Tankless water heaters are a great option for efficiency and unlimited hot showers. Unfortunately, when they aren’t working correctly, they can be quite challenging to determine what is causing an issue.
I recently had a client call where the issue was with a new Rinnai tankless water heater installed (by her son) and would randomly be not heating the water during showers. This was pretty disheartening to the client, as long, hot showers were the main reason behind the purchase of the equipment! We initially found a fault code of 11 and 12. Both codes were related to ignition failure. I had the client turn on the shower and we observed the unit operation. In this instance, it appeared that the unit had a hard time igniting and when it did ignite, it seemed to have trouble keeping a steady flame.
I checked the outgoing gas pressure and insured that it was within manufacturer spec. This specific unit is actually non-adjustable and set at the factory. I then checked the incoming gas pressure and it instantly sent my manometer into “O.L. or “outside Limits”. This meant that the pressure was so high, that my meter was unable to read it! Turns out, this client had just built the home and had all new gas lines and meter installed. The solution here was to have the gas company return and install a regulator and adjust it to allow for a lower incoming appliance gas pressure.
I also had a client with similar issues, but the tankless water heater was not showing any code, it would just randomly stop heating water. After a series of tests and running through all the parameter settings, it was determined that the client had a failed shower mixing valve, allowing cold water to back-feed into the hot water line. This was determined by shutting off the cold-water supply to the tankless water heater and turning on the hot water at any sink faucet. If cold water comes out, you have a bad mixing valve.
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