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Old 01-24-2010, 06:57 PM   #1
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Sequence of operation hot water heater


What is the sequence of operation supposed to be for a hot water heater? I replaced a thermostat today on my unit. The old thermostat turned on the lower heating element first. When the th temp is satisfied, it would switch it to the top heating element. The new thermostat does it the opposite way. It turns on the upper one first, and the lower one turns on after the upper thermostat is satisfied. They never run simultaneously.

Is the upper or lower element supposed to come on first?
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Old 01-24-2010, 07:14 PM   #2
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Sequence of operation hot water heater


Summary: Use a small amount of hot water, only bottom element comes on.
Use a lot of hot water, bottom first, then top, then bottom again, then off.

The lower element is supposed to come on first since the incoming cold water goes to (should go to) the bottom. If the top half of the tank is still hot the lower element will go off again when the bottom half is heated.

If you use so much hot water that incoming cold water gets near the top then the bottom element goes off and the top element goes on. Then when the top half is heated the top element goes off and the bottom element goes on. Finally the bottom element goes off when satisfied.

Are you sure that the top element consistently comes on first on your heater?

Are you sure you connected the thermostat correctly?

There are two thermostats, one near each element. The one on the bottom turns the power on and off. The one on top selects bottom element or top element. Except for some special purpose extra fast water heaters requiiring higher amperage circuits, both elements are not supposed to be on at the same time.

If you installed a new water heater and connected up the water backwards, that is connected the incoming cold water to the connection atop the tank labeled "hot" then nothing happens until the incoming cold water gets to the bottom. By this time the top of the tank is also cold and the top element comes on first. The performance of the heater is also poorer because this way there is a lot more commingling of hot and cold water within the tank since the cold water tends to sink.

Gas water heaters have just one thermostat, near the bottom. While all the heating takes place at the bottom where the gas burner is, a gas heater can heat the whole tankful in the time it takes the electric heater to heat just the top half.
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Last edited by AllanJ; 01-24-2010 at 07:36 PM.
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Old 01-24-2010, 07:53 PM   #3
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Sequence of operation hot water heater


Thanks

The wiring diagram and setup of the water heater is like the diagram I attached. If you look at terminals 1 and 2, you will notice they are normally closed, so the upper element comes on first. When the upper thermostat is satisfied, it closed the contact points and power is sent to terminal 4. Terminal 4 powers the lower element. The diagram shows the water heater as utilizing the upper element first and lower element second. It seems like it would be a better way to heat the tank, as the water in the top of the tank is used first on demand. Therefore, when someone turns on the faucet, the water is hot even though the bottom of the tank is not. It makes since to me, but I don't know why the old thermostat turned on the lower element first. The new and old stat were both 59T. The only thing I can think of is that the contacts were stuck closed keeping terminal 4 closed. I tried searching online and some people say the top should come on first and some say the lower should.
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Old 01-24-2010, 08:01 PM   #4
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Sequence of operation hot water heater


This is with a cold tank that the upper limit comes on first. In a normal operating situation, the bottom would probably kick on first, but the top element takes priority. Basically, if I use hot water, cold water will be shot to the bottom of the tank. This will turn on the bottom element. If I use too much water that the upper thermostat calls for heat, it will switch from the bottom to the top because the upper element takes priority. Once the top is satisfied, the bottom element will come on.

The bottom element does come on first I guess. I forgot to mention that it is a cold tank.


I guess my question is pertains to which element is first stage heat.
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Old 01-25-2010, 07:24 AM   #5
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Sequence of operation hot water heater


Looks confusing lets see if I got it?

When the tank is started from cold not being on for a long time the top element is ST stage when the upper water gets hot the relay cahges the power to the lower element 2nd stage.

Now once its hot and in use the lower element becomes the 1st stage.
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Old 01-25-2010, 11:29 AM   #6
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Sequence of operation hot water heater


Staging probably isn't a good term to use either. I think Allanj had it right the first time. I just forgot to mention to him that it was a cold tank. Basically, the upper element will always come on if the upper thermostat is calling for heat. Therefore, if both thermostats are calling for heat, the upper element will come on first. The reason the bottom thermostat comes on first during normal operation (hot water in tank) is because the upper thermostat is satisfied. The lower thermostat calls for heat once the rush of cold water comes into the tank. If you just use a gallon or two, the lower element will probably be the only one that turns on. If you take a shower and use a lot of water, the lower element will come on first. Once the water at the top of the tank drops below the setpoint, the upper element will switch on and lower will switch off regardless of whether the bottom thermostat is satisfied or not. I think my old thermostat had sticking points. It was showing continuity on the 4 terminal, when it should not have.

Basically, on a cold tank, the upper element will come on first. A tank that is already brought up to temperature will kick on the bottom element first.

Last edited by pomelo; 01-25-2010 at 11:31 AM.
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