Since installing Rinnai RC80i tankless water heater, the performance of the dishwasher has diminished. The dishes aren't getting clean. I suspect the water temperature is not hot enough. The thermostat is max'd at 120 degrees and by the time hot water gets to the dishwasher, there's probably several degrees of temperature loss. I'm wondering if I should attempt to install the MCC-91-2W Commercial Controller that will enable me to raise the thermostat. However, what about the risk of burns at the showers? In comparison, what's the water temperature at the normal setting on non-digital storage tank water heaters, which is what I had when I wasn't having this problem?

Follow this steps and fix it. God bless you
Drain a tank:

1. Shut off power at the electrical service panel and post a sign warning others not to turn it on.
2. Close the cold-water supply valve and open a hot-water faucet somewhere in the house (to speed draining).
3. Attach a garden hose to the drain valve and run it outside the house.
4. Open the drain valve and allow all water to drain out.
5. To refill the tank, close the drain valve tightly and open the cold-water supply valve. Also open a nearby hot-water faucet. When a steady stream of water flows from that faucet, the tank is full and the faucet can be closed.
6. Once the tank is full, turn the electrical power back on

1. Turn off power at the electrical service panel and post a sign warning others not to turn it on.
   
2. Remove the access panel on the electric water heater.
   
3. Disconnect one of the element wires and set a multimeter to RX1000 (resistance times 1,000). Touch one probe to an element mounting bolt and the other to each element terminal screw, in turn. If the tester displays anything but infinity ( open circuit), replace the element.
   
4. If necessary, set the multimeter to RX1 and touch the probes to the terminal screws. If there is any resistance reading at all ( closed circuit), then the element is good. If not, replace it. Both upper and lower elements are tested in the same manner.
   
5. To remove the element, first drain the heater.
   
6. Disconnect the remaining element wire. Remove the mounting bolts holding the element in place. Remove the element.
   
7. Replace the heating element with one of the same model, shape, and rating. Make sure you also replace any installation gaskets.
   

1. Disconnect power to the heater at the electrical service panel and post a sign warning others not to turn it on.
2. Remove the upper access panel and push the reset button.
3. Replace the access panel and turn the power back on. If the water is hot, the reset was the problem. If not, you'll need to turn the power off again and reopen the access panel.
4. Use a multimeter to test the cutoff terminals for continuity. If faulty, replace with an exact replacement part.