Condensing gas water heaters and the vent that changes
Why condensing tanks and tankless units need listed plastic vent and a condensate drain, and why the old metal flue usually cannot stay.
A conventional atmospheric gas water heater sends hot flue gas up a metal vent. A lot of the fuel’s heat leaves with that gas. A condensing heater adds heat-exchanger surface and pulls so much heat out of the flue gas that water vapor in the gas turns to liquid.
That liquid is the “condensing” in the name. The heat that would have gone outdoors stays in the water, which is why these units post higher uniform energy factors than atmospheric tanks.
The liquid is acidic, the vent runs cool enough for listed plastic, and the old galvanized B-vent is usually the wrong pipe. People get hurt, and warranties get denied, when a condensing heater is hung on the vent the previous tank used.
This page is the distinction. Everyday gas-versus-electric choices are in gas or electric. Carbon monoxide, combustion air, and draft hoods are in venting and carbon monoxide. Tankless flow behavior is in how a tankless heater works.
What the second heat exchanger is doing
Natural gas combustion produces water vapor. In an atmospheric tank the flue gas stays hot enough that the vapor remains a gas all the way outdoors. In a condensing unit the flue gas is cooled below the dew point.
Each pound of vapor that condenses gives up a large slug of latent heat to the water. The uniform energy factor moves from the 0.60s, typical of older atmospheric storage heaters, toward the 0.80s and 0.90s for condensing storage and higher still for some condensing tankless models.
Compare those numbers only inside the rules in uniform energy factor. A higher UEF is not a reason to ignore vent material.
Because the flue gas is cooler, buoyancy is no longer the main way it leaves the house. A fan pushes it. The fan is why these heaters can use longer, more complicated vent runs than an atmospheric draft hood, and why they sit dead during a power outage.
It is also why a blocked intake, a blocked exhaust, or a failed pressure switch shuts the burner down instead of spilling combustion products into the room the way a failed atmospheric draft can.
Shutdown is the correct failure. Repeated resets are not a cure. Write down the code and read the manual for that board.
Vent material is a listing, not a preference
Manufacturers specify PVC, CPVC, polypropylene, or stainless systems, sometimes with restrictions by run length, by ambient temperature, or by which generation of the heater you own. The installation manual includes an equivalent-length table: an elbow counts as several feet of straight pipe. Exceed the total and the fan cannot prove draft, or the heat exchanger runs out of its design. Do not “make it fit” with a tighter elbow the table does not list.
You generally cannot reuse the atmospheric B-vent. The metal pipe was sized and listed for hot flue gas. Cooler, wet, acidic exhaust does not belong in it, and the connections are wrong.
A masonry chimney that served the old tank is not automatically a chase you can drop plastic into. Some installations use the chimney only as a sleeve for a listed vent system, with the manufacturer’s termination at the top and the clearances the listing demands.
That is a designed vent, not a hose dropped down a flue. If the house has other appliances still connected to that chimney — a furnace, a boiler — orphaning the water heater can leave the remaining appliance with a chimney that is now too large.
A heating contractor has to look at the whole venting system, not only the water heater. This is one of the cleanest reasons a condensing replacement is not a Saturday DIY project. The boundary is repeated in when to call a pro.
Terminations have clearances to windows, doors, grade, and inside corners. Those clearances are in the manual and the fuel-gas code. Pointing an exhaust at a bedroom window, or terminating under a deck where exhaust can pool, is how a high-efficiency heater creates a carbon monoxide problem the old draft hood did not.
Intake and exhaust on two-pipe systems must not be cross-connected. The manual’s diagram is the one to follow, including which pipe is which.
Condensate is a drain problem with chemistry
The liquid leaving a condensing gas heater is acidic because the flue gas contains dissolved combustion products.
It is not the neutral condensate ENERGY STAR describes for a heat pump water heater. Those are different machines. A heat pump’s condensate can often go to an ordinary drain.
A gas condensing heater’s condensate often needs a neutralizer — a cartridge of alkaline media — before it enters a drain the local code will accept, and it should not sit in metal pipe that the acid will eat.
The trap has to hold water so flue gas cannot leak back through the drain. A dry trap after a long vacation is a real service call: exhaust odor in the room, or a pressure switch that will not close.
If the condensate line clogs with debris or with algae, a properly designed unit locks out rather than overflowing into the burner. The symptom is “no hot water” plus a code, sometimes after months of quiet operation.
Clearing a hose you can see, at the floor, is reasonable. Opening the heat exchanger because you assume the clog is inside is not. The symptom sorting for tankless lockouts, including condensate, is in tankless symptoms. Storage condensing tanks use the same idea with a different cabinet.
Do not route this drain to a gutter, a windowsill, or a surface where people walk. The water is small in volume and unpleasant in chemistry.
A floor drain, a condensate pump to a proper receptor, or the termination the plumber designed are the options. In a freezing climate the exterior portion of a condensate line can freeze and shut the heater down on the coldest night of the year.
That is a design detail, not bad luck. Heat tape and insulation have to be appropriate to the listing. A hardware-store heat tape wrapped at random is how plastic vents get melted.
What you are paying for
The equipment costs more than an atmospheric tank, and the installation often costs more than the equipment difference, because of the vent and the drain. What water heaters cost separates equipment ranges from installed ranges so this is not a surprise in the driveway.
Operating cost can be lower because less gas leaves up the flue. Whether that payback is real in your house depends on how much hot water you use and what a therm costs. Multiply the EnergyGuide therms by your rate. Do not adopt a national-average dollar figure as your bill.
Atmospheric heaters still exist and still pass code when they are installed with combustion air and a sound metal vent. If the current vent is correct, the gas line is adequate, and the room is a drafty garage, an atmospheric replacement can be the rational buy. If the room is a tight interior closet, or the old vent is failing, a sealed-combustion condensing unit can be the safer architecture even before the efficiency math.
Sealed combustion draws its air from outdoors through a dedicated pipe, which removes the heater from the room’s air. That is a safety feature, not only an efficiency feature. It still has to be piped with the listed material, at the listed length, with the listed termination.
What not to improvise
Do not cement a vent joint with ordinary plumbing PVC cement if the manual specifies a different cement or a gasketed polypropylene system. Do not add a screw through a plastic vent “so it cannot come apart” if the manual forbids screws that penetrate the pipe.
Do not pitch the vent backward so condensate runs to the termination and freezes, unless the manual’s pitch diagram says that is the design. Most diagrams pitch back to the heater so condensate returns to the drain.
Do not connect the condensate hose to the temperature-and-pressure relief discharge. Those are different fluids with different rules. The relief valve’s pipe is covered in permits, pans, and straps.
If you inherited a condensing unit and you do not know what the plastic pipes are, read the labels on the pipes and the manual before a contractor offers to “replace it with a normal one and hook the old flue back up.” Going backward can be legal. It can also mean a new metal vent, new combustion air, and a different set of clearances. It is a design change, not a shortcut.
Sources
U.S. Department of Energy, Energy Saver, “Which Water Heater Is Right for You?” (energy.gov), for the efficiency and venting contrast between storage and higher-efficiency equipment. ENERGY STAR residential water heater pages (energystar.gov/products/water_heaters).
Manufacturer installation manuals for vent tables, equivalent length, and condensate treatment: Navien (navieninc.com), Rinnai (rinnai.us), A. O. Smith (hotwater.com), Rheem (rheem.com). International Fuel Gas Code / IRC fuel-gas chapters, as adopted locally, for termination clearances. The model’s listing is the controlling vent specification.