Your plumbing vent stack can freeze up in extremely cold weather, letting harmful sewer gases build up in your home.

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Plumbing Vent Stacks: More Important Than You Think

Your home has a few different exhausts – something for the furnace, maybe a fireplace chimney, a dryer vent, a kitchen fan. The one that usually gets the least attention is the plumbing vent stack. And that’s because it usually doesn’t need much care. But during extremely cold weather, you might have a problem. If it freezes shut, sewer gases may build up in your home.  And it may make it harder to flush your toilet. Here’s what you need to know:

What does it do?

The vent stack allows sewer gases to vent harmlessly through your roof. Most homes just have one stack, and all of the drain lines are connected to it somehow. This is typically a cast iron pipe but in newer homes may be made of PVC.  (Cast iron has the benefit of being quieter, but also conducts more heat.)  Each of the drains in the home (sinks, toilets, tubs, etc.) will also have a trap – typically a curve in the pipe that is filled with water – that acts as a plug to keep sewer gases from escaping from the drain. Plumbing vents keep the air pressure in the drain system normal so that water is not siphoned out of these traps.

a diagram showing a plumbing drain system, including vent stack to the roof and drain line to the city sewer

A diagram showing the plumbing drain system (in black) with a vent to the roof and drain to connect to the city sewer line.

 

plumbing p-trap diagram

This is a p-trap. Water in the bottom of the trap forms a seal to prevent sewer gases from backing up through the drain.

 

What can go wrong?

In extremely cold weather, water vapor in the vent can freeze on the top of the stack, and may close it off completely. When this happens, the pressure in your drain system may be disrupted, causing the water traps to empty. Then, with no vent to the roof, the gases may build up in your home. Besides smelling awful, sewer gas also can contain harmful compounds like hydrogen sulfide, ammonia, and carbon monoxide.  Even if the exhaust vent doesn’t freeze shut, it may be restricted, and this may cause toilets to have trouble flushing or affect other drains in your house.

a plumbing vent covered with ice

Ice has built up on the top of this plumbing sewer vent.

What can you do?

  • From the ground, or if you can see the cap from a window, visually inspect to see if you can spot ice build-up.  Don’t go on your roof.
  • If your toilet is slow or gurgles, this can also be a sign that the vent is plugged.
  • Insulation is key.  If you can access the pipe in your attic, wrapping it with insulation will keep it warmer and prevent ice build-up.
  • Running hot water from one of your faucets will create warm vapor that can help melt the ice.
  • Opening a door or vent to your attic can warm up the stack, but will increase your heating bill temporarily.
  • Periodically add warm water to drains that aren’t used often, to keep the trap full. This is especially important for floor drains.
  • Make sure the cap is clear of snow and leaves.
  • If this is a recurring problem for your home, you may need to extend the stack further from the roof, or add heat tape to the stack within your attic.  Adding an insulated cap may help also.

an insulated cover for a plumbing vent pipe

Learn more about protecting your home during extreme cold weather.

 

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