Browse Forums General Discussion 1 Jun 11, 2021 9:52 am Hi, Just moved into new house and noticed raintank overflows during mild rain. The house has 2 tank 6000lt(tank1) + 3000lt(tank2). Each tank has 2 x 100mm feeding in 3000lt tank has 1 x 100mm outlet to street Tanks are side by side, and connected via 50mm pipe. I noticed when tank1 is full it cant discharge via 50mm into tank2 quick enough and overflows. Not sure if undersized? Trying to get builder to rectify, but not sure how to calculate the correct pipe diameter between tanks. Roof area ~ 224m2, tank1 60% roof space tank2 40%, location Bexley NSW. Appreciate any help. Cheers Re: Rainwater tank pipe diameter 2Jun 11, 2021 1:23 pm Thank you for the good information. I assume that the tanks have the same wall height. Yes/No? Unfortunately, practically every rainwater harvesting system installed is either sub standard, not compliant or both. Your installation is a sad reflection on the lack of knowledge in the industry. Do you know who did the installation? You undoubtedly will also have other issues you are unaware of. If you post some photos of areas I request (if asked), I can also advise you of those issues. FACTS: Your rainwater harvesting system from the overflow outlet to the stormwater LPOD is part of the storm water system and subject to the same regulations. Eaves gutter roof drainage must be sized to drain without overflowing during a minimal 1:20 Average Recurrence Interval (ARI). Your 1:20 ARI looks to be 200 mm/hr. The ARI figure is based on an average rainfall intensity over a 5 minute duration and so your minimal 5 minute 1:20 ARI average rain intensity is most probably 3.33 L/m. Unfortunately, rainwater harvesting regulations are inadequate and poorly written but roof stormwater drainage regulations take into account the roof slope and abutting walls to factor additional rainfall during wind driven rain. If for example a downpipe harvests a roof area of 30 sq mm (the roof area draining between the gutter's high points) when looking at the plan area (roof space) and if the roof has a standard roof slope of 22.5 degrees, the 30 sq m must be multiplied by 1.21 to determine the roof catchment area of 36.3 sq m. The roof catchment area is the figure used to determine roof drainage compliance and your 224 sq m roof space equates to a roof catchment area of 271 sq m. Q. But the amount of rain that falls on the roof only falls on 224 sq m so why isn't the roof space area used to determine the amount of water diverted to a tank? Aha, good question and I am glad that you asked. You have two 100mm pipes diverted to each tank (4 pipes in total) from a number of downpipes and during wind driven rain, the downpipes would be draining differing volumes of water even if each downpipe was servicing equal roof areas...which they don't! If a system is harvesting every downpipe to a single pipe, then the roof space area is used for water tank calculations but I always advise home owners not to base hydraulic calculations on the minimal rain intensity during a 1:20 ARI and allow for a minimum 20% safety margin. Ok, assuming a 22.5 degree roof slope (you need to check this), we now have the following figures to work with: House roof catchment area: 271 sq m. Note that the gutter overhang area must also be used in calculations. Tank 1. Roof catchment: 163 sq m (543 L/m). Note that the roof space of 134.4 sq m calculation if wrongly used still comes to 448 L/m. Tank 2. Roof catchment: 108 sq m (360 L/m). Total water diversion from roof space during a minimum 1:20 ARI: 224 sq m x 3.33 mm/min = 746 L/m. Total water diversion from roof space for every 1mm of rain per minute: 224 litres. ISSUES: Two tanks gave you the opportunity to have a superior settling tank system where all water discharges to one tank which then supplies decanted water to the second tank that the pump draws from. The overflow pipe is always fitted to the first tank as the second tank will never fill above the height of the first tank. It is also best practice to use a floating inlet hose to transfer water from the first tank to the second tank. 50mm low balance pipe. A 50mm short flooded pipe will flow with a high velocity flow rate but this depends on the hydraulic head. The head is determined by the different water levels between the two tanks but if there is little difference in the water levels, there will be little hydraulic head to generate flow. A 50mm balance line is big, I usually recommend a 32mm. The overflows are not the fault of the 50mm balance line, it is doing its job. The fault rests with the system's design (if it even was designed) and its massively inadequate overflow capacity. OVERFLOW PIPE: You say that there is one 100mm overflow pipe on tank 2 and so I assume that there isn't an overflow pipe on tank 1. A horizontal outlet is less efficient than a vertical drain pipe. The water enters the pipe through a fitting that invariably has a smaller entry than the pipe's internal diameter (ID), for example, a 90mm pvc storm water pipe has an ID of 86.2 but the fitting's ridge that the pipe abuts is most commonly 84mm ID. I mention this because the 100mm DWV overflow pipe you have will be SN6 (a stiffness rating) with a 104mm ID but the ridge it abuts will be closer to 100mm ID. AS/NZS 3500.1 lists the following discharge rates through a 100mm horizontal opening. Water level above the invert (bottom of the opening). 75mm 2.78 L/sec. 166.8 L/m. 100mm 4.67 L/sec. 280.2 L/m. 125mm 5.72 L/sec. 343.2 L/m. 150mm 6.60 L/sec. 396.0 L/m. There is a current bad trend to having high set overflow pipes to maximise a tank's storage capacity. This has largely been borne by consumers finally realising that the tank's capacity is often less than advertised once an overflow pipe has been fitted. As you can see in the chart, the outlet's flow capacity increases with deeper water depth above the invert and so having mitigation above the overflow outlet allows for higher overflow discharge rates. HOWEVER, mosquito proof wire mesh fitted to a tank's overflow usually has 0.955 mm apertures which have an open area of usually no more and often less than 55%. This presents a flow restriction but note that the chart shows flow rates through an unmeshed outlet! Your single overflow pipe is hopelessly inadequate, it and additional overflow pipes will also need modifying to increase their flow capacities. