The water jet pump wont turn off or sometimes turns on by itself

If there's a damper on the exhaust, check it for proper operation.Or bypass it to see if it works fine then, if so you need to replace it.

Check for crossover.
Bad check valve.
http://waterheatertimer.org/Crossover.html

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

You need to have the flow switch looked at. The low flow of the water to the lavs and the kitchen sink, they all have as small demand next to t bath or shower,
Tom

Did you turn the breaker completely off, then back on? When a breaker trips it is in a middle position, and nust be turned off to reset it, then it can be turned back on.

Are they white flakes? Sounds like you have lime deposits turning loose in the plumbing.

Was the unit purchased and installed new? Maybe the privious owner had a hard water problem.

In either case, you may have to invest in a water softener.

HTH,

Mike...

The tap fitting on the shower may have failed. Some of the modern fittings have a more complicated internal mechanism than the traditional taps and can fail rendering the tap inoperable. Turn off the water at the mains. First take off the tap handle and then with a spanner carefully unwind the tap spindle. Be very carefull not to use excessive force in case you damage the pipework behind. If it does not come ralatively easily leave it for someone else. When you have this out have someone crack the main valve and you should have hot water coming out. If the fitting itself is the problem you may be able to take the spindle and washer out of a standard brass garden tap and use it as a temporary fix untill you get a propper replacement. Just another thought, you may have an isolation valve on the hot water line to the bathroom. Could a kid have turned it off?

The heater will only fire if it has water in it. If you just got the house, make sure the gas/electric to the heater is on. Turn off the pump and turn it back on.. See if that works. good luck

im not a great expert in this field but try putting some dish washer tablets in water when running to see if they can clean out anything,about 3/4 tabs

grohe is a faucet all by itself ,,, I would recomend to have you call there repair dept. and explain this to them, you said you put in the new cartridge and thats about all there is to it, when you put on the handle make sure it is not over tightened, sometimes it pulls on the cartridge and makes it come on by itself good luck

Some circulation pumps have a 3 phase mixing valve (cold,warm,hot) to regulate the water pressure in the building; no way to know unless you trace the plumbing. Basically, the hot water temperature could be lost because the circ-pump is cooling it with the cold water supplied to it. This is used to balance the pressure (aka Pressure Balancing Circ-Pump), so heater is really working overtime to heat the water. If the pump has two in and one out that's more than likely your problem; but you should consult a "old-school" plumber in your area.