Unik 17' black water tank upgrade - Fiberglass RV


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Old 06-22-2020, 08:02 PM   #1
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Trailer: 1981 Trillium 5500
Quebec
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Unik 17' black water tank upgrade

I'm currently helping my cousin to rebuilt his 1979 Unik 17ft fiberglass trailer. Due extensive floor rot, it's a total gut and rebuilt interior job.
This trailer has a wet bath (shower/toilet combo) The toilet base incorporates a small above-floor black water holding tank. I think some Scamps and BigFoot trailers have a similar set up.
There are no other underfloor tank.
According to our rough measurements, the black water tank under the toilet holds about 5.5 US gallons, which isn't much (my Trillium tank holds 15 gal).
While we're replacing the entire plywood floor, we thought it would be a good opportunity to install a proper, higher capacity, under floor black water tank.
All RV/trailer toilets I've seen are mounted directly above the holding tank, and flushing sends the content straight down in the tank, and the tank is mounted under the floor between the main frame members. In this Unik, the way the toilet and shower pan is made, the toilet sits right in line with the trailer frame. Under the trailer, the tiny tank drains from the bottom, through a hole in the floor near the edge of the trailer side, on the exterior side of the trailer frame. If we install a regular black water tank, to only way to connect it to the toilet would be by using two 90deg elbows in the pipe right under the toilet: one sideway to clear the frame, then one down to the tank.
Would you think using two elbows cause problems with the toilet (clogging or whatever) ?

We're still debating if installing a BW tank is worth the cost and effort, or if it's even possible. Any comments appreciated, including opinions on the 5.5 gallons limitation if we decide to reinstalled everything as it was originally.

Thanks
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Old 06-23-2020, 10:46 AM   #2
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You can't go wrong with more capacity and you should be able to do it in the same area.

Good luck with your choice.
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Old 06-23-2020, 02:18 PM   #3
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Originally Posted by Carl V View Post
I'm currently helping my cousin to rebuilt his 1979 Unik 17ft fiberglass trailer. Due extensive floor rot, it's a total gut and rebuilt interior job.
This trailer has a wet bath (shower/toilet combo) The toilet base incorporates a small above-floor black water holding tank. I think some Scamps and BigFoot trailers have a similar set up.
There are no other underfloor tank.
According to our rough measurements, the black water tank under the toilet holds about 5.5 US gallons, which isn't much (my Trillium tank holds 15 gal).
While we're replacing the entire plywood floor, we thought it would be a good opportunity to install a proper, higher capacity, under floor black water tank.
All RV/trailer toilets I've seen are mounted directly above the holding tank, and flushing sends the content straight down in the tank, and the tank is mounted under the floor between the main frame members. In this Unik, the way the toilet and shower pan is made, the toilet sits right in line with the trailer frame. Under the trailer, the tiny tank drains from the bottom, through a hole in the floor near the edge of the trailer side, on the exterior side of the trailer frame. If we install a regular black water tank, to only way to connect it to the toilet would be by using two 90deg elbows in the pipe right under the toilet: one sideway to clear the frame, then one down to the tank.
Would you think using two elbows cause problems with the toilet (clogging or whatever) ?

We're still debating if installing a BW tank is worth the cost and effort, or if it's even possible. Any comments appreciated, including opinions on the 5.5 gallons limitation if we decide to reinstalled everything as it was originally.

Thanks
I think you will have flushing problems with 2 elbows. There's no pressure like in a house toilet when flushing so it is all gravity. Gravity runs down not sideways. All you could do is put the elbows together over a bucket and see if it would flow with stuff in it. Remember it won't just be liquid and it's the solids that plugs a toilet. Sometimes even a straight drop hesitates. We had an RV that the pipe was down but had a very slight maybe 30 degree curve and it liked to stick sometimes.
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Old 06-23-2020, 02:28 PM   #4
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Originally Posted by Jann Todd View Post
There's no pressure like in a house toilet when flushing so it is all gravity.
Exactly my thinking, and my concern!
Not a lot of water pushing down on it when flushing.
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Old 06-23-2020, 11:02 PM   #5
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Originally Posted by Carl V View Post
If we install a regular black water tank, to only way to connect it to the toilet would be by using two 90deg elbows in the pipe right under the toilet: one sideway to clear the frame, then one down to the tank.
Would you think using two elbows cause problems with the toilet (clogging or whatever) ?
ABS fittings come in long turn or wide sweep for carrying liquid; the sharp elbows are only for venting air. RV toilets don't send much water down the drain, so I would anticipate problems even if you used a wide sweep.

