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<pre>* Cubic feet per minute (CFM) or litres per second (L/s) of outside air brought into the boat* CFM per person: CFM/p* CFM per unit floor area: CFM/ft2* Air changes per hour: (ACH)</pre>
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= Ventilation Calculation =
== Summary == Ventilation is required to replenish oxygen removed by people and sources of combustion, and to dilute odours and pollutants. The most effective method is an integrated HVAC system with air distribution and local controls in each cabin. But without pass-throughs for air ducts an integrated ventilation/humidification system is impossible. A water system for distribution minimises the scope of pass-throughs in water-tight bulkheads but like many design decisions it forces trade-offs and the use of local ventilation.
== Method ==
People prefer a relative humidity of 30 to 50% and find anything much higher to be very uncomfortable.
Unfortunately choosing in favour of a water system for distributing heating and air cooling has further consequences. A water system minimises the scope of pass-throughs in water-tight bulkheads but like many design decisions it forces trade-offs. It made makes an integrated ventilation/humidification system impossible.
The alternative to running fairly large air vents the length of the boat is local ventilation in the main zones of the boat. This is far from ideal. In both summer and winter the air intakes will be working against the air conditioning and heating systems, respectively, and deck-mounted dorades for intake and return air are multiple hull openings. The ventilation system must be designed carefully to minimise these risks of water entering.
If you plan to spend your time in hot humid climates, you should consider a solution that incorporates a dehumidifier.
=== Ventilation Rates ===
Ventilation rates can be expressed in several ways:
Standards for ventilation differ, and have varied over time subject to lobbying, energy efficiency doctrines and the emergence of sick building syndrome. A reasonable yardstick is somewhere in the range of 0.5-1.25 ACH or, more precisely, 1.0 ACH translating to around 1.66 CFM per 100 cubic feet of cabin volume. You can double check this to ensure at least 15 CFM/p.
Maximum air velocity in ventilation ducts and vents should not exceed 2.6-3.3 ft/s (feet/second) (0.8-1.0 m/s) to minimise noise and differentials in air pressure. In comparison, air ducts for combustion systems can run as high as 40-66 ft/s (12-20 m/s).
=== Working an Example ===
Let’s work a complete example. Assume a salon of 1280 cubic feet. At 1.0 ACH this requires 21.3 CFM: