Tankless issues

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sctclimbs

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I bought a rental property with a tankless WH. The WH serves two separate units.
When the upstairs tenant is showering the water will come on and will get hot but then go cold and even stop flowing water completely for a period of time combined with water hammer, then some back on but not be as hot.
Ok, now on top of that, a lot if not all of the fixtures in each unit are crossed with hot on the right and cold on the left. The shower valves are single handle and only work if you open them up a little Is a cross over occurring at a mixing valve? Does the crossover even explain the the reported symptoms? Wondering if that shower valve is only opened a little that it isnt drawing enough hot water and the unit drops below the minimum and shuts down. WOndering if the lines were crossed when the tankless unit was installed, would it even work if that were the case?
 

Dana

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There is a minimum hot water draw at which it will flame-out. It varies by model. Lowering the output temperature of the tankless by 5-10F increases the amount of flow through the tankless mixing down to the ~105F shower head temp.

Shower anti-scald valves will sometimes interact with tankless units due to the pressure drop across the heat exchanger in the tankless when the water pressure is too much higher on the cold side than on the hot. This is more problematic at higher flows, in which case RAISING the temperature of the tankless 5F might correct the symptom.

Since the incoming cold water can vary in temperature by 20-30F (or more) over the course of a year, that affects the hot-side flow, so the problems may come & go with the seasons.
 
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