When a client asks about tankless water heaters, the conversation eventually lands on one fundamental question: condensing or non-condensing? Both technologies deliver endless hot water on demand. Both are significantly more efficient than a standard tank water heater. But they work differently, they install differently, and they are not the right choice for the same jobs. This guide explains exactly what each technology does, how the installation differs, and how to decide which one belongs on your next job.
How a non-condensing tankless works
A non-condensing tankless water heater uses a single heat exchanger. Cold water enters the unit, the burner fires, and hot combustion gases pass over or through the heat exchanger, transferring energy into the water. The exhaust gases exit the unit hot — typically between 300°F and 500°F — and are vented directly outside through a metal vent pipe.
Most of the energy from combustion goes into heating the water, but a meaningful portion exits with the hot exhaust and is lost. The Uniform Energy Factor (UEF) on non-condensing gas tankless units typically falls between 0.80 and 0.82 — meaning 80 to 82 cents of every dollar of gas consumed becomes hot water. The rest goes out the flue.
Because the exhaust exits very hot, non-condensing units require Category III stainless steel concentric venting that can handle high temperatures without degrading. This is more expensive than plastic venting but is a well-understood, widely stocked material that most installers work with regularly.
How a condensing tankless works
A condensing tankless water heater adds a secondary heat exchanger to the process. After the combustion gases pass through the primary heat exchanger and give up most of their heat to the water, they pass through a secondary heat exchanger where the remaining heat is extracted — including the latent heat released when water vapor in the exhaust condenses into liquid. By the time the exhaust gases exit the unit, they have been cooled to around 100°F or below.
This extra heat recovery is where the efficiency gain comes from. Condensing gas tankless units typically achieve a UEF between 0.93 and 0.96 — meaning 93 to 96 cents of every dollar of gas consumed becomes hot water. That is a substantial improvement over non-condensing.
The condensation process creates two important installation differences. First, because the exhaust exits cool, it can be vented through PVC or CPVC pipe rather than metal — a significantly less expensive material that is faster to work with. Second, because the unit actively condenses water vapor from the exhaust, it produces liquid condensate — mildly acidic water — that must drain somewhere. A condensate drain connection is required at the installation location.
Key differences at a glance
| Condensing | Non-condensing | |
|---|---|---|
| Typical UEF range | 0.93 – 0.96 | 0.80 – 0.82 |
| Heat exchangers | Primary and secondary | Single primary only |
| Exhaust temperature | Approximately 100°F or below | 300°F – 500°F |
| Venting material required | PVC or CPVC — less expensive | Category III stainless steel — more expensive |
| Condensate drain required | Yes | No |
| Unit cost | Higher | Lower |
| Venting material cost | Lower — PVC is less expensive than metal | Higher — Category III stainless steel costs more |
| Long-term energy cost | Lower | Higher |
| Secondary heat exchanger material | Stainless steel — corrosion resistant | Not applicable |
The venting difference explained
Venting is where the installation cost difference between condensing and non-condensing shows up most clearly — and where the two technologies require completely different materials.
Non-condensing venting. Because exhaust exits at 300°F–500°F, non-condensing tankless units require Category III stainless steel concentric venting rated for high-temperature exhaust. This is the same type of venting used on many furnaces and boilers. It is more expensive per foot than PVC and requires more care during installation to maintain proper clearances and support. The maximum vent run varies by manufacturer and model — typically 30–50 feet depending on the number of elbows in the run.
Condensing venting. Because exhaust exits at approximately 100°F or below, condensing tankless units can vent through standard Schedule 40 PVC or CPVC pipe. PVC is significantly less expensive than Category III stainless steel, faster to cut and join, and available at virtually every supply house. Maximum vent runs on condensing units are typically longer than non-condensing units — many manufacturers allow up to 150 feet with 3" pipe. However, condensing units require both exhaust and intake venting as a sealed combustion system, and the condensate produced in the exhaust pipe must drain back to the unit or to a drain at the low point of the vent run.
Retrofit consideration: If an existing non-condensing tankless is being replaced and metal venting is already in place, a non-condensing replacement can reuse that venting — eliminating the need to run new vent pipe entirely. Switching to condensing on that same retrofit means new PVC venting and a condensate drain, which adds labor and material cost. Confirm the condition and compatibility of any existing venting before committing to either unit type on a retrofit job.
The condensate drain — what it is and what it requires
Condensate from a gas condensing tankless is mildly acidic — typically with a pH between 3.5 and 5.5, similar to coffee or orange juice. Most plumbing codes require that condensate be neutralized before it enters a drain — particularly if the drain connects to a municipal sewer system. A condensate neutralizer (a small inline device filled with limestone chips) raises the pH to an acceptable level before discharge.
