# Forum Home Renovation Plumbing  water tank overflow

## wannafix

hi all, i have a 3000ltr tank in my back yard. When we get heavy rain the tank overflows and runs into the neighbours yard. Their is a 90mm inlet to the tank and an overfow which goes out to the street through a 90mm pipe as well. When it rains heavy, the overflow pipe can not get rid of this excess water quick enough and the tank overflows. This is a new house and the tank was already installed. What can i do to fix this problem. Would love some advice.

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## Danny

I come across this type of situation a lot but a bit more information is needed before I can reply. 
You say the tank is in your back yard. Is it fed by a wet (charged) system? 
If it is a wet system fed by a single 90 mm pipe as stated, do other downpipes also connect into this pipe? 
Do you know the height difference between the bottom of the overflow pipe where it exits the tank and the bottom of the overflow pipe where it overflows to the street? 
What is the total length of the overflow pipe? 
In the meanwhile, have a look at the mesh on the overflow outlet. These often get clogged. If this is the case, then consider removing the meshed fitting and replacing it with a flap valve.  Flap Valves | Rain Harvesting  
Even when clean, overflow outlet mesh still presents a restriction in the overflow pipe. Typically, the metal mesh 'open area' is less than 50%, something the 'experts' who frame regulations have failed to realise. 
The trend towards tanks with high set overflow outlets is also a cause of tank overflows.

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## ringtail

x 2 with danny, very common problem for the overflow mesh to slime up. I'm fortunate in that my tank sits right next to a spoon drain so my overflow pipe is very short. I got rid of the overflow mesh  and fitted a frogflap to bottom of the overflow pipe. I also siliconed a small nut onto the flap to make sure it closes again after heavy rain

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## wannafix

Hey mate 
I'm not sure what type of system it is but would assume it is a wet system. I believe all of the down pipes from the house feed into one 90mm pipe which runs up the side & into the top of the tank. The tank sits in a lower part of the yard.  
The water exits the overflow pipe & drops 1.9m where it runs approx 30m to the street and would probably drop about 1m in its travels. 
We currently have the flap valve so its not getting clogged at all.  
The overflow outlet sits right at the top of the tank, the top of the 90mm pipe is equal to the top of the tank and a great deal of the water runs out of the top of the tank during heavy rain . Maybe this is my problem. How far from the top should the overflow pipe exit the tank?  
Thanks for your help!

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## Danny

You have confirmed my suspicions. What you have is a rainwater  harvesting system that is a dog and whoever installed it wouldn't have a clue. Unfortunately, there are many people in the same situation  as yourself. However, all is not lost! 
Firstly, your situation as is (the bad news). 
You have a wet system. This runs with the pipe full of water at positive head and consequently generates a much higher flow rate than a same size downpipe or tank overflow that operates with weir flow and draws a vortex. Both ends of the wet system pipework should also be fitted with < 1,000 micron mesh screens to prevent mosquitoes accessing the water and  breeding. 
Here in Victoria, the tank's overflow capacity has to be equal to or greater than the inflow capacity. Some other states and regions, particularly Brisbane, do not have such commonsense regulations. I do not know the regulations in Newcastle but a quick call to the council should (???) give you the right info. If the plumbing is not compliant, maybe you could get the builder to rectify.   
The vertical pipe going to the top of the tank is called a vertical riser. A vertical riser does not generate sufficient velocity to  transfer all debris to the tank and the lack of an air/water interface  when water remains in the pipe after it stops raining promotes anaerobic conditions, leading to  stagnation plus the trapped debris in time will often cause blockages.  
There should also be drain valves and inspection points fitted to allow draining and cleaning. If not, I would also query the council as to the applicable regulations. Unfortunately, some councils also overrule plumbing regulations and plumbing regulations are not uniform throughout Australia. Regulations pertaining to rainwater harvesting probably suffer the most.  
90 mm PVC pipe has walls 2 mm thick and should not have been used, particularly if it is also buried at some point.  
A 100 mm overflow should also have been used instead of 90 mm. 100 mm carries a bit over 40% more volume than a 90 mm PVC pipe, which is actually 86 mm internal diameter. There are also ways to increase a tank's overflow rate of discharge without changing pipe size but is not advisable when restricted to using 90 mm PVC pipe.  
A lot of tank manufacturers are (amazingly) clueless and the trend to high set overflows negates a tank's mitigation capacity amongst other problems they can cause. You may find the thread below interesting and notice the 'advice' given to the O.P. by the tank manufacturer!!!  http://www.renovateforum.com/f194/wa...urgling-94945/  
Can you advise as to the number of downpipes connected to the 90 mm inflow pipe? Given NewCastle's 1:20 Average Recurrence Interval, I would be staggered if there was not a build up of water into the gutters if more than two downpipes were connected; unless there were overflow systems (3 way fittings) in place, the downpipes were harvesting small roof areas or there was more dynamic head than usual.  
Once you digest all this, we can work on a suitable solution. There are several.  :Secret:

