As far as any sewer smell or back pressure that won't be an issue.
Your sink trap will take care of any sewer gas and there should be no back pressure.

I do not see any problem with your idea.
But I am an amateur plumber at best.
Perhaps someone more qualified can respond.

 

You can remove the flange, bush the toilet line down to a 2" line. Then plumb it as a normal sink. This would include adding a vent and P trap. You might be able to add an air admittance valve if your code folks allow them.

 

You might be able to add an air admittance valve if your code folks allow them.

I'm an amateur. If I don't vent it and don't add a air valve, what would the result be? Would it basically drain slow and gurgle when it drains? Any idea how to find out if Calgary (Alberta Canada) code allows the air admittance valves?

Also, instead of busting up the floor and existing 4" toilet drain, I'm just wanting to put some type of adapter on it to convert it to a 2" sink drain line. Is this available at hardware stores? Looks don't matter since I will be putting in a cabinet style vanity and sink, so the plumbing will be hidden. Though at the same time I don't want to do something that is against code or obviously wrong, since it will be visible if the drawer is opened and I don't want potential issues one day when I sell.

Thanks

 

I work for a plumbing wholesaler so I know a tinybit. I don't feel that this is an issue but yes you will need to adapt. I'm trying to imagine what you have going on here so bear with me. What kind of material is your toilet drain into the ground? PVC or cast iron? If it's PVC, they make what's called a bushing that is basically a donut with 4" 'outside diameter' and 2" inside diameter. I don't think pressure should be an issue either & if you're worried about smell, I would add a p-trap on your vanity.

 

Got it, thanks.

 

Toilets are sometimes vented on the main stack. But, if his toilet is actually vented as the image I posted shows, the it would probably be acceptable to connect the sink drain to the toilet vent and use it as the the drain and vent. This would be a typical wet vent. Something like this might work-

 

So my question. Can I connect a sink directly to the toilet floor drain in your picture? I removed the toilet, and it is in that area where I want a sink now. I already ran a hot water line to that area, and have the cold water line (from the old toilet). Can I simply buy a 4" to 2" adapter, silicone it into the toilet floor drain, and then connect my sink to it?

Thanks

 

If you don’t have a vent as shown above the trap seal you will need a p trap, which is not allowed, and it could siphon out and stink. An air admittance valve will suffice.

 

Just a stupid question but why don't you just reuse your old sink drain?

 

Because I had a partition wall where the old vanity was - I knocked out this wall. So basically the old drain was plugged at the floor and I then covered it with tiles. Plus where this sink was is on the opposite side where the toilet was, so if I were to use it I'd have to run ~10' of extension to get the drain to where I now want the new sink. Being the area of the new sink was a toilet, I simply want to use that toilet drain for my new sink

 

Here's what I would do, based on all of the recommendations and advice here.
I'll assume you are putting the sink against a wall.
Plumb the drain with a tee and plug it. Run the drain into the floor drain as planned. If you need to add an AAV you will have the tee there to do so.
It is unlikely you will need one since the overflow of a vanity sink will provide air.
And the toilet pipe already installed has a vent already.

 

We wanted to convert the bathroom into a toilet room,is it even possible to do this without getting an unpleasant smell?

 

Well, what I can tell you is the best way to see this in practice, of course, it is not advisable to redo the purpose of the rooms because there were previously brought sewer pipes of a certain structure and this can affect the effectiveness of the devices. We also redid the room and wanted to make a toilet room, put the toilet and noticed that there was a very bad smell from the sewer, it's all because of the structure of the pipes. We solved this problem by buying a toilet bowl on the advice of this site strongest flushing toilets with a check valve that delays unpleasant odors, this does not affect the functionality.

 

Thank you so much for your advice my husband and I are already going to buy a toilet on your advice, we found a model from a quality company ceresit. We really hope that there will be no problems with unpleasant odors, and we are already ready to do a full repair of the new toilet room. Thanks for the help

 

Why do people keep asking if there will be a smell?

There is no difference between a toilet and a sink, or anything else that drains water from your house. It all does to the same place. "Sewer gases", or just smelly air, can basically be in any of your drain pipes at any time. This would be an issue for every sink in your house, and they've already thought of a solution many years ago. It's called a "trap", because it traps some water in the pipe to block any air from coming through. All your sinks have to have them. It has nothing to do with the fact you're installing over a toilet drain. On a toilet the trap is built into the toilet itself, on a sink you have to plumb it in separately.