Float valves: the failure mode that floods a paddock

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Research. Nothing here has been through a season on our place yet. What follows is how these valves fail, how the flow ratings differ, and what has to go in the line alongside one. When ours has seen a summer and a freeze, this page gets updated with the date.

A float valve is a twenty-dollar part sitting between your well pump and a flooded paddock.

The failure that matters

A float valve fails in two directions and only one of them is quiet.

Failed closed — a bent arm, ice, a swollen seat — means the tank does not refill. Bad, but visible. You find an empty tank.

Failed open is the one to design around. Grit under the seat, a cracked float that filled with water and sank, an arm knocked out of line, a perished diaphragm. Nothing announces it. It simply runs, all night.

The numbers get large fast. A modest 5 gpm for twelve hours is 3,600 gallons; a full-flow valve at 15 gpm is over 10,000. On a well the pump runs near-continuously for hours, and the pump is the expensive part of that system, not the valve. On rural water it is a bill. Either way you get a bog where the animals stand.

The valve is not really the risk. Unattended flow is the risk.

Full flow against standard

Standard. The small brass ones that look like a toilet fill valve. Roughly 2 to 5 gpm at household pressure — small orifice, and the float has to lever against line pressure to close it.

Full-flow. Larger orifice, more mechanical advantage. 10 to 20 gpm and up.

It matters because a herd empties a tank in minutes and then stands there. Refilling at 2 gpm, the last animal in gets what is left. With a few head and a 100-gallon tank a standard valve is plenty. Full flow earns its keep only where stock drink it down faster than it comes back.

Here is the symmetry nobody mentions: a valve that fills four times faster floods four times faster. Full flow is a reason to be more serious about the shutoff, not less.

Check the pressure rating too: cheap plastic bodies are often rated below what a pump delivers at cut-out.

Where you mount it

Above the water, always. The outlet has to sit above the highest water level in the tank. If it discharges under water and the supply loses pressure, tank water siphons backwards into your line. That is the reason air gaps exist.

Set the shutoff level a few inches below the rim as well. A valve adjusted to fill to the brim overflows every time an animal puts its head in.

Bracket or bulkhead. A bracket over the rim is reversible and moves to another tank. A bulkhead through the wall is tidier and stronger, but it is a hole, and a hole in a poly tank is permanent.

One spec that misleads: "1/2 inch" on the box is the inlet thread, not a flow rating. Two valves with identical threads can flow very differently.

Animals and ice

A float ball on a wire arm, at head height in a paddock, is the most vulnerable thing you will install. Horses chew them. Cattle rub on them. Anything with horns finds the arm eventually.

Cover it — a hood bolted over that corner of the tank, a pipe sleeved over the arm, or the valve in its own box plumbed to the tank low down so the stock never see it.

Ice does the same damage differently. The float freezes into the surface and the arm stops moving. Seized shut, the tank does not refill. Seized open, it runs until something stops it. An exposed valve body above the waterline can freeze solid and split, and you learn about that on the thaw.

Which makes the float valve and the tank heater one decision, not two. There is a separate piece on tank heaters; the sizing rules there are their own subject.

The shutoff is not an accessory

A ball valve upstream of the float valve, within reach of the tank. A union next to it so the valve comes out without cutting pipe.

Three reasons. You cannot service a float valve without one. You cannot stop a flood without one. And the shutoff at the house is four hundred yards away in the dark.

Add a second at the source so you can isolate and drain the run for winter, and a hose bib upstream so you can fill a bucket without taking anything apart. A few dollars of fittings and half an hour while the trench is open. It is the gate latch argument applied to plumbing.

What we are putting in

Sizing is not the open part. At the scale this place will run, standard flow is enough, and the decision that goes with it is to order the ball valve, the union and a cover alongside it, because those never get added afterwards. What we do not have is a number for the line pressure at the tank, measured there at pump cut-out — which is also the figure that says whether a plastic body is rated for this line at all.

Fitting one puts that measurement in this section, with the valve, standard or full flow, what it cost, how the arm and float were covered from stock, and the date. That is a purchase, and a purchase leaves the tag alone. Whether the cover held is decided by a summer of animals rubbing on it and by a January that freezes a float into the surface. The Research line at the top of the page stands until ours has seen both.

Before you buy

• Buy the shutoff, the union and a cover in the same order as the valve. You will not go back and add them.
• Size on how fast your stock empty the tank, not on the biggest flow number offered.
• Mount the outlet above the waterline and keep it there. Air gap, every time.

— Chris D., McEwen, Tennessee