Ok, folks, here's the moment you've all been waiting for (I flatter myself)...
We've been using our flush composting toilet for 4 months and I'm here to give an update and post a video of the build. The video is really hokey and repetitive cuz I've never done one before, but at least you'll get to see what we did.
First for the update:
During the summer & fall, it worked like a charm!! That's it. The whole update. No problems whatsoever. And then there was winter.
When winter set in, I had to come up with yet another brilliant system to ensure our holding tanks didn't freeze. Here's what I did:
I noticed that the exhaust port of our propane-burning furnace was pumping out burning hot air. I accidentally left a plastic thing near the exhaust and it melted, so, pretty hot.
Our trailer already has the cold-weather package but it's only good down to about freezing so I needed to keep the underneath of the trailer warm to prevent the fresh, black and grey water tanks and water lines freezing.
So I bought some flexible aluminum ducting like you put on your dryer (not the plastic stuff!), long enough to reach from my exhaust port, and stretch all down the center of the underneath of the trailer, with detours looping under areas like the kitchen and bathroom, and ending under the black water tank and drain portal. The opening of the duct would exhaust out into the open air so as not to build up carbon monoxide under the trailer.
Then I wrapped pipe insulation around the duct from the exhaust port down to the ground and under the trailer a couple feet. Then I salvaged some rigid ducting and a couple elbows that were about 2 inches bigger diameter and enclosed the flexible duct (that was a bit*h, I'll have to figure out an easier way to do that) down to the end of the insulation. Then I used baling wire to attach the opening of the duct over the exhaust port and turned on the heat to make sure the airflow was unobstructed and that the insulated portion of the duct wasn't going to get too hot. It didn't; I could easily keep my hand on it at the highest temp it could produce.
Next, I went under the trailer and checked that the temp of the uninsulated duct wouldn't be so hot as to melt the goodies on the under side of the trailer--that was all good too.
Now comes the messy part. My daughter and I stacked bales of straw all around the outside of the trailer to close the gap between the trailer walls and the ground, to retain the heat from the furnace exhaust. We draped them with an old insulated pool cover I got for free, to keep them dry. We had laid down scrap 2x4's covered with tar paper ahead of time to keep them from absorbing rain and snow melt from the ground. Yes, I could have bought some fancy insulated skirting and spent hours measuring and cutting and making it beautiful--and maybe I will do that at some point but we were running out of time and money so...
Anyway, that worked beautifully for a while, then I realized that moisture from the hot air was condensing in the duct and causing ice dams. So I went underneath and poked some holes along the bottom of the duct to let condensation drain out. I wasn't that worried about CO2 escaping the duct work at that point cuz I didn't notice at first that the ice dams had formed and our CO2 sensor never sounded to warn us, so the CO2 that was escaping from the seams in the duct was not entering the living space. Besides, hot air rises, right? The hot air and fumes were traveling along the top of the duct and the holes were in the bottom.
The hole-poking completely solved the problem and we were back in business! Our tanks never froze and we were able to drain the black and grey water tanks at will. Right up until something crawled into our furnace air intake and we had to switch to space heaters until we could get it resolved. But up until then, it worked great!!
And now, the moment you've ACTUALLY been waiting for...here's the video of the flush composting toilet build: