracting air while the other is bringing in fresh air.

Unlike a fan designed for window mounting, it is necessary to separately purchase grilles and ducting. We used the following:

1 x 140mm square gravity grille with 100mm outlet. This is for mounting on the outside of the board, and closes automatically when the fan is not in use.

1 x 100mm circular grille. This is for the air intake in the ringing room.

4 x 100mm duct clamps. These are large jubilee clips used to attach the ducting to the fan and grilles.

1 x 3m length of 100mm flexible PVC ducting.

Board Layout

View from inside the ringing room (not to scale)

Side view (not to scale)

Due to limited space, allowance was made for the second fan to be be fitted slightly higher than the first one. The fans can be removed from their mountings by releasing two retaining clips, allowing them to be reversed to change the airflow direction without unscrewing the mounting from the board. The red dotted lines in the diagram show the position of the retaining clips when open. If the fans were mounted level with each other, then it would not be possible to open them.

The window is hinged on the right hand side, so additional space is needed on that side to ensure that the board can be fitted into the recess while the window is open.

The window opening is 835mm high x 410mm wide. The board was made using 18mm marine ply and was supplied CNC cut to the exact size. This proved to be too exact, and needed to be made slightly smaller to allow it to be easily removed. To make the best use of the material, the supplier offered a discount if the direction of the grain was not specified. Given that this is not really relevant for plywood, we suggest that this option is always used.

The board was treated with 18mm Ronseal wet rot protector and painted with black paint to provide weather protection.

Although only one fan was being fitted, holes were made for both outlets to simplify the fitting of a second one at a later date. The centre of the holes needed to be at least 85mm from the edge of the board to ensure that the edge of the grilles did not foul the the rebate on the outside edge of the window frame. The circular grill intended for use on the air intake has been used to temporarily block the spare hole, and will be replaced with a second gravity grille.

The board after cutting holes for the outlets, treating for rot, and painting.

The board with grilles fitted.

Fixings

To hold the board in place while allowing it to be easily removed, we used four brass Fitch fasteners, normally used to secure sash windows. These are effectively rotating circular wedges. The rotating parts were fitted to the board and the slots into which they locate fitted to the window frame. As the board is 18mm thick and the rebate for the opening pane is 45mm deep, additional pieces of wood were fitted at the top and bottom of the board to increase the thickness to 50mm - the additional 5mm ensures that the catches hold the board securely in place.

Completed Assembly

The board with the catches, fan, grilles, ducting, connection box and wiring fitted. A short length of the ducting has been used to connect the fan to the outlet, the remaining length allows the intake to be at a high level.

The board in position with the window open.

With the fan in use and the other two windows closed, the CO2 levels indicated that the ventilation was still not adequate. Opening the windows a small amount allowed sufficient fresh air into the ringing room to resolve this, while still allowing the temperature in the ringing room to be kept at a comfortable level. To prevent direct draughts, temporary mesh curtains have been fitted over the windows.

Fitting a second fan

We subsequently decided to fit a second fan, increasing the capacity to 7 air changes per hour. While the single fan was sufficient for the numbers of people expected at our weekly practice and service ringing, this would allow larger events such as association meetings to take place. A switch was also fitted so that it was still possible to operate the unit with just a single fan working.

While the fitch clamps hold the completed assembly securely, a short chain has been fitted between the top of the board and the window frame in case they become loose. The chain can easily be removed, due to the window being an emergency exit.