I have been advised, that I can put salt into a trap to prevent the water from freezing in building I do not use much during tough part of the winter. I know I need to remove all water from pipes where possible, but I would like to avoid manually sucking the water from traps, especially from toilets.

My questions:

  • can I use road salt, or is it safer to use the salt used for human consumtion? In other words - is something wrong in road salt that could damage the pipes?

  • what is correct dosage of road salt (for sink, toilet) if I do expect about -20 degress celsius as the lowest outdoor temperature possible (small building).

  • 4
    thete is nothing wrong with salt ... it just happens to promote corrosion ... why don't you use RV antifreeze?
    – jsotola
    Oct 12 '20 at 7:56
  • 2
    Because I have 50kg of road salt I do not need.
    – matousc
    Oct 12 '20 at 9:09
  • 11
    Maybe I'm wrong here, but I'd think the drain lines are the least of your worries. If your drains function properly, you'll have a little bit of water in each trap and otherwise completely empty pipes. There should be enough room for the water to expand in the pipes as it freezes without breaking the trap. The supply lines, however, are full of water from one end to the other all the time and if they freeze, there is nowhere for the expansion except out of the pipe through a break. Also, draining the traps can allow sewer gas into the building which you don't want.
    – FreeMan
    Oct 12 '20 at 12:29
  • 10
    @FreeMan traps freeze, and it's a really easy way to make a right mess of a toilet (ceramic is not a big fan of expanding and contracting.) Traps commonly freeze at the surface first, so that plugs the rest of the trap and further freezing results in pressure rising in the trapway.
    – Ecnerwal
    Oct 12 '20 at 16:20

Your plan is OK except that -20C is about the lowest you are going to get protection with NaCl (i.e. common salt). Here is a graph of freeze point vs. concentration. You really cannot get a higher concentration than about 23% since excess salt precipitates out.

I think you'll be better off using an anti-freeze product.

Freeze point vs. concentration

  • Good point, but building interiors rarely approach actual outdoor temps due to solar gain. If the coldest outdoor temp will be -20C, salt should do fine.
    – isherwood
    Oct 12 '20 at 13:43
  • 2
    I'd probably try a bit of anti-freeze intended for automotive windscreens. That stuff tends to hold till -25 and is cheap and easy to get by. But if you go that route, a bit of pure (cleaning) alcohol might not be a bad idea either.
    – Mast
    Oct 12 '20 at 16:51
  • 4
    @Mast Most people use "RV" antifreeze for this purpose. It's environmentally safe and good to -45C or -50F. It's also relatively inexpensive.
    – jwh20
    Oct 12 '20 at 16:56
  • 1
    @Mast wouldn't the alcohol evaporate rather quickly, though?
    – user35915
    Oct 12 '20 at 17:37
  • 8
    @Mast Do not use automotive antifreeze. It's pretty toxic, and pouring it down the drain is illegal in most places. There are antifreeze solutions specifically made for toilets, so use those. Oct 12 '20 at 20:25

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