Clearance needed between shower drain bottom and waste water tank?

[Timeline]

Found these on Amazon, very low clearance needed, but in the reviews people say they need regular cleaning.
https://www.amazon.com/Wirquin-Prof...62edd&pd_rd_wg=O6Tb9&pd_rd_i=B00KEUQVGS&psc=1

 

[DrDanger]

You might consider a HepVo valve in place of the "P" Trap. Driving will slosh alot of the water out and heat will evaporate the rest faster than you think, and then stink. HepVo gives you a few more inches of clearance to drop the pan another 3-4".

Well, you were of course correct. The water did slosh out, leading to some unpleasant smells. I threw in one of these <https://www.homedepot.com/p/Oatey-2-in-Seal-Drain-for-Showers-and-General-Purpose-43745/315169787> and that has worked, but for those currently designing/building just do yourself a favor and use a HepVo.

 

[DrDanger]

Theoretically the shower won't back up until the gray tank is 80% full, but I am not sure how fast it will actually drain. I finished the tile and whatnot yesterday so my first test was today. It was draining slowly and backing up into the shower until I went out and opened the drain for the gray tank. I don't know if this means the gray tank was too full, or if I need to put a vent on the shower catch tank itself. More testing to follow.
If this system doesn't work, I have a macerator pump that I will use to drain the shower tank and pump it into the top of the gray tank via a clean-out inside.

UPDATE: grey water never backs up into the shower even when the grey tank is completely full WOOHOO! This means I don't have to use the pump after all. Also, I am not sure why the shower drained slowly at first, it is totally fine now. I may still drill a small vent hole or two in the side of the shower catch tank just to prevent negative pressure from pulling the water out of the shower P trap.



Now if I could just get my water pump to give me enough shower flow to allow the tankless water heater to work without running the faucet at the same time that would be great...

 

[Trooper2]

UPDATE: grey water never backs up into the shower even when the grey tank is completely full WOOHOO! This means I don't have to use the pump after all. Also, I am not sure why the shower drained slowly at first, it is totally fine now. I may still drill a small vent hole or two in the side of the shower catch tank just to prevent negative pressure from pulling the water out of the shower P trap.



Now if I could just get my water pump to give me enough shower flow to allow the tankless water heater to work without running the faucet at the same time that would be great...

The tankless heaters we all buy for skoolies are similar to what was available for houses in 2005 -2006. Technology is pretty old school compared to the new ones it's why the price point is so low.

I installed a few off grind units back then that used water flow to generate the ignition spark ( great idea but not for low flow) on those units I removed the tiny generator and installed a battery pack. If there was still a flow issue I would start with getting a little more flow from the shower head by removing restrictors, then upsize the pump if it still had issues.

A little bit of tuning and you'll be showering with hot water in no time.