Condensing vs Non-Condensing Tankless Water Heaters
This is a venting decision wearing an efficiency costume. On a long flue run it is not close.
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Condensing wins on most installs. It extracts more heat from the flue gas, reaching 0.93 to 0.98 UEF, and the exhaust is cool enough to vent in PVC rather than stainless category III pipe. Non-condensing units cost less to buy and need no condensate drain, which only wins on a short, simple flue run.
The verdict
Buy condensing unless your vent run is short, straight and already in stainless, and you have nowhere for condensate to go. The reason is not the efficiency difference, though that is real. It is that PVC vent pipe costs a fraction of stainless category III pipe per foot, and on a 30 or 50-foot run that difference frequently exceeds the price gap between the two units.
Which is why the whole-house gas market has largely moved to condensing and every gas unit in our tankless roundup is one.
Where condensing wins
- Vent material. PVC, which any plumber can source and cut. The Rheem RTGH vents up to 150 feet in 3-inch PVC and 60 feet in 2-inch.
- Vent length. Long runs are practical, which matters when the heater cannot go near an exterior wall.
- Efficiency. 0.93 to 0.98 UEF against roughly 0.80 to 0.85 for non-condensing.
- Rebate eligibility. Utility programs commonly set a UEF threshold that non-condensing units do not clear.
- Lower exhaust temperature, which eases clearance requirements near combustible materials.
Where non-condensing wins
- Purchase price. Consistently less than the condensing equivalent at the same BTU input.
- No condensate. Nothing acidic to drain, no condensate pump, no neutralizer — which genuinely matters where there is no drain within reach.
- Simplicity. One fewer heat exchanger and one fewer thing to fault.
- Replacing like with like. If a non-condensing unit is coming out and the stainless flue is sound, reusing it is cheap and quick.
| Condensing | Non-condensing | |
|---|---|---|
| Typical UEF | 0.93 to 0.98 | About 0.80 to 0.85 |
| Vent material | PVC, CPVC or polypropylene | Stainless category III |
| Vent cost per foot | Low | Several times higher |
| Maximum vent length | Up to 150 ft in 3-inch PVC (Rheem RTGH) | Shorter, model dependent |
| Condensate drain | Required | Not required |
| Heat exchangers | Usually two | One |
| Purchase price | Higher | Lower |
| Exhaust temperature | Low | High |
A condensing unit of each kind
Pick 1 — Most whole-house gas jobs
Rheem Prestige RTGH-95DVLN
The condensing gas tankless most houses should start with: 5.5 GPM at a 70°F rise covers two showers in a cold-inlet climate, and the 12-year residential heat exchanger warranty is the longest on this list.
| Fuel | Natural gas (LP variant sold separately) |
|---|---|
| Gas input | Up to 199,900 BTU/h |
| Flow at a 70°F rise | 5.5 GPM |
| Maximum flow | 9.5 GPM at a 35°F rise |
| Efficiency | Up to 0.93 UEF, stainless condensing heat exchanger |
| Minimum activation flow | 0.4 GPM |
| Venting | Up to 150 ft of 3-inch PVC; 60 ft of 2-inch PVC |
| Dimensions | 18-1/2 in W x 9-3/4 in D x 27-1/2 in H |
| Warranty | 12-year heat exchanger (residential), 5-year parts, 1-year labor |
What is good
- The 70°F-rise figure is published, so you can size it honestly instead of guessing from the headline number
- Condensing design vents in PVC, which is far cheaper to run than stainless category III pipe
- 0.4 GPM activation means a trickle at a bathroom faucet still fires the burner
- Long PVC vent runs let you put it where the gas line already is
What is not
- Needs a gas line sized for nearly 200,000 BTU/h — many existing 1/2-inch runs are not
- Condensate has to go somewhere, which means a drain or a condensate pump
- Annual descaling is not optional on hard water
Skip it if: Skip it if your gas meter and line cannot carry 199,900 BTU/h. Upsizing the gas line is often the single largest cost in a tankless conversion, and no amount of unit quality fixes a starved burner.
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Specifications above from Rheem RTGH series specification sheet (PDF); Pace Supply listing, RTGH-95DVLN, 199 kBtu/hr. We have not tested this unit ourselves.
Pick 2 — Highest published efficiency
Rinnai SENSEI RX199iN
The efficiency pick, and the one to buy if the unit has to hang outside: 0.98 UEF, 11.1 GPM at full fire, and it runs on either natural gas or propane indoors or out.
| Fuel | Natural gas or propane |
|---|---|
| Gas input | 199,000 BTU/h |
| Maximum flow | 11.1 GPM |
| Efficiency | 0.98 UEF |
| Installation | Indoor or outdoor |
| Warranty | 15-year heat exchanger (per retailer listings) |
What is good
- 0.98 UEF is the highest published figure of any unit on this page
- One model covers natural gas and propane, which matters if you may convert later
- Outdoor installation removes the venting cost entirely in mild climates
- 11.1 GPM headroom means it throttles rather than straining in a three-fixture draw
What is not
- Outdoor installation needs freeze protection and a power supply at the wall
- Still a nearly-200,000 BTU/h gas load
- No integrated recirculation pump in this model
Skip it if: Skip the outdoor install if your winter design temperature goes well below freezing for days at a time. Outdoor tankless units rely on an internal freeze protection heater and mains power — a multi-day outage in a hard freeze is how they split.
No live price feed on this site, so we show none. #ad
Specifications above from Rinnai RX199iN product page. We have not tested this unit ourselves.
Pick condensing if...
- Your vent run is long, or has to go up through the house rather than straight out a wall.
- There is a floor drain, a laundry standpipe or a condensate pump location within reach.
- You want to qualify for a utility efficiency rebate.
- You intend to keep the house long enough for the efficiency difference to matter.
Pick non-condensing if...
- You are replacing an existing non-condensing unit and the stainless flue is in good condition.
- There is genuinely nowhere for condensate to go without a pump and a long run of tubing.
- The vent run is short and straight out an exterior wall.
- Purchase price is the binding constraint and the install is otherwise simple.
The condensate question, properly
Condensing units produce acidic condensate — a few gallons a day in heavy use. It needs to reach a drain. If gravity does not get it there, that means a condensate pump, which is a small powered unit and one more thing that can fail. Some jurisdictions also require a neutralizer cartridge before the condensate enters the drain.
None of that is difficult. It is just the thing nobody mentions when quoting the price difference between two units, and it is worth pricing before you choose.
What goes in with it
A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.
- Flush kit
The annual descale is not optional on a tankless unit, and you need a pump, hoses and a bucket.
- Descaler solution
What actually dissolves the scale once the kit is hooked up.
- Recirculation pump
Fixes the cold-water delay that makes new tankless owners call a plumber.