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I got a service call to check out a hot water tank that was blowing fuses.
Customer had had water heater recently changed due to blowing fuses high limit trips. Before I got there he had replaced the thermostat and high limit control on the new water heater, because the new water heater was also blowing fuses and tripping the high limit.
I replaced a fusible disconnect at water heater location with a 60a pullout disconnect. I did this because the fuses and bus in the old disconnect showed heat damage. Tested resistance on elements 18 ohms each, current when heating 13a, voltage 240v. So I left thinking all good. It’s still tripping the high limit once or twice a day. Any point in replacing that thermostat high limit again? What makes them trip anyway? Is it just an overcurrent tripping on heat?
 

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if the HO changed any thing .
i would double check thermostat and element wiring.
also check both tstat temp settings
also possible that a new tstat is bad

New tstats arent always the same wiring
incorrect wiring could leave an element direct connected
they wont notice how hot the water is unless they run some just before or after it trips
i have found it usually takes about 30 mins to come up to temp and shut off, you may have to wait and do that to find the problem

down here they are designed to run one element at a time with 2 tstats.
the upper tstat controls its element and the lower tstat.
the lower tstat only controls the lower element
both tstats have a red button that is heat activated and opens a contact to shut off the current for hi temp
the upper element should run until tstat is satisfied, then switch power to the lower tstat
the lower element should run until lower tstat is satisfied and switch off

there is internal plumbing that a lot of ppl arent aware of
the cold inlet side for the water actually has a tube extending to near the bottom of the tank
the hot outlet is at the top
hot water rises, cold sinks
this way you get the hottest water in the tank, even if the lower element is currently running
 

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Blowing fuses and tripping a high limit are 2 different issues.

1. Blowing fuses - Chances are the fuses were blowing because of excess heat from a bad connection with the damaged bus you replaced. What is protecting the circuit in front of your 60A pullout you put in? Are the breakers/fuses and wiring sized properly for the rated load? What is the rating of the elements of the hot water tank? 3000W or so? 2P20A breaker with #14 wire would work, though I usually wire with at least #12 in case they ever go to the 4500W elements.

2. High Limit- Trips based on tank temperature, nothing to do with current flow. High limit is usually set for 170F I believe. Is it seated properly against the tank? The thermostat may have an adjustable range, say 110F-160F. Basic operation is from a cold tank the top element will run first till the upper thermostat turns off (heats the upper half of tank), then the lower stat will turn on the bottom element and heat the rest of the tank/maintain tank temp till the lower stat turns off. If the high limit is tripping, then the tank is overheating, an element isn't shutting off. If the high limit didn't trip, it would keep heating until the pressure relief valve opened...

Very rare to find a residential tank that runs both elements at the same time.
 

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Is this a ECO tripping?

Chances are the dip tube is cracked or broken and cold water is bypassed and the units is over heating where the limit is.

if the unit was over pressure or temperature in a unsafe condition the TPSV will open.

If a old tank scale build up can cause this as well but rarely. Usually the efficiency is noticeably reduced.
 

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both tstats have a red button that is heat activated and opens a contact to shut off the current for hi temp
I always thought there was only one thermal cutout and it was above the top element.
 

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Is this a ECO tripping?

Chances are the dip tube is cracked or broken and cold water is bypassed and the units is over heating where the limit is.

if the unit was over pressure or temperature in a unsafe condition the TPSV will open.

If a old tank scale build up can cause this as well but rarely. Usually the efficiency is noticeably reduced.
by ECO do you mean brand?
he said newly installed new tank was doing the same as the old tank
then the ho changed tstat
then called him
i suspect he solved the fuse blowing when he replaced the disconnect
 

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Sub transient reactance X”d worshiper.
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by ECO do you mean brand?
he said newly installed new tank was doing the same as the old tank
then the ho changed tstat
then called him
i suspect he solved the fuse blowing when he replaced the disconnect
ECO=energy cut off switch, always the very top one on tanks. 190F is the set point. on electric or gas models.

the lower hi limit is a little lower but that depends on manufacture and size of tank.
 

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When that happens I usually find tiny leaking plumbing entering tank, water on top of tank evaporates quickly so may not be noticed, but drips on t/Stat makes it keep calling for heat

Twice was bad pressure reducer, pressure in tank would build up at night and T&P valve would drip, not piped so waer dripped on top of tank
 
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