Several weeks ago while we were having our hot water heater replaced with a tankless unit I shared a shocking photo.

While the picture may look quite a bit like a set prop piece from a dream sequence in A Nightmare on Elm Street, it was actually the deteriorated metal flue ducting that had long served its purpose of directing the exhausted gasses from our water heater up and out of the chimney. Though it had done its job for years, its structure rusted away to the point where it was becoming a potential hazard.

I was quite relieved to remove the whole vent duct when we replaced the water heater (new heater is direct vent with PVC), but this left a hole in the brick where the old vent entered the flue.

To resolve this issue I mixed up a little mortar we have leftover from the repointing project last year and used a half brick to patch the area up.

But while I was patching up the hole, like any normal person, I decided to use the opportunity to stick my phone in the flue to snap a few photos and just to see what I see. What I saw was even more shocking than the old vent duct. The other vent pipe for the furnace had been nearly completely eaten away by rust, leaving a half collapsed shell of itself.

The level of deterioration was so severe that it got me thinking and worrying about the possible danger this vent might be putting our family in. Rather than sit idly by and potentially asphyxiate during the first cold days of the year, I figured I could remedy this situation on my own by replacing the pipe with a new one.

First off, allow me to point out that this is a single wall stainless steel rigid vent duct. This means that the replacement of this item is actually pretty straight forward, and also very inexpensive. But this is only because the vent duct is not within 18" of any combustible surfaces between the furnace and the flue. If it were within 18" of anything, like a framing member, drywall, or a giant hoard of paper bags Wendy happened to be hiding in the basement, we'd need to be using one of the double walled vent ducts that's made for such situations since the exhaust from the furnace heats this pipe up so significantly.

I picked up all of the parts I'd need at Lowe's using off the shelf straight 3" duct lengths and elbows. I also bought a roll of foil tape since I apparently can't be trusted with half used rolls of tape and have misplaced all of my four rolls of half used foil tape in various places throughout our house.

I started the task by disassembling the existing vent duct. It was being held in place with a single screw into the furnace vent outlet and removing that screw allowed me to free the whole assembly.

When it came time to remove the duct from the furnace, most importantly I wanted to ensure nothing from inside the duct fell down into the furnace when I jostled things. I knew there might be a lot of junk that had accumulated in the vent duct, and the last thing I wanted was that junk making its way into the furnace exhaust blower and playing the role of Tasmanian Devil with the innards of our already aged furnace. To prevent this potentially catastrophic event, as I carefully lifted the vent duct I slid a piece of metal between the duct mouth and the exhaust outlet. The piece I used was the side piece from a nearby computer that's just laying around in the basement (you know, cause everyone has laying around computers).

After successfully removing the duct without dumping disgusting stuff into the furnace, removal of the rest of the pipe was super easy. Just a quick cut of the tape around the point where the pipe met the flue, and the whole thing slid right out.

I was careful to keep the whole thing intact so that I could measure each length of replacement duct to mimic the lengths and curves of the original. I also didn't want to do anything to dump any of the accumulated junk all over the basement floor.

If you're every working with duct elbows that you need to get the angle just right on, and you want it to stay in place when they're set, there's a very easy trick to accomplish this. Using a small length of foil tape, once the angle is set, run the tape lengthwise across all of the elbow's seams. This essentially pins the whole thing in place. Then just cover the whole elbow in tape to seal up any gaps left by the joints.

After making up the new vent duct, I sealed up all of the seams and joints with foil tape and fit it in place on the furnace and into the flue.

Feeling good that I was being proactive in protecting our family from CO poisoning, I decided to open up the old length of duct to see what sort of shape it was actually in. Who knows, it could have been just fine and I went to this length without cause. However, upon breaking it open (it was so decayed that is broke in my hands) my suspicions were confirmed. The interior of the vent duct was almost completely filled with rust, dirt, and hard debris that must have slowly fallen into the duct over the years.

There is no way the duct was able to successfully vent the furnace as it was intended, there was easily 75% or more blockage in some places. I'm so very happy to have taken care of this issue now rather than when it became a real problem.

For good measure I also installed a new Nest Protect CO/Smoke detector in the basement just to be sure we're alerted if there's ever an issue.

This is one of those nagging maintenance items that it's very important to stay on top of. Too often the utility items in a home are "out of sight, out of mind" until you have a problem. But a little preventive maintenance saved us a much bigger problem down the road. It saved our furnace, it reduced the risk of a fire, and it helped protect the health of everyone living in the house.

Okay, so who's going to go check their furnace and water heater now? Maybe just for peace of mind?

Comments 13


11/10/2014 at 8:48 PM

I rent so don't have that kind of stuff to deal with, and when something does come up, call the apartment manager, which I had to do recently.

My electric hot water heater simply stopped heating water one day, and the tepid water never recovered to hot, so called her and she got it dealt with.

However, when I finally find a house to buy (in the search phase) I will do my best to stay on top of things like you just did.


This is one of those joys of homeownership that I have a serious love/hate relationship with.

11/10/2014 at 8:59 PM

Three cheers for staying safe! Glad you caught that in time.

11/11/2014 at 6:57 AM

I've got a pile of fireplace issues to work through over the next year. 4 fireplaces, and we don't feel safe using any of them yet.

Projects for safety don't have as much "wow factor," but they definitely are necessary.


Ugh, that's the worst feeling. Fireplaces are so wonderful, but only if you feel safe using them.

11/11/2014 at 9:12 AM

Hang a fire extinguisher in that room, too, so that one day if you come to check on the alarm & find a fire you can do something about it!


Good call on that. We've got one at the base of the basement steps just in case. We hope we'll never need to use it, that's for sure.

Laura C
11/11/2014 at 9:38 AM

Nice work - that old duct looks pretty alarming. What are your thoughts on the Nest CO/smoke detector? I was thinking of getting one, but never did because of all the problems they had when first released.


I bought one of the Nest detectors when they came out since it's a combo detector that's wifi enabled, and I like it for what it is. However, we've not bought several because we're holding out for the alarm system compatible version.

Franki Parde
11/11/2014 at 1:46 PM

An "ounce of prevention..." franki


So very true.

11/13/2014 at 3:30 PM

Yuck. That's rather disgusting. Though to be honest, the last time I cleaned out the cold air return it didn't look much better. Miss Molly is a border collie and they shed like crazy. Speaking of which, I need to do that again and put in new filters (no central air yet, so no filters in the summer).

Yep, weekend project!


I have a weird obsession with changing the filters and looking at all of that disgusting buildup. Makes me feel good that they're doing their job or something.

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