Dehumidifier Size for Basement: Pints, Square Feet and Drain

What the pint rating measures after the test changed, how to correct it for a cold New York basement, and why the drain matters as much.

Dehumidifier Size for Basement: Pints, Square Feet and Drain

A basement dehumidifier is sized by a number on the box, and the number
means less than it looks. It is a pints-per-day figure measured in a test
chamber at conditions a New York basement rarely matches, under a procedure
that changed a few years ago and lowered every rating on the shelf. Picking
the right size means reading that number for what it is, then correcting it
for the room you actually have: how big, how damp, and, more than anything
else, how cold.

What the Pint Number Measures

The capacity rating is how much water the unit pulls out of the air in a
day, at a fixed temperature and relative humidity in a laboratory. The test
conditions used for years were warm and sticky, closer to a Florida summer
than a Catskills basement. The procedure now in force runs the same test at a
cooler temperature, chosen to be nearer to where these machines actually sit.
Because a compressor dehumidifier removes less water from cooler air, the same
machine now carries a noticeably lower pint figure than it did before the
change. The machines did not get worse; the label got more honest.

This matters when comparing a unit bought years ago with one on the shelf
now, or reading an older sizing chart. A chart written around the old ratings
will point at a number that, under the current label, describes a larger
machine than it meant. When a chart and a label disagree, check which test
each one is talking about.

Sizing by Area and Dampness

The standard sizing charts work on two axes. One is floor area. The other
is how damp the space is, described in plain terms: a musty smell in humid
weather; damp spots on the walls or floor; walls that are wet to the touch;
standing water or seepage. Each step up in dampness moves you up a capacity
class, and each step up in area does the same.

Two things about a New York basement push it further along the dampness
axis than the smell suggests. The first is that the walls below grade sit
near soil temperature all summer, so they are a condensing surface even when
nothing is leaking, which is the whole subject of
condensation on basement walls. The second is that
most older basements take in humid outdoor air through the rim joist and the
bulkhead, so the moisture load is not the room’s own, it is the yard’s. A
basement that reads as merely musty in June is often in the wet-walls class
by August. Size for August.

Erring large costs little in practice. A unit with capacity to spare
reaches the target and cycles off; a unit that is too small runs without
stopping, never gets the air’s dew point below the wall temperature, and
wears out doing it. The one real cost of oversizing is the fan noise of a
bigger machine, and a bigger machine that cycles is often quieter over a day
than a small one that never rests.

The Cold Basement Problem

The rating conditions assume a warm room. A New York basement in early
summer, and a walk-out basement in the Adirondacks for most of the year, sits
well below that. A compressor dehumidifier works by chilling a coil until
water condenses on it, and the colder the room air already is, the less
margin the coil has before it frosts over. Below roughly the low sixties
Fahrenheit, capacity falls off steeply and cheaper units spend much of their
time in defrost, drawing power and removing nothing.

So for a cold basement the pint figure is the wrong first question. Look
for a stated low-temperature operating range and an automatic defrost that
actually stops the compressor rather than letting the coil ice up. Below the
range a compressor unit can work in, a desiccant dehumidifier removes water
regardless of temperature, at the cost of more electricity per pint and a
warm exhaust. The other route is to warm the room a few degrees, which is
what wall insulation does permanently and a heat pump water heater does not.
A heat pump water heater dries the basement as a side
effect but cools it while doing so, which helps in July and works against a
compressor dehumidifier in April.

Set It by the Dew Point, Not the Dial

The setpoint is a relative humidity target, and the usual advice is to
hold the space below the level at which mold grows on cool surfaces, which
is generally given as the low fifties. Lower than that is wasted electricity
in a basement; higher is the musty smell coming back. Verify with a separate
hygrometer placed away from the machine, since the unit’s own sensor reads
the air it has just dried.

Continuous mode, where the compressor runs regardless of the reading, is
for the first days of drying out a wet space or for a unit that is too small
for its room. Left on permanently it is the sign of a sizing mistake, not a
setting. If a correctly sized unit cannot reach the low fifties on a normal
day, the moisture is arriving faster than the machine can remove it, and the
answer is upstream: outside water, then air leaks, in the order the
crawl space article lays out. A
blower door test finds the leaks that matter, and a
bathroom fan upstairs that runs long enough, on a timer or a
humidity sensor, stops shower moisture settling into
the coolest floor of the house.

The Drain Decides Whether It Runs

Every dehumidifier fills a bucket, and every bucket eventually stops the
machine until someone empties it, usually on the day the family is away. In
a basement the drain is the second most important specification after the
capacity.

