No wall is made from just mortar; mortar is used to hold another material such as blocks or stone together. It's also not at all clear how enough of the "mortar" ended up inside the exhaust pipe to block the flow of exhaust gasses. Without further information, it's impossible to give advice. If, however, you're really experiencing carbon monoxide inside the house, this is a very dangerous situation and a qualified furnace repair person is probably your best bet.

I know it's not just lime mortar but as far as I can see and/or reach from the inside, it is. I'm sure from the outside, it's brick. The mortar just kept crumbling as I removed the old flue pipe and inserted the new one. I wasn't sure if a furnace repair guy would do something about the wall - as it is a wall. Plus, the wall basically needs fixing before it can even hold up the flue pipe (and keep from crumbling into it any more).

lastusername:

This is not a DIY project. Hire a professional and do what he recommends.

You need to carefully remove the bricks from the crumbling part, clean them off. clean out all the crumbly stuff back to solid material and re-do the wall's mortar .. basically you rebuild the wall.

The tricky part is that you are removing the house's foundation to do this ... it's a job for professionals that know how and where to shore up the joists, rebuild the wall, and set the house back down on the new wall.

It sounds as if the area around the original exhaust pipe was patched/reinforced with mortar. There also must be an opening through the brick to allow the pipe to extend to the outside. If you clean out the entire passageway from the inside to the outside before inserting the new pipe, there should not be any problem with debris clogging up the new pipe. How much repair will be necessary after doing this is another question entirely and may well be a job for a mason. Without any information about the type of furnace or the temperature of the gasses, it's impossible to know if something other than mortar could be used for patching/repair.

In regard to the "many cubic feet of crumbling mortar". Is this really the mortar holding the bricks together or is it an interior coating over the brick walls? If it's the first, you really do need to consult someone knowledgable about foundation repair. If it's the second, the crumbling coating was originally entirely cosmetic and is not effecting the overall strength of the foundation. It is, however, masking the brick and concealing any problems with it.

If I call a pro, where should I look? The "chimney" on top of the house is - I think - purely decorative. The actual flue leads elsewhere; would it still be considered a chimney? On homeadvisor.com, "Furnace / Forced Air Heating System - Repair or Service" doesn't seem to cover the flue. Should I look at "Repair a Brick or Stone Fireplace"?

"Furnace/Forced Air Heating System-Repair or Service" certainly covers the flue.

I highly recommend homeadvisor.

Ah, OK, I see. I was looking in the problem checklist part for crumbling mortar (which the chimney one has and the furnace doesn't), and for the furnace category I assumed that the "smells funny" referred to leaking of the fuel and "leaking" to water leakage. I suppose either of those will do here. Thank you.