This Heating Mistake Sets You Up For Major Water Damage

Water damage from unused ducts in the winter. Source

Do you have a zoned duct system? If so, you may be tempted to turn off the heat in unused areas of the house to cut heating costs.

However, few homeowners realize that not using air ducts during winter can potentially cause expensive water damage.

Don’t believe us? It happens, just like it happened to this family that had 6 gallons of water drained from their ductwork.

We’ll explain why water can form in your ductwork and what you can do to prevent it.

How can unused air ducts cause water damage?

It all comes down to warm air coming into contact with a cold surface, creating water in the process.

You know how water beads up on a cold glass of sweet tea on a hot day? That happens because warm, humid air comes into contact with the cold glass. The same thing happens to unused ducts in a zoned system during winter.

You see, when your zoned system turns off heat to a particular area, electronically operated “dampers” block warm air from the ductwork that feeds those areas.

The damper inside this duct rotates to control how much or how little conditioned air reaches an area/room. Source

Without warm air flowing through the ducts, they become cold. And when warm air from inside your home finds its way into those cold ducts, moisture forms. Over time, moisture collects and water accumulates inside the ducts.

You’ll then have 2 big problems on your hand:

  • Water damage in your home—Since ductwork isn’t designed to be watertight, that water will quickly find tiny openings to leak out of and wreak havoc on your home. Or, if conditions are right, that water can collect and freeze, only to thaw out and create water damage later.
  • Mold infestation—Mold growth can occur anywhere there is moisture and dust/dirt (that is, inside your ducts).

So, how is some warm air getting into unused ducts and causing all these problems?

According to the NAIMA (North American Insulation Manufacturers Association), warm air gets into unused air ducts in 2 places:

  • Dampers. Zoning dampers aren’t 100% airtight and allow small amounts of warm air into unused ducts.
  • Return air registers. There’s no conditioned air blowing out of the registers connected to unused ducts, which also means there’s nothing stopping the air inside your home to travel inside the return air registers and coming in contact with cold duct surfaces.

How do you prevent moisture in unused ducts?

The best answer is this: Don’t cut heat completely from any one part of your house.

Instead, just lower (not cut off) the heat in unused areas of the home. Lowering the heat will keep the ducts warm enough while preventing condensation. And you’ll also still lower your heating costs a bit.

Water damage isn’t the only problem, either

Turning off the heat completely in unused parts of your home also causes other problems, including:

  • Higher chance of breakdown due to short cycling (system constantly turning on and off)
  • Higher utility bills
  • Decreased comfort for heated areas


Well, your furnace or heat pump is sized specifically for your home. And if you completely cut off a zone from being heated, that heating system is now oversized. An oversized heating system causes the above problems.

So, again, don’t completely cut off heat to some parts of your home. Just lower the heat.

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