Question...Over the past 18 months, after our home was built, we learned our two tankless water heaters have been running propane through a natural gas orifice. We discovered the issue after hearing a loud noise coming from one of the water heaters. Could this have damaged the water heaters? Thanks in advance

  • Is there no conversion adapter installed? If I recall correctly it's just a flow-rate difference and the appliance has to be rated for both fuel types.
    – MonkeyZeus
    Dec 20, 2019 at 19:49

4 Answers 4


Nat gas orifice is significantly larger than propane orifice , so the heaters have been exposed to more heat /flame than design ; that could be a problem. I learned this the hard way; hooked up a nat gas grill to a propane bottle. On the lowest burner setting flame came out around the space between the top and bottom of the grill. In round numbers the gas molecules take up the same volume ( Ideal gas law) . In the nat gas molecule you have one C and 4 H to burn , in a propane molecule you have three C and eight H to burn. So the nat gas orifice is larger to let more molecules through , it lets the same larger number of propane molecules through producing more heat/flame.


Damaged? Probably not, just inefficient and more prone to the flame blowing out.


From memory the propane orifice would be larger so by using the natural gas your heater would not get as hot as it would with the correct orifice so I do not think it would hurt anything.

Added : Orifice size for natural gas range from # 54-31 (drill bit sizes) Propane orifices range #62-48 so I had it backwards The larger the number the smaller the hole. In this case there was two much going to the heater so it may have shortened the systems life , however since the air mixture was probably off it may not have been burning efficiently and wasting gas.

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    Ed, I think it's the other way around, he said they had propane going TO an appliance designed for natural gas.
    – JRaef
    Dec 20, 2019 at 20:44
  • The natural gas orifice I thought was smaller than the propane, maybe I remember it wrong , but I have converted stoves , bbq’s and furnaces that only required an orifice change, some also required the jets in the burner to be changed also.
    – Ed Beal
    Dec 20, 2019 at 21:27

An appliance designed for natural gas has the burner orifices (flame holes) designed for the pressure and flame temperature of a natural gas system. Propane burns with twice the energy of natural gas, so if you put propane through a burner designed for the lower pressure / lower yield gas, it will burn significantly hotter and potentially more dangerously. It will also likely waste a lot of heat because the heat exchanger in the water heater was designed for a specific amount of flame contact. So the hotter / larger flame coming from the propane will just go up the flue as waste heat. It MIGHT possibly cause damage to the heat exchanger, only the water heater mfr could tell you for sure, but it's a sure thing they will not tell you that it is OK.

Of concern here is not only this, but if someone did that with your hot water heater, could they have plumbed the propane into your entire house? If so, there is significant risk of that excess heat causing damage to the fire box of your heater, which can then lead to gasses like carbon monoxide getting into your living space and poisoning you! There have been news reports of whole families being killed as a result of substituting propane for natural gas to heat their homes (without making the necessary changes).

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