The upper thermostat only comes on in high demand situations. During
normal operation, the bottom one comes on first. Once the bottom
thermostat is satisfied it shuts off. In theory, if there is never high
demand the upper will never come on.
Lets say water is hot and high demand starts. At the beginning, the upper
thermostat (UT) and lower thermostat (LT) are satisfied.
- Water draws from tank top and is replaced by cold to the tank bottom.
- LT hits low limit first and lower element kicks on.
- At this point UT is still satisfied and it's element stays off.
- High demand causes UT to hit low limit,
- Simultaneously the lower element kicks off and the upper on.
- When the UT is finally satisfied it shuts it's element off.
- The lower element kicks back on until the LT is satisfied.
Both elements are never on at the same time. That's how the old fangled
ones worked anyway.
In his case where something is wrong and the water is cold in the tank
because it's been off, the tank should try to operate as high demand and
be applying power to the upper element right off.
I've never seen one wired as you describe. The typical scenario is a single
pole double throw upper thermostat that sends power to the upper element.
Once the upper stat is satisfied, it switches and sends power to the lower
stat, which in turn sends the power to the lower element. As the hot water
is being drawn off the top of the tank, the upper temperature drops and the
upper stat disconnects power from the lower and sends it back to the upper
element. Here is a diagram of the wiring:
If you are an ex appliance person then you should know THE BOTTOM ONE WILL
COME ON FIRST under normal operation. As an appliance service person, what
percent of the time did you find the bottom element bad compared to the
upper one? Maybe 80-90? The bottom element comes on to some extent to heat
ALL water. The top doesn't. Sediment is not always the cause of lower
The condition I itemized is normal operation.
Look at the full document with text from the same reference:
It even says bottom on first on the second page under "Normal operation" in
the first paragraph. The second paragraph states on high demand it switches
to the top. Then it switches back to the bottom to complete heating before
totally going off.
If the tank has been off then that is an Initial Startup state. That's on
Under normal operation, the lower element comes on first because cold water
is being pumped into the bottom of the tank. Not because hot water is being
drawn off the top and it gets colder than the bottom.
So is RBM right? Well, yes and no. It would come on first if the tank was
cold to start with.
As a second reference to support what I say, I have personally put two
meters on the elements simultaneously and watched how the elements
You need to re read the article. It very clearly says that the upper heats
first, then the lower. If you look at the diagram, it's apparent that the
lower thermostat doesn't even get the second leg of the 240 volts until the
upper is satisfied
I think you two guys are actually saying the same thing and somehow
missing each others points. Let me summarize how I think they work:
Only one element is on at a time.
The upper element has priority, so if the water at the top of the tank
drops below the thermostat setting, that element will be on, disabling
the lower element.
If water is drawn, provided the upper water temp is still above the
upper thermostat setting, the lower thermostat will drop below it's
turn on point from the incoming cold water at the bottom of the tank,
turning on the lower element.
If you continued to draw a lot of water, eventually the supply of hot
water will diminish enough so the upper thermostat will drop below
it's setting. That will turn off the lower element and turn on the
upper one. The obvious intent here is to try to now heat the water
at the top of the tank which is going out first.
Yes it does clearly say that - under the Initial Startup section. In a
properly working tank this only happens once - when the tank is started
from a cold/ambient state. All other times it follows the Normal
"the bottom thermostat closes and the element will
begin to heat the cold water."
It depends on the initial condition.
Yes, the upper must be satisfied first before the lower will come on.
Under NORMAL operation, repeat, NORMAL operation, the lower will come on
If your tank is hot and no elements are on and you put a meter on the
lower and upper element and turn on some hot water in the house until the
water heater kicks on...if the upper element comes on first you have a
bad lower element, defective thermostat, or broken dip tube.
** There is a difference between which must be satisfied first and which
comes on first.** The upper must always be satisfied first. The fact it
it is usually already satisfied when the lower kicks on. Examples would
be normal useage of tap water or normal heat loss to surroundings. Run a
washer with hot water or a shower (high demand) and you will see the
bottom come on, bottom kick off and top on, top kick off and bottom on
then both off.
The only way you will be convinced it to do the things with the meters as
I have done on multiple electric water heaters.
The OP has no hot water. I gave him the sequence of events to try to
understand how the heater functions. You replied to me, telling me that the
lower element heats first. You're wrong. Then you try to justify your
position by claiming "under normal operation". "Normal operation" doesn't
begin until the upper element first heats the water.
I attempted to specify both situations referring to NORMAL operation and
the OPs case.
I did not want to mislead anyone, including the OP, that any time the
lower element comes on first that there is something wrong. COnsider OP
makes the repair, lets it heat then checks things out. Runs some hot
water and "Crap, the damn lower element is coming on first."
It's not likely that both heating elements would go bad at the same
time. If one shorted, the one with the popped breaker, disconnect it and
reset the button. The bottom element should still work or vice versa.
One is bad, the other is still good. Determine the bad one. and run with
a single until you can repair it.
If the upper element was bad, which there is no indication of, the lower
element wouldn't get any power , since the upper thermostat must be
satisfied first, before sending power to the lower thermostat
You were addressing someone that doesn't have my experience. You didn't tell
him to disconnect the top and jump it, you implied that it would "run with a
single". You don't have to hold my hand, but it would probably help the OP,
if you clarified what you meant
And the chances that this IS the problem are extremely remote.
The proper short-term solutuion is to totally disconnect the lower
element which will allow the upper element to heat the top of the
tank, providing roughly 1/3 the reserve of hot water and taking
roughly 3 times as long to recover as normal - but he WILL have hot
water, and it will NOT trip the overtemp safety untill he can get the
lower element replaced.
Yeah, in my case there seemed to be a chain rection: my lower element
silted up, the lower thermostat then went intermittant and failed, the
upper thermostat then stuck on permanently, and finally the overtemp
popped. Tank clean-out and two new thermostats (probably about $30) and
all is well - until the tank gets a hole or the dip tube falls off or an
element dies* ;-)
* well, the lower one. Upper one I can replace, lower one is corroded in
and wasn't moving even with 6' of breaker bar on it...
Do any manufacturers sell elements with the thermostat combined? Seems
srange to me that the normal ones don't measure the temperature of the
water, but of the exterior of the tank; there must be quite a bit of
lag-time between the water heating up and the thermostats shutting the
From John Wood manual:
When hot water is being used, cold water enters the bottom of the
heater (either through the bottom inlet nipple or the dip tube), the
bottom thermostat closes and the element will begin to heat the cold
When a significant amount of hot water has been used, the upper
thermostat will take priority and heat up the top portion of the
heater. Once heated, it will flip/switch power down to the lower
thermostat and heat the lower portion.
High Limit Control:
All electric water heaters are supplied with a high limit control
switch. This switch is a safety device designed to shut the unit off
if it over heats and the water reaches an unsafe temperature. Power to
the thermostats and elements is completely cut off when it trips. The
high limit control can be reset by firmly pushing on the red button
above the upper thermostat. An audible click can be heard when it
resets. If the high limit control trips frequently it is an indication
of additional problems. Contact a qualified technician for service.
The red button is the high limit. If the high limit or the upper
element goes open you get NO HEAT. If the lower element goes open you
get reduced capacity and slower reheat.
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