Very odd macerator issue....possibly a new one!!

[Dean1969]

Hello everyone
Here's one for you.....
We have a DIY ensuite (put in by previous owners). Miles from waste stack, so SaniPro services toilet, sink and shower cubicle.
This died about a year ago, and I replaced like for like with no issues (apart from trying to find the electrical connections behind a stud wall, but that's another story).
All working - absolutely no problem.
We do not use the toilet for 'solids', so it's only dealing with water anyway...not even any paper goes down it.
When we go away for any length of time I always switch off water at stopcock (we've had some disasters in the past!), and recently I've also taken to switching macerator off at isolator switch too (belt and braces in you like).
So - we come home from a weekend in London to find shower tray full of water, which has then overflowed the lip and caused a load of damage to floor and ceiling underneath.
But the mains supply was OFF, and the macerator was OFF!!
Never happened before......
All I can think of is that the water sitting in macerator has somehow siphoned back into shower...but I can't reproduce it......the toilet cistern was full of course, but even if you flush this with macerator off...you just don't get enough volume of flow back into shower to fill up the tray.
The toilet could be leaking into bowl BUT the supply was off, so the most that could come through is one cistern full.
Where has the water come from, and what would cause it to randomly flow back........???
Cheers for any advice...
Dean

 

[rpm]

Check cistern for dripping water into the pan or taps dripping, if I am correct this water has filled the pump and backflowed into the shower tray.
Never isolate power to the macerator, water yes but not electrics.

 

[leakyBYnature]

check the NRV

 

[Dean1969]

check the NRV

NRV?

 

[rpm]

check the NRV

Wouldnt release enough water.

 

[rpm]

NRV?

Non release valve. one way valve and holds water back.

 

[rpm]

Dean, turn things off as you did before and slowly add buckets of water down the pan to see if you get back flow into the shower. Slowly mind.

 

[Dean1969]

Thanks for all the advice guys. Very much appreciated.
I've tried to reproduce as suggested by rpm and water certainly flows back into the shower from the macerator tank (if it is off). The little 'flap' valve on the macerator tank end of the shower waste is in good nick but of course is not really designed for total seal so water does get by if tank fills right up...point of least resistance.
So......this is clearly what is happening...but I'm still confused about the volume of water. With mains supply off, the available water would be residual in macerator tank + toilet pan + (maybe toilet cistern if leak into pan) + (maybe water in plumbing between macerator tank and main soil waste......).
I think that maybe it would only be enough if the latter was included. This suggests maybe some siphoning? I checked for NRVs suggested but no convinced there are any (it's difficult as most of plumbing is inaccessible and I didn't have it installed...was there when we bought house). There is what I think is an air inlet branch in the loft space......which looks like it is intended to act as a siphon break? Could this have stopped working or be blocked?
Cheers
Dean

 

[rpm]

Thanks for all the advice guys. Very much appreciated.
I've tried to reproduce as suggested by rpm and water certainly flows back into the shower from the macerator tank (if it is off). The little 'flap' valve on the macerator tank end of the shower waste is in good nick but of course is not really designed for total seal so water does get by if tank fills right up...point of least resistance.
So...this is clearly what is happening...but I'm still confused about the volume of water. With mains supply off, the available water would be residual in macerator tank + toilet pan + (maybe toilet cistern if leak into pan) + (maybe water in plumbing between macerator tank and main soil waste...).
I think that maybe it would only be enough if the latter was included. This suggests maybe some siphoning? I checked for NRVs suggested but no convinced there are any (it's difficult as most of plumbing is inaccessible and I didn't have it installed...was there when we bought house). There is what I think is an air inlet branch in the loft space.***..which looks like it is intended to act as a siphon break? Could this have stopped working or be blocked?
Cheers
Dean

First up Non-release valve should of been a Non-return valve last night ( I was tired) and the little "flap" only works properly when there is a rush of water added ie Toilet flush.

With main supply off I wonder if the supply comes from a cold water tank?

 

[Dean1969]

First up Non-release valve should of been a Non-return valve last night ( I was tired) and the little "flap" only works properly when there is a rush of water added ie Toilet flush.

With main supply off I wonder if the supply comes from a cold water tank?

Got that - no worries
No cold water tank or tanks of any description in our system. We have a combi boiler.
The plumbing from macerator to soil goes a long way though.....from front of house up into loft space...right around interior perimeter of house...then down at back of house where sewer is.

 

[JusKel]

Well then you have all that height above full of water slowly draining back. If you leave the power on then you might hear the macerator kick-in all by itself without being used trying to clear it again.