The condensate volume from a residential condensing tankless is modest — typically one to three gallons per hour during operation — but it flows continuously whenever the unit is running. The drain connection must be accessible, clear, and slope continuously to the drain point. Freezing of the condensate drain line is a concern on outdoor-installed condensing units in very cold climates — plan heat tape or a freeze protection strategy on any outdoor condensing installation in a northern climate.
How efficiency affects the real-world energy bill
A 0.96 UEF condensing unit and a 0.82 UEF non-condensing unit running identical hot water loads use different amounts of gas to produce the same output. The condensing unit uses approximately 15% less gas to deliver the same amount of hot water.
How much that matters in dollars depends on three things: how much hot water the household uses (high-use households save more), the local cost of natural gas (higher gas prices mean faster payback on the efficiency premium), and the climate (cold climates where temperature rise requirements are highest and the unit runs harder see the largest efficiency gain from condensing).
For a household with moderate hot water use in a warm climate, the annual savings from condensing may be relatively modest. For a large household in a cold northern climate running high daily hot water demand, the difference can be substantial and the payback period on the unit price premium may be relatively short.
When condensing is the right choice
- New construction — new venting is being run regardless, and PVC is less expensive than metal. The condensate drain is a minor addition on a new build where plumbing is already open
- High hot water demand — the more gas the unit burns, the more the efficiency difference matters in dollars per year
- Cold northern climates — high temperature rise requirements mean the unit works harder, longer, and the efficiency advantage is most pronounced
- Clients focused on energy efficiency or utility rebates — most utility rebate programs require a minimum UEF that only condensing units meet
- Long vent runs — condensing units typically allow longer maximum vent runs than non-condensing units of the same brand
When non-condensing is the right choice
- Tankless-to-tankless retrofits with existing metal venting — reusing compatible existing venting eliminates significant installation cost
- No condensate drain available — if adding a drain to the installation location is impractical or cost-prohibitive, non-condensing eliminates the requirement entirely
- Warm climates with moderate hot water use — the efficiency premium of condensing has the longest payback period in these conditions
- Outdoor installation in very cold climates — condensate drain freeze risk is eliminated with a non-condensing unit
- Budget-sensitive jobs — lower unit cost and potentially lower installation cost if existing venting can be reused
Frequently asked questions
Can I use PVC venting on a non-condensing unit?
No — never. PVC cannot withstand the exhaust temperatures of a non-condensing tankless (300°F–500°F) and will fail, creating a fire and carbon monoxide hazard. Non-condensing units always require Category III stainless steel or equivalent high-temperature rated venting. Only condensing units — where exhaust has been cooled to approximately 100°F or below — can safely use PVC or CPVC venting.
Does a condensing unit require more maintenance?
Not significantly. Both types require periodic descaling in hard water areas. A condensing unit adds a condensate drain line that should be inspected periodically to confirm it is flowing freely and not blocked. The condensate neutralizer cartridge requires periodic replacement. These are minor maintenance items that add modestly to the overall maintenance picture.
Does the condensate drain require a neutralizer?
In most jurisdictions, yes — condensate from a gas condensing unit is mildly acidic and local codes typically require neutralization before it enters a drain connected to a municipal sewer. Confirm local code requirements before installation. A condensate neutralizer is a small, inexpensive inline device that most manufacturers offer as an accessory.
Is condensing always better?
It is always more efficient — but whether it is the better choice for a specific job depends on the venting situation, condensate drain availability, climate, usage patterns, budget, and total installation cost. On a retrofit where existing metal venting can be reused and no condensate drain is available, a non-condensing unit may be the more practical and cost-effective choice even though its UEF is lower.
Can a condensing unit be installed outdoors?
Yes — most manufacturers offer outdoor condensing models. On outdoor installations in very cold climates, the condensate drain line must be protected from freezing. Some manufacturers offer outdoor-rated models with freeze protection for the condensate system. Confirm the unit's freeze protection rating and plan accordingly for the installation location.
What does UEF mean and how is it measured?
Uniform Energy Factor (UEF) is the federally standardized efficiency rating for water heaters, replacing the older Energy Factor (EF) rating. It measures the percentage of energy consumed that is converted into usable hot water under a standardized test protocol. A UEF of 0.95 means 95% of the gas consumed becomes hot water. Higher UEF means lower operating cost for the same amount of hot water delivered.
Shop tankless water heaters at PlumbersCrib.com
- Rheem tankless water heaters — condensing IKONIC series and non-condensing RTG series
- American Water Heaters — condensing Adapt SC X3 series
- Stiebel Eltron electric tankless — point-of-use and whole-home electric
- Intellihot commercial tankless — high-demand commercial applications
- Takagi tankless — remaining inventory available while supplies last
Not sure which type is right for your job? Contact our team or call 877-881-5501 Monday through Friday.