What Can Cause A Shower Trap To Empty Out?

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My family owns a single story house that no one has lived in for a few months. It will be put on the market in the spring. In the meantime, we've run into a strange issue. The house is 300 miles from where I live, so I'm troubleshooting this from afar.
I stayed at the house during the holiday season and noticed that the first time I flushed the toilet, the shower drain gurgled. I know that that can happen if the trap is empty, so I ran water in the shower and then flushed the toilet again. No more noise for the rest of the 4 day stay.
I should point out that the sewers on their street were replaced very recently and you can see where they dug up the front lawn to attach the sewer from the house to the new sewers. My dad and sister stop by the house every now and then but I do not know if the toilet had been flushed between the time the sewer work was done and when I flushed the toilet. The assumption I made at the time was that the trap had been sucked dry during the sewer work and once I refilled it, all was well.
Well, my sister just called and told me that she was at the house, flushed the toilet (the first time since Christmas) and heard the gurgling in the shower. She ran water in the shower, flush again and heard no more noise. (She was not aware that I had had the same experience about 3 weeks ago.)
As best she could, she checked the trap in the basement and didn't see any signs of leakage.
Since the house was empty for a few months before the sewer work was done, and we don't recall the gurgling happening prior to that, I'm wondering if the issue is related to that work. Could they have done something to the sewers at the street that is causing a pressure related situation and sucking the trap dry? Is a descendant of Daniel Bernoulli squatting in the house when we are not there? ;-)
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The traps could be drying out by evaporation. The place has been empty in between gurgles, consequently no water was going down the drain during those times.
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On 01/16/2016 2:20 PM, John G wrote:

+1 as first choice if there's no visible leak
Although only a month to six weeks seems pretty quick, the water surface level only has to below the bottom level of the U by a fraction for there to be an air break so it's not like the trap has to be completely dry.
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On 01/16/2016 2:49 PM, dpb wrote:

Oh, meant to add --
If it's going to be vacant for a while again, after the shower has drained so it's not flowing put a tablespoon or so of vegetable oil, baby oil, or anything similar down the drain. That'll cover the surface with a lower-rate evaporating film. If it doesn't then gurgle next time, you'll have uncovered the culprit in all likelihood.
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On Saturday, January 16, 2016 at 3:50:05 PM UTC-5, dpb wrote:

It's even shorter than that. 20 days since the last "holiday tenant" left the house.
It's a shower only fixture, so since there is no tub overflow, I'll suggest to my sister that she figure out a way to seal the drain. (Rubber pad, pot full of water on top) and see what happens.
That said, I have to assume that some of you have a guest shower that goes 20 days without being used. Are your traps empty after 3 weeks?
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On Sat, 16 Jan 2016 15:41:02 -0800 (PST), DerbyDad03

I have a shower that goes as much as 6 months without being used. I use the toilet next to it every day. This is the toilet I posted about months ago that makes metal-flapping noises (from somewhere) at the end of the flush cycle, but it never makes gurgling noises from the shower. I never smell anything either.
I think the last time I used the shower was April, the night and the morning before surgery. They gave me two foil-wrapped sponges, soaked in something, one for the night before and one for the morning before, to wash my neck with (where the incision would be) and I thought, or the instructions said, that a bath would not be clean enough.
I didn't get an infection so I guess they worked.
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On 1/16/2016 6:52 PM, Micky wrote:

The above implies that you are full of sh*t but not very clean.
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wrote:

bedroom and bath are on the first floor. On the second is a bath room. The tub is seldom used, but I use the toilet up there almost every day. About once a week if I think about it, I run some water in the tub just to keep the trap full.
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On Saturday, January 16, 2016 at 10:59:08 PM UTC-5, Ralph Mowery wrote:

But do you have to? Do you get the gurgling if you don't?
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I don't know about the gurgling or if I have to. I just do run some water in it to make sure the trap stays full so I don't get the smell if it would go empty.
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On 1/16/2016 11:00 PM, Ralph Mowery wrote:

We have a few unused fixtures at work. Cooking oil solved the problem.
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On 01/16/2016 5:41 PM, DerbyDad03 wrote: ...

It does seem a little short but much would have to do with the actual conditions in the house as far as evaporation rate and what the actual trap design is; they're various shapes and mayhaps this one is "more flatter" than others. It's also possible there's some suction being applied owing to venting but my bet would it's not the issue and might as well eliminate the quickie, easy as the culprit first.
As for the question; not here, no, we don't in specfic. There was one in the basement in the house in VA but it was a floor drain with full 2" drain so the size of the trap was quite large compared to a fixture drain so no comparison.
I'd still posit as my best guess it'd behaved the same way while the house was occupied if the shower (and probably a shared lavatory?) were _never_ touched but that it didn't occur owing to there being the occasional use or even just routine cleaning, etc., that would freshen the trap without your even thinking of it or recalling that such did happen now as it was so routine.
But, yes it is possible there's a new phenomenon altho I'd really be hard pressed to think of something outside the house; there being a blocked vent from a bird nesting or somesuch would be higher on my list than that (see above)...
Good luck...
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On 1/16/2016 3:49 PM, dpb wrote:

If it's evaporation, you may be able to keep the water in by covering the drain with some thing. Bit of rubber, duct tape, saran wrap.
I'm unsure if a couple drops of mineral oil into the drain will coat the surface of the water and help reduce evaporation. Maybe?
- . Christopher A. Young learn more about Jesus . www.lds.org . .
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On 1/16/2016 2:20 PM, John G wrote:

brand new building the traps in the floor of the HVAC equipment room would dry out every few months. We could always tell by the smell when we came in.
Bill
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Slow leak P-trap, in my estimation.
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On Sunday, January 17, 2016 at 9:35:59 AM UTC-5, snipped-for-privacy@gmail.com wrote:

Trap is accessible from below. According to my sister it is completely dry with no signs of leakage anywhere in the area.
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On Saturday, January 16, 2016 at 1:55:17 PM UTC-6, DerbyDad03 wrote:

My janitorial company has cleaned a couple of buildings where the restroom floor drains emitted sewer gas if water wasn't poured in them at least once a week. I don't know what happens to the water in the trap. One building's restrooms were on the second floor, so a leak would be evident. The other was on a slab. Adding a non-evaporative chemical in the drain didn't help b ut for a short while.
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On Sat, 16 Jan 2016 11:55:10 -0800 (PST), DerbyDad03

Could be a blocked vent. I had gurgling once in my 2-flat. Didn't ever localize it, but heard it when emptying the kitchen sink, so the gurgling was probably coming from the sink trap. It was pretty loud. Got a plumber and he went right to the roof after hearing it, and power rodded it. Pulled out a rubber ball slightly smaller than the vent. Probably the work of my tenants, who went on the roof occasionally. Anyway, that solved it. If the vent is accessible to squirrels, wouldn't surprise me if one got in there and died.
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On Sun, 17 Jan 2016 07:41:38 -0600, Vic Smith

I was getting ready to post the same thing. A blocked vent or lack of a vent is the likely problem. Running water down a drain nearby (emptying a sink or flushing a toilet) sucks the water right out of the shower's trap. That's the first place to look.
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