Sewer Smell When My Furnace Comes On? A MN Tech Explains

So you’ve noticed a nasty, sewage stench throughout your home every time your furnace kicks on. And you’re desperate to get rid of it (we don’t blame you).

First, you’ll want to rule out a dead animal in your ductwork (a dead animal and sewage gases can produce similar smells).

If you’ve ruled that out, let’s move on to the most likely answer: If you get a sewer smell when your furnace starts, the problem is most likely that your furnace is pulling in sewer gases via return air duct leaks.

Unfortunately, this is both a plumbing problem and an HVAC problem. Don’t worry, though. We’ll explain both problems and what you need to do to get rid of that sewer smell altogether.

Sick of the stench and just want it gone ASAP? Just contact us and we’ll send over a professional immediately.

The HVAC problem: Return air duct leaks

Most homes use air ducts to cycle air to and from the furnace. Those ducts can be split into 2 sections:

  1. Return ducts that pull air into your furnace to be heated.
  2. Supply ducts that deliver warm air into your home.

Think about it this way: return ducts “suck” air in while supply ducts “blow” hot air out.

Properly designed return ductwork will suck in air from conditioned areas of your home. But leaks in those return ducts allow air to be sucked in from unconditioned areas of the home (attic, basement, crawl spaces, between walls, etc). 

And if there are escaped sewer gases wafting about in those unconditioned spaces, voila! Your furnace can now suck in those gases and spread it throughout your entire home (lucky you).

What to do: Have a professional seal your air ducts

If you have duct leaks, you’re likely dealing with higher-than-average utility bills. The good news is that sealing your duct leaks can save you 20% on heating and cooling bills.

In addition to saving you money, duct sealing also ensures your return ductwork can’t pull in nasty sewer gases anymore.

Sealing your ducts prevents other duct leakage problems, including:

  • An excessively dusty home
  • A higher chance of introducing mold spores into your home
  • Higher utility bills
  • Uneven temperatures, stuffy areas

But let’s go back to your particular problem: sewer gases being distributed throughout your home. Why are sewer gases wafting about in unconditioned spaces of your home in the first place? 

Well, that leads us to the plumbing part of your problem…

The plumbing problem: Escaping sewer gases

In normal operation, you should never smell sewer gases in your home. As soon as you flush your toilet, empty your garbage disposal or turn on your shower, that wastewater is directed out of your home via enclosed sewage piping.

But sometimes, problems in that piping system allow sewer smells to escape and invade your home. These problems include:

  • Dry P-traps
  • A clogged drain
  • Leak or cracks in piping
  • Backed up main sewer line
  • A loose toilet wax ring

So, how do you know which one is your issue? We’ll walk you through some DIY steps you can take before calling in the professionals.

What to do: Find and stop sewer gases from escaping

Before you call a plumber to inspect your pipes and find the escaping sewer gases, try these DIY fixes:

Check any drains near return ducts for dry P-traps

Step 1: Go to each water appliance that is located near a return duct in your home (such as a basement toilet or sink) and shine a flashlight down the drain. If you don’t see a few inches of water, you have a dry P-trap and can continue to Step 2 below. 

A P-trap is basically a thin layer of water designed to block any sewer gases from wafting back up the drain and into your home. But over time, that water can slowly evaporate (especially if the drain isn’t used often).

So, if you don’t see a layer of water in your drain, your P-trap is dry and allowing sewer gases to float back into that unconditioned space. And if there are leaks in nearby return vents, the gases will get sucked into your ductwork and eventually spread throughout your home.

Step 2: Pour several gallons of water down the drain until you can see a layer of water standing in the P-trap.

Step 3: If that drain rarely gets use, pour 4 ounces of mineral oil down the drain. This slows down evaporation.

Check for clogged drains near return ducts 

If a drain near a return duct clogs above the P-trap, the decomposing matter in the clog can create some serious sewer gases. And if there are leaks in the nearby ducts, those gases can get sucked in and blown into your home.

Check for slow or clogged drains. If you’re up or a DIY drain clearing solution, try these solutions.

Can’t clear the drain on your own? Or have you tried the above steps but still smell sewer gases when your furnace comes on? Have a plumber inspect your drains and water appliances. They’ll be able to locate the escaping gases and get rid of that sewer smell.

Live in Minnesota or Western Wisconsin and need a tech or plumber?

Whether you need a plumber or an HVAC tech to solve your sewer smell issues, we can help.

Just contact us and we’ll send over the professionals you need.

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