[Daniel Holzman]

This topic has been discussed at least three times in the last year or so on this forum, you may want to do a search. But let's start with this. You have a 40 gallon heater, assume it is at 120 degrees F. Your shower probably runs at about 2 gallons per minute, unless you have the low flow 1.5 gallons per minute showerhead (check the head to see the rating). At two gallons per minute, after 15 minutes you will have used up approximately 30 gallons of hot water, if you keep it at 120 degrees, and shower at 120 degrees (that is what mine is set at). So you will have used up 3/4 of your hot water, which would have been replaced by cold water in the tank, and due to normal mixing, the water coming out of the tank will be cooler, i.e. start to cool off, which is what you report happens. So why does this surprise you?

If you want to check the flow in the shower, get a 5 gallon bucket and time how long it takes to fill. Measure the temperature of the water at the showerhead with an accurate thermometer. This will tell you all you really need to know to determine if everything is normal, which I suspect it is.