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Airstream Tankless Water Heater Too Hot?

If your tankless water heater is outputting water significantly hotter than the maximum temperature you’ve set, it’s possible that the inlet water temperature is to blame. In our experience, when the inlet temperature is over approximately 90 degrees, the water heater will fail to regulate the output temperature correctly.

We took delivery of our Airstream in mid-March of 2022, when the weather in Texas was relatively cool and comfortable. On our first trip with it in early April, the mornings were chilly enough to justify a sweatshirt for an hour or two. I took my first Airstream shower on that trip and had no complaints about the water temperature. That initial trip was just a one nighter, so Jill opted to shower when we got back home. Our next trip a few weeks later was also very short, but Jill remembers using the shower and feeling as though the water temperature wasn’t as consistent as she would have expected. Still, no major problems reported.

But then we went on a week-long trip in early June when the temperatures in Texas were scorching. This is when showering became decidedly irritating. Every couple of minutes, the water temperature would go from lukewarm to comfortable to burning hot. I’m not speaking figuratively here. Regardless of the maximum water temperature we set on the Girard tankless water heater control panel, the reported output temperature would go as high as 140 degrees! Showering became a game of dodgeball, only instead of dodging balls, we were dodging blasts of scalding hot water.

I found many similar complaints online as I searched for answers. The most common advice I read was to set the maximum water temperature to whatever you find comfortable for showering, and then turn the shower temperature dial all the way to hot. The folks making this suggestion say that tankless water heaters can’t handle mixing hot and cold lines of water to achieve a desired output temperature. This line of reasoning conflicts with what is stated in the Girard owner’s manual1, but we gave it a try anyway. We set the maximum temperature to 95 degrees – the lowest allowable temperature – and ran the shower with the dial turned all the way to hot. The results were no different than before. Within a few seconds, the Girard control panel would show a continuously rising output temperature until it hit the magic number of 140 degrees which triggers error E6: “Outlet Water Temperature has exceeded 140°F (60°C) for 3 sec.” This cycle repeated endlessly. The shower dance we learned involved waiting for the water to warm up, washing fast as soon as it was comfortable, slapping the shower head out of the way when it got too hot, turning the temperature dial down, waiting for the water to get comfortable again, washing fast, then starting the cycle over again when it got too cold.

I eventually found a forum thread where someone indicated that very hot water inlet temperatures can screw up the water heater’s ability to regulate output temperature. The water temperature out of the ground where we were staying at the time was about 95 degrees – at its coolest. If we ran the water after the hose and water filter had been baking in the sun for hours, it was much, much hotter. Unfortunately, I didn’t measure those peak temperatures, but according to Google water in hoses can exceed 140 degrees in very hot environments. Based on this, we had another working theory for why our tankless water heater was trying so desperately to burn us. But until we traveled to an environment with lower ground water temperatures, we had no way to test this theory.

A few weeks later we made our way to the mountains of Colorado, at an elevation of nearly 8,000 feet. The outside temperatures there ranged from the mid 50s at night to upper 80s during the day. The water out of the tap was about 64 degrees. And I’m happy to report that our hot water temperatures suddenly became consistent and haven’t exceeded the maximum set on the control panel. Our showers became pleasant again!

In conclusion, when the ground water temperature is higher than about 90 degrees or so, you apparently cannot expect your tankless water heater outlet temperature to conform to the maximum temperature set. This is really disappointing and directly conflicts with Girard’s statement on its website that “parents will love the ability to pre-set the hot water temperature to a lower temperature and not worry that the water may get too hot.” That said, I feel better at least understanding the limitations of our water heater. I hope this post helps some other poor RV soul struggling to understand why their water heater isn’t working as they expect it to!

  1. The Girard manual says “The model GSWH-2 can be operated in two different ways: 1. Operate like a Tank Water Heater. The user turns on the hot water and add cold water to achieve the desired Hot water temperature. 2. Select the desired temperature by adjusting temperature setting up (^) or down (v). The UCP settings are from 95° (F) to 124° (F). The unit will maintain the set temperature.

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