I have an older natural gas floor model heater. I've been turning the garage into a an activity room for the family.

The heater works fine. No issues. After 30 yeas or so, however, the gas fittings to the heater, venting pipes, etc. look cruddy. Especially compared to the modest remodelling in the room.

My issue is that I have cleaned and painted the heater, changed the supply line, some of the venting pipes, etc. All looks new and sparkling...except for 2 feet of the 3" b pipe vent that goes from the room, through the ceiling, through the roof.

I can't really change this last section without cutting into the ceiling, redoing the flashing on the roof and so on. The winter is just not the time of year for this in Canada.

My question. This last section is 3" B venting. Is it okay to slip a section of 4" single venting pipe (or other sheet metal) around the 3" b venting pipe (up to the ceiling) to give it a semblance of good appearance? The 'cover' pipe would be only cosmetic. OR...will this produce some unforseen problem?

thanks