Condensing vs non-condensing (gas)
A condensing gas water heater extracts extra heat from its exhaust until water vapor condenses, reaching UEF ratings around 0.9+; a non-condensing unit vents that heat outdoors. Condensing units need a condensate drain and can use PVC venting.
The efficiency difference is roughly 10–15% on gas use. Condensing exhaust is cool enough for plastic vent pipe, which often simplifies venting; the acidic condensate requires a drain and sometimes a neutralizer cartridge.
Essentially all ENERGY STAR certified gas tankless units are condensing designs — the certification threshold is hard to meet without condensing heat recovery.
| Flue gas dew point | About 130 °F for natural gas — condensing means cooling exhaust below it |
|---|---|
| Exhaust temperature | Roughly 100–140 °F condensing, against 300–500 °F non-condensing |
| Efficiency ceiling | Non-condensing designs top out near 85%; recovering the latent heat is worth about 10 points |
| Certified gas tankless | UEF 0.95–0.98 on this site — the whole certified list is condensing |
| Vent material | PVC, CPVC, or polypropylene for condensing; AL29-4C stainless for non-condensing tankless |
| Condensate | Roughly ½–1 gallon per therm burned, at pH 3–5 — acidic enough to attack metal drains |
The mechanism: getting the water back out of the exhaust
Burning natural gas produces carbon dioxide and water vapour, and turning that water into vapour consumed energy the burner will never see again — unless it condenses. That latent heat is about 10% of the fuel’s total energy content, and it is the entire difference between a good non-condensing appliance in the low 80s and a condensing one in the mid 90s. No amount of burner tuning gets past that boundary; the vapour has to be condensed.
A condensing heater does it with a second heat exchanger placed in the exhaust path, fed by the coldest water in the system — the incoming supply. Because that water is at 50–70 °F, it can pull the flue gas below its roughly 130 °F dew point, at which point the vapour turns to liquid on the heat exchanger surface and releases its latent heat into the water. Exhaust leaves at 100–140 °F instead of 300–500 °F.
This is also why condensing efficiency depends on inlet temperature. A unit fed 50 °F groundwater condenses aggressively; the same unit on a recirculation loop returning 110 °F water may barely condense at all, and its real efficiency drops toward the non-condensing number. Hot-water recirculation is convenient, but it costs some of the efficiency you paid for.
What it changes about the installation
Cool exhaust can travel in plastic. Condensing units vent in PVC, CPVC, or polypropylene at a few dollars a foot, in longer runs than a chimney allows, and through a sidewall. Non-condensing tankless units run exhaust hot enough to require AL29-4C stainless, which costs an order of magnitude more per foot and often makes the “cheaper” heater the more expensive installation once a 20-foot run is priced.
The offsetting requirement is the condensate. Roughly half a gallon to a gallon of liquid water per therm burned drains out of the unit, at a pH somewhere around 3 to 5 — acidic enough to corrode cast iron and steel drain lines over time. Many jurisdictions require a neutralizer cartridge, essentially a tube of limestone or magnesium oxide chips, between the heater and the drain. The cartridge is cheap and needs its media replaced every year or two.
Two practical failure modes follow. Condensate lines that run through unheated space freeze, block, and shut the heater down on a lockout code in the middle of a cold snap. And any condensing installation needs a drain within reach or a condensate pump — a constraint that decides where the unit can go as firmly as the gas line does.
When non-condensing is still the right choice
Do the arithmetic on your own use rather than on the efficiency percentage. A ten-point efficiency gain on 130 therms a year saves roughly 15 therms, about $22 at $1.45 per therm. Against a $300–700 price premium that is a 15-to-30-year payback on the appliance alone, which is longer than the appliance lasts.
What flips it is the venting. If the existing route needs 20 or 30 feet of vent, the stainless a non-condensing unit demands can cost several hundred dollars more than PVC — enough to make the condensing model cheaper on day one, before any fuel saving. The reverse case is a short run straight through an exterior wall, or a unit mounted outdoors in a mild climate with no vent at all, where a non-condensing heater is genuinely the sensible buy.
For storage tanks the calculation is different again, because the alternative to a condensing gas tank is not a slightly worse gas tank — it is a standard atmospheric one at UEF 0.58–0.64, roughly 30 points lower, with a payback measured in a handful of years rather than decades. That is the gap the certified gas storage list on this site occupies, at UEF 0.86–0.93.
Frequently asked questions
What is a condensing water heater?
One with a second heat exchanger that cools the exhaust below its roughly 130 °F dew point, condensing the water vapour out of the flue gas and recovering the latent heat that would otherwise leave the building. It raises efficiency by about ten points, lets the unit vent in plastic pipe, and creates acidic condensate that has to be drained.
Is a condensing tankless water heater worth the extra cost?
On fuel savings alone, rarely — roughly 15 therms a year, about $22, against a $300–700 price premium. It becomes worth it when the venting is long, because PVC costs a few dollars a foot and the stainless a non-condensing unit requires costs many times that. Price the complete installation, not the appliance.
Can a condensing water heater use PVC vent pipe?
Yes, if the manufacturer’s instructions list it. Exhaust leaves a condensing unit at 100–140 °F, within the rating of PVC, CPVC, and polypropylene. Use only the material and diameter the instructions specify, follow the maximum equivalent length table, and seal every joint — the vent runs under positive pressure, so a leaking joint puts flue gas in the house.
Do I need a condensate neutralizer?
Check local code, but usually yes if the condensate discharges into metal drain lines or a septic system. The condensate runs at pH 3–5 and corrodes cast iron and steel over time. A neutralizer is an inexpensive inline cartridge of limestone or magnesium oxide chips; the media needs replacing every year or two.
Why does my condensing water heater drain water?
That is the point of the design — burning gas produces water vapour, and a condensing unit deliberately condenses it out of the exhaust to recover the heat. Expect roughly half a gallon to a gallon per therm burned. A unit that suddenly stops draining, or that locks out in cold weather, usually has a frozen or blocked condensate line.