I need help fixing a furnace vent on an old chimney

[Evan Ryan]

I suspect the vent is leaking. There are some dark spots on the drywall ceiling below the old chimney.

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I dont know why these images turned 90 degrees left, it happened when I uploaded them.

Picture 1 is the chimney with the vent on top.

picture 2 shows the metal plate beneath the vent. This plate could also be letting in water, it doesn't completely cover the chimney and it's stuck to the chimney with an adhesive but I think I can pry it up. I pulled it up, but I'm not taking it all the way off until I have a plan.

picture 3 shows me tilting back the vent. The vent still has an adhesive holding it to the metal plate but it's not strong.

The vent does have a pipe or cylinder of metal going down into the chimney.

the Borg doesn't have a new vent with a base that will cover the 17 1/4" x 17 1/2" chimney.

i went to a roofing supply store, waited around, Lost patience and left.*

back to the Borg, and an employee recommended I use roof sealer. He suggested applying it from the neck of the vent down over the chimney. Like hot fudge on a sundae.

i would appreciate any suggestions.

*ture detective season 1, when Matthew mcconaughey is burning the midnight oil on the case and all the cops except woody harrelson are completely uninterested in his theories. I thought, they're detectives, wouldn't they at least be curious?
the store was not for a diyer, fine, I expected to be made a fool because I'm inexperienced, but I'm sure any of the 6 people who were ignoring me could've helped me figure out a plan, and sold me something.
cest la vie, it's Friday and I'm also to blame for not being aggressive. Just wasn't feeling it today.

[Brad Adams]

Any reputable hvac company should be able to make you a new cap to fit the masonry chimney with a riser on top that the existing chimney goes through. Add a new storm collar and top cap and your set to go. I usually make them out of stainless. DO NOT put roof coating on it.

[Evan Ryan]

Thank you brad, I thought covering it in roofing goo seemed like a bad idea.

[Tom M King]

There are many possibilities for leaks on that roof other than just the cap. I would think that any leaking around that cap would get absorbed by the masonry in the chimney. The mortar packed around the base of the chimney is suspect to me.

[Evan Ryan]

How can I assess leaky masonry?

The furnace vent was the first thing I saw that could use some attention or replacement.

Could I clean up and reseal the vent that's up there with silicon, or something? Will all that rust be a problem?

[Jason Roehl]

I see a few issues there.

1. The sheet metal cap needs to extend over the edge and down the chimney a couple inches with sealed corners. I would have that replaced and paint it with a quality DTM acrylic.

2. The vent cap boot is no longer sealed to the sheet metal cap. Butyl rubber is what I'd use--after a new sheet metal cap is installed.

3. The chimney itself should be evaluated by a mason. It may need to be parge-coated.

[Evan Ryan]

Thank you all for taking the time to respond.

It's an old chimney and it's been repurposed to vent the furnace. if I parge coat the chimney and replace the vent cap I might not root out the problem of poor venting for the furnace. I'm going to call hvac.