Passive Ventilation Design Parameters Guide

Composting toilets can be accessorised with a wind-driven vent for passive ventilation. This can be employed as a redundancy measure for power outages or fan failure, or as the primary means of ventilation to make composting toilet operation completely power-free.

Passive ventilation requires a good degree of air movement. Wind-driven vents perform best when installed in areas with high wind, such as coastal and mountain locations. However, if you're in a low wind location, you can maximise the efficacy of passive ventilation with the following design parameters:

Vent Stack Pipework Size

Optimise the performance of the wind-driven vent by creating the right conditions for solar-thermal climb, which creates an updraft.

• Use pipework that is at least 100mm wide. 

Solar-thermal gain needs surface area. 100mm pipework provides sufficient surface area and internal laminar flow to create the necessary updraft. It is impossible to get passive ventilation in smaller pipework.

Vent Stack Length

The vent stack needs to be sufficiently sized to catch air currents. Ideally, it should be extended above the apex to access crosswinds. However, this may require a large vent stack - at a minimum, extend your vent stack to:

• 1m above the roofline if located at the eave.
• 600mm above the roofline if located near the apex.

Anything below these recommendations will experience luff (stilling of air movement).

Vent Stack Position

Maximise the vent stack's sun exposure to create solar-thermal climb.

• If possible, place the vent stack on the northern side of the building.
• If not on the northern side, ensure at least 1m of the vent stack above the roofline is exposed to the sun. 

1m is the minimum exposure to heat energy necessary to create solar-thermal climb. If the vent stack is on the southern side, the duration of sun exposure relative to the pitch of the roof and shadowing needs to be evaluated.

Vent Stack Colour

Maximise solar heating.

• Paint the entire vent stack black (or another dark colour to suit your cladding/roofing).

Need help choosing a system? Call WCTNZ® on 0800 022 027 for free advice on system specification and setup. Advanced design consultancy services are also available.

Copyright © 2025 Waterless Composting Toilets NZ Limited (WCTNZ®). All rights reserved.

This content has been reviewed and approved by Dylan Timney, Managing Director of WCTNZ®, who brings over 17 years of composting toilet expertise and 16 years of experience in building and eco-construction in New Zealand.

Last reviewed: July 10, 2025