Tank question

russell · Nov 28, 2008 7:23 pm

Quick question, I asked our builder about a water tank for the garden and whether the whole roof area is harvested. He said normally they just have one downpipe out of 6 or 8 going into the tank and reckoned that should fill a good sized tank easily?

Would that be right?

(Surely it's not the tank size but the usage rate that matters.)

I would like to potentially water a big vegie patch with it so am wondering whether I need to get him to divert more water to the tank and how much hassle and cost that will involve.

Michelle · Nov 28, 2008 7:34 pm

I would imagine it just means less down pipes, so more water goes into the tank.
That would have to save you money?

I_BUILD · Nov 28, 2008 7:52 pm

It doesn't necessarily mean less downpipes because the guttering needs to have a certain amount of downpipes so the guttering will not overflow. Less downpipes can be done in some cases by using rain heads but it would be best to connect up as many major downpipes as possible.

donuts · Nov 28, 2008 7:57 pm

I think we are getting 3 or 4 downpipes connected to the tank. From memory of having a tank man sit down and look at my plans and explain it to me, it doesn't look like it should be too hard to get at least half of the roof to drain into the tank if it is planned for at design stage?

Casa2 · Nov 28, 2008 7:59 pm

My understanding is that only 1 of the 6 to 8 downpipes feeds the tank. If this is the case you either have a very small tank (say 3000 litres) or you'll have trouble filling it.

ECOECO · Nov 28, 2008 8:15 pm

Tank size, catchment, and usage are all important...

A very good analysis of rainwater harvesting.

http://www.eng.newcastle.edu.au/~cegak/ ... mbes51.pdf

TheGardenWhisperer · Nov 29, 2008 10:11 am

They can be filled very quick sometimes! as you will see in the first clip

https://forum.homeone.com.au/viewtopic.p ... 327#133327

Mel&Em · Nov 29, 2008 10:47 am

Your tank will fill up depending on the surface area available on your catchment area of your roof. You get 1 litre of water for every mm of rain on every square metre of catchment area.

So, if you had an area of roof that measured 10m by 10m that's 100 square metres. Provided that all of this area flowed into one downpipe into your tank, for every mm of rain you would get 100 litres. That means a 2500 litre tank would need 25mm of rain to fill.

Hope that helps

russell · Nov 29, 2008 10:48 am

Sounds like I will have to get him to divert more water ... is that likely to be expensive or make him grumpy?

Rough calculation, we have probably 230 sq m including eaves. A 10 mm rain should yield 2.3 cubic metres = 2300 litres. Divide by 7 downpipes and you only get 330 L so a 3000 litre tank needs 10 events like this to fill it.

The builder reckoned it should fill a 3000 gallon tank easily ... I guess he actually meant 3000 litres.

Looking at this, http://www.diggers.com.au/pdf/DroughtPr ... Garden.pdf

They reckon about 570 litres per square metre of garden area, for the growing season. For a 50 square metre vegie patch, that's 28500 litres. A bit of a stretch with a 3000 L tank! 7-10kL would be closer to the mark I'd think.

Average rainfall where I'm building is about 650 mm, yielding 150,000 litres from the whole roof area. Divide by 7 downpipes, that's 21,000 litres, which falls a bit short. Get two downpipes into it and we should be laughing though.

Does that sound right?

Yak_Chat · Dec 02, 2008 7:48 am

It's more case of what is your monthly rainfall.

Central vic.

As a guide - central vic is in the 30 to 60mm per month.
So that gives you (lets take a conservative average) 45mm per month - allowing for climate change reductions.

45mm x 200m2 = 9000 litres per month falling on the roof.

re how many downpipes - it really is based on the roof area that the downpipe of coming from. Not all downpipes will be gathering from the same roof area size. They are installed based on the fall and how easily they can be connected to another one.

What you really need it sufficient storage to meet your needs during the dry periods when the rainfall is less, and your water usage is up - ie the summer vegie growing season.

to see what "historically" data says about rainin your area check here - but upsize from these figures because it will change.

http://www.bom.gov.au/climate/averages/

Steve

chris... · Dec 05, 2008 8:34 am

russell

They reckon about 570 litres per square metre of garden area, for the growing season. For a 50 square metre vegie patch, that's 28500 litres. A bit of a stretch with a 3000 L tank! 7-10kL would be closer to the mark I'd think.

Remember you can only store 3kl the rest goes to waste.
My catchment area is half the carport roof and it fills a 2500L tank easily, my veggie patch is about 30sqm and I easily empty it when it doesnt rain for a while. I'm going to get another tank