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I'm looking to install a Natural Gas Tankless (condensing) water heater (likely Navien 240). I am planning the drain line (which will have a neutralizer on it to reduce the acidity). The installer & distributor say the condensate will be 2 Gallons per hour (which seems to be the manufacturers maximum number). I live in a desert area with a very low humidity (<15% normally) so 2 GPH seems excessive to me.

  1. Does anyone have any perspective on likely actual condensate volume?
  2. I'm planning on piping it away from the heater closet (on outside of house). If it is going through a neutralizer, is there any issue with letting the water flow into landscaped areas to water plants?
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    Is there some actual problem you're trying to solve here? What's wrong with the 2 gph maximum value specified by the manufacturer? It's practically certain that in practice, the actual amount of condensate will depend on numerous factors, but probably the most significant is simply how often the heater is operated, and that this amount will not exceed the manufacturer's stated maximum but very well could be less. "Any perspective" is not a real question, lacking focus and clarity. And posts with multiple questions are also considered unsuitable on the Stack Exchange network.
    – Peter Duniho
    Apr 24, 2021 at 5:20
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    Low humidity environment does not reduce the condensate output of a condensing tankless water heater or combustion furnace. The condensate is a product of combustion of the hydrocarbon fuel: hydrocarbon + oxygen > carbon dioxide + water.
    – Jim Stewart
    Apr 24, 2021 at 11:29
  • Will this water heater be on or in an outside wall or will it be inside at the location of the current tank? I would bet that this tankless heater would produce so little condensate water that this would have a negligible effect on the chemistry of soil it would drain into. Is a neutralizer necessary for tankless condensate? Is the neutralizer designed to protect cast iron drains or is it required to protect septic systems or to satisfy regulatory requirements?
    – Jim Stewart
    Apr 24, 2021 at 12:17

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Humidity will have little to no effect. The condensate is a combustion byproduct, and will be determined almost entirely by how much the heater runs - when the heater is running, expect pretty much the rated output. When the heater is idle, expect none. As such, volume per day will depend on your hot water usage.

What, if any, treatment is required before irrigation use will depend in part on your local conditions. I know some desert soils are very alkaline and might neutralize acidic condensate quite well themselves. Beyond that, some experimentation is likely called for. A small reed bed filter (see virtually any permaculture reference book/website) or the like might be beneficial before feeding to more sensitive plants - condensate is often a bit weird due to minor combustion byproducts other than water.

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