We need a little more info on the heater. Is it gas or electric is it a tankless one. Brand info (Manufacturer) would be helpful too Gary

The re-set button on electric water heaters is typically a separate part of the upper thermostat block that functions as a high limit cut-off by using a bi-metal disc ("Snap-Disc") to interrupt power. The disc can fatigue, though rarely. The back of its housing should be in contact with the tank. The elements themselves should have no voltage leaking to ground (tank). Test that first. If unfamiliar to you, or lacking a tester, hire an electrician. I will assume you have set the thermostats to a reasonable level and verified output water temp. with a thermometer, that the heater itself is not in a high ambient temperature location, and that all wiring connections are tight.

I'm not sure I completely agree with faucetman... some additional information would be helpful. How big is the water heater? 40 gal, 50 gal, larger? Under what conditions does it run out of hot water? Is it when only one of you take a shower? Is it when two of you shower after each other, etc? Is it when you do laundry or some other large demand? I suspect that something else is going on - I suspect that the bottom element is burned out and shorted out - or possibly the thermostat is malfunctioning. My first guess would be the lower element. You do need to fix this problem now. The reset button is a safety device - it works when the normal operational controls fail. It is a second-line defense against catastrophic tank failure. You don't want to count on that to continue to prevent serious damage... it's kinda like the emergency brake in your car...you don't want to drive the car knowing that the regular brakes don't work and count on the emergency brake to stop you. As FaucetMan pointed out, you do have one more layer of protection, the TPR valve- that can cause flood/steam damage if it releases but at least it won't level your house like an exploding water heater can. As I said, I think you will find that the lower element is burned out - but you need to get it fixed now and stop resetting it. The link below shows what happens when a water heater lets go. It doesn't happen often because of the safety devices, but it's a good reminder of why there are a couple of safety devices on the heater and that we shouldn't ignore a failed operating control. Here is a link that might be useful: Water heater exploding

That being said, if your pump pushes more water than the heater can accommodate, a bypass must be installed or you will have a blowout, whether it it be a gasket or pipe, or you could damage some other parts in the heater.