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Rainwater tank pipe diameter 3Jun 16, 2021 3:44 pm Thank you for your detailed response. I have attached the hydraulic drawing. As you assumed roof is 22 degrees, tanks same height, 50mm is balance line. 100mm Discharge on tank 2 is at the maximum height. Plumber (builder) has advised overflow is normal in heavy rain, im not convinced. I asked to lower the discharge point(100mm?) to provide some buffer however this will reduce tank capacity, im ok with that, however hydraulic engineer would probably not approve. Another option is a second balance pipe? Or increase the discharge capability? Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Re: Rainwater tank pipe diameter 4Jun 17, 2021 7:03 pm You need to increase the discharge capacity and this includes having an overflow pipe on tank 1. HandyManny Plumber (builder) has advised overflow is normal in heavy rain, im not convinced. And you are right. Having tanks in a stormwater system overtopping with about 470 lpm going to ground during a minimal 1:20 ARI is not right by any stretch of the imagination. The plumber either has no idea or is evacuating terminological inexactitudes. (Thanks for that Winston)! The person who designed the system also lacks required knowledge. Best practice would have incorporated a settling tank system with a single 150mm vertical riser and an internal 150mm standing bellmouth on a single 100mm overflow pipe to supercharge discharge capacity during a high intensity rain event. To learn how little the person who designed the system and the plumber know about the art, ask them "What is the minimum flow rate entering the tanks during a 1:20 ARI?" and then ask them "What is the discharge capacity of the single 100mm overflow pipe and what flow restriction does the mesh present?". Unfortunately, rainwater harvesting systems designed with insufficient overflow capacity are not uncommon. HandyManny 100mm Discharge on tank 2 is at the maximum height. There may be 125mm above the overflow fitting's invert so those figures I posted last time should be used but the mesh will reduce the discharge flow rate from about 343 lpm to about 275-280 lpm and much more if the mesh has started to gum up. It is difficult to keep the overflow mesh perfectly clean. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ TANK 2 The 100mm overflow pipe would satisfy this tank's requirement if the outlet was unmeshed. I suggest removing the mesh and fitting a dual meshed air gap to the vertical overflow pipe. https://www.youtube.com/watch?v=sE8IChfIFCA P.S. NOT impressed by the demo tank's overflow design! TANK 1 Fit additional 100mm overflow pipe. Remove mesh and fit an air gap to the vertical overflow pipe. This will provide a total overflow capacity from the two overflow pipes of close to 690 lpm. With the roof space diverting a minimum 746 lpm during a minimal 1:20 ARI, the balance line will send some water to tank 2 with tank 1 having an additional overtopping head but a new overflow on tank 1 requires an absolute bare minimal discharge capacity of 400 lpm. I suggest having more. I would as a minimum fit an additional 50mm unmeshed outlet and connect it to a DWV 100mm x 50mm 45 degree reducing junction above the air gap. This is easily done when also fitting the air gap. With the 50mm outlet at the same invert height as the 100mm outlet, it would have a discharge capacity of 1.65 lps (99 lpm) with 125mm of water above the invert. To supercharge the new 100mm overflow pipe's discharge capacity, just fit the overflow outlet a little lower down and fit an internal 90 degree standing elbow. A vertical (standing) pipe drains more efficiently than a horizontal one, with 125mm of water above the crest, a 100mm vertical pipe will drain at 7.38 lps (443 lpm). Heating and bellmouthing the pipe's 'reservoir wall' even a little will further increase the pipe's overflow capacity, for example, a 150mm bellmouth will discharge at 7.51 lps (450 lpm) with only 40mm of water above the crest! Bellmouthing also allows Tank 1 to have a higher (delayed) overflow level than Tank 2. You can see in the photo below why the outlet hole has to be drilled lower down but note that it is a big tank. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ A couple of additional notes: The builder or plumbing engineer will have almost certainly used the NCC (Part 2) roof drainage regulations because they are laxer than the Australian Standards. The NCC Part 2 has the regulation pasted below. "3.5.3.5 Downpipes — size and installation Downpipes must— (a)not serve more than 12 m of gutter length for each downpipe; and (b)be located as close as possible to valley gutters; and (c)be selected in accordance with the appropriate eaves gutter section as shown in Table 3.5.3.2a to Table 3.5.3.2c". If you look at downpipes 4 and 5, there is one heck of a distance between them and the gutter length serving those two downpipes could/would exceed 12 m. The upper roof downpipe that discharges to the garage roof drains a concentrated flow of water to the end of the gutter furthest away from where the downpipe is located. This is poor design, particularly when the tank 2 top meshed inlet is close to the gutter's mid point where the downpipe should have been fitted. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. 18 74211 DIY, Home Maintenance & Repair But if it is a ground level open pit, then it is not a charged system. No surprises there. The pipes have obviously been altered and there would be a reason for this.… 3 24965 Grate, thank you! RexChan if thats the reason i could sleep well without thinking about additional cost. But 1st i'll need to read about NRV cleaning/replaing stuff. I… 7 25270 |