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Composting toilet in a Passive House?

This is probably only a hypothetical question as I doubt I could talk my wife into a composting toilet but…If we have a tight home with balanced ventilation, and if we had a composting toilet that has a small 5W fan continuously exhausting air - how would we provide make-up air?

Asked by Scott Tenney
Posted Jul 24, 2014 7:27 PM ET


4 Answers

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Aerobic decomposition is enabled by an independent air supply and off-gassing is vented vertically (mechanically or passively), above. Avoid back and down drafting by venting the bathroom independent of the house's primary ventilation system. The toilets draft performance will depend on intake and exhaust locations, stack height, etc and should consider the weather-side of the house, prevailing winds, sun and temp affects.

Answered by flitch plate
Posted Jul 25, 2014 12:00 AM ET


A 5-watt fan is likely to have a very low airflow -- less than 50 cfm. (One source recommends a 122-cfm fan for a Clivus Multrum composting toilet). Such a small volume of makeup air is likely to be available through ordinary cracks in the building envelope.

The Clivus Multrum pulls air into the chamber and up the stack from two locations: an opening near the clean-out door, and through the toilet seat area when the toilet seat is up. See the illustration below.


Clivus Multrum ventilation 2.jpg
Answered by Martin Holladay, GBA Advisor
Posted Jul 25, 2014 7:17 AM ET


Scott, with a composting toilet, don't underestimate the frequency with which you will have to clean out the "composted" material, a pretty down and dirty process for many people. With intermittent use (two people) on my power boat, I have to do this about every month or so. Presumably with a larger unit, this period will lengthen, but with additional and/or continuous use, may even be more frequent.

I like my unit, and prefer it to a holding tank, but it does have its down-side.


Answered by Anthony Ratliffe
Posted Jul 25, 2014 3:18 PM ET



I'll probably get shot for saying this because it brings up some complications and I'm a Zehnder dealer in the West. But... I've gotten used to life on the edge.

We have a project in Bellingham WA where the design was to include a composting toilet who's exhaust was picked up by the homes ventilation system provided by a Zenhder ComfoAir 350 HRV. One of the reasons this could work with a CA 350 is that the case leakage, or "cross flow leakage" (one airstream leaking into the other) is less than 1%. Obviously here is a case where you wouldn't want your exhaust coming back at you... There are other HRV's that have as high as 52% crossflow leakage. Something that you would really not want in this application. So some equipment will work, some will not.

So if you can talk your wife into a composting toilet, it can be done. The ventilation system would want to be designed by a factory engineer. Design is still a free service so at least you'd have some qualified back-up for this family adventure.

Thanks! Seeing questions like this makes me smile.

Answered by albert rooks
Posted Jul 26, 2014 12:08 AM ET
Edited Jul 26, 2014 12:16 AM ET.

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