Condensate pumps

Why there is no drain down there

Water only runs downhill, and a crawl space is already the bottom of the house. A dehumidifier sitting on a vapour barrier under a joist has its drain outlet somewhere around ankle height, and the nearest thing that will accept water — a laundry standpipe, a utility sink, a rim-joist penetration to the outside, a sump — is usually above it, often by ten feet or more. Gravity draining, which is what every dehumidifier manual recommends first because it has no moving parts and cannot fail, simply isn’t available.

Older basements are the same story for a different reason. Many were built with a floor drain, and many of those drains were later capped, plumbed into a system that now needs a backflow device, or found to discharge somewhere a modern code inspector takes an interest in. Plenty of finished basements have a perfectly good drain sitting behind a stud wall nobody wants to open.

The alternative to a pump is emptying a bucket, and a bucket is a machine that stops working the first weekend you go away. Anything running unattended in a crawl space needs a permanent water path. That is the whole job.

What the pump actually does

A condensate pump is a small reservoir with a float switch and a centrifugal impeller in the bottom. The dehumidifier’s drain hose runs into the tank by gravity over a very short drop. Water collects, the float rises, and at a set level the switch closes and the impeller runs until the float drops again. Discharge goes out through a barbed fitting into vinyl tubing, up to wherever you are sending it.

The Little Giant VCMA series — the pumps that dominate this category — use a 0.5 gallon tank and a vertical float switch, which is what keeps the footprint small enough to sit beside a dehumidifier under a joist. The discharge adapter has a check valve built into it, and there is a thermal cut-out in the motor. That is essentially the entire machine.

It runs in bursts, not continuously. That single fact is what makes the sizing arithmetic below come out the way it does, and it is the piece most sizing advice skips.

The specification that is always misread

Every condensate pump is advertised on a gallons-per-hour figure. The VCMA-15UL says 65 GPH. The VCMA-20ULS says 80 GPH. Those numbers are true, and they are also measured at a lift of one foot — which the listing states, in the specification bullet, in the same sentence, and which almost nobody reads.

A pump does not deliver a fixed flow. It develops a pressure, and the higher you ask it to lift, the less of its flow survives. Push it to its shut-off head and delivered flow is zero: the pump runs, the impeller spins, and nothing comes out the top. Little Giant publishes that number too — 15 feet for the VCMA-15UL, 20 feet for the VCMA-20 series — and it is a far more useful figure than the GPH.

So a pump “rated 65 GPH” is rated 65 GPH at one foot and zero GPH at fifteen feet. What it does at the eleven feet you actually have is somewhere in between, and the manufacturer does not publish the curve, so we are not going to draw you one. Two honest points and an unknown middle is what the data supports.

Which raises the obvious question: if you can’t know the delivered flow, how do you size the pump? The answer turns out to be that it barely matters, and the reason is the interesting part of this page.

How much water does a dehumidifier actually make?

Work it from the capacity rating, which is a tested number, rather than from anything on the pump’s box. One pint is 0.125 US gallons, and a pints-per-day rating spread over 24 hours gives you a continuous flow you can compare directly against a pump’s GPH. Here is that arithmetic for five real units from our corpus, running flat out.

The finding: you need a far smaller pump than you think

Look at the fifth column. The largest dehumidifier in our entire corpus — a 158-pint crawl-space machine, running continuously at its full tested capacity, which no real installation does — produces 0.82 gallons per hour. The cheapest Little Giant in the category is rated 65. That is a headroom factor of 97×.

Turn that around and it becomes the useful statement. For the pump to fall behind the biggest dehumidifier we could find, the lift would have to destroy 99.0% of its one-foot flow. You do not need the manufacturer’s curve to know that it doesn’t — not at any lift below the shut-off head, where flow is still positive at all. A typical 50-pint basement unit asks for 0.26 GPH, which is 0.33% of the rating.

The practical consequence is that GPH is essentially never the binding constraint on a residential dehumidifier install. Buying a bigger number because your dehumidifier is big is solving a problem you do not have. What can genuinely stop water reaching the drain is the other figure entirely.

Lift is the constraint. Buy on shut-off head

Shut-off head is a wall, not a gradient. Below it you get some flow; at it you get none. The VCMA-15UL stops at 15 feet and the VCMA-20 series at 20 feet, and those four feet decide whether a lot of installations work at all.

