When a client asks whether they should go condensing or non-condensing for their tankless water heater, the answer affects venting material, installation cost, efficiency, and long-term operating costs. This guide breaks down the real differences between the two technologies so you can make the right call for each job — and explains how Takagi's lineup maps to that decision.
The Core Difference: One Heat Exchanger vs. Two
Both condensing and non-condensing tankless water heaters heat water on demand using a gas burner. The difference is what happens to the exhaust gases after the primary heat exchange occurs.
Non-condensing units use a single heat exchanger. After the burner heats the water, the remaining exhaust gases — still very hot, up to 300°F+ — are vented directly outside through Category III stainless steel flue pipe. That residual heat is wasted out the vent.
Condensing units add a secondary stainless steel heat exchanger. The hot exhaust gases pass through it, dropping from around 400°F down to 100–120°F before venting. As the gases cool below the dew point (approximately 135–140°F), water vapor condenses — releasing additional latent heat that preheats the incoming cold water before it reaches the primary exchanger. Less energy is needed to finish heating the water, which is where the efficiency gain comes from.
Efficiency: What the Numbers Actually Mean
Condensing: 90–96% UEF (Uniform Energy Factor). Takagi's condensing T-H3 series achieves 0.93–0.95 UEF depending on model.
Non-condensing: 80–85% UEF. Takagi's non-condensing T-KJr2 carries a 0.81 UEF rating.
In practical terms, a condensing unit converts roughly 93–95 cents of every dollar of gas into usable hot water. A non-condensing unit converts about 81 cents. For a household using significant hot water daily, the annual energy cost savings from a condensing unit typically run $40–80 per year, with a payback period on the premium of 5–15 years depending on gas prices and usage.
Venting: The Installation Cost Factor
This is often the deciding factor on the job site — and it cuts both ways.
Non-condensing venting: Requires Category III stainless steel flue pipe to handle the 300°F+ exhaust. Stainless steel venting runs $15–30+ per linear foot vs. $3–6 per linear foot for PVC, a difference of $400–800 on a typical installation. The venting is heat-resistant and durable but costs significantly more on longer runs.
Condensing venting: Because exhaust exits at only 100–120°F, PVC, CPVC, or polypropylene pipe can be used. The Takagi T-H3-DV-N accepts 4" default venting up to 100 equivalent feet, or 3" optional up to 70 equivalent feet, using polypropylene (recommended), solid-core PVC, or Category III stainless steel. Longer vent runs are easier and cheaper to accomplish.
The retrofit math: When replacing an existing non-condensing unit that already has stainless steel venting in place, the existing vent can often be reused — removing one of the condensing unit's main cost advantages. Factor this into the total installed cost comparison.
Condensate: What Contractors Need to Plan For
Condensing units produce liquid condensate as a byproduct — typically 1–2 gallons per hour of operation. This condensate is acidic (pH 3–5) and must be handled properly:
- Condensate drain line must be routed to a floor drain, utility sink, or exterior. This rules out installation in attics or locations where a drain line can't be reasonably run.
- Condensate neutralizer is required in many jurisdictions to raise the pH before it enters the drain system. Neutralizer cartridges run $20–40 and need replacement every 6–12 months.
Non-condensing units have no condensate — the exhaust is hot enough that no liquid forms. Simpler installation in that regard, and no ongoing neutralizer cost.
How Takagi's Lineup Maps to This Decision
Condensing — Takagi T-H3 Series
The T-H3-DV-N is a 199,000 BTU indoor condensing unit using HRS35 copper alloy for the primary heat exchanger and 316L stainless steel for the secondary. It achieves 0.93 UEF with a maximum flow of 10 GPM, and supports Easy-Link for up to 4 units or Multi-Link cascade up to 20 units. The T-H3S runs at 180,000 BTU with 0.95 UEF, and the T-H3J at 160,000 BTU with 0.94 UEF — both condensing. The T-H3 series carries a 15-year limited warranty on the heat exchanger in residential applications and a 5-year warranty on all parts.
Non-condensing — Takagi T-KJr2
The T-KJr2 is a compact 140,000 BTU non-condensing unit, Energy Star qualified, suited for residential applications and up to two bathrooms in warmer climates. It carries a 0.81 UEF and a maximum flow rate of 3.9 GPM at a 60°F rise. Lower upfront cost and simpler installation — no condensate drain, no neutralizer — but requires Category III stainless steel venting and delivers lower efficiency.
Side-by-Side Comparison
| Condensing | Non-Condensing | |
|---|---|---|
| Heat exchangers | Two | One |
| UEF efficiency | 90–96% | 80–85% |
| Exhaust temp | 100–120°F | 300°F+ |
| Venting material | PVC, CPVC, PP or Cat. III SS | Category III stainless steel only |
| Venting cost | Lower on new installs | Higher — SS pipe required |
| Condensate drain | Required | Not required |
| Neutralizer | Required (many jurisdictions) | Not required |
| Unit cost | Higher | Lower |
| Best for | New builds, long vent runs, max efficiency | Retrofits with existing SS venting, attic installs, tight budgets |
Which to Specify and When
Specify condensing when:
- New construction — PVC venting is cheaper to run and easier to route through new framing
- Long vent runs — condensing units tolerate longer equivalent footage at lower cost
- Client is prioritizing energy efficiency or Energy Star certification
- Cold climate — condensing technology recovers more heat when incoming water temperature is very cold
- Multi-unit cascade installations where efficiency gains compound across multiple units
Specify non-condensing when:
- Replacing an existing non-condensing unit — existing Category III stainless venting can be reused, eliminating the condensing unit's venting cost advantage
- Attic or interior wall installation where routing a condensate drain line is impractical
- Budget-constrained project where lower unit cost matters more than long-term efficiency
- Warmer climates where incoming water temperature is already moderate — condensing efficiency gains are reduced when the water doesn't need as much heating
Shop Tankless Water Heaters at PlumbersCrib.com
- Full lineup: Browse our tankless water heater collection including condensing and non-condensing models from Rheem, Stiebel Eltron, and Intellihot.
- Spec help: Contact us for sizing help or questions on a specific job.
- Free Shipping: All orders over $199 ship free within the contiguous United States.