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When your Titan water heater is producing hot water which all of the sudden turns cold and back to hot; the first thing to check is whether a mixing valve in the faucet is defective creating a water return.
The simplest way to identify a return of water in your house is by going to the Titan Tankless Water Heater. You should have a shut off valve on the cold side or on both the hot and cold side. (If you don’t have shut off valves please see alternate instructions below).
Close one or both shut off valves, to cut the supply of hot water and open the hot water faucet. There shouldn’t be any water flowing (only the trapped water between the faucet and the water heater.)
Once you open a hot water faucet,( you must ensure that the faucet is all the way to the hot side) if water continues flowing after about a minute, at that point it means there is cold water entering into the hot water pipes. That would indicate the mixing valve from that faucet is not working properly. If water does not flow that valve is working properly and you would have to complete the same process for each other faucet in the house until you determine which faucet is not working properly.
If you do not have shut off valves on the cold or hot side of the Titan Tankless Water Heater, it becomes a two person job. You will also need a bucket as well to catch water coming out of the pipes. First thing would be to shut off power (breaker) to the Titan Water Heater, then remove the water heater from the pipes (some trapped water will come out of pipes). One person would have to open hot water faucet while the other person checks to see if water comes out of the pipes on the left (hot side). No water should be flowing from that pipe. If there is a steady flow of water then that faucet is not working properly. If there is no water flowing you would have to complete the same process for each other mixing valves in the house to be able to determine the faucet that is not working properly.