Ok, so we are having one of our bathrooms remodeled. About 2 weeks ago the plumber came out and replaced all the piping for that bathroom. Then suddenly our adjacent bathroom had no hot water (well luke warm water, but we are in Phoenix and it is 111 degrees, so even the cold water is luke warm right now).

These two bathrooms share a wall, and from what I can tell the piping comes down from the ceiling and splits off to the two bathrooms.

There are two other faucets in the house - one is in the garage next to the hot water heater, that faucet seems to get the hottest water, and the kitchen faucet, (which is about 10 feet from the hot water heater) which is warmer than the bathroom, but when I used a thermometer to test the temperature was only putting out 90 degree hot water, and that is only for a couple of minutes before it cools back down.

We thought it was the top heating element, and replaced that. Nothing changed. So we thought that the thermostat was bad too, and changed that as well. Nothing changed. The bottom heating element and thermostat seems to be reading fine (using a Fluke to test for power, etc).

Any suggestions? I don't think that this is related to the plumbing changes, but I am at a loss.

We set the thermostat for the same temp as it was previously, but we were thinking of turning it up and seeing if that helped?

Since you have a fluke, you can measure the resistance of the lower element before replacing. You MUST shut off power to the heater first! Then you check for voltage with the fluke meter, if voltage is not present remove one wire from the lower element terminal, set your fluke meter to ohms or the inverted U seting and read across the two element terminals. You should get a reading between 10-20 ohms depending on what you wattage rating is for that element. I can tell you in my experience the lower element is the failure (of the two) the majority of the time.

Call the plumber back. It is either something he did in the repiping or it's a broken or deteriorated dip tube in the water heater.