Seems to vary, on occasion, in my house. Generally it’s roughly a certain level, but sometimes it’s noticably lower, and this seems to possibly be correlated with a sewer odor. (I’m aware that in general one function of the water in the bowl is to block odors and gases from the sewer from coming up through the pipes, so it’s possible that the low level is not enough for this purpose. But I’m not sure this is actually correlated.)
Question is what controls this?
the toilet drain is like an S rotated 90 degrees, with one end of the letter being your house drain system and the other the toilet bowl. the bowl should refill, after the flush, to the point where water will slowly spill (without making the bowl flush) into the drain.
you are correct that it has to be full to block the sewer gas, as do all drain traps.
until you remedy this you could add water slowly to the bowl with a container, to bring it up to the full level, or hold the toilet handle down partly to start water running but not enough to start a flush.
low bowl can happen if your roof vent isn’t providing enough air, this might also cause poor flushing.
your toilet tank valve may not be staying open long enough to refill the bowl. a flapper valve that leaks air may close too early.
I have had this problem on occasion and it was because of a blockage in the feed pipe leading from the toilet to the main sewer. A full or partial blockage occurs in the feed pipe, causing sewerage to back-up toward the toilet. Eventually pressure builds up and dislodges the blockage, causing a sudden flow toward the main sewer and a partial vacuum which sucks water from the toilet bowl.
I’m afraid it’s a case of opening the sewer manhole covers and get pushing with the rods to remove any blockage.
I seen strong winds lower the level in my toilets. Probably creating a suction by blowing over the stack vent.
I forget how the first part of your name is pronounced. Help?
Psmith
Atmospheric pressure. Known as the “Toilet Barometer”. You should notice a lower level in winter storms, high winds (low pressure) and “normal” otherwise.
I use my toilet to predict coming snowstorms. Better than the local news fool! 