• Gravity drain. A hose from the unit’s drain port to a
  floor drain, sink or sump pit, running downhill the whole way. Simple and
  silent, and only possible if the unit sits higher than the drain.
• Built-in pump. Some units carry a small pump that lifts
  the condensate to a sink or a window-height outlet. This is what a basement
  without a floor drain usually needs.
• Separate condensate pump. A small box with a float that
  collects from the hose and pumps out on demand. The same device that carries
  water away from a heat pump air handler, and the same things go wrong with
  it: a clogged inlet, a stuck float, a line that freezes where it exits the
  house, which is the subject of
  the condensate freezing article.

Whichever route, check the drain hose can be reached and cleaned. A hose
that slimes up over a summer restricts flow, the unit’s tank fills instead,
and it shuts off exactly as if the bucket had never been connected.

Placement in the Room

A dehumidifier dries the air that reaches it. Put it near the middle of
the space, clear of walls on the intake and exhaust sides, with interior
doors open so the air can circulate and the exterior door and windows closed
so outdoor air cannot. In a basement with several rooms, one large unit in a
central spot with doors propped usually beats two small ones. Tucked into a
corner behind the furnace it dries the corner.

Whole-House and Ducted Units

A finished basement, or a house where the humidity problem is not confined
to the basement, is where a ducted dehumidifier earns its place. It sits with
the air handler, draws from a return, and treats the whole conditioned space.
It is sized the same way, in pints per day, but at a larger scale, and it is
installed rather than plugged in. It is also the option that a
basement finishing job should decide on before the
walls are closed, since the ducts and the drain go behind them.

Reading the Label Against the Room

What It Costs to Run, in Principle

A dehumidifier is a small heat pump that dumps both the heat it removes
and the heat its compressor makes back into the room. That is why a basement
gets a little warmer with one running, which helps the walls. The electricity
it uses is its rated wattage multiplied by the hours it actually runs, and
the hours are what sizing controls: a unit that reaches setpoint and cycles
off spends far fewer of them than one that cannot. The current rate per
kilowatt-hour is on your utility bill, and the arithmetic from there is
yours. What the sizing gets you is the smaller number of hours.

Size for the August basement, not the June one. Correct the pint figure
for a cold room. Give it a drain. Then set it to the low fifties and let a
hygrometer across the room tell you whether it is winning.

Frequently Asked Questions

Why do dehumidifier pint ratings look smaller than they used to?

The test procedure changed. The old rating was measured in warm, humid chamber air; the current one runs at a cooler temperature closer to a real basement, and a compressor unit pulls less water from cooler air. The same machine now carries a lower number. When an older sizing chart and a new label disagree, check which test each refers to.

Is it better to buy a dehumidifier that is too big or too small?

Too big. A unit with capacity to spare reaches the target and cycles off; one that is too small runs constantly, never gets the air’s dew point below the wall temperature, and wears out doing it. The main cost of oversizing is the fan noise of a larger machine.

What should a basement dehumidifier be set at?

A relative humidity target in the low fifties, which keeps cool surfaces below the level at which mold grows without wasting electricity going lower. Check it with a separate hygrometer placed across the room, since the unit’s own sensor reads the air it has just dried.

Why does my dehumidifier stop working when the basement is cold?

It works by chilling a coil until water condenses on it. In cold air the coil frosts over instead, and the unit spends its time defrosting. Look for a stated low-temperature operating range and real auto-defrost, or use a desiccant unit in a space that stays below the compressor’s range.

How do I size a dehumidifier for a wet basement in New York?

Use the floor area and the dampness class on a standard chart, then step up. New York basement walls sit near soil temperature all summer and condense humid air on their own, so a basement that smells musty in June is usually in the wet-walls class by August. Size for August.

Do I need a drain, or is the bucket enough?

In a basement, the drain is the second most important specification. A full bucket stops the machine, usually while nobody is home. Use a gravity hose to a floor drain or sump, a unit with a built-in pump, or a separate condensate pump, and keep the hose reachable for cleaning.

Where should the dehumidifier go in the basement?

Near the middle of the space, clear of walls on the intake and exhaust sides, with interior doors open and exterior doors and windows closed. One large central unit usually does better than two small ones in separate rooms. Behind the furnace in a corner, it dries the corner.

Will a dehumidifier fix damp basement walls on its own?

It lowers the air’s dew point, which stops condensation on the walls while it runs. If humid outdoor air is still coming in through the rim joist and bulkhead, or water is reaching the wall from outside, it runs without stopping. Deal with outside water and air leaks first, then size the unit for what is left.