Which Damper for my stove?

so this is an Avalon Olympic insert?
(be hard to put a damper in- Pics?)

Even with 28' of chimney you shouldn't need a damper. The Avalon Olympic should be able to control the burn with just the air control on the stove itself for at least an overnight 8 hr burn. Do the dollar bill test on the door gasket to make sure it's sealing. Put a dollar bill in the door and lock it shut. It should be held firmly if you try to pull it out. Try this in several spots around the door. If it pulls out easily you should replace the gasket or adjust the door tighter. Be sure you're using a hardwood like Oak and filling the stove up. That's a big stove that will take 24" wood so you should be able to get some big pieces in there. I have the Travis Industries equivalent, the LOPI Liberty (freestanding), with about 25' of double wall DVL to the support box and another 6' of Class A chimney to the cap and I don't have any issues holding a fire overnight without a damper.

If you have 18 inches of exposed double wall stovepipe before you go into the fireplace flue, I'd invest in a condor probe thermometer. Insert it at `16 inches or so, and check what flue temps you are running. (Or just put a magnetic thermometer on the outside of the pipe to get a good idea - if single wall multiple temp by tow, if double wall by three for a rough idea). I'm not familiar with your insert and don't know if it is cat or non-cat. If it's cat, you should be able to burn with flue temps around 350-450 ideally. If you are running 600 to 700, I'd say try a damper. With any stove, if your flue is running hotter than your stove and you have a problem with short burn times and not being able to slow the fire down as much as you'd like, I'd try a damper. If you have ICC pipe, then you can use their damper, which is in a 6 inch pipe section. If another manufacturer, then you should use the damper section they make, or if there isn't one, just get a damper and drill your present pipe and insert it in the pipe....probably near the top of that 18 inches, but I'm no expert. You could check with either your pipe or your stove manufacturer.

You can be fine with a 28 foot chimney, but you can also have way too much draft, depending on your climate and exposure. I had to go with a damper. While others with my stove could get cat burns with stovetop 450 and flue 350, I could not get my flue temp under 600-700 in bitterly cold weather, and could not get a slow cat burn. Too much draft. My fire was very controllable, just couldn't shut it all the way down, and lost heat up the chimney. So, if that is what is happening with you, by all means try a damper. You can always remove it if it doesn't help, and it might make all the difference.

You do have insulated liner? And block off?

I have about a 27' stack on my liberty. I'm lucky to have a 5-7 hour burn, the stove doesn't hold hot coals. After the fire's dying down there's hardly any coals. My dealer ordered me a key damper which made no difference. They now have a new air control on order, hopefully that will help.