I just finished testing how well our black and gray water tanks cross-feed with a Valtera twist-on valve. Schoolsout2 may find these results most useful, because they have the same motorhome as ours, but others may find it helpful too - especially if you boondock a lot.
Our black tank holds 51 gallons and our gray tank holds 103 gallons. When boondocking, our black tank fills up long before our gray tank, because we have two macerator toilets (reprogrammed for low water flow) and the half-bath sink drains into the black tank. We rarely take showers when boondocking so the only water that flows into the huge gray tank is from brushing our teeth in the rear bathroom, and from washing dishes in the kitchen sink (We primarily use paper plates) In other words, not much water goes into the gray tank.
The first thing I did was drain the tanks until they indicated empty. We have external ultrasonic tank level sensors, so the indications are fairly accurate. I filled the black tank with fresh water (through the cleanout valve) until it indicated 3/4 full. It was actually a little more than 3/4 full, because it took me a minute to get to the spigot and shut the water off. I closed the Valtera valve and opened the black and gray tank's dump valves. As expected, a lot of water started flowing out of the black tank and into the gray tank. I could hear it and see it in the 45 degree clear elbow I installed. When the water stopped cross-feeding, I went inside and checked the tank levels. Both tanks now indicated 1/4 full.
That means that roughly half the black tank's capacity cross-fed into the empty gray water tank. Obviously, if the gray tank already had some water in it, not as much black water would have flowed into it, but as I said earlier, our gray tank rarely has more than 15 or 20 gallons in it - even after several days of boondocking - and I didn't start this test with a full black water tank. A full black water tank would have forced more black water into the gray tank.
The amount of black water that will transfer into the gray tank will obviously depend on how much gray water is already in that tank. Even so, I think this is going to significantly increase the amount of time we can boondock without having to dump. Not bad for a $26.00 investment. I could be wrong, and if so, I'll deal with it, but I'm really not concerned about solids not flowing out of the gray tank. Our macerator toilets chew things up before they get to the black tank anyway, and the only "stuff" I've ever seen flow out of our black tank through the clear 45 degree elbow are liquids.
I hope this helps.
Kev
