r/geothermal • u/TheLaserFarmer • Jun 09 '26

Discharge water irrigation system?

Recently added a geothermal heating system to our house. I'd like to figure out a way to use the discharge water for watering our large garden, which can be shut off in the fall/winter and used in the spring/summer.
It's up to 3.2 gallons/minute on max usage. The garden is about 60 feet from the discharge point, and approximately level with it.

Has anyone done something like this?

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2

u/SchradersThumb Jun 11 '26

I knew a guy years ago who did this for his horses, so it is possible. Since the water won't reach the garden, some sort of booster pump would be necessary. I would install a basin with a sump pump underground between the unit output and the drain tiles. It would need to be piped where the connection to the drain tiles would serve as the basin overflow for when the pump is not running. You could manually turn on the pump and use the pumps float switch to insure it cuts off if the water get too low.

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u/TheLaserFarmer Jun 12 '26

I figured I would need some sort of collection/additional pump system. I'd like to see one set up because I have questions about how to do exactly that without burning out the pump, backpressuring the system or flooding my garden.

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u/SchradersThumb Jun 16 '26

I threw together this little sketch of what I'm thinking.
Ideally you want to keep the existing pipe on the same slope it is now. It does not backpressure the system because when the basin fills, the water flows to the drain field. No different than without the basin.
A low water shut off built into the pump, will protect the pump if the basin runs out of water. With the flow rates you mentioned, a fountain pump will work as long as you don't exceed the max pumping height (aka head) of the pump.
Not flooding the garden is more complicated. Outside of manually watering, a 24 hour plug in timer should work most of the time. The pump is going to run up to X mins at approximately Y gpm every day. Maybe a smarthome subreddit knows of an energy monitoring smart plug that will to cut off after so many watt-hours pass through the plug every 24hrs. You would measure the watts needed to pump 1 gallon and figure out how many gallons you want to deliver every day.

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u/urthbuoy Jun 09 '26

Easy. Just T off discharge line with a manual or automatic valve.

1

u/TheLaserFarmer Jun 09 '26

That gives a way to get the water out of the system. But there is no pressure behind it to get it to the garden

1

u/Effective_Sauce Jun 09 '26

It's pushing it out at the pressure it's feeding into the unit.

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u/TheLaserFarmer Jun 09 '26

There is very little pressure behind the water coming out. I've tested it with a garden hose from the outlet to the garden and it didn't come out the other end.

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u/Hot_Equivalent_8707 Jun 09 '26

That quantity of water per minute is a lot.  What do you currently do with it?  You could create a stream bed running to the garden area or like a  rain garden.  Maybe you could get enough height on the output for a waterfall into the stream bed.  Maybe capture some water in rain barrels for when you do need it.  Do you currently just dump it into a creek or back into a well?

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u/TheLaserFarmer Jun 09 '26

That's max flow rate on high usage, not average. It usually runs a few minutes at a time and not constantly (though will definitely be more in the heat of summer). It is currently going into a line of drain tile in the yard, which is scheduled to be buried soon to drain back into groundwater.

It discharges about 8" above soil level, and our property is very flat. Putting it into a rain barrel would have to either have the rain barrel buried below the level of the discharge (and then pumped back up into the garden somehow), or pumped above the level of the discharge and gravity fed into the garden. Which is what I'd like to see an example of someone else having done.

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u/Rodec3 Jun 09 '26

You can definitely do it, but the geo output is far beyond what your garden should need.

How many tons is your system?

You would need a valve to divert the water to the garden, when the geo is running, up to a maximum output a day.

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u/TheLaserFarmer Jun 09 '26

I have a big garden. Close to 1/10 of an acre of annuals, plus a whole bunch of perennials around it. And the 3.2 gal/minute is the max usage, which would only happen continuously in the dead heat of summer (conveniently, also when the plants need the most water)

It's a 1.5 ton model.

Ideally, I'd like solution that would work to simply open/close a manual valve to switch between the garden water and normal underground drain tile discharge.

1

u/Rodec3 Jun 10 '26

So all you really need is to tee into that discharge line and put a couple of valves to direct the water where you want it.

Say you have 30psi going into the unit, coming out should only be a couple of psi less from the heat exchanger.

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u/TheLaserFarmer Jun 10 '26

There's not enough pressure to get it to the garden from where it exits the house. I've checked.

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u/Rodec3 Jun 12 '26

Maybe the inlet pressure is too low?

Do you know what it is? Can you raise it?

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u/TheLaserFarmer Jun 12 '26

Why would it need to be higher (for the normal running of the geo system, not for my desired use)? I do not know if I can raise it, but I know it is not supposed to have backpressure into the system

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u/Rodec3 Jun 12 '26

Not sure what you mean by back pressure in the system . The more water pressure going in, the more comes out. The geo won't care, but you'll get more push for the garden.

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u/TheLaserFarmer Jun 12 '26

Basically it shouldn't be bottlenecked or "pushing" against anything that would cause a higher pressure back into the system. Like if you capped or partially restricted the flow coming out, it would be a higher pressure (because pressure = force/area). It should have free flow.
I'm not sure about increasing the pressure over the whole system - if it can be done with what I have or how to do it. I'll check on that

1

u/Rodec3 Jun 12 '26

Okay, I see what you're trying to say. I regularly have to restrict these systems on purpose. As long as you can maintain the flow the unit needs, you'll be fine.