Do I need to remove my pre-fab firebox before installing an insert?

No, if you remove the fireplace there would be nothing left to put an insert into.
You will need to find an insert that is approved to be placed into a Pre-Fab fireplace and one that will fit. It will also require a fully insulated Stainless Steel liner that goes from the insert all the way to the top.

Is there any chance you could just install a new chimney and a freestanding fireplace in another location? You would be much happier.

Hi. I would say, no. I had something that looked like that and the installer put an insert in there. See what model and size zero clearance fireplace you have, measure, go to local stove shops and see what fits and clearances, etc.

That wood right above the fireplace might have to go.

I have a similar pre fab box and put a Jotul c350 in it. It is a small insert.

I also have a similar pre fab , I've got a Lennox ( Country) C160 in it, its a very small insert with a 1.6cuft firebox, yes you need a full stainless steel liner but not necessarily a insulated one, think it depends more on what part of country you live in and length on the run. i live in a mild climate and have a short liner run, my liner is not insulated

However i think if i had to do it all over again i would of installed a rear vented free standing stove, that way you can have a bigger stove, longer burn time. would require a new hearth to meet the stoves specs

Elderthewelder said:

yes you need a full stainless steel liner but not necessarily a insulated one, think it depends more on what part of country you live in and length on the run.

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Are you sure about that? Just because yours isn't, doesn't mean that isn't supposed to be. These codes are International Codes, some places don't have an inspector looking it over, but codes should still be followed.