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Old 12-19-2019, 01:35 AM   #1
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THOR #17154
Black tank backed up - how could this happen?

Iím going to post this in a couple of forums hoping for a good answer...

Today when we connected to city, all went as expected (the Kantleak does, and is leaking into the FW tank). So I watched the FW tank to ensure that I turned city off before it overflowed. I have a new valve that weíre going to install this week.

When Ms. WT went into the back bathroom the sink was overflowing. Somehow, the back black tank got filled up and water came up thru the sinkís drain. A check of the back black level showed 4 of 4 (full). When I did an emergency dump of the back black the sink drained and went to 1 or 4 lights.

So - am I correct in ASSuming that the back bathroom sink and the patio sink drain into the back black tank?

No faucets were turned on. The toilet flush pedal was not pushed down. That said - What could have caused the black tank to fill? I emptied it when we left the RV park this morning - so it was empty before I connected to city tonight.

What would cause the back black to fill? Is the Kantleak able to leak into the black? That seems far fetched...
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Old 12-19-2019, 02:02 AM   #2
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Not sure about your particular model but our ACE and the Gulfstream we had before this one the bathroom sink drains into the black tank. Very common on many MH's. Sorry I Don't have any idea why the tank filled. There are a couple folks on this forum with your model that may have some insight.
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Old 12-19-2019, 02:14 AM   #3
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THOR #1469
Contact Thor and ask them for the fresh and waste water diagrams for your coach so you can see how things are plumbed
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Old 12-19-2019, 09:26 AM   #4
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THOR #15553
Yes, if you have a rear bath behind the axels some manufacturers do in fact drain the sink into the black tank. It'd the old water don't run uphill to get to the gray tank.

Are you sure you weren't hooked to the black tank flush valve??
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Old 12-19-2019, 11:44 AM   #5
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If there were truly no faucets turned on: The blackwater tank should not have filled...
We're missing an important part of the story.
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Old 12-19-2019, 11:52 AM   #6
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I've had a couple Thor motorhomes and the bathroom sink is plumbed into the Black Tank. There are many theories as why this is the case...

- Helps to keep more water in the black tank for more efficient dumping
- To reduce the amount of water going into the grey tank
- Easier plumbing


As previously mentioned... if you were hooked to the San-T-Flush port and not the fresh water fill that would cause the problem.

Just looking at how these rigs are plumbed, I can't see how filling the fresh water tank could get water into the black tank.

When the fresh water tank is full, there is an overflow at the top of the tank that allows the excess water to come out right next to the fill port. The fresh water tank can't be pressurized from filling such that it could push water past the pump.

Even if the fresh water tank could become pressurized from filling, there would also need to be a serious plumbing problem for fresh water to get into the black tank.

There is a Vacuum Breaker for the San-T-Flush system that prevents the black tank from siphoning back to the San-T-Flush hose port. There have been times when that has failed or was plumbed incorrectly and it typically causes a leak on the coach floor (my two coaches had the Vacuum Breaker behind the shower wall.
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Old 12-19-2019, 11:56 AM   #7
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THOR #17154
Quote:
Originally Posted by cavie View Post
Yes, if you have a rear bath behind the axels some manufacturers do in fact drain the sink into the black tank. It'd the old water don't run uphill to get to the gray tank.

Are you sure you weren't hooked to the black tank flush valve??
Iím almost 100% positive no faucets were on and the toilet pedal wasnít depressed. The only reason I canít say 100% is that Ms. WT was the one who walked into the bathroom and not me. But sheís pretty anal about things like that and said the faucets were off.
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Old 12-19-2019, 12:00 PM   #8
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Quote:
Originally Posted by Bob Denman View Post
If there were truly no faucets turned on: The blackwater tank should not have filled... .
I agree 100% with you. Weíre perplexed. Thatís why Iím posting in several groups - this should not have happened. Ms. WT says that no faucets were on (and water would have been running from the tap if they had).
Quote:
Originally Posted by Bob Denman View Post
We're missing an important part of the story.
We canít think of anything else.
We will turn the water on again when it warms up around here...
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Old 12-19-2019, 12:11 PM   #9
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THOR #17154
Quote:
Originally Posted by Judge View Post
I've had a couple Thor motorhomes and the bathroom sink is plumbed into the Black Tank. There are many theories as why this is the case...

- Helps to keep more water in the black tank for more efficient dumping
- To reduce the amount of water going into the grey tank
- Easier plumbing


As previously mentioned... if you were hooked to the San-T-Flush port and not the fresh water fill that would cause the problem.

Just looking at how these rigs are plumbed, I can't see how filling the fresh water tank could get water into the black tank.

When the fresh water tank is full, there is an overflow at the top of the tank that allows the excess water to come out right next to the fill port. The fresh water tank can't be pressurized from filling such that it could push water past the pump.

Even if the fresh water tank could become pressurized from filling, there would also need to be a serious plumbing problem for fresh water to get into the black tank.

