There is a very invasive smell in my Mum's house.
There are no pets, gas etc but the smell has surfaced since she had a new heat exchanger type furnace put in. The house has a crawl space.. but it does not smell like a mould.
The smell has impregnated clothing, furniture and hits you when you enter.. after a while you get used to it but when my Mum comes to stay she brings the smell with her
Anyone heard of anything like this?

Is there a possibility that an animal has crawled into the system and died there?