When Elevation Necessitates Pumping Downhill: Problems and Solutions

It’s best to avoid it, but if a system must pump downhill, make sure to protect the pump from prematurely burning out

When Elevation Necessitates Pumping Downhill: Problems and Solutions

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Learn from my mistakes: Once upon a time a developer created a subdivision with houses built at the top of a steep bluff, and the soil tester tested in the flat farm field 45 feet straight below the houses. 

Here comes a novice installer (we’ll not use any names to protect my identity), who as usual places the tanks near the home at the top of the bluff. The system had to be pumped to the mound because the mound was designed to be pressurized. 

A month after the owners moved into their new home their alarm was going off. The pump had failed — burned out. But how? It was only a month old, it must be an inferior pump, I figured, so I replaced it; and another month went by with the same result, a second pump failed within a month's time.

After researching the system design with the pump company they realized that the pump was pumping 42 feet straight down. And that is a bad thing for several reasons.

When we talk about total dynamic head, pumps are designed to accommodate the fact that it will have some resistance (vertical lift, friction loss in a force main, etc.). As you pump uphill, the vertical elevation is actually a good thing for the pump as long as the pump is sized properly for the correct elevation difference. If the pump had no resistance it would burn out. Like mine did twice.

By pumping downhill I had negative total dynamic head, the pump had no resistance, and ran so fast it burned out in a month.  

A major pump manufacturer whom I interviewed for this article said that a second problem besides no resistance for a pump is pumping downhill like that after the pump turns off will siphon the rest of the water out of the tank. A long siphon is bad because it will actually spin the impeller right off the pump in some cases. 

Different solutions

This article is specific to pumping downhill because the system is not a gravity system, it is required to be pressurized or dosed. 

The obvious first solution is not pumping downhill. When you can, locate the pump tank down at the lowest point and pump up into the system. This is by far the best option, when it is possible. 

A second (partial) solution is using an anti-siphon fitting on the force main so after the pump shuts off it doesn’t siphon. This is a great way to protect the pump. But that’s only protecting siphonage, not pump burnout.

A third option, if the system is not pressurized but merely dosed, would be a siphon method. Rissy Plastics makes a “Flout,” a type of siphon that doses water down grade without the use of a pump or any electricity. This is a great method of dosing when the dose tank is higher than your system elevation. This method will only siphon a predetermined amount and stop, unlike the type of siphoning mentioned above.

You can use the Flout also as replacement siphons in some old siphon tanks where the siphon loses its prime. Some installers replace failed siphons with pumps, but in some cases the Flout could be used and save the customer adding electric to the tank.

The siphon method works almost like a pump by dosing after the water builds up to a certain point in the tank. Where in a pump tank the water would trip the pump float, in a siphon the water trips a dose of water into the system. 

How we fixed the problem

Well, this was already in place. People lived in the house and tanks were already installed and we did not want to burn out another pump. So how did we correct the issue?

The pump manufacturer told us to take a ball valve and install it in the force main in the pump tank after the pump. We installed a ball valve and turned it about three-quarters of the way closed to add resistance to the pump. We drilled a 1/4-inch hole in the force main as well. If I was pumping uphill, I’d drill that same hole as a weephole for drainback. What this hole did was act as the siphon break so after the pump shut off the hole caused the force main to
not siphon. 

The pump is still running decades later.

So not having enough resistance is a problem for pumps. If you have a really low TDH (let’s say 5 TDH or less), look at the pump curve. You might notice most pump curves stop at 5 TDH; I had two systems in the past week that would have ended up with TDH just under 5. In those instances, we always add the ball valve and close it halfway or more to restrict the flow, thus increasing the TDH, which aids in the longevity of the pump. One manufacturer also told me you could restrict the flow to increase TDH by installing a foot of 1-inch pipe after a pump (then returning to the actual force main size.) I really prefer the ball valve option because I like the flexibility to adjust if we need to.

I’ve been using the ball valve option when needed with excellent results and long-lasting pumps. Even the pump that burned out twice in two months has lasted decades once we added the ball valve.

So try not to pump downhill if you don’t have to. But if you have to pump downhill, make sure to protect the pump from prematurely burning out.

About the author
Todd Stair is vice president of Herr Construction, Inc., with 34 years’ experience designing, installing, repairing, replacing and evaluating septic and mound systems in southeast Wisconsin. He is the author of The Book on Septics and Mounds and a former president of the Wisconsin Onsite Water Recycling Association.


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