A leaky furnace vent pipe can cause an unpleasant odor. There are different sources for these leaks, and the most common of these is from a frozen or cracked pipe. This will often result in a leaking of the entire vent line. In some cases this could lead to a huge fire, and in other cases there could be little danger if the problem is diagnosed and repaired.
The two main areas that are damaged by leaks in furnace vent pipes are the insides of the vents themselves and the brickwork surrounding them. In the former case there is a situation where the leak was caused by frost, which melted the insulation around the pipe. The insulation would have to be removed in order to access the internal pipe. If this is done then it is possible to force the insulation out and have a good look inside. This will often reveal a crack or hole. In the case of the wall surrounding the pipe, if the pipe had been cracked in that area it would not have leaked until it had dried out, but the insulation would have remained intact. In both situations the only way to repair the damage is to freeze the affected area.
It should be noted that heating ducts do not usually cause heat loss to a particular room. This occurs because the heat is deflected around the room rather than being received by the room itself. A little imagination is all that is needed in order to think of the most suitable solution. A simple fix for a simple problem can often be made with some adhesive or a small screwdriver. Some minor damage to the insulation may need to be replaced, but the repairs may only be as costly as the repair, which will be vastly reduced when the insulation is replaced.