Our Escape has a large tank which is partly under the toilet and partly under the adjacent bed. I don't know if something like that is a prospect for your layout. The usual answer for small spaces is a cassette toilet or a porta potty.
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Old 06-24-2020, 06:22 AM   #6
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Apparently some RV toilet manufacturers (Dometic is one) in their installation manual, specifically advises against installing the toilet in any other way than directly above the tank with a direct flush, at least for some of their models. Reading on various forums, some trailers (mainly fifthwheels) have a couple 30deg or 45 angles in their pipes to the holding tank, and some have reported it's causing problems. One mentioned having two 90deg elbows (to everybody's surprise), and complained about being clogged all the time.
So I guess I have my answer.

Another option would be a marine-type macerator toilet. Pretty expensive though.
And yes I think if we could find a nice cassette toilet that would fit, we would look into it. Probably less capacity than the current setup but at least it can be emptied without bringing the trailer to the dump station.

Thanks to all.
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Old 06-24-2020, 09:34 AM   #7
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Originally Posted by Civilguy View Post
Our Escape has a large tank which is partly under the toilet and partly under the adjacent bed. I don't know if something like that is a prospect for your layout. The usual answer for small spaces is a cassette toilet or a porta potty.
I forgot to mention that our tank is located above the floor, so the toilet sits elevated on it like a pedestal.

Also, composting toilets have a really good reputation. You have to empty the diverted urine often as keeping the solids dry is key to successful use.
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Old 06-24-2020, 09:48 AM   #8
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I forgot to mention that our tank is located above the floor, so the toilet sits elevated on it like a pedestal.
Ok so it's a similar arrangement. But there isn't much space to install a larger tank.

I'll look at composting toilets.

Thanks!
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Old 06-24-2020, 02:00 PM   #9
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Ok I just saw a 17ft Boler restoration where the toilet has a similar set up, and the tank extends way forward under the adjacent cabinet (dinette bench I guess). We may be able to go the same way with a long and narrow tank, except in the Unik the hot water heater is right next to the toilet/shower stall wall. This would require relocating the hot water heater. Which would require a FG job patching the shell where the water heater is now, and cutting a new hole wherever we reinstall it.
Hmmm...
Debating cost vs effort vs usefulness....
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Old 06-26-2020, 07:41 AM   #10
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I managed to use a 45* offset adapter in my small bathroom in our house, after a reno the toilet didn't clear a floor joist, it gave me just enough offset to clear the joist, you might get the same, and with Just an offset and frequent flushing might be ok
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Old 06-26-2020, 03:43 PM   #11
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Originally Posted by Joe MacDonald View Post
I managed to use a 45* offset adapter in my small bathroom in our house, after a reno the toilet didn't clear a floor joist, it gave me just enough offset to clear the joist, you might get the same, and with Just an offset and frequent flushing might be ok
Thanks Joe.
I also had to use an offset flange on my house's toilet, to clear a joist also. Typically offset flanges allow a 2in displacement. In the trailer's case, we need 5 inches at the very least.

Right now we're looking into a holding tank that extends under the dinette seat. Simple to install, but as I mentioned it involves moving the how water heater, and glassing the shell where the water heater was. Not sure if I want to embark on this.

Hey, you got me thinking (different subject here) but do you still have your 5500? Have you listed it in my registry?
The registry is here:
https://airtable.com/shr4bWwL6yN2zjyAD

You can register by filling this form (no personal data collected, it's about the trailers, not the owners):
https://airtable.com/shrJg5Nd0S13LxN2A

Thanks!
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Old 06-26-2020, 03:45 PM   #12
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You use a lot more water in a home and don't have to store it after flushing.
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Old 07-02-2020, 09:48 AM   #13
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Hey Carl, not registered yet, doing some upgrading right now, then will list with pictures
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Old 07-02-2020, 10:39 AM   #14
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Sounds good! Thanks!
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