I have a tenant in a 1/1 apartment with a Rheem 30 gallon water heater.
Heater was newly installed in Feb 2015 and has been working fine.

The only fixtures that use hit water are lav sink faucet, shower, kitchen sink faucet and the dishwasher.

Tenant has never used the dishwasher.

In the morning, after turning on the shower, the water stays hot for 4 minutes, then lurk warm for another 2 minutes or so, then goes cold. This is the pattern for the past two weeks, prior to that it worked fine. No other fixture was using hot water.

I checked the water heater thermostat and both the upper and lower are set to 120.

The shower head is a 2.5 gpm head, so even ten minutes of pure hot water would only deplete 25 gallons.

Could this be caused by a faulty shower valve? Or more likely a defective water heater? Would one of the elements going bad cause this?

The lower element is most likely bad.

if it was a faulty shower valve, the water will still be hot in the sinks.

If an element is bad does it mean I need to disconnect everything, and take the entire tank back to the plumbing supply store? Or is it replacing a part?

I will do more diagnosis tomorrow.

I seem to remember hearing that you can expect to get 70% out of a hot water heater?

Next time the shower water begins to cool, immediately check hot water at another faucet and verify that it is the same temperature.

It is EXACTLY what a bad lower element would do.

Do you scrap your car when the ashtray gets full? Returning the heater because of a bad element is exactly the same thing.

No but since the water heater was purchased in March 2015 it is still under warranty I think, whatever that means.

I was told the 30 gallon heater is more like a 23 gallon by the plumbing supply store. They said the newer model is even less due to more insulation being put into the tank. But then when I bought the WH the insulation is a big coil of fiberglass outside of the WH which I don't use in our warm climate.

How do I verify if the lower element is bad?

I haven't been over to run the shower until it depletes and time it and then test the lav sink faucet's hot side yet.

But I did open up the two panels to the WH over the weekend.

Both thermostats were set to 120. I changed it to 130 - just to see if it makes a difference.

I turned power off then put a multimeter to the electrical connection on the top of the WH. I am getting 240V.



I then measured the resistance on the top element's two screws. 17.5 Ohms. The element is 3500W.



I also measured the resistance between the bottom element's two screws. 17.5 Ohms. Same reading.



I also noticed the lower element has some sort of spots on the black paper...what is that is it normal?

What is the definitive test for the elements?

I pushed the RESET button on the upper element a few times, not sure if that does anything.

Also my understanding is only one element heats at one time. The upper or the lower so there must be some electronics that switches from the upper to the lower?

The TANKS are the same size, so with thicker insulation, the tanks will not cool down as quickly, so you will get as much, or possibly a little more, water with a newer tank.

Nothing as far as you are concerned, they will replace the tank if it is leaking, but if it is an element they will give you a replacement when you bring the old one in, and the new one will probably be a PoS like the old one.

I do not use ohms for testing. I use a voltmeter to see if the element SHOULD be heating and an ammeter to see if it IS heating. the low thermstat setting would seriously reduce the amount of hot water received, and your ohmeter reading would indicate that the elements are good although I would test them with the wires disconnected.

The reset button is a 2 pole safety switch, opening the circuits #1 from #2 and #3 from #4, if an overheat occurs.

You must remove at least one wire from the element for an ohm test. 17.5 almost sounds as though you were getting an amp reading instead of ohm, which is approximate amp reading on a 4500 watt element.

I did test them with both wires disconnected. I took the pics before I did anything. Then I disconnected both conductors, tested, then reconnected.

I am not sure if my tests are valid, though.

steve, thanks so much, that picture explained a lot. I was a bit confused when I opened the panel why there are so wires...but seeing the picture I get it now. I printed a copy of the picture as a reference.

m&m, may be my multimeter was acting up? I know I had the test mode set to ohm/resistance.

When I did the tests I had the breakers off the whole time. So I couldn't be reading amps or voltages. I only had the breakers on when I opened up the top electric connection "well" and made sure I had 240V there. Then I shut the breakers off to do the rest.

The heater has two elements, 3500W each. So 3500W/240V = 14.6A. That's why I was able to get away with #12 conductors on a pair of double pole 20A breakers.



I am going back to do more tests.

This is my plan.

(1) Turn on shower and time how long it takes to deplete hot water. Once it's done I would turn on sink hot water lever and see if that's hot or cold, and the kitchen sink faucet too. May be I will pull up the tub spout diverter knob and see if the tub spout is delivering hot water.

(2) Open up the heater panels and repeat the ohms tests with breakers off, just to make sure.

(3) Turn breakers back on and do a voltage test across the thermostat and reset button according to steve's annotated pics to make sure I have the right voltage. That should tell me if I have a faulty thermostat or reset switch right?

Is there anything else I should test while I am there?



Edited 1 times.

If problem not resolved.

Suggest following check:

1. Secure all power to hot water heater
2. At TOP of heater - remove metal plate where main power FIRST connects to heater
3. Check connections where main house power first attaches to heater wiring.
4. Wire nuts at this connection may not be properly connected / deteriorated etc.
Note -- This is especially a problem with 1960 / 70's houses which may have aluminum wiring.

It is possible for above connections to allow power/amps to register on a meter at wiring in lower boxes - but fail under / during load.

Also note that in the original photos of one of the lower box's, the top left 120 V wire and the top yellow wire appears to not be correctly connected.

WC, the water heater was installed Feb/March 2015. I have checked the electrical connections at the top of the water heater and the wire nuts are fine. Actually I was the one who wired it. The voltage at the wire nuts are as expected, 240V. The wiring was also done at the same time. I pulled new THNN #12 copper conductors from the panel to the WH location through metal conduits. So I know everything from the panel to the top of the WH is fine.

After that is where things get a bit murky.

Please double check that the hot and cold connectors are installed correctly , and not reversed.


Tenant may have modified the showerhead by removing the flow restrictor. Test the showerhead by filling up a 5-gallon bucket while timing with a stop watch.

Cooler outdoor temperature and/or colder inlet water temperatures may have made this ongoing problem suddenly seem to "appear".

Sum,

I'm not a plumber, but I an wrestling with a similar problem. Please read this thread:

[www.plbg.com]

Bottom line: My tenants complained of insufficient hot water after 6 months of normal operation. Water heater is three years old and still under warranty. Problem was intermittent. Replaced the gas valve, but the real problem was probably a simple temperature sensor. After replacing that sensor, the water heater seems to be working properly, but we'll have to wait a while to know for sure.