Surging Hot/Cold Electric Tankless Water Heater

[Jim Em]

Recently I bought a electric tankless water heater, I have a hot/cold surging problem. PLEASE NOTE: this is an ELECTRIC TANKLESS SYSTEM.
The system uses well water.

OK, the inner workings of the heater
There is no way to set the temperature
There is no digital display on the heater
It is a dumb heater
It uses 2 breakers of 50 amp each
It is wired to the breaker panel using 2 #8's wires, total of 6 wires

The tankless heater is manufactured by: Tankless Inc.
Model: T-24-4
It is used but never used. The receipt attached to it said it cost $850 and was bought in 2018

When I take a shower it goes from luke warm to hot so I am constantly fiddling with the shower controls.

I have looked all over the web to solve this problem.

Is there anyway to help solve that problem?

I have come up with a theory, that I install a five or so gallon "Mixing Tank" to help with the surging problem so the water comes out with less or no surging?

Finally, I bought a soft water system that is not installed yet.

Or did I buy a $100 (what I paid for it) boat anchor?

 

[Tuttles Revenge]

Does the same surging happen at a sink faucet? How many GPM is the shower head rated at? What type of shower mixing valve are you using; 1-pressure balance turning from cold adding more hot water or 2-thermostatic where one handle sets the temp and the other sets the volume of water?

My best guess is that your shower is asking for more hot water than the water heater can produce instantly so that when its overwhelmed, it stops.. then starts again.

If you can get constant hot water from a single faucet at a certain flow rate, but then increasing the flow of just hot water creates the surge.. then that would be the most logical answer. Test max flow by removing the aerator after testing at low, med and max flow with aerator in.

 

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[Fitter30]

Heres the company's web site. Contact info bottom of their page
https://www.stiebel-eltron-usa.com/...s-whole-house-tankless-electric-water-heaters

Surprised 50 amp breakers hold 12 kw amp draw 50 amps. They don't list your heater just the newest model. They use a flow control valve on the inlet on some models and scr to control amperage/ temperature. If water flow varies look at flow control valve if its just temperature might be a sensor problem.