Measure the vertical rise from the pump’s discharge fitting to the highest point the tubing reaches — the highest point, not the destination, because tubing routed up over a beam and back down has to be lifted over the beam. Horizontal run costs you very little: at flows around one gallon an hour through 3/8″ tubing the water is barely moving and friction losses are negligible compared with the vertical rise. It is the height that counts.

Then leave margin, because the delivered flow near shut-off head is small and the unpublished part of the curve is exactly where you would be operating. Staying comfortably under the rated head is the honest way to handle a number the manufacturer hasn’t given you.

The safety switch is the part that matters

A condensate pump will eventually fail, and when it does the tank keeps filling. In a crawl space that means water onto a vapour barrier, under insulation, and into the exact conditions you bought the dehumidifier to prevent. In a finished basement it means water onto a floor.

The fix is a float-operated overflow detection switch: a second float, higher in the tank, wired to cut the dehumidifier off if the water gets past the normal pump-out level. The appliance stops making water, the pump stops being overwhelmed, and you find out because the humidity climbs rather than because something is wet. Little Giant states the switch “can be set to shut down the appliance or sound an alarm”, and notes that basic wiring knowledge is required.

🔴 This is the one specification worth paying for, and it is the difference between two pumps that look nearly identical. The VCMA-15UL listing says, in capitals, “DOES NOT INCLUDE OVERFLOW DETECTION SWITCH”. The VCMA-20ULS includes it. The price difference is $9.00.

Tubing, check valves and the run itself

The VCMA pumps discharge through a 3/8″ barbed adapter into vinyl tubing. The adapter has a check valve built in, and it is doing real work: without one, everything in the vertical run drains back into the tank the moment the pump stops, which refills the tank, restarts the pump, and gives you a machine that cycles forever without ever emptying itself. If you ever replace the adapter, replace it with one that has the valve.

A few things that decide whether a run keeps working:

• Rise first, then run. Get the vertical done near the pump and let the horizontal section fall very slightly towards the discharge. A dip halfway along collects water and, in a cold crawl space, freezes.
• Support the tubing. Unsupported vinyl sags into traps. Clip it to a joist every few feet.
• No kinks at the top of the rise. The tightest bend is usually where the tubing turns over at the high point, which is also where a kink costs you the most.
• Terminate somewhere you will notice. A discharge into a sump or a standpipe you can see beats one into a cavity you cannot.
• The tank inlets are not filtered. The VCMA tanks take water through inlet holes in the lid; debris from a dusty crawl space goes straight in and ends up around the float. Keep the lid seated.

Why pumps fail, and why you will buy another

A condensate pump is the moving part in a system whose other components mostly sit still. Take the largest unit again: if it ran at rated capacity all year it would move 7,209 gallons, in 0.5 gallon sips — on the order of 14,418 start-stop cycles. That is an upper bound and a real installation will be well under it, since no dehumidifier runs at its rated capacity continuously. It is still the right order of magnitude for understanding why this part wears out and the vapour barrier doesn’t.

The failure modes are consistent:

• The float sticks. Biofilm, dust and mineral scale build up in a warm, wet, dark tank. A stuck-up float runs the pump dry; a stuck-down float overflows. This is the most common failure and the most preventable one.
• The check valve stops sealing. Grit under the seat gives you backflow and a pump that short-cycles for months before anyone notices.
• The motor gives up. Thermal protection saves it repeatedly, and then one day doesn’t.
• The tubing traps or freezes. Not the pump’s fault, but it presents as a pump failure and gets one replaced unnecessarily.

Almost all of it is answered by one habit: pull the lid once a season, lift the tank out, rinse it, and check the float moves freely. Ten minutes, and it is the difference between a pump that lasts and a pump that doesn’t. Do it at the same time you change the dehumidifier filter and you will actually do it.

What we can’t tell you

We have never installed any of these pumps. There is no test rig here, nobody has run one to failure, and every specification on this page is the manufacturer’s own, transcribed and attributed rather than verified. What we have done is take those published figures, take the tested capacity of real dehumidifiers, and do the arithmetic in public so you can check it.

The specific gap worth naming: we do not have the pump curve. Little Giant publishes flow at one foot and shut-off head, and nothing between. Every claim on this page about intermediate lift is therefore bounded rather than precise — we can tell you that 0.82 GPH is far below any point on any curve that reaches 20 feet, and we cannot tell you what the pump delivers at exactly eleven. Where a number would have required inventing one, there is a sentence instead.

If you are still choosing the dehumidifier itself, size it on pints per day rather than on a coverage claim in square feet, and see the desiccant page if the space runs cold enough that a compressor will struggle.