When a toilet flushes slowly there are really on two possible causes. The water is either slow going out through the trapway of the toilet bowl drain or, the water is too slow going from the tank to the bowl and the siphon of the water out the drain never occurs. The siphon is the process that rapidly clears the bowl and its contents with the gulping of air down the drain at the finish. When the water enters the bowl to slow the water just slowly runs through the toilet and doesn’t effectively remove the waste. Sometimes the waste just swirls around and only part of the waste leaves with each flush.
Siphon Flush
If you look at the side view of many toilet bowls you can see the outline of the trapway which sweeps up from the bowl then curves downward along the back of the bowl then continuing downward curves again towards the front of the bowl and goes out the outlet at the bottom between the two bolts that secure the bowl to the floor. The initial upward sweep and then the downward curve is what controls the water level in the bowl, and the trap way is a couple of inches in diameter. When the toilet is flushed a large volume of water should enter the bowl. Some toilets have a siphon jet which is a hole at the bottom of the bowl pointing into the trapway which water pours out of helping to push water into the trapway. The large volume of water should completely fill the trapway setting up a siphon, which sucks the water and waste out of the bowl ending with the air gulping into the trapway. If the water volume entering the trapway is insufficient to set up the siphon action the water just flows over the top of the trapway like water over a dam, this action is what results in the slow flush.
Bucket Test
When a toilet flushes slow, one of the first things you can do is the bucket test, which will test the function of the bowl. Get a pail and fill it with the amount of water that your toilet is designed to flush with. Quickly pour the water from the pail into the bowl, and see how the toilet flushes. If the toilet flushes poorly then you should suspect a clogged trapway and have the toilet augured to remove the clog. If the toilet flushes well then you know the flow of water from the tank to the bowl is the problem and you should direct your efforts in that direction.
How To Improve Flow From The Tank To The Bowl
When faced with a tank to bowl flow problem one of the first things I look at is if “in tank bowl cleaners” are being used. The bowl cleaners often clog the internal water passages of the toilet causing problems. I remove the bowl cleaner from the tank before doing anything else and in some cases this alone fixed the problem. I check the level of the water in the tank to make sure that the tank fills to the water level line on the tank or, the overflow tube. If there is no mark then 5/8” below the top of the overflow tube should be used. Check the flapper to make sure that it stays open long enough when flushing and does so consistently. Replace the flapper if there is any doubt. Make sure the fill valve has water going through the bowl refill tube attached to the overflow tube and the bowl is refilling to the proper level. Slowly pour water into the bowl to see if the water level in the bowl increases. If part of the water from the flush is devoted to filling the bowl to the proper level it can cause a weak flush. Finally take a piece of coat hanger wire into each of the holes under the rim of the toilet bowl and make sure that the siphon jet is clean as well. Frequently mineral deposits can clog the holes around the rim and the siphon jet causing water to flow slowly into the bowl. Checking these items on your toilet may restore good flushing to a slow flushing toilet. If these steps do not improve the flush then the toilet may require replacement.