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## Steffen595

your tank is too small for the catchment area? If it never really empties its just big enough? If its constantly overflowing, all the junk stays in the tank and then the water leaves. My tanks hardly overflow because I take water out all the time, so they catch just as much water and dirt as necessary. Even if you have leafbusters and diverters, there is always some junk entering your tank.

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## wannafix

Thanks Danny, 
It is great to talk to someone who knows what they are talking about. I agree with you that the tank suppliers have no idea and just bury their head in the sand. I will be checking with the council to see what our regulations are and hopefully our house builder will come to the party. I will also check how many downpipes run into the tank and get back to you as i am not holding my breath on the builders. Thanks again, I appreciate your help and will be in touch.

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## Danny

Hi wannafix, 
I will give you as much information as I can so you have a clear understanding of what is actually happening. Hopefully it will also help others. 
Some of the terms appear strange at first but most are self descriptive and easily learnt. The term 'wet system' throws people but it simply applies to supply pipework that remains 'flooded' after it stops raining. 
There are a large number of errors made by 'experts' when designing and installing rain harvesting systems. In Victoria, we have arguably the strictest regulations in Australia and yet I estimate that approximately 40% of installations I see are non compliant and over 90% are sub standard. Most non compliance concerns insufficient overflow capacity while substandard installations commonly includes the follow: 
Top meshed inlet sited directly above the pump's draw off outlet, impacting vortex rings then causing sediment resuspension which is then drawn through the pump's impeller. 
The pump's draw off outlet fitted too close to the bottom of the tank. This is very common. 
Unfiltered water entering the tank which quickly leads to excessive sediment build up.  
First flushes fitted at the top of vertical risers. The number of websites I see with photos of this is remarkable!  
First flushes with insufficient capacity. 
Wet systems not mosquito proofed. 
The choice of downpipe been the one directly under a TV antenna (bird roost). 
Harvesting roof areas in the rain shadow of a large tree. 
Pumps that start up everytime water is drawn. Large stand alone pressure tanks can reduce pump start ups by as much as 90%, saving energy while reducing pump wear and tear plus they don't advertise to the neighbours whenever you use the toilet!!!  :Badair:  
Unwise use of 90 mm PVC pipe. 
Wet systems not fitted with drain and inspection points. 
Last year I saw an example of three tanks sited on a sloping sand base next to a house. When it rained, all the sand washed away, undermining the base of every tank. The tanks were also linked with small 25 mm balance pipes which meant that the two higher tank's water level self levelled to the height of the lowest tank which in turn was the only tank fitted with an overflow pipe. The tanks were also fed by wet systems connected to multiple downpipes plus an additional downpipe fed direct to the upper tank's meshed inlet.  
The three guys that the (award winning) plumber left to do the job even glued the bottom of a first flush kit which prevented the inside of the flush pipe from been accessed and cleaned out once it blocked up (as most do).  
As per legislation, the homeowner paid for a compliance certificate which wasn't worth the paper it was written on because the Plumbing Industry Commission failed to act on any of the above and other not mentioned problems. The revenue raised annually from compliance certificates must be staggering! 
Re the link in my last post, that particular problem is not exactly the same as yours as the linked overflows from the two tanks was causing an additional problem but the problems of multiple inflow and high positioned overflow outlets are common to both situations. The solution to that unusual problem ended up been quite simple. 
It won't take much to fix your problems and give you a good system. If you also identify any additional issues from the above, just post the ones that apply.

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