Top Toilet Makes Bottom one Flush


Old 09-19-03, 05:16 PM
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Top Toilet Makes Bottom one Flush


When the water shuts off after filling the upper floor toilet, it causes water to be sucked out of the bowl in the toilet on the lower floor and the tank refills. They share the same pipes and vents.

Any idea why this is happening? I did have sewage back up into the first floor toilet and had to replace the wax o ring.

Could a bad seal in the first floor toilet be causing this? I also replaced both toilet flappers and fill valves and closed the upper toilet shutoff valve as much as possible to lessen the water pressure.


Last edited by Ragnar; 09-19-03 at 06:39 PM.
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Old 09-19-03, 06:33 PM
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Test the water pressure around 1 or 2 in the morning off of a hose bibb and run at least 15% of the hot water out of tank.

This will tell if you have high water pressure.

Sounds like water pressure is causing your problem more than anything in this matter.

Post back so we have a figure to work with.
Old 09-19-03, 06:44 PM
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Some of the syptoms you describe are hard to understand... You say that when the upper toilet stops filling, the BOWL of the downstairs toilet empties... That sounds to me like the sewer is again stopped up which is causing the lower toilet bowl to siphon out... However, that doesn't explain why the downstairs TANK would refill? That tank should not empty unless someone has lifted the flapper, so that is not happening... Please explain in as much detail as possible what happens to the lower toilet... Also, if there is a tub or shower downstairs, check it for sounds of gurgling or excessive water when the upstairs toilet is flush several times in succession... Also, partially closing the valve to the upstairs toilet will not decrease the pressure to that fixture or to any other fixture for that matter... It only decreases the volume... At best, it will make the upstairs toilet fill slower and will make it a little quieter filling up in some cases...
Old 09-19-03, 08:30 PM
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More information

Thanks for the quick response.

When the upstairs tank stops filling, the lower bowl begins to refill for a few seconds.

Now, if I run the upstairs cold water while the upstairs tank is refilling, the lower bowl does not refill. Not sure if this is because I have lessened the water pressure by using the sink.

This has been going on for over a year so I'm thinking the sewer is not backing up again. Any chance the sewer pipe or vent is partially obstructed by something?

There is no lower level shower or tub unfortunately.

I'm a novice when it comes to plumbing. What do I need to check the water pressure at the water heater?

Old 09-20-03, 06:37 AM
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I'd say replace all or some of the lower tank parts, like the fillvalve and flapper, these can be to problem.
Old 09-20-03, 11:08 AM
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Don't check the water pressure at the water heater; it will give a false reading due to thermal expansion.

Follow my first posting and you will get your answer.

Sounds to me high pressure is causing the fill valves to actuate by simple means of sharp jolts of pressure in line.

Example: Flushing other toilets ......sinks.......washing machine. etc etc etc.

Follow plumber 2000 on the advice because old fill valves wear out, causing this same situation.

Anything above 60lbs of pressure is considered my area 80lbs is recommended a pressure reducing valve and expansion tank at the water heater.
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