maybe the previous tank was insulated.
maybe you have a mixer valve upstream of the toilet that needs hot
water increased slightly.
maybe the bathroom exhaust fan is blocked or broken allowing increased
maybe your new toilet is leaking cold water into the tank so much that
it makes the tank unusually cold.
maybe you have increased usage due to guests or additional humnidity
from their showers.
maybe the leak can be found by you keep reading.
"Toilet leaks can waste as much as four to five gallons of water per
minute and cost you the consumer up to $100.00 per month in increased
water and wastewater bills.
Follow these simple instructions to find out if a leaky toilet is
robbing you of precious gallons and dollars.
Toilet leaks occur in two ways and are often very difficult to detect.
The most common toilet leak and often hardest to detect is caused by a
deteriorated or defected flush valve (flapper) ball at the bottom of
the toilet tank. If the flapper or ball valve does not seat properly
and form water tight, seal water will leak around it into the toilet
bowl. Often, this leak will occur without being heard. To test for this
type of leak, add a few drops of food color or place a colored dye
tablet (available from many hardware stores or possibly your water
utility) in the toilet after it has stopped filling. Do not flush the
toilet. Wait about twenty minutes: if the food coloring or dye appears
in the toilet bowl, the flapper/tank ball or drain seat is leaking and
needs to be replaced.
The second most-common type of leak is caused by an improperly adjusted
or broken fill (ball cock) valve. If the float is set too high or if
the shut-off valve fails to close completely, water will continue to
enter the tank and flow into the overflow tube. This type of leak can
be seen simply by taking the tank top off and observing if water is
flowing into the overflow tube once the tank is full. "
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where much more help is also available