I purchased a house and moved in about 5 days ago to it. It has an AQ 125-BNQ tankless with an install of June 2001 based on writing on the side of the tank.

I've noticed when I shower, it takes about 30 seconds or so to get the hot water flowing. For about 2 minutes, there's no issue, then suddenly all hot water is lost and the water turns ice cold. No adjusting of the shower nozzle helps. Approxmiately 1 minute later, hot water returns just as suddenly as it was lost, and then there's no issue. No other water is being used in the house at the time.

Any idea what the problem might be? The home inspection showed no issues with plumbing or anything. Just wondering why I completely lose all hot water for about a minute.