Long story short, new hot water heater won't heat water. Now for the long version, I work for a Housing Authority and a few weeks ago a tenant complained about not having enough hot water. One of my co-workers checked it out, finding nothing wrong he replaced the thermostats just to be sure. Next day, same complaint so we put in a new water heater. Next day, same complaint. We checked out voltage, ohms, wiring, thermostats, elements and found nothing wrong. Talked to the place we bought the heater and they gave us new thermostats. Still not enough hot water. My co-worker then replaced the breaker in the electrical panel. After a week we find out the heater is still not working. We assumed it was working since we did not hear from the tenant. Today, I called and had a plumber look at and they could not find anything wrong but thought maybe one of the valves in the tub or kitchen was letting the hot and cold water mix. I had the plumber replace the top element as it was not heating the water hot and still barely warm water after 2 hours. We then installed another new water heater and I got a txt that it is not working after several hours. I am stumped and so is the plumber and my co-workers. I don't see how a bad valve in the shower control can keep the water from heating up if no one is showering. Any ideas, anyone? My co-worker took the original heater apart to see if anything was wrong and found nothing wrong.

Any chance there's a leak in the hot water piping(underground pipe?) and the water isn't in the tank long enough to get "hot"?

No signs of a leak, no underground plumbing

Turn off the cold water feed to the heater and see if water comes out of a faucet when turned to "hot". If you have water coming out of the hot side of the faucet, with the heater feed turned off, then there's cold water piping cross connected with the hot piping. Has the water temperature been checked at the heater, with water being drawn from the drain valve. This will show the water temperature in the heater, that wouldn't be affected by a cross connection.

put a 5 gallon pail under the relief valve discharge and pop the relief valve. fill the bucket. what temperature water is in the bucket? should be nice and hot..

1. If the thermostats and elements were tested properly and were working, you DO NOT "replace them just to be sure".
2. Before replacing ANYTHING you find out what the REAL problem is.
3. We do not have enough information to diagnose what the REAL problem is without being there, but since a water heater is a relatively "dumb" piece of equipment, it should be fairly simple to figure out, IF the plumber is halfway competent.

I got to thinking about a possible leak and drove up to the apartment today (almost an hour drive) and low and behold there was a leak under the apartment so we shut off the water to the heater and called the plumber to come and fix the leak. I'm surprised the plumber didn't think about that possibility. I will know in the future. Thanks for the input.

quote; low and behold there was a leak under the apartment

What happened to the "No signs of a leak, no underground plumbing"?