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Condensing vs. Non-Condensing tankless water heaters


As Tankless water heaters become more popular here so are increasing choices of types and manufactures.

One term that is referred to lately is Condensing Tankless Water Heaters, so what is behind that?

As the term applies tankless water heaters don’t have a water storage tank like the traditional water heaters. The water in a tankless unit is heated as it flows through its heat exchanger which is heated through combustion, more intense that your regular water heater.


In a typical combustion we burn the hydrogen content of the fuel and as the result hot gases are produced. One of these hot gases is steam or better water vapor. Now when these steam gases are cooled they turn into water that we generally refer to as condensation.


The condensation water is acidic (pH 3-5) and hence corrosive. It will corrode steel metals and other materials.


A Non-Condensing tankless water heater will push these hot gases through its vent to the outside. They will then cool down outside the unit. These gases have temperatures around 300 degrees F and must be vented through non-corrosive venting materials that can withstand the heat. Here is special stainless steel category 3 venting used, which is expensive.


In the combustion process fuel was burned and that energy was used to heat the water, although not all of it. Because the hot exhausting gases contain heating energy that is not used to heat the water and are wasted by exhausting them to the open. The hotter the exhaust gases are the less energy is used in heating the water and hence lower efficiency.


Non-condensing units have efficiencies around 80%, meaning around 20% of the heat is wasted and exhausted.


Condensing Tankless Water Heaters


Condensing Tankless Water Heaters extract the additional heat from the exhaust gases through various means and therefore exhaust cooler gases, usually around 100 degree F. As mentioned before cooling the exhaust gases produces condensation, in this case inside the unit. Since the exhaust gases are now much cooler a less expensive venting material can be used, mostly standard PVC schedule 40 is used as it can easily withstand the heat and the corrosively of the gases. Since we have captured the residual exhaust heat to heat the water we have achieved higher efficiency in the high 90% (up to 98%).


As a note, a Condensing unit does not always condensate, surrounding air temperature and air humidity are impotant factors. The manufacture claimed efficiencies are achieved mostly in labs under controlled conditions and real life values maybe lower.


Since the exhaust gases have been cooled inside the unit the condensation water is collected now inside the unit. The heat exchanger is the heart of the tankless water heater. It has now to be of higher quality and non-corrosive material to withstand the corrosiveness, usually made of special stainless steel alloy.

The collected condensation water has to be neutralized before it can be drained to the outside. This can be done through special filtration or dilution.


Conclusively Condensing Tankless Water Heaters are very efficient (up to 98%), real energy savers and use inexpensive venting material. They are slightly more expensive to manufacture but are over all on par with non-condensing tankless water heaters that need to use expensive stainless steel venting materials and are less efficient (about 80%).



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