Buyer's guide

Well pumps: sizing, constant pressure, and what actually fails

Undersized head kills more pumps than anything else. And the constant-pressure convenience carries a repair cost most buyers never hear about.

Get the well log first

Before anyone quotes you a pump, get the driller's well log. It tells you the well's total depth, the static water level, and — most importantly — the pumping water level, which is where the water settles while the pump is actually running. That last figure is what the pump has to lift against, and it is often 50–150 feet below the static level.

Without it, sizing is guesswork, and the guess is usually wrong in the same direction.

Head, not horsepower

The most common well pump mistake is choosing on horsepower and treating head as an afterthought. Head is the vertical lift the pump can produce, and it has to cover:

• the pumping water level (how far down the water sits while running)
• the pressure you want at the fixtures, converted to feet — roughly 2.31 feet per psi, so a

50 psi system needs about 115 feet

• friction losses through the pipe run, which grow with distance and shrink with pipe diameter

Add those up and buy a pump comfortably above the total, not marginally above it. A pump running continuously at the edge of its curve is a pump that fails early. Our recommender flags models whose head is marginal for a stated well depth for exactly this reason.

Constant pressure: worth it, with a real caveat

A constant-pressure pump uses a variable-frequency drive to vary its speed and hold steady household pressure regardless of how many fixtures are open. No more pressure dropping in the shower when the washing machine fills. It also soft-starts the motor, avoiding the hard electrical and mechanical shock of across-the-line starting, which genuinely extends pump life.

It is a real comfort improvement and most people who have it would not go back.

The caveat is repairability, and it is rarely discussed at the point of sale. On the popular integrated designs, the drive electronics live inside the pump, down the well. Field reports cluster failures around years four to six. When it goes, the pump has to be pulled — and pulling a pump is the expensive part of any well repair, often several hundred to over a thousand dollars in labour alone before parts.

A conventional 3-wire pump puts its control box above ground at the wellhead, where a capacitor failure is a cheap part and a screwdriver.

So the honest framing is: constant pressure buys comfort and pays for it in expected repair cost, and the trade gets worse the deeper and less accessible your well is. A 120-foot well in an open yard is a different calculation from a 400-foot well under a deck.

There is a middle path many people miss: a conventional pump with an external variable-speed controller mounted above ground. You get constant pressure with the electronics somewhere you can reach. It costs more than a plain setup and less than a pulled pump.

Sand is a design input

If your driller mentioned sand or silt, or your last pump died early without an obvious cause, say so before anyone quotes.

Pumps with floating impeller designs let abrasive particles pass through rather than grinding against fixed surfaces, and they can be rebuilt rather than replaced. In a sandy well this is the difference between a pump that lasts fifteen years and one that lasts five. It is a specific design feature to ask for by name, not a general quality tier.

The pressure tank matters more than people think

An undersized pressure tank makes the pump cycle on and off constantly, and cycling is what wears pumps out. If you are on a conventional pump, the tank is not an accessory — it is part of the system's life expectancy.

Rule of thumb: drawdown capacity should give you at least a minute of run time per cycle at your typical flow. Bigger is better and tanks are cheap relative to pumps.

Constant-pressure systems need much less tank because the pump modulates instead of cycling.

What to ask a well contractor

• What is the pumping water level from the well log, and what total head are you sizing to?
• What is the drawdown capacity of the pressure tank you are quoting?
• Is the control box included in this price, or separate?
• If this is a constant-pressure system, where do the drive electronics live?
• What is the labour cost to pull the pump if it fails out of warranty?

That last question is the one that changes how the quotes compare, and almost nobody asks it.

Power and water are the same problem

If you are on a well, a power outage means no water — not just no lights. This is the single strongest practical argument for backup power on a rural property, and it changes how you size a battery: a submersible pump's starting surge is one of the hardest loads in the house, so you size on continuous kW, not on kWh. Our battery recommendations account for this automatically when you tell the recommender you are on well water.

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