A toilet water surge could happen in the tank or bowl, and when it happens in the bowl, it's often a sign of a blockage in the waste or vent pipes. If your house is on a septic system and you see a water surge in one or more toilets, the likely cause is a blockage in the drain field or septic tank, especially if the surge happens during a rainstorm or a winter freeze.
A water surge in the tank is often due to a faulty fill valve. As posters on the Plbg.com forum recommend, the easiest way to handle this is to replace the valve.
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Waste and Vent Blockages
When water surges from a toilet bowl, the most common cause is a blockage in the waste line or, if the problem happens only when you flush the toilet, a blockage in the toilet. A toilet plunger will break up such obstructions 90 percent of the time as long as you use it properly. Be sure to fill the cup with water so you aren't plunging air.
If you don't have any luck, try taping off all the other drains in the bathroom to focus the force of the plunger on the obstruction. If plunging doesn't work, you may need to use a toilet auger or even remove the toilet, but don't put Drano or any other caustic drain cleaner in the toilet.
The surge could also be caused by a blockage in the vent pipe. When water flows from another fast-draining fixture, such as a different toilet or the washing machine, it pushes air in front of it. If the air can't get out through blocked vents, it may have no other exit than through the toilet. If this is what's happening, the water surge will be accompanied by air bubbles, as described by Scott English Plumbing.
Vent blockages are usually caused by debris or ice buildup in the main vent stack that exits through the roof. You can clear debris from the vent opening manually by climbing on the roof, but if ice is the problem, your best bet is to locate the vent pipe inside the attic and heat it with a hair dryer to melt the ice.
Septic System Backup
If your house is on a septic system and the toilet starts to back up during a rainstorm or winter freeze, stop using the toilet and call a plumber or septic pro to evaluate the situation. If you have a gravity-fed system, the drain field could be flooded or frozen, and since water can't drain there, it's backing up into the house. This is a problem no one wants to have, but it's one you can't ignore, and it's best to deal with it quickly.
Some septic systems rely on a transfer pump to empty the tank. If the pump malfunctions, the tank can't empty and water has nowhere to go but back into the house, usually surging first from toilets in the basement and on the ground floor. The pump may need servicing, but before you call a plumber, look for a tripped breaker in the electrical panel. Resetting it may solve the problem.
Surges in the Toilet Tank
When water surges in the toilet tank, the first thing to do is try readjusting the toilet float to lower the water level. If you aren't able to stop the surge this way, the fill valve is probably worn out and needs to be replaced. This is a simple job you can do yourself and is a good opportunity to switch out your old ballcock valve for a more efficient cup-style float valve, such as the inexpensive Fluidmaster 400A.