How to Pipe a Condensing Water Heater Condensate Drain
Why this matters
A condensing water heater, tank or tankless, pulls so much heat out of its flue gas that the water vapor in it condenses, and that water has to go somewhere. The condensate line is a few feet of small plastic pipe, and it causes a share of condensing-heater callbacks out of all proportion to its size: a line that froze outside and locked the heater out on the coldest night, a doubled trap that air-locked and flooded the cabinet, an unprimed trap that let flue gas into the utility room, or acidic water eating a cast iron floor drain for years. This HowTo is that line from the heater's outlet to the drain. The rest of the install (gas, venting, water connections) is owned by How to Install a Tankless Water Heater and the maker's instructions.
What you are draining, and why it matters for the pipe
Condensate from gas combustion is mildly acidic, because carbon dioxide and traces of nitrogen and sulfur compounds in the flue gas dissolve into it. It is commonly in the range of pH 3 to 5. That is weak enough to handle with gloves and eye protection, and strong enough over years to corrode copper, galvanized steel, cast iron drain lines and the concrete around a floor drain. So:
- The line is plastic, PVC or CPVC or the tubing the maker supplies, never copper or steel.
- The water may need neutralizing before it reaches the building drain, depending on the local rule and the maker (below).
- The volume varies. A condensing heater makes the most condensate when the incoming water is coldest and the burner runs long, so a line that copes in summer may run near full in winter.
Where neutralizing is required
Neutralizing is required where the local plumbing rule or the heater maker requires it, and that varies. The condensate disposal sections of the adopted codes (IPC 2021 Section 314 and IMC 2021 Section 307, or UPC 2021 Section 814, as adopted) govern, in substance, where condensate may discharge and how it connects to the drainage system; check the current adopted edition and any local amendment, because some jurisdictions add a neutralizing requirement the model code does not carry. Many heater makers require or strongly recommend a neutralizer in every installation, and their instructions are part of the listing.
Even where neither requires it, a neutralizer is the right call when the condensate drains to cast iron or galvanized drain pipe, to a septic system, or to a floor