Installing black water holding tank

When I upgraded to a larger tank, the dump/exit flange was already installed. I just made sure it was in a good location for the trailer. And of course that the tank itself would fit properly and could be held in place properly (cut out some old and added some new angle iron across the frame for that)

You make your own cut outs in the tank top for the vent and toilet pipe.

When I dropped the old tank, after removing the toilet and old flange, the vent pipe came down with it. Just cut it off and leave it sticking up in the trailer. Then use the old tank as a pattern for the two holes needed in the top of the new. Glue in the 2 new flanges. Measure how much of the vent pipe you cut off (to get the old tank out). Glue a coupler and the needed extension to the old existing vent pipe left in the trailer. Push the new tank under the trailer, and glue in the modified vent pipe into the top of the tank.

Trickiest part of mounting the new was lifting it up in place with the glued in vent pipe. Two people will help. Before securing it tightly, adjust the toilet hole to line up with the hole in the floor.

Now measure the length of pipe needed from the tank top flange to the floor flange. Mine screwed in at the tank so getting this length right so the floor flange set level with the floor was a little tricky.

In my case, we doubled the size of the black tank, making life a lot easier with extended stays.

you might have to wait until the old tank is down before you can figure out the fittings on the top. I would definitely look into whether the manufacturer can or does provide the tank with the 3" drain already plumbed to a flange. I believe the tanks mostly are side or center drain so hopefully it can be purchased that way. As far as the toilet or vent pipe openings, those are going to have to be measured and cut on site. On mine I glued the fittings on with standard ABS glue. I let them cure for a week before I even thought about installing the tank.

Hopefully your tank comes with the fittings, if not I would use one of these glue on fittings instead of a rubber slip in fitting for the vent. Something like this fitting has a little flange at the bottom so the vent pipe can't slip down into the tank.

Here's a few pointers that I would suggest based on my ONE black tank replacement.

Take a digital picture of the sensor wiring or take good notes.

After you remove the toilet and the flange screws from the floor, the flange most likely screws into the top of the tank. Use a hammer and punch to get the flange turning and unscrew it.

Go up on the roof and remove the vent cap add make sure the vent pipe is free to slide up and down. If it's glued to the tank, it's going to need to come down as the tank comes down.

A new piece of 1 1/2 inch black ABS pipe is only a few bucks at Home Depot. If your vent is just a straight pipe then just hacksaw the old one off and replace with a new pipe.

The flange hole is 4" I bought a $15 hole saw kit from Harbor Freight for this task. Kit


Measure 10 times, you only get to cut once.

Cut the toilet hole first making sure it is far enough away from the edge of the tank that you can mount the flange. Maybe mount the tank and verify the location or mark the location through the hole in the bathroom floor. BUT take the tank down to do the cut. This way can can make sure you're not going to drill right into a corner of the tank or something. I didn't do this and it was close enough to make me remember this tip.

I would glue the toilet fitting on the tank and mount the tank under the floor making sure to center the tank opening under the hole in the floor. Then figure out a way to mark or verify the vent location.

One thought would be to dust the top of the tank in the vent location with baby powder and mount the tank. Then drop a few drops of water from the center of the opening in the roof to land in and mark the powder.

Or maybe you can drop a plumb bob down the vent hole and mark the center of the vent.

Glue the vent flange on the tank and let it dry before installing the vent pipe. On mine I lifted the tank into place and my neighbor lowered the new vent pipe down from the roof. I then snaked my hand up on top of the tank with the ABS glue and smeared it into the fitting and slid the pipe in. I then cut the extra length off the pipe from up on the roof.

Those are the things I can think of. Good luck

Thanks for your reply Avion! Just want to order everything at the same time...cross your fingers for us! :)

karma13 wrote:
We are installing a new black water holding tank and toilet in our TT. The tank is 26x62x7....3" drain and 1" vent. What fittings/grommets will we need for installation?

I'm not sure exactly how yours is set up, but the vent sounds a little small. I would have thought it would be 1-1/2". You would use ABS pipe to pipe it to the roof, staying away from any air intake. The 3" discharge would have the flanges and slide valve. I use the Valterra products for this. I would use 3" ABS piping to the drop, wherever that that is. You would have a hose adapter on the end of the 3" drop. I used a stainless steel clamp for the adapter on my camper so that if I ever have to remove the cover, I don't have to cut the pipe. You will need an adapter to a closet flange on top of the tank, however that is mounted to your floor. There is no way anyone will be able to tell you the fittings you will need without knowing the exact routing and situation that you have.

Remember to grade the 3" drain at more than 1/4" per foot. 1/4" per foot is code, but your trailer may not be level when you are dumping so go a little more grade to keep from having issues.