We can't get our hot tub to heat. The heater light comes on but the heater still won't heat.

these units have an inlet filter i would check that first, 2nd i would turn on 3 hot water sources and see after doing see if it corrects last try resetting the unitit. the 3 - 220 circuits are for the heating elements.

Gas or electric?
Electric water heater ... then test for crossover, check for water leak, replace thermostats, test elements
Gas water heater ... test for crossover, check for water leak
http://waterheatertimer.org/Crossover.html

http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

http://waterheatertimer.org/How-to-test-water-heater-element.html

water leak: turn off all taps and put ear against pipe and listen for water leak... check if water meter advances when all water taps are off.

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Water heater can be added to any incoming water line, either on hot or cold.
http://waterheatertimer.org/How-to-install-under-counter-water-heater.html
Under-counter water heater located near tub will supply hot water just like ordinary household water heater. But it will not reheat water that circulates inside the spa tub.
Otherwise, a spa heater is made with electric element that can work with 120V or 240V. Water from tub circulates through tank that holds element. Element must be fully immersed in water.
To retrofit spa tub with this heater would require cutting water line near pump, running both ends of water line into tank with element.
Then of course you would need to retrofit adjustable thermostat to control when element is on. And integrate element operation into air control switch at tub that can override thermostat so element is not heating when tub is not in use.
Unless there is a pre-packaged kit to retrofit heater, I think adding heater would be difficult.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

You say the hot water runs out after one shower.
You posted under electric heater.

In case you have a gas heater:
With gas heater, the flame quality could be bad because of restricted air flow or dirty burner, the control valve and thermostat could be bad, the air intake could be clogged, or the water heater is full of sediment and the flame is burning all the time and unable to heat much water.

With electric heater, the lower element is probably burned out and bottom filled with sediment, but thermostats could be bad, or you could have shorted wire.

Narrow down the suspects
If water heater makes some hot water then upper element is working.

Three remaining suspects are upper thermostat, lower thermostat, lower element, and possibly a loose wire

Check screw terminals for signs of burning and loose wires. Especially check lower element.

Let's test lower element.
Turn off power.
Remove both wires from lower element.
Test across both screws with a continuity tester.
Continuity tester is available at hardware store, or you can make one with battery, wire and a flashlight bulb.

The continuity tester has a battery and when light comes on, it says the circuit is complete.
Test1) Test across both screws on lower element with continuity tester.
If light comes on then element is good, but element has to pass one more test.
Test2) Test each screw on element to any bare metal part of water heater.
If light does NOT come on with second test, then element is good.
Light ON first test. Light OFF second test. Then element is probably good. But test is not fail-proof without using a meter.

I'm betting the lower element is burned out and you need to replace lower element. And you may need to vacuum out the sediment.
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

Now if lower element is good, then the easiest thing is to do is replace both thermostats. Buy generic thermostats at hardware store for $25. After replacing both thermostats, the water heater will probably work. If there is still a problem, then the lower element was not as good as test showed, and it should be replaced also.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

If you are still having a problem, answer back and we'll give more details.

If I understand correctly, you have an electric water heater.
The water heater is dual element and has 2 access panels on water heater.
You run out of hot water quickly.
And the reset is stuck down.

First of all, if the reset button is tripped, the water heater will produce NO hot water.
The reset, or ECO (electricity cut off), is designed to cut off all power to water heater.
So if you have some hot water, then ECO reset is fine.
The thermostat may be bad, but the ECO is fine

Next in line:
If your upper element is burned out, then the water heater will not make any hot water.
So if you have some hot water, that tells us the upper element is working.

The three remaining suspects are upper thermostat, lower thermostat, lower element

Let's test the lower element.
Turn off the power.
Remove both wires from lower element.
Test across both screws with a continuity tester.


The continuity tester has a battery and when light comes on, it says the circuit is complete.
Test1) Test across both screws on lower element with continuity tester.
If the light comes on then the element is good so far, but element has to pass one more test.
Test2) Test each screw on element to any bare metal part of water heater.
If the light does NOT come on with the second test, then element is good.
Element has to pass both tests. Light ON first test. Light OFF second test. Then element is probably good. This is not a fail-proof test but it is a good indicator.

I'm betting the lower element is burned out and you need to replace lower element. And you may need to vacuum out the sediment.
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

Now if lower element is good, then the easiest thing is to do is replace both thermostats. After replacing both thermostats, the water heater will probably work. If there is still a problem, then the lower element was not as good as the test showed, and it should be replaced also.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

If you are still having a problem, answer back and we'll give you more details.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.

Tankless water heaters normally take a little longer than a regular heater to provide hot water because they have to start and get warmed up. I'm guessing the heater uses a pilot light and that the short amount of hot water when you turn on the tap is hot because when the water is sitting still in the heater the heat from the pilot light is enough to get it hot. The tub faucet will get hot a lot faster than the shower because it lets a lot more water through and empties out the pipes faster than the shower. You could probably speed things up by letting the tub faucet run on completely hot for like 10-15 seconds and then turning on the shower.

sounds like the heating element is going bad, have someone come out and change it, it will be fine

you probibly ran the hot water tank out of hot water shut off the hot water comming out of your fausets for a few hours and check it again u could also try to reset your tank by pulling off the front cover and hitting the reset button and my last thought is sadly you burnt out your heating elements and they need to be replaced good luck man

turn up t- sta 5 degrees and let it cycle. test with thermometer.repeat until proper setting is obtained ,good luck