Condensing vs Non-Condensing Tankless Water Heaters: How to Choose
Every gas tankless water heater falls into one of two families: condensing and non-condensing. Both give you endless hot water. They differ in efficiency, venting, installation and price. Here is how to choose.
How they differ
A non-condensing tankless heater uses a single heat exchanger and sends hot exhaust gases out through a stainless steel or special category III vent, or no vent at all on outdoor models. A condensing unit adds a second heat exchanger that pulls extra heat out of the exhaust, cooling it enough that water vapor condenses. That recovered heat is why condensing units reach efficiencies around 0.93 to 0.96 UEF, versus roughly 0.81 to 0.82 UEF for non-condensing models.
| Condensing | Non-condensing | |
|---|---|---|
| Efficiency | About 0.93–0.96 UEF | About 0.81–0.82 UEF |
| Venting | PVC or CPVC, flexible routing | Stainless/category III, or outdoor (no vent) |
| Condensate | Needs a drain and usually a neutralizer | None |
| Unit price | Higher | Lower |
| Examples | Navien NPE-2, Rinnai Sensei RX | Rinnai RE/REP, Navien NPN, NHW |
When condensing makes sense
- You use a lot of hot water, so efficiency savings add up.
- The heater goes indoors and inexpensive PVC venting simplifies the install.
- You want built-in recirculation, such as Navien A2 models with ComfortFlow.
When non-condensing makes sense
- Mild climates where the unit can mount outside, with no venting at all.
- Replacing an existing non-condensing unit with compatible venting already in place.
- Lower up-front budget.
Sizing: BTU and GPM
Size for the fixtures that run at the same time. Add their flow rates (a shower is often about 2 GPM, a kitchen faucet about 1.5), then check each model’s flow at your required temperature rise. Incoming water in northern states can be 40°F or colder, so a 199,000 BTU unit that delivers about 11 GPM at a 35°F rise delivers much less at a 70°F rise.
Installation notes
Gas tankless heaters need adequate gas line size and pressure, a dedicated vent, and installation by a licensed professional following local codes and the manufacturer’s manual. Professional installation is also usually a condition of the manufacturer warranty.
Browse tankless water heaters, or if you also heat with hot water, look at combi-boilers. Questions on sizing? Contact us.