What is wrong when the heat is up on high and hardly any heat is coming out

Hope this helps..I have also attached a link that may be useful.

No Hot Water
REASON REMEDY No power Check fuses or circuit breaker, replace or reset. Check with utility if necessary Wrong wiring connections at heater terminal Correct according to manufacturer's instructions Leaks around heating elements (1) Tighten element (2) If necessary, clean and replace with new gasket Loose wiring connection Locate loose connections. Clean carefully (remove all oxidation). Reconnect properly Lightning Inspect: (1) fuse (or circuit breaker). (2) Heater elements, and (3) thermostat, replace if necessary, see "grounding instructions" below High voltage Check utility for correction Short Locate short circuit and correct Wrong piping connections Reconnect pipes to proper fittings according to manufacturer's instructions Undersize service wiring Check with utility co. or electrician Improper calibration Replace thermostat Faulty thermostat Replace Faulty thermal cutout Replace Open thermal cutout Reset Top heating element burned out Replace
http://www.bradfordwhite.com/sites/default/files/product_literature/48494-A.pdf

If all the items that you mentioned are electrical, then you have an issue with the supply to the fuse box. It could be a loose connection at the box, the meter or on the transformer at the utility connection. Since this is high voltage you are better off contacting an electrician because while you can repair your own stuff without a license, only someone with a license can do it for you.

Well when it is heating up the pressure will rise
mine set at i bar but will go to 1.5 when hot

I do not know how old this unit is or what model but many water heaters are not sophisticated enough to measure the temperature, but heat based on water flow and temperature rise. To start, tankless water heaters turn off every time you turn the hot water off. Lets use an example. You use the facilities, wash your hands then turned off the water and take a shower. You will bring the already heated water (from washing your hands) from the pipe up to the shower. Then the cold water that comes into the pipe from the reactivation of the water heater comes up the hot water pipe with warm then hot water just behind.

If this is not your problem, keep reading. Every unit I have been called to see were installed wrong. Check your installation manual to see if the problem with the system started with the contractor.

First, check the temperature of the water from the heater. If it is too hot and you are mixing too much cold water in, the unit will turn off because there is not enough water flow to keep it running.

Second, turn off the valve at the water heater and turn on the hot water at a faucet. If the water does not stop running, there is a "plumbing cross over" usually caused by a failing pressure balance valve in a shower.

Third, if the unit has not been serviced, go to bosch-climate.us to find a tankless water heating professional in your area. Totally Tankless LLC

It could be dyslexia, but I think you want to know what may be wrong with your boiler in this case!
This is a VERY old design. The diaphragm is on the water section, this operates a switch to turn ON the pump, and a further device causes the diverter valve to divert boiler water from CH duty to HW duty.
From your description it appears that the boiler is shutting down on internal boiler water temperature (too high), your 'briefly too hot' HW symptom may be caused either by the boiler side of the secondary heat exchanger being blocked/caked in sludge, OR the DHW side of it being scaled. Either way a new or replacement (descaled) secondary heat exchanger MAY inprove things. It's not easy on these old clunk-pots, and EVERY disturbed seal will subsequently leak, so get a good supply of o-rings and fibre seals before you start ("you" being the competant fitter you employ)

Before calling service technician, copy following link for troubleshoot resources:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

C4 can range from failed PC Board to lime build-up inside heat exchanger
Error 15 is 'heat exchanger is too hot', clogged heat exchanger, restriction in cold water inlet, temperature set point too high.
If you recently changed temperature setting, then that might be cause.
Tankless require monthy-to-yearly maintenance to avoid common error codes.
Delime tankless and clean water filter, keep temperature low.
Never install tankless water heater is best way to avoid complications of expensive water heater that is maintenance heavy, hard to diagnose, expensive to fix, etc.
Never expose tankless to more than 11 grains hard water.
http://waterheatertimer.org/pdf/Rheem-CondensingTankless-maintenance-2012.pdf
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf

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

Most electric hot water heaters have two thermostats, one near the top of the tank and one near the bottom, and are covered by removable metal cover plates. The thermostats are pressed firmly against the bare metal wall of the hot water heaters tank.

The top thermostat usually has a high limit switch that will trip if the water gets too hot. When it trips it shuts off the electricity to both the upper and lower heating elements.

To reset the high limit switch there is usually a red button that you must press. When the upper limit switch trips it is often an indication that something else has gone wrong with the heater.

When the top of the tank is hot the upper thermostat removes power from the upper heating element and transfers the power to the lower thermostat and heating element. If the lower thermostat is defective, then the lower portion of the tank will not be heated and the supply will be greatly reduced.
Check for power at the upper thermostat terminals where the power is sent to the lower thermostat and heating element. If there is no power then the upper thermostat should be replaced. If there is power then check for power at the lower heating element. If there is no power at the lower heating element then replace the lower thermostat.
If there is power to the lower heating element then it should be getting hot. If it is not, replace it. Another possibility is a broken dip tube. Check for a broken dip tube and replace if necessary.

Hope this helps out

Probably a waterlogged expansion tank. The tank provides a springy air cushion for the water to push against as it expands, and the air pushes back as the water contracts. This scheme minimizes the pressure change that would otherwise be caused by expansion and contraction of the water, giving the water somplace to go when it expands. If the air cushion is gone ... big pressure swings.

Hi,
You are right you definetly have some issues there and it would seem that the "short" either messed with the brain of your heater or the critter is wired wrong.
To be totally honest for me to sit at a computer and try to assume what exactly went wrong would be totally unprofessional.
If you were my customer I would be telling you to call me the next time it acted up and let it in that state so that I could come and do some testing ti figure what the problem is... Once you reset the rpoblem goes away and I am out o luck to find it.
See if you can find someone that will come and treouble shoot that thing while it is in the failed state, that will let you know for sure hwat the issue is.

I hope that this will help you to solve your problem!

Thanks for using Fixya!!

Heatman101

When an electric water heater fails to heat water, either the power to the water heater has been interrupted or there is a problem with the controls or the heating elements. 1) Be sure electricity is being delivered to the appliance. Check the main switch on the water heater and the circuit breaker (or fuse) that serves the water heater. 2) Check the high-temperature cutoff in the water heater. Open the panel, and push the reset button. 3) If these steps don’t solve the problem, call an appliance repairperson.