It’s important to note that this discussion is relevant only to gas tankless water heaters and not to electric tankless water heaters.
When natural gas is burned in the combustion chamber of a tankless water heater to provide the heat to yield the instant and endless hot water that we all desire, there are key differences between condensing and non-condensing water heaters that are important to understand. Our goal here is help you better understand these two types of systems when choosing the right option for your tankless gas water heater. Depending on your home venting system and construction, either a condensing or non-condensing is a great choice. Let’s check out the differences:
What is a non-condensing tankless gas water heater?
Traditional water heaters, whether utilizing a storage tank or tankless, are non-condensing. The non-condensing system is a proven and dependable technology utilized in majority of US homes built even recently. However, US standards changed in 2015 to require significantly higher energy efficiencies. While not absolutely mandating condensing water heaters, the new energy efficiency standards gave a significant push to condensing water heaters. Most existing homes currently have non-condensing water heaters (usually storage tank based), but you may well choose to move to a condensing water heater or condensing tankless water heater when choosing to replace your water heater.
Non-condensing tankless water heater systems requires the following components:
- Stainless steel vent piping (expensive)
- Electronic ignition
- Indoor combustion air
- Venting outside of house
- Fan induced draft
- Small diameter flue
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Dimensions: 17.5” D x 30” H x 13” W
Weight: 50 lb.
Flow rate: 7.5 GPM.
Demand type: Medium-demand a small to medium occupancy of 2-3 bedrooms household.
Application: Residential centralized or single point-of-use.
Read our review of Rinnai V75IN non-condensing Gas Tankless water heater.
How does a non-condensing tankless gas water heater work?
When the natural gas is burned to heat the water, a significant amount of heat is generated. A non-condensing water heater, uses metal (stainless steel pipes) venting to manage the superfluous heat by venting it to the outside air. Typically, this vented exhaust may be in the range of 300°F. This presents both an opportunity and a problem.
Advantages and disadvantages of a non-condensing tankless gas water heater
The problem with non-condensing tankless water heaters is that the exhaust is both hot and corrosive. As a consequence, venting is comparatively complex in that it must utilize materials (like stainless steel) which are heat resistant and resist corrosion, and the venting must be done in such a way to remove any fire danger or unwanted heating of your house.
The opportunity is that there is still significant heat in the gases vented to the outside; wouldn’t it be great if there was a way to utilize this heat to make your tankless water heater more efficient? A condensing water heater is a more efficient solution to capture the heat that would otherwise be vented.
What is condensing tankless gas water heater?
A condensing gas tankless water heater, like all tankless water heaters, instantly heats water as it passes through the unit and (generally) does not have a tank in which water is stored. Flame from the gas in a combustion chamber directly heats water as it passes through the unit. While this heats the water, the combustion exhaust still is quite hot, and, to improve efficiency we would like to utilize that remaining heat to further heat water. Condensing water heaters implement this extra step to improve the efficiency of the tankless water heater. This second step significantly drops the temperature of the exhaust to be vented. In a condensing tankless gas water heater, the exhaust gases of combustion are cooled and are mechanically ventilated either through the roof or through an exterior wall. At the high combustion efficiencies of a condensing tankless water heater, a drain must be supplied to handle the water condensed out of the combustion products, which are primarily carbon dioxide and water vapor. Often this will require the installation of a small pump to evacuate the condensed water if the condensed water must be elevated to drain.
Since the exhaust gases from a condensing water heater are quite cool, venting is usually simplified and uses cheaper piping (such as PVC).
A condensing tankless gas water heater systems requires the following components:
- Inexpensive, small diameter vent piping (e.g., PVC) venting outside of house
- A drain to evacuate water condensed out of the combustion products (an possible small pump)
- Electronic ignition
- Indoor combustion air
- Fan induced draft
Our top-rated gas condensing tankless water heater: Rheem RTGH-95DVLN 9.5 GPM Gas tankless water heater
- 94-Percent Energy efficient with stainless-steel condensing heat exchanger
- Intelligent electronic controls designed to increase energy efficiency and safety
- Third party efficiency listed by AHRI
- Up to 38-Feet of 3-Inch PVC Pipe or 5-Feet of 2-Inch PVC Pipe
Dimensions: 27.5” H x 18.5” W x 9.75” D
Weight: 79 pounds
Flow rate: 9.5 GPM
Demand type: High-demand suitable for a home with 4 bathrooms.
Read our review of Rheem RTGH-95DVLN condensing Direct Vent Gas Tankless water heater.
How does a condensing tankless gas water heater work?
When the natural gas is burned to heat the water passing through your tankless gas water heater, a significant amount of heat is generated in the primary combustion chamber where that heat is used via a heating element to produce the instant, endless hot water for which tankless units are known.
Condensing water heaters use a secondary process to extract heat from the hot combustion products by using the cool temperature of the incoming water to extract heat by condensing the water out of the hot combustion products. This process increases the efficiency of the tankless water heater significantly, although it results in liquid, acidic water being produced which must be disposed of.
Advantages and disadvantages of a condensing tankless gas water heater
Condensing water heaters deal with both the problem and the opportunity created by hot exhaust gasses. In short, condensing water heaters recirculate the hot exhaust gases to heat your water more efficiently. With respect to the venting problem, the condensing cycle causes the exhaust gases to cool significantly (to about 100° F) such that they can be vented with a lower cost materials (like PVC piping) and they pose less of a problem with respect to unwanted heating or fire safety. At installation, venting of condensing tankless gas water heaters is usually much simpler than their non-condensing tankless equivalents. Using the heat out of the recirculated exhaust gases also makes condensing heaters much more efficient in terms of both energy use and reduction in environmentally harmful gases.
Non-condensing water heaters (whether tankless or utilizing a storage tank) are typically 80% to 85% efficient at best. Conversely, condensing tankless water heaters may be up to 98% efficient with reduced emissions of environmentally harmful gases.
So, depending on the venting system in your home, either a condensing or a non-condensing tankless system is a great choice. We suggest:
- If you are doing a new construction, go with condensing model.
- If you are planning to install your tankless gas water heater in a home currently using a non-condensing water heater (tankless or with a storage tank), you may prefer to choose a non-condensing model as you likely already have appropriate venting in place.
- However, since tankless gas hot water heaters require much less space than storage tank water heaters, it is almost always quite easy to do a direct replacement of a centralized gas storage tank water heater with a centralized condensing or non-condensing gas tankless water heater. Typically, the altered venting of a condensing water heater is easy to deal with. Our tankless gas condensing water heater, which is in our garage, is vented through a simple PVC pipe attached to our garage ceiling and exiting through a side wall and the condensate draining goes through a small sump pump and drains into the drain of our laundry sink. Going from a centralized system to a multiple point-of use tankless system can be more of a challenge.
We feature both condensing and non-condensing models. Check out our recommended list of tankless gas water heaters for home use.