A: Hank, with any mystery there are the usual suspects involved. When you run out of hot water, the usual suspect in most cases may be your water heater.

Now, your water heater is probably working fine, and that may be throwing you off track. However, you're failing to see the obvious issue. With 2 new mixing valves installed, you have basically doubled the water use in your shower when both valves are running, and that can drain your water heater about twice as fast as one mixing valve.

Many homeowners fail to realize that when upgrading a shower to multiple heads or installing larger whirlpool tubs, the house water heater may need to be upgraded as well, or else you can easily run out of hot water.

Plumbing is all about flow rates, and water heaters need to be sized correctly to accommodate those flow rates. In your case it seems that you overlooked the supply and demand of hot water needed for your new shower setup. You can confirm this by running both showers and when you feel them cool down, check the outgoing hot water supply line from your water heater. If the hot water supply line is also cooler, your mystery is solved.

I hope you have a little money left over from the original job, because the solution is to have a properly sized larger water heater installed by a licensed plumber -- or you can even have a second water heater installed "in line" or "in series" with your present water heater. Either way this will give you a larger reservoir of hot water and your new showers should work fine. A licensed plumber can also check out if your present water heater is set properly and operating correctly.

Follow local codes, never raise a water heater thermostat to unsafe levels, and always follow the manufacturer's instructions. Bottom line is that whirlpool tubs and multiple shower valves use more water than standard fixtures, and it may take some extra cold cash to get more hot water!