There is a Vacuum Breaker for the San-T-Flush system that prevents the black tank from siphoning back to the San-T-Flush hose port. There have been times when that has failed or was plumbed incorrectly and it typically causes a leak on the coach floor (my two coaches had the Vacuum Breaker behind the shower wall.
I agree. The city hose is up front. The back black, with this flush inlet is in back. Absolutely no way I made the mistake of connecting city to black flush. Theyíre 20í apart, clearly labeled, and (I started a diff thread about this) I believe that my back black flush is installed backwards and I couldnít pump antifreeze into it for winterization.

The only thing I can think of is the faucet being on would cause the black to fill and overflow the sink. Thatís the ďlogicalĒ answer. And I hope itís the correct answer, as that would mean thereís nothing physically causing the problem.
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Old 12-19-2019, 12:48 PM   #10
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THOR #8860
Whatever you do: DON'T touch the toilet; until you've got the tank drained!
(That's the normal "Low-point. The seal in it is keeping you from a disaster!)
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Old 12-19-2019, 02:16 PM   #11
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I am not familiar with your coach/floor plan, but some of the bath and a half plans have 2 black tanks with separate dumps. Any chance you are only dumping the front black tank?
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Old 12-19-2019, 05:01 PM   #12
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I agree. The city hose is up front. The back black, with this flush inlet is in back. Absolutely no way I made the mistake of connecting city to black flush. Theyíre 20í apart, clearly labeled, and (I started a diff thread about this) I believe that my back black flush is installed backwards and I couldnít pump antifreeze into it for winterization.

The only thing I can think of is the faucet being on would cause the black to fill and overflow the sink. Thatís the ďlogicalĒ answer. And I hope itís the correct answer, as that would mean thereís nothing physically causing the problem.
You think we have never heard of somebody doing that!
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Old 12-19-2019, 11:46 PM   #13
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I have the same model as you and yes, the aft bathroom and deck sink plump into the rear black tank. I would highly recommend that you contact thor customer/warranty support and ask for a set of prints/schematics for your rig. Safe travels -
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Old 12-20-2019, 02:13 AM   #14
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THOR #17154
I talked to Thor. Yes, indeed, the rear bathroom toilet AND sink drain into the back black.

The main bathroomís toilet and sink drain into the main black.

Everything else, to include the outdoor sink, drains into the grey.

We putzed with the water system today. My overflow problem could not be recreated to I have to ASSume that we screwed up and the back bathroom faucet has to have been inadvertently left on.

We replaced the Kantleak valve today. It was a little difficult to pull the old valve out. The corners of the handle were sharp and cut into the hand. A glove fixed that. After the replacement the ďnormal,Ē ďcity fixtures,Ē and ďcity fill tankĒ worked like a charm. The fix also stopped whatever leak there was and the water pump stopped running after it built up sufficient pressure. So, while Iím not at all pleased with the Kantleak quality Iím pleased that itís now fixed (till the damned thing breaks again!).
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Old 12-20-2019, 10:06 AM   #15
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So.... does that mean your water pump was also on while you were filling your fresh tank? If you left the bathroom faucet on, the pump would also need to be for it to fill up your black tank.

I am always worried about water leaks.... at home and in the RV. I always turn the water off at home if we are gone for more than a day and I turn off the pump in the RV when we are not using water. I even started shutting off the camp city water if we are gone for the day.

If something started leaking or let loose, I don't want to come back to a mess and water damage.
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Old 12-20-2019, 12:05 PM   #16
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I'm glad to hear that you're sure that you've got a handle on the situation now. :Lessons learned, can be painful: but useful for the future.
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Old 12-20-2019, 12:23 PM   #17
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THOR #17154
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Originally Posted by Judge View Post
So.... does that mean your water pump was also on while you were filling your fresh tank? If you left the bathroom faucet on, the pump would also need to be for it to fill up your black tank.

I am always worried about water leaks.... at home and in the RV. I always turn the water off at home if we are gone for more than a day and I turn off the pump in the RV when we are not using water. I even started shutting off the camp city water if we are gone for the day.

If something started leaking or let loose, I don't want to come back to a mess and water damage.
Iím confident that the pump wasnít on. I say that because with the Kantleak issue, we thought that the leak was causing the water pressure to never remain stable enough for the pump to stop. It ran continuously. So while we had the Kantleak issue we made sure that the switch was off. And itís in the wet bay, where I was mucking about with the water system. Iíd have heard it.

Iím pretty sure that while filling the FW thru the city inlet that the rear bath sink faucet was on, causing it to fill the back black and over flow the sink.

All water systems appear good now that we replaced the Kantleak valve.
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Old 12-20-2019, 03:37 PM   #18
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Replacing a Kantleak valve because it leaked. That ain't right!
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Old 12-20-2019, 05:10 PM   #19
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Replacing a Kantleak valve because it leaked. That ain't right!
I think they call that an "Oxymoron".

Or alternately: a major pain in the backside!
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Old 12-20-2019, 05:31 PM   #20
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"Everything else, to include the outdoor sink, drains into the grey"

That is a major change from one model year to the next - the schematics definitely show the patio sink draining into the rear black tank along with the toilet and rear bathroom sink - see attached picture -

The reason I bring this up is that my patio faucet got turned-on somehow and I nearly had the same issue -

Thor emailed me 10 sets of schematics from plumbing to sidewall & roof layouts - they have come in handy on many occasions -

Safe Outlaw Travels
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