Condensate Pump Not Working: The Jug Test

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the maker’s manual

Condensate Pump Not Working: The Jug Test

Short answer

Pour a jug of clean water into the reservoir and watch. It runs and empties, it hums and does nothing, it sits silent, or it empties and starts again a minute later, and each of those four points at a different part. Slime is behind several of them, and a pump is one of the few things on a heating system a homeowner really can replace.

A condensate pump is a small plastic box that exists for one reason: the water coming off a coil has to go up to reach a drain, and water will not do that on its own.

It can also stop your whole system, because many of them carry a safety switch that shuts the heating or cooling off rather than let the box overflow. So a dead pump often gets reported as a dead air conditioner.

Before you start

What to carry: a jug of clean water, a torch, a small screwdriver, an old towel and some mild soap. Half an hour.

Gas or electric? The pump itself is always electric and usually plugs in like a lamp, so you can unplug it completely. What it sits under matters though: a gas furnace with two white plastic pipes makes its own condensate all winter, and an air conditioning coil makes it all summer.

Find it first. Look on the floor beside the furnace or air handler, strapped to the wall near a mini split head, or above a ceiling cassette. It has a tube coming in, a thin tube going out and up, and a power lead.

One thing not to do, and one thing to notice. Do not bypass the safety switch. Those two extra wires leaving the pump go to your furnace or air handler and shut it down when the box fills. People disconnect them to get the cooling back on, and then the next blockage puts water through a ceiling instead of stopping the system.

If the system has stopped and the pump is full, that is the switch working. It is doing exactly what it was fitted to do, and clearing the pump usually brings everything back on by itself.

What it actually is

A tank, a float, a motor and a one way valve. Four parts, and three of them fail in ways you can see.

Water drips in and collects in the reservoir. As the level rises the float rises with it, and at the top of its travel it switches the motor on. The motor pushes the water up the thin tube to wherever the drain is, the level drops, the float drops, and it stops.

The small valve on the way out is there so the column of water you just lifted does not run straight back down into the box when the motor stops.

That is the whole machine. Everything below is one of those four parts.

The jug test

One jug of water tells you which of the four parts to look at, and it costs nothing.

Check 1

Pour clean water in and watch what happens

Take the lid off, or pour into the inlet, and put in enough clean water to raise the level properly. Then stand there.

It runs and empties. The pump is fine. If you still have water on the floor, the problem is further along: a kinked tube, a sag holding water, or a blocked end.

It hums but nothing moves. The motor is getting power and the impeller cannot turn. That is slime, and Little Giant’s fix is to clear the blockage in the impeller housing.

Nothing happens at all. Either it has no power, or the float is stuck down and never asks the motor to start.

It empties, then runs again a minute later, over and over. The check valve is letting the lifted water fall back down. This is the whole answer to a pump that will not stop running.

Cleaning one properly

Check 2

Unplug it, empty it, and get the slime out

Condensate is not clean water. It grows a pale slime that coats everything, sticks the float and blocks the inlet screen, and it builds up over a season.

Unplug the pump. Pull the inlet tube and the discharge tube off, with the towel underneath because both hold water.

Lift the lid, tip the reservoir out, and wash it with warm water and mild soap. Swirl it, empty it, rinse it. Clean the float itself and make sure it slides freely up and down its stem, because Little Giant says a dirty float can be too heavy to work.

Flush the discharge tube through with water from a bottle, and look for algae plugs and for any sag where it runs flat. It has to fall the whole way after its high point, the same as any other drain.

Put it back together, plug it in, and do the jug test again.

Little Giant says to inspect and test the whole setup every 6 months, more often in heavy use.

When cleaning does not fix it

Check 3

Power, check valve, then the pump itself

Power. Many pumps plug into a socket on the side of the furnace, which is dead when the furnace switch is off. Others are on a switched outlet that somebody turned off. Try a lamp in the same socket.

The check valve. A small, cheap part in the discharge line. Fit it the right way round, because backwards it stops the pump entirely.

Too much lift. Every pump has a maximum height printed on it. One that was fine for years and now struggles is usually slime rather than height, but a pump fitted to reach a drain above its rating will never work properly.

A frozen outdoor end. If the tube discharges outside, a cold snap can plug it with ice and back the water up. It clears when it thaws and comes back every cold night until the line is moved or insulated.

The motor. They do wear out, and swapping one is genuinely a homeowner job: unplug, swap the two tubes over, move the safety wires across the same two terminals, plug in, jug test. There is very little on this site I would say that about.

Where the water was going instead

If the pump failed and you have water somewhere, it is worth following it before you forget about it.

Water sitting in a furnace cabinet is not just a mess, because the control board and the blower motor are in there. Water from a mini split runs down the inside of a wall where nobody sees it for weeks.

The other thing to check is whether the pump was the cause or the symptom. A pump that fills faster than it can empty is sometimes telling you the coil above it is icing up and then melting in a rush: why a coil ices and how to thaw it.

And if the machine above it is a condensing furnace, its own trap is upstream of the pump and blocks just as readily: the trap, the hose and the three sources of water.

Questions people ask

Why does it keep running and never stop?

Either the float is stuck up, or the check valve is letting the water you just lifted fall straight back in. The jug test separates them, because a stuck float runs with an empty tank.

How often should it run?

On a humid day, in short bursts, again and again. In winter under a condensing furnace, a lot. Silence in the middle of summer is the thing to be suspicious of.

Can I pour bleach in to kill the slime?

Little Giant says warm water and mild soap. Sauermann, another pump maker, says that without a pump cleaning product you can use bleach diluted to 5% in water, but never petroleum products or solvents such as acetone. And whatever you pour in ends up wherever that tube discharges.

Mine gurgles and hums every time.

Normal. A short hum with a gurgle as it lifts the water is what a working pump sounds like. A rattle rather than a gurgle points at the check valve.

Does every system need one?

No. A pump is only there when the drain is higher than the machine or too far away. Plenty of systems drain by gravity and have nothing to go wrong.

The pump is fine but water still appears.

Then it is arriving faster than it is leaving, or it never reached the pump. Check the tray and the line feeding it before you look at the pump again.

Sources: Little Giant VCMA Series condensate pump owner's manual 998086 (float, high water switch, check valve, cleaning, troubleshooting), and Sauermann "How and why should you clean the condensate removal pump" (cleaning products)