We have had an intermittent problem with a pungent odour coming into the house smelling like rotting garbage or eggs.
We have had the furance, ducts&chimney cleaned, bleach poured into the sump pump and still we get the smell.
The temperature in the house is 23C today.
One area of attention was the chimney allowing smells to come into the house when the outside pressure changed and the airflow started coming down back into the chimney and into the house. It was cleaned and we thought the smell had stopped.
Because it is intermittent, it has become impossible to associate one action exactly to when the smell occurs.
This morning we were awoken by the smell again.
The smell was also around the actual furnace.