I used a fireplace a while back that had a pipe with holes along its length suspended near the top of the firebox. It was loosely hanging on some sort of support and easy to knock off with logs. The owner didn’t remember why it was there and suggested something to do with emissions. Does anyone know what this was?
Are you talking about an open fireplace, or an enclosed stove?
I have a wood stove (glass doors, you can see the fire) with several pipes with air inlet holes in them running along the top of the stove. They are fixed in place, but with pins that can wear out and fail to hold them in place. There’s a control lever that lets air go straight up the stovepipe when lighting the stove or otherwise opening the door, or when pushed in closes a damper at the top back of the stove which forces the exhaust to instead come forward across those air inlet holes before finding its way out, so that remaining combustible gases in the smoke get burned with the aid of the extra oxygen and of the second pass over the fire: which simultaneously gets more heat out of the stove and reduces the polluting effect of the exhaust.
I don’t think any of that would work with a standard unenclosed fireplace; but maybe you’re describing a closed fireplace insert?
And maybe the owner of that stove doesn’t know that they need to, or that they can, replace the pins that hold the tubes in place? ETA: or maybe it’s a poor design and just leaves the tubes loose?
I can’t find a good picture of the stoves that shows the air tubes in place – you have to look in the door from below or you don’t see them – but here’s some images of replacement tubes:
https://www.google.com/search?client=firefox-b-1-d&tbm=isch&oq=&aqs=&q=lopi+wood+stove+air+tubes
Further ETA: I did find one picture of a stove with something similar built in that wasn’t a tube at all; so there may be a batch of varying designs out there doing the same sort of thing. But I do think there’d be a control, because if you try to light a stove with the air being forced over the tubes for a secondary burn, there’s probably not going to be enough draft before the stove gets hot for the chimney to draw. Once it’s burning well, then there should be plenty of draw. So if the owner doesn’t understand the tubes, they may also not be using the control necessary to make them work. Or maybe somebody’s coming up with a design that’ll draw enough for a secondary burn even with a cold stove, I don’t know.
Could it be part of the damper assembly that has become loose over years of use?
Modern fireplaces should have a combustion air supply, but my experience is that this was just a pipe at the back of the fire box. Doesn’t seem like it would be a good idea to have it high up near the damper.
I have seen insert fireplaces that had ducting around the firebox and a blower to aid heating the room, much more common with gas fireplaces. Not anything with holes that could be dislodged though.
Could it have been used to hold hooks for hanging things?