Why can I have an indoor pizza/wood oven, but not a BBQ or smoker? (or can I...?)

Why pizza/wood ovens, but not BBQ/smoker?

Fire is not fundamentally a problem indoors; there are certainly safe ways to do it, like fireplaces.

The things that make fire dangerous are lack of containment and lack of ventilation coupled with significant size. If it's at all uncontained, it's a fire hazard, and if there's not enough ventilation then you can get a smoke-filled home, carbon monoxide poisoning, and all manner of unpleasant/deadly things.

Wood-fired pizza ovens are much more like fireplaces than anything else. The fire is well-contained. They have chimneys, so that all of the byproducts of the fire are safely sent outside, and new fresh air is pulled in.

Standard barbecues/grills/smokers usually fail on the ventilation aspect. They don't have a way to attach a chimney so that you can reliably send all the nasty stuff outside. They may also be insufficiently well-contained. For example, a lot of charcoal grills make it relatively easy to throw sparks outside the grill, which is pretty bad indoors.

Indoor smokers do exist, though. Generally they're either large, with serious ventilation, or they're small stovetop things that just burn a tiny amount of wood chips in a tiny volume, so it's safe without ventilation but not exactly comparable to a full smoker.

The reason the big ones need serious ventilation is that you're deliberately holding smoke (and associated gases) inside, and you can't let that slowly build up in your home. On top of that, it's at a much lower temperature, probably 225-250F compared to a wood-fired oven at probably 600-900F, so a chimney won't even be as effective because you don't have as strong convection from all that hot air trying to rise.

Can I just seal off an oven and turn it into a smoker?

No. If you don't send the smoke out through the ventilation system, it'll be going into your home. (You might have experienced this yourself, if you've ever started a fire in a fireplace and accidentally left the flue closed. It can fill the room with smoke impressively quickly.)

I know you said partially close, but if the oven hasn't been designed for it, you're just asking for trouble. You don't really have a good way to make sure that smoke (and probably carbon monoxide) is held within the oven enough to make it a good smoker, but is not pushed out into your home. This is fire/home safety. You really don't want to be asking for trouble here, and depending on where you live, it may well be illegal.

So it's really unlikely that you can even make this conversion, and if you're doing it yourself you can't trust it anyway, and the absolute best case is that you manage to somehow build yourself an indoor smoker and you've probably had to make compromises that make it not work as well as a pizza oven. So you might as well just buy an indoor smoker. (But you still might not be as happy with it as with a regular outdoor smoker - see dlb's answer.)

Please, Please, Please do not try to do indoor smoking without equipment made for it. It is Dangerous! DANGEROUS

Devices that use open flame inside your house take advantage of a chimney to let the smoke out. Pizza ovens are just funky shaped fireplaces. In a fireplace, the hot fire causes the smoke and nasty gasses to rise up the chimney, drawing in fresh air from the front. This creates a one way flow. It is similar for just about any combustion driven heat device. Block that flow out of the chimney, and the smoke and Carbon Monoxide stay around. Since they are heavier than air, they will settle at the lowest point as they mix with cooler air and cool down.

Smoking meats requires lower heat than is normally found in a fireplace. A lot lower. At this point the smoke is not going to rise as readily or quickly. Here is where you face a second problem: A column of cool air blocking your chimney. If this happens, NONE of the nasty stuff escapes and your house fills with particulates and CO and CO2. THIS CAN KILL YOU!

I love smoked meats too, but I would not put yourself or loved ones at risk for it. It may be inconvenient to use an outdoor smoker, but its a lot more convenient than suffocating on Carbon Monoxide.

Please don't attempt to smoke meats indoors unless you have a specifically designed indoor use smoker that has stuff to help clear the nasty stuff out and not kill you.

Now for BBQ: There are ways to do BBQ indoors. so long as you are not trying to trap combustion by products, and follow safety rules, you can do some pretty tasty stuff. I use an induction cooktop and a cast iron grill to sear the meat and get the lovely hash marks, then finish it off in the oven broiler. That's just me though. There are gas ranges with grills, there are oven BBQ recipes and techniques. You can spend hours on allrecipes looking at different techniques for this meat or that. That said, ALL of them need adequate kitchen ventilation. Not as much as my prior apocalyptic ramblings, but you do need to ventilate the kitchen.

TL:DR Smoke meats outside and stay safe!...oh, and enjoy

Opinion, but there are multiple indoor options, like stove top and oven smokers, gas and electric grills with stove top ventilation systems such as those by Jenn-Air and other makers. These are not really that uncommon.

They do however come with drawbacks such as better ventilation requirements and many are relatively expensive. They are limited use items that are space eaters, you give up space for something which often is not actually used enough to justify. I owned one, and the result was I had a stove-top the size of an 8 burner stove, with a very loud exhaust which was always oily, and could only use either the burners or the grill, not both at the same time due to power draw and the grill top always seemed to cause burns. I an not sure why it was easier for people to understand not to touch burners, but they could not understand the grill might also be hot, but that was the case. When you add in the mess, for most people, the outdoor option is simply cheaper and more convenient.

There's no reason you couldn't have an indoor BBQ or smoker, but you're going to have to either design it for such use, or buy one of the (rather expensive) commercial options.

A pizza oven is basically a wood stove where the exhaust gasses go around a box on their way out the chimney and heat it up. You can find a wide variety of wood-fired oven and stovetop designs over the last few hundred years that were fine to use indoors, they all work pretty much the same way.

A barbecue has the difficulty that it's not sealed. As such it has a few problems for using it indoors, none of which are insurmountable, but they do need to be taken into account. Firstly a barbecue can throw sparks. You don't want sparks landing in your shag carpet. Your indoor barbecue will need to have a hearth of brick, stone, concrete, metal, or some other non-combustible substance out to beyond the maximum range it might throw a spark. You can reduce the amount somewhat by putting a shield or spark curtains like you see on open fireplaces around it, but you're going to need at least a little bit. Secondly, a barbecue indoors will need a chimney. Said chimney will need to be designed to draw properly with the amount of heat your barbecue produces. Depending on where you want the barbecue and what the venting options are, this can prove quite a challenge. But, never fear, modern technology to the rescue! Rather than relying on natural convection, you can install a high-temperature exhaust fan. You can test that it's actually pulling all the exhaust gasses out of the room with a little powdered chalk. Needless to say, unless you're an experienced tinkerer who's got a lot of practice with fires and fans, by the time you get it right it will probably have been quicker and cheaper to just buy one of the models designed for indoor use.

An indoor smoker is rather easier, you just need one that seals up air-tight. Have it draw its fresh air from outside and have its chimney go outside. Get one of the models that uses a fan to control the airflow anyway and it should work fine. The only issue is going to be the smoke escaping into your house when you open the door to check on the cooking process. There are two solutions. Either don't open the door until the cooking process is done and the fire is out. (Difficult, but likely not completely impossible) Or install the smoker in an enclosed room with a good exhaust fan so that the smoke can be cleared out quickly before it leaks into the rest of your house. Once again, unless you really know what you're doing, building this will likely cost you more than just getting one of the units designed for indoor use...

For either type, you're definitely going to want a Carbon Monoxide detector nearby. But then, it's usually good to have those anyway if you ever burn anything indoors... Or even if you don't.

If you can find a surplus laboratory fume hood somewhere and have that installed, that would likely provide adequate ventilation for a barbecue or small smoker, plus whatever other projects you might have that could produce unsavory gasses. It would probably cost you as much as an indoor smoker, but would give you more flexibility in terms of how it could be used.

Cooking fires are set up to achieve complete combustion of the fuel, producing the maximum amount of heat from the minimum fuel, by burning efficiently at high temperature and supplied with plentiful or even excessive oxygen. Complete combustion of carbon-based fuel is effectively smokeless and produces mostly carbon dioxide and steam. Too much CO2 indoors will displace the oxygen in a room and asphyxiate you to death, but small amounts leaking into an otherwise reasonably ventilated room will just make you feel drowsy, so a properly managed cooking or heating fire can tolerate a "good enough" but imperfect chimney and hearth that are open to the room.

To produce smoke, on the other hand, you need deliberately imperfect combustion, at lower temperatures and with insufficient oxygen. This will tend to produce carbon monoxide, which is not just an asphyxiant like CO2, but is highly toxic, and the nasty thing about toxic effects is that they build up over time. Any attempt to run a process that produces it in an enclosed space, where gas can collect*, is thus inherently very dangerous. Carbon monoxide is a silent killer and any amount of leakage back into the room, no matter how small, can gradually build up and poison you to death, and you won't even be able to tell whether or not it's happening until it's too late and you doze off forever.

In a reasonably ventilated room with a proper chimney, a well-built fire producing CO2 that hasn't killed you within twenty minutes likely won't ever. On the other hand, a smoky fire producing CO that hasn't killed you in twenty minutes might very well do it in twenty-five.

*Forget anything you've heard about being lighter- or heavier-than-air. This is technically true but, in practice, gases also mix.)