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In a helpful comment on another QA, community member RedSonja wrote:

In [the] future, if you are going to leave the toilets unused for a long time, a spot of cooking oil on the water surface will stop it evaporating as quickly.

I've never heard this tip before, and sounds like a simple solution to a common problem.

Can one simply add a bit of cooking oil on the surface of toilet water to reduce the rate of evaporation? Will this work for sink and shower drains too?

About how much is a "spot" of cooking oil?

Are there any problems with doing this?

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    It's not that common of a problem, TBH. The only time I've used oil in a toilet is when mopping out all the water and refilling with enough mineral oil (old-school - these days, use non-toxic RV antifreeze) to make the trap seal while winterizing an unheated camp. I don't live in a dry desert area, and I've never seen a toilet that's evaporated enough water to break the seal in any house that's still occupied, even if it's one left for months. In a drier area it's not inconceivable. Unoccupied (and unheated, not prepared for it) houses toilets usually break the first winter from freezing.
    – Ecnerwal
    Aug 1, 2022 at 13:59
  • I would recommend instead filling the trap with nontoxic antifreeze designed specifically for in-house (and exterior hot tubs, etc) use. You should be able to find this at any hardware store. Aug 1, 2022 at 17:23
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    "Don't put grease down drains" is for significant quantities, like a pot of deep-frying oil, or - even worse - fat that solidifies at room temperature, like the rendered fat from cooking an entire pound of bacon. A drop of liquid oil a toilet bowl full of water isn't going to hurt anything.
    – nobody
    Aug 2, 2022 at 0:47

2 Answers 2

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I've heard this a few times and have never tried it because grease down a drain is not a good thing. I grew up in a house with a grease trap and guess who had to clean it before I was in the first grade.

A much better solution that I have used when leaving my house or months at a time was to stretch some clingy Glad wrap around the top of the bowl. Never had an evaporation problem. Just remember to remove it before doing any business.

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  • After a long trip home, the bathroom would probably be needed first thing. Having a good memory is quite important.
    – crip659
    Aug 1, 2022 at 13:29
  • If you're somehow unable to see the shine coming from the cling foil, you can always put a centered sticker (of any kind) on it.
    – MiG
    Aug 1, 2022 at 14:05
  • "Don't put grease down drains" is for significant quantities, like an entire pot of deep-frying oil, or (even more likely to be a problem) fat that solidifies at room temperature, like the rendered fat from cooking an entire pound of bacon. A drop of liquid oil a toilet bowl full of water isn't going to hurt anything. If you do any cooking or dishwashing at home you probably put at least a drop of oil down the drain every day.
    – nobody
    Aug 2, 2022 at 0:51
  • Grease that doesn't solidify unless at very low temperatures is not a problem for any drain. Animal fat and coconut oil: bad. Vegetable oils: no problem.
    – user19565
    Aug 2, 2022 at 19:46
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Yes this is because oil doesn't evaporate into volatile vapors nearly as fast as water at room temperature and it blocks access between water and the air.

Yes, this works for other traps as well.

Enough to cover the entire surface of the water exposed to air.

You will end up flushing that oil down the drain when you start using those toilets/sinks/showers again, this can cause contribute to grease buildup in your drains and related clogs.

The drainline might not have humid enough air to prevent evaporation, so over a long enough period of time it may still evaporate and lead to a smelly trap anyways.

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