Grunaire unit trips breaker immediately when we try to reset it.

Something on that circuit had a direct short.

tripping the circuit means the unit is has a ground, "means some peeled wire inside is touching its metal case"

All resets and breakers trip with high heat.
Do you mean the tankless unit is tripping the 40 amp circuit breaker?
Or the thermal cut-out, located on unit, is tripping?

Is this a new installation?
Or was the tankless working before and stop working?

Copy following link to check wiring.
AE 9.5 requires 40 amp breaker and 6 gauge wire.
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

If the unit is tripping the thermal cut, -out, then product manual says to call Bosch or service technician.
Call Bosch 866-330-2729
http://waterheatertimer.org/pdf/Bosch-Powerstar-AE3.4-7.2-9.5.pdf
Thermal cut-out can trip if water flow is too low.
Try changing power selector screw from Hi to Lo so only 1 element is ON. See diagram page 6 of manual.
Adjust the 'outlet service valve:' The manual recommends that a ball valve be installed between hot outlet and faucet. Adjust the valve so more water flows to faucet.
Remove aerator, clean aerator, don't use low flow restrictions or low flow shower heads, replace leaky faucet cartridge on single-handle faucet.

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

Have a short in unit. Need to check continuity of each component to ground to determine where short is. Also make sure breaker is good.

see this step: God bless you
Check the house circuit breaker to see if it tripped?
Click here if you own a Vacuflo Central Vacuum Unit continuously trips your circuit breaker
If it is tripped, then turn off your central vacuum unit and reset the circuit breaker. Then turn the central vacuum unit on. If the circuit breaker trips again, then refer to the following:
Determine if the house circuit your vacuum unit is on has sufficient voltage required.
If your unit has a reset button, press the button and try again.
Cross the low voltage wire from the relay at the unit with a small screw driver. If you hear the relay click you need to replace your motor.
Click here to view Central Vacuum Motors
Click here to view How to replace a motor
If you do not hear a click, then you need to replace your relay.

Do you have anything plugged in? If they keep tripping with nothing plugged in I would call a electrician to troubleshoot or replace the breakers. They may still be under warranty from Siemans.

If the unit is new then the unit may be over-sized for the electrical circuit. If the unit is not new then the compressor may be failing

Hi,
The compressor is bad and is causing the circuits to trip....

Here is a tip that I wrote about that problem...

http://www.fixya.com/support/r4662409-dehumidifier_running_but_no_water_in

heatman101

sometimes gfi outlets go bad and trip try testing with a heavy draw machine in it or just replace it and see if this fixes the problem

If the breaker itself is more than 15 years old, replace it. If it continues to switch off, check the amp draw of both the inside and outside unit.