Appliances on board that use LPG, coal, oil, wood or other carbon based fuels require an adequate supply of oxygen to burn efficiently. Inadequate ventilation will starve the burners of vital oxygen resulting in poor burning. Poor burning can produce highly toxic carbon monoxide. Carbon monoxide is a killer at worst and can cause severe brain damage at best.

Room sealed appliances take their combustion air supply from specific external flue ducting. However all other appliances, flued or not, will take their combustion air supply directly from the cabin space.

The formula used to calculate ventilation takes into account the number of people on board as well as the number and type of appliances installed. It uses the output rating of open flue solid fuel appliances and the input rating for all other appliances, distinguishing between those that have flues & those that don’t.

This rating is kW, or kilowatts, and this can usually be found on the manufacturers plate on the appliance, in the operating instructions or from the internet.

Be honest with yourselves and list the maximum numbers of people who potentially will use your boat. It’s better to be cautious and safe when dealing with carbon monoxide.

BSS recommend fixed ventilation is split as equally as practicable between high level and low level outlets. The BSS examiners will not include vents that can be closed without the use of tools and on adjustable vents such as roof mushrooms, the vent will be measured in its most closed down position.

High level is ideally in the cabin roof and commonly takes the form of mushroom vents which often can be adjusted. Ensure yours have a screw stop installed so they cannot be fully closed.

Low level venting can be achieved by letting in cold air through louvre vents in doors, bulkheads or by ducting from a higher level. Cabin bilges should also be open to unrestricted ventilation to prevent the formation of mould from excessive moisture and to protect against the build up of gas in the unfortunate event of system leak.

The BSS examiner will carefully examine any fixed ventilation for fly screens or louvres & carefully measure these potential restrictions so as not to render calculations inaccurate. If you are fitting out a sailaway canal boat, ensure you accurately calculate the amount of ventilation you require as to retro fit ventilation if you fail your BSS would not be time consuming and messy!

The commonest form of roof ventilators are known as mushroom vents. They are adjustable on a screw thread and are available in chrome, stainless or brass finishes. They are prevented from being removed by a nut on the end of the thread, inside the roof space. Check periodically that the nut has not vibrated loose, especially if you have an electric vent attached to the mushroom.