If the flange is embedded in the caulk, cut and try.

Whomever set it may have run the caulking first and then embedded the vent cap.

Thanks, that is a good suggestion. The flange did not look caulked around the sides and bottom, but there might be some goo in there. The top of the stand-off box and the top of the flange were caulked, but the sealant pulled off very easily. I can run a blade around it and see what happens.

I am still looking for an answer on how these things are normally connected to the duct. Are they typically a slip fit so this should just slide off if I get the flange loose? Should I have posted this in the heat and AC forum?

You will need to remove the fan assembly inside the house as well. Do not forget the electrical connections. They will be located inside a cover in the upper left side (IIRC). There will also be 3 smaller screws that hold the inner tube to the outer shroud. If it has been in use for a while, they may be hidden under grease and assorted other yuckyness. Once you have those removed and the screws on the outside removed, you should be able to pull from the outside, or shove from the inside to remove it. On the outside vent, there will most likely be a second flange about an inch from the visible one. You may need to remove some of the siding adjacent to it to get the whole thing out. If you start on the inside, you should be able to get a better view of the outer. Hindsight on my part, I wish I would have removed it instead of cleaning it up and keeping it as I really don't think it works that great and the Iowa winters seem to find their way in through it as it does not seal up very tightly.

Good luck!

"I am still looking for an answer on how these things are normally connected to the duct. Are they typically a slip fit so this should just slide off if I get the flange loose?"

Should be taped with metal foil tape.

Parts of this installation could be 60 years because that is the age of the house. The vent is probably over where the original gas range was located. When the the kitchen was renovated, a deep soffit was installed over the current refrigerator location. In addition to that, the original wood siding was covered with asbestos-cement. All or part of this is newer. It is hard to say exactly what was done when.

The bottom line is that the duct from the surface-mounted fan to the damper pictured on the outside wall is about 30" long. It is beginning to sound like I should take the fan off the inside and look in there from that end. A mirror with a long handle might be handy.

I was planning on rehabbing it partly because I thought it would be easier. It is beginning to seem like the easier part might not be true. Since I have good ventilators over the cooktop and the wall oven, however, I should probably consider removing it. It is redundant for the most part and not compleletly airtight. I need to plug a lot of leaks in my home's envelope.

I forgot to add, thanks for you carefully-considered responses.

If you remove it, there mnay be issues:

a. You might not be able to find replacement siding to match what you've got.
b. If you already have spare siding and can get the old stuff off to the nearest joints without damaging the adjacent ones the color probably won't match, so you'll have to repaint your whole house.
c. The old siding might have asbestos in it.

Thanks:
a. Replacement GAF siding is a close enough match so that you would have to be looking for a mismatch to see one. I have some original ones around too. I hope that I have enough to replace what I need to because I don't know what a minimum order might be if local suppliers don't stock it to sell in smaller quantities.

b. Paint matching is paint matching, on cement siding, cars, and on wood siding. I think I can get it to match pretty darned well, but whether it will age at the same rate as the surrounding stuff will be interesting to see. I have two gallon cans of the SWP mix in the garage with 1 1/4 gallons in them from the last painting 5 years ago (before I bought the house). The base paint is still available though who knows if it is still the same stuff. The listed pigment mix might help them match the slightly faded color. It will be what it will be. I have other repairs to do right now that are more urgent than the vent.

c. I have not done any determination but I assume that it does have asbestos in it. It is not a big deal as long as I do the work myself. Local disposal involves bundling it up for regular trash pick-up wether I am putting out a few pieces or if I am tearing it all off.

Sounds like you have it all worked out! T'wer me, I'd rip it out and stuff unbacked insulation in the run and/or use expanding foam. Otherwise it's just more heating and cooling loss.

I have way to much to do to think about re-siding. It might be closer if there were not wood siding underneath to deal with. I have to foam the roof and crawl space before the walls. Before I can foam the roof, I have to re-roof. Before I can do the crawl, I have to fix the floors and plumbing.

Removing the cement would be relatively easy. but I like the stuff. It holds paint like nothing else.

I have the same siding on my house and had to order replacements from GAF last year. Their minimum order is a pack of 12 pieces. It cost $150 plus tax. Free shipping. Good luck with your project.

I will have to see how far what I have on hand goes and start looking for a local source of used material.