Does anyone know if it is alright to leave some residue water in the pipe in subzero winter?
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Not Sure about residue water. However, I had the same issue years ago and all the water was drained by contractor and drain lines and pumps that need water was displaced with pink non-toxic propylene glycol antifreeze.
Also, you can leave faucets slow dripping so water can run in pipe. Running water does not freeze.