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The 2 most common causes of this occurrence are either a dirty filter on the cold water inlet to the heater or water temp that's set to high. If you hadn't told me this was set to 120 I'd suspect the latter immediately. So I wonder if the temp sensor on the unit is working properly. Is the full on hot water actually 120? The filter is a cap on the side of the fitting where the cold water line into the heater is connected. To clean it you need to turn of the water to the unit then open a hot water (only) faucet and let it run util it stops. The unscrew the filter cap while you have a bucket handy to catch any water that might run out when you take the filter off. Of course you need to unplug the unit or turn off the gas before you do this.