Have a Whirlpool Electric Water Heater which is failing to heat water.
Have replaced the heater, the elements, & the thermostat. Water heated for a month or so, and now won't heat.
Whirlpool has been of no assistance what so ever.
Not sure it's a plumbing problem or electrical problem.
Heater is in a first floor condo.

Have you confirmed power to the thermostat, most heaters are 240. I am confused by the statement "Have replaced the heater, the elements, & the thermostat" Did you really mean that you bought each of these separately?

This sounds more like a electrical problem. Start at the breaker box and check from there. Reset the breaker and check to see if you have power to the heater. Have you pushed the red reset button on the upper thermostat? Let me know if there's power to the heater. Good luck, Tom

All Electrical aspects of heater are fine.
When close cold water heater valve above heater and let it sit; 15 minutes later; could hear elements clicking, and water did get steaming hot, but when open cold water inlet valve water got cold again, with water running at kitchen and bathroom sinks. Not sure if this is an accurate test? In a 6 unit; 3 story, condo building, we are on first floor, when cold water inlet is open , can hear water running in bathroom, when water is running in kitchen. No leaks. Wonder if water is going to other unit.

What size water heater is it? Must be a commercial water heater, most likely 100 gallon if it is feeding all six units. Also, I'm betting you have a recirc pump on the system. Going to be hard to give accurate advice here without knowing more about the system.

In a 6 unit; 3 story, condo building, Are you saying that one heater supplies the hot water for 6 units? What's its capacity?

When close cold water heater valve above heater and let it sit; 15 minutes later; could hear elements clicking, and water did get steaming hot, but when open cold water inlet valve water got cold again,
Whoa! You have just described a bad dip tube, (see image). As you can see by the image the dip tube directs the cold water down to the bottom of the heater where it's heated by the lower element and raises to the top where the upper element maintains it. If the dip tube's shot the cold water will stay in the upper part of the tankand you'll have the exact situation that you describe. I can't believe that nobody's flashed on this eariler.
Shut the water and power off to the heater and open up the cold water inlet.
The dip tube will have a flared end that you can pull up and check it. If you don't see it then you have located your problem.
Solution? Replace the dip tube. TIP: Do not use heat near the cold water inlet it will melt the flare and tyou'll be in the same position as before. Use a brass nipple to come out of the cold water inlet.
Good luck and let me know what you find, Tom

I apologize.
Haven't used this website for several years. Not sure I am responding correctly.
Each unit within the 3 story building has it's own water heater.
Our water heater is a 40 gallon tank, purchased at Lowe's.
We purchased and installed the heater, changed the elements, the thermostat, etc. then purchased all of these items again. Yes, we installed a 2nd 40 gallon water heater, another thermostat, and several elements.
We tested all of the elements before placing them, and they all tested positive.
In regard to the Dip Tube, can this part be purchased at a Lowe's, or a Home Depot store, or must it be purchased through a plumber; plumbing supply store?
There is no recirculation pump.
We have a Home Depot bathroom faucet; model # BP 150.
We have the maintenance guy for the condo association paying us a visit tomorrow, to see if this may be an 'internal' problem, a problem within the entire building, opposed to just our unit.
Thank you all for your time and for your suggestions and recommendations.
Truly appreciate it!!

Yes, we installed a 2nd 40 gallon water heater, another thermostat, and several elements. you installed two heaters and still had the same problem?
Do this for me . Take a look at the cold water inlet and tell me what you see coming out of the cold water inlet. Is it a 4 or 6 inch brass nipple or is it a soldered piece of copper pipe? Let me know soonest. Tom

You installed two heaters and still had the same problem?

Not technically.
First heater supposedly had been clogged, had some form of blockage, possible sediment and something with aeration. Yet temp of water still was inconsistent, and frequently only luke warm.

(Should have said this from start; I'm only individual making inquiries (typing/sending posts) on behalf of my spouse who is attempting to fix this heater; he was at the condo, is no longer, they have postponed any fixing until tomorrow when condo maintenance guy is due to arrive.)

What difference do the pipe fittings make (forgive my ignorance, that's why I'm posting)?

I need to know if there's a soldered fitting or a brass nipple connected to the cold water inlet. Please check the dip tube and let me know if you still have one. Tom

I need to know if there's a soldered fitting or a brass nipple connected to the cold water inlet. Please check the dip tube and let me know if you still have one. Tom

Will try to inform you tomorrow, late afternon.
If it is the Dip Tube, where do I find this?

you installed two heaters and still had the same problem?

Not technically.
First heater supposedly had been clogged, had some form of blockage,

The reason we need to know about the fitting/connection on the top of the water heater is because they may be clogged. If the water heater was replaced, but if they reused the fittings connecting to water heater, this may be your problem.
Can you post a picture of the fitting connections at water heater? This may help us help you.

The reason that I want to know is that if there's a soldered fitting on the cold water inlet the heat of the torch could melt the plastic flare of the did tube allowing it to disengage from the tank. So let me know what's there. I'm waiting! Tom

We had a plumber who was hired by the condo association look at the water heater.
There was a similar problem at another building.
Problem was diagnosed, because tenants heard water through the walls.
Plumber states that the problem is a pipe within the condo walls.
So, after all this time and effort, it appears it wasn't the water heater after all.
Thank you all for your time and suggestions.
Truly appreciate it.

Glad problem has been located. Take care.