My father-in-law has a vacation house that is subject to frrezing. He drains (blows out with air) the hot and cold water lines when it is left vacant. There is now a very noticable sulphur smell in both hot and cold water, especially in the hot wat. He has flushed that quite a bit, but the smell remains. There is no noticable smell from the hose bibs.
What would be generating this smell in the water lines? I understand this issue in well water, but not from the city.