Garden water tank plumbing

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neb

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#268156 2-Mar-2020 17:03

As part of the Casa de Cowboy rebuild, we're looking at adding a slimline water tank to hold (non-potable) garden water. This is based on the current multi-month stretches with no rain and the fact that it's only going to get worse in the future, we want to have a reserve of water to keep the garden alive when water restrictions come in. The plan is to add a 3-5,000L tank at the side of the house where we're digging out for the basement, and hook it up to the water pipes that go out into the garden:

That's about a 1m gap, wide enough for a range of long, narrow tanks, and despite it looking like a really bad position it's actually the least awful place to put it. Apart from the potential issue with removing a lot of the soil near the house piles, which an engineer is looking into, the other big thing is how to connect it up.

Option 1: The outflow from the tank is connected to the garden water pipes via a tee connector with a backflow preventer (for the mains side) and we run either mains water or tank water depending on what's available.

Option 2: Mains water is fed into the tank (again via a backflow preventer) and the outflow goes to the garden water pipes.

With option 1, we can use the mains water pressure most of the time until we have to switch to tank water, but there'll have to be some switchover mechanism downstream from the tank to select which input we use.

With option 2 we've only got one lot of plumbing downstream of the tank, but we'll need to run a pump every time we water.

At the moment I can't see any strong thing pushing for either 1 or 2, does anyone have any thoughts?

Scott3

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#2430257 2-Mar-2020 17:33

I'm not sure if having a towns supply filled tank exempts you from water restrictions. (Yeah I that you could shut off the water when the restriction comes into effect, and use only stored water).

If you are doing non-potable water, you might as well combine it with rain water harvesting, or gray water collection. If you are going to feed this to toilet flushing, cold laundry use etc, you likely need to use a licenced plumber, get consents & notify your water supply company.

With regards to Teeing into outdoor water circuit, I think it would be better to have dedicated non-potable taps (in addition too potable taps if you choose). This avoids dealing with backflow protection, and makes it easy to label the non-potable taps.

Backflow protection is a fairly detailed subject. Any device will need regually tested if you plan to relay on it for the safety of your drinking water. In the chemical industry we allway's used RPZ's to protect the potable side of the system:

https://www.aqualine.co.nz/store/reduced-pressure-zone-rpz/rpz-backflow-preventers/

neb

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#2430261 2-Mar-2020 17:42

We won't be refilling it from mains water, we'll use it as a reserve of stored water in case water restrictions come into effect. It's purely for rain water collection, we get a lot of rain in winter and want to store it for the dry season. The garden taps are purely for the irrigation system, there's a single pipe going from the water main to those, so pretty easy to isolate.

Another thing is that the tank is way down below the house so just the effect of gravity should prevent any backflow (although obviously we'll fit a protector), and in addition to that there'll be a valve at the tee to switch from tank to mains water, so it shouldn't be a big deal apart from what's required to meet the regulatory requirements.

nickb800

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#2430293 2-Mar-2020 19:12

I think it'll be a lot simpler if you keep the mains system separate from the rainwater system. Backflow is a big health risk, so you will likely need building consent and an expensive Backflow preventer. Being down a hill doesn't necessarily stop Backflow if there's a loss of pressure supply in the mains network.

Way simpler to have a separate rainwater supply tap - you can probably skip building consent then

neb

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#2430305 2-Mar-2020 19:45

nickb800: I think it'll be a lot simpler if you keep the mains system separate from the rainwater system.

There's no way to do that, it would mean only running the garden watering from the tank, which would quickly run down - my mother has a ~5000L tank and that never lasts past about mid-February, even less the last few years. The tank is intended as a backup when mains water becomes restricted, not as the sole source of garden water.

nickb800

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#2430329 2-Mar-2020 19:47

Run a separate pipe from the tank to your existing garden tap, and install a separate tap next to it?

neb

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#2430336 2-Mar-2020 19:56

nickb800: Run a separate pipe from the tank to your existing garden tap, and install a separate tap next to it?

There's around 30m of pipe buried, in some cases several meters deep, under terraces, retaining walls, concreted areas etc, leading to multiple outlets. It has to be a shared connection.

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#2431534 3-Mar-2020 06:40

Having a tee between the town supply and tank supply heading to the tap would tip you into medium risk Backflow preventer territory, which generally means annual inspections = $$

So perhaps a float valve feeding your tank from town supply, and the tank running direct to the tap. You do loose effective tank capacity that way though. Ideally you would turn the mains feed off ahead of heavy rain in order to capture as much free water as possible.

Here's some info on Backflow options - including float valves http://www.level.org.nz/water/water-supply/system-layout-and-pipework/backflow-prevention/

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Zeon

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#2431594 3-Mar-2020 09:48

Only in some situations do you need an inspection for a backflow limiter. I also don't think you need a building consent?

We have a water tank that is fed by town water and a backflow preventer. Then a pump pulls off off this tank and we thus get amazing water pressure :)

The plumber installed it and council gave a code of compliance without any issue on the tank. Only thing they were worried about were non-return valves on the showers, which considering the setup are completely meaningless as there isn't the possibility of backpressure in our setup.

We do have a rain water tank which is used for toilets and garden. There is a seperate tap for it for the garden and it has its own pump. The plumber installed a valve to connect the 2x systems in case the rainwater tank was low. I guess that could then cause the showers, kitchen etc. to be fed by the rainwater tank if we turned off the pump from council water.

Rainwater can smell bad and is quite dirty so not keen on that!

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WaffleMakerMan

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#2431682 3-Mar-2020 11:11

Have a tap and pump at the tank and just run a hose from your irrigation to it during restrictions?
Fully integrated would be nice but the cost would outweigh the benefit I think. Even for irrigation you are supposed to have back flow prevention theres alot of rules around this stuff when I looked into it a while back. That and having it seperate completely removes the risk of contamination to your water supply and the street.

Can be done though just depends on how much you want to sink into it.

kotuku4

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#2431795 3-Mar-2020 13:31

Scott3:

If you are doing non-potable water, you might as well combine it with rain water harvesting, or gray water collection. If you are going to feed this to toilet flushing, cold laundry use etc, you likely need to use a licenced plumber, get consents & notify your water supply company.

Rain water harvesting is fine if set up and maintained correctly. Be prepared to flush the tank as required. And dose/treat/boil if needed for back up water supply. All water will age and degrade, in pipes and tanks.

Recommend against using grey water in any water tank. Grey water is septic waste water and needs full treatment and dispersal area, away from food crops and limited human contact. Our council requires fully engineered system, resource and building consents for "grey water" treatment and reuse.

neb

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#2431948 3-Mar-2020 15:30

WaffleMakerMan: Have a tap and pump at the tank and just run a hose from your irrigation to it during restrictions?
Fully integrated would be nice but the cost would outweigh the benefit I think. Even for irrigation you are supposed to have back flow prevention theres alot of rules around this stuff when I looked into it a while back. That and having it seperate completely removes the risk of contamination to your water supply and the street.

Can be done though just depends on how much you want to sink into it.

That was another option that someone suggested, just physically switch over the connection from the tank to mains water and back as required, which would avoid all the regulatory requirements. In terms of consent it's not such a big deal, we can roll it into the build, it's mainly the cost vs. convenience since crawling under the house to switch a connection over will be a pain. After that the pipes are buried under walls/concrete/etc so there's no place to do the switch.

neb

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#2431952 3-Mar-2020 15:36

Forgot to add, we'll probably be using one of these, unless someone has experience with them that indicates it doesn't work, or knows of something better. Again, the motivator is the lack of easy access to the tank, so having it mostly self-cleaning will be good.

wellygary

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#2431953 3-Mar-2020 15:39

neb: Forgot to add, we'll probably be using one of these, unless someone has experience with them that indicates it doesn't work, or knows of something better. Again, the motivator is the lack of easy access to the tank, so having it mostly self-cleaning will be good.

Err, why are you bothering??

You originally said this was non-potable gardening water....has it changed to now be drinking water?

IMHO, you're spending money for nothing....

neb

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#2431959 3-Mar-2020 15:54

wellygary:
neb: Forgot to add, we'll probably be using one of these, unless someone has experience with them that indicates it doesn't work, or knows of something better. Again, the motivator is the lack of easy access to the tank, so having it mostly self-cleaning will be good.

Err, why are you bothering??

Because having a tank in a mostly inacessible position fill up to the drainage point with sludge and gunk, and even more so for the sludge to get into pipes and valves and whatnot, would be a pain.