[jandrese]

Maybe this is something to have the landlord check? In practice I don't think most gas furnaces are as well sealed as advertised. Many years ago when I lived in a house with gas heat with a dodgy pilot light I got a pretty good feel for how the thing was put together and it was absolutely not hermetically sealed.

[Brian_K_White]

It's not supposed to be sealed, it's just supposed to have draft. (the flue always vacumming from convection)

[throw0101c]

> It's not supposed to be sealed, it's just supposed to have draft.

It depends on the model. High (>90%) efficiency, condensing furnaces are completely sealed off from the house: they take in air from the outside (via PVC pipe), combust, extract the heat, and exhaust the results (via PVC pipes).

* https://www.youtube.com/watch?v=lBVvnDfW2Xo

Older lower efficiency (<85-90%) furnaces suck in air from the house, exhaust into metal pipes (PVC would melt because not as much heat is extracted from the fumes), can be back-drafted and you have to worry about CO.

Basically: if the furnace has vents in which you can see a pilot light, it's the older style.