Thanks for the fan finder link. To use that site, I need to know the duct size I have. Might there be a standard size or will it be easy to tell once I start taking it apart?

Maybe someone here who knows for sure will answer you, but I'd think you could just pull it out and see how large the duct is. Or maybe you could send your picture to the Broan folks (if you don't see a model # on it) and ask them what the likely duct size would be.

Here is a link that might be useful: Broan customer service

If you remove the cover assembly the model of the fan should be in the vent housing. You can then use that model number to research the original fan and figure everything out.

If you find no model number, just measure the size of the fan housing, or the size of the hole cut in the ceiling, and use that to find a comparable fan.

I'd venture it's a 4" duct.

For a replacement vent, you'll want to match the size of the vent housing that is recessed in the ceiling, and if you can't get one the same size, consider getting one that is slightly larger. You can easily enlarge the hole in the drywall ceiling but installing a smaller housing and then patching the drywall might be more difficult.

Most fans are "new work" fans, meaning the fan housing is secured to the ceiling joists before the drywall is installed. If you find a model number, go on the manufacturer's website and you can usually find a pdf with installation instructions. That might help you figure out how to remove the fan without destroying the drywall around it.

Thank you all. I followed your advice and found the model number inside. It is Broan model 652. I posted this same question on the Broan website, and they suggested I call Technical Service Department at 800-637-1453. I will be doing that Monday, thanks again!

Ok, so I called Broan tech service department, shared my existing model, mentioned I needed something very similar, and they said QTR080L would be a suitable replacement. I could not find it in the big box stores, so forced to order it online. It arrives, I unbox it, remove the old one, and I notice existing unit has a 3 inch vent, whereas the one Broan recommended is 4 inch! I also find asbestos suspect insulation and aluminum wiring. Seems I will be staring at plastic covered hole in my ceiling for awhile. Someone on Broan's website asked if you can get an adapter to reduce to 3 inch, and Broan replied it was not recommened. Looks like I will be replacing the entire duct now or returning this and back to searching for something similar. Ugh.

I had the same problem with the 652 / 663 . nothing fit but I picked up a broan c350bn motor assembly at home depot for 15.00 . pull the fan off and take the motor off the assembly. the new motor mounts on the opposite side so you need to take one bolt off the new motor mount and put it in the opposite direction and put the nut back on. then do the same for the other side. it needs a small washer or two on each bolt to take up the extra space. now put it on the old assembly after you remove the old motor. take the lock nuts used on the old motor and tighten the new motor to the old assembly. push the fan back on and it works great now. it is a 50cfm motor and the old one was 60cfm .

I have the same fan!!! How do I change the light bulb!!?? Like it’s a real question I have tried everything!!

If you haven't figured out the light bulb, there are two small screws under the grill by the broan labels. These come out with flat head screwdriver to remove housing cover exposing light bulb.

I have the same one and I can’t figure out how to change light bulb. So aggravate.

Ours in Broan Model 652, which looks like the above pictures. As Father Thomas wrote above, there are two slotted screws which go into the fan/light assembly. Those screws go through two keyhole slotted tabs which are part of the grill assembly. You'll need to shine a flashlight up into the fins to see them. In ours, the screws are 3-slots away from the "B" of both Broan badges.

Loosen the screws enough so that the entire vent cover can rotate counterclockwise a little. This movement will align the screw heads with the wider opening of keyhole slotted tabs, thus allowing tabs to pass down over the screw heads, for removal of the grill assembly. From there, you can easily access the light bulb.

To reassemble, align the screw heads with the larger opening of the keyhole slotted tabs, raise the assembly so that the screw heads pass through the tabs, then rotate the assembly a tiny bit clockwise so that the screws are aligned in the narrow ends of the slotted tabs. Tighten the screws enough to snug it up to the ceiling, but not so tight that it bends the aluminum.

While you have the grill off, you might want to clean the fan and vent, which is fairly simple. Remove the 1/4" hex screw which is located near one of the long screws loosened to remove the grill. When the hex screw is removed, the entire fan/light module will swing down on a “hinge”. That motion results in unplugging the fan and light from the receptacle inside the 12X12 housing. Then slide the lowered fan/light module from the hinged peg, to completely remove the assembly from the housing.

With the serial number, I ordered the same fan off of Amazon. I had someone just replace the motor and the outer cover. Tried pulling the whole fan out and after buying and returning multiple fans that would not fit in the space my handy man suggested this and it worked perfectly.

Jennifer, will you share with us the specifics about the fan you ordered from Amazon? Also, did it require any modification?

This is the fan I ordered from Amazon. I searched by serial number and they had the same fan. My old fan was original back to 1990 so I was surprised they still made the same fan. And all he did was replace the motor and the cover. He left the housing in place. This eliminated cutting into the ceiling to replace the housing. There is living space above me so no access there.
I had bought and returned at least three fans that should have fit but they didn’t without cutting into the ceiling.
Search for your serial number and see what comes up.
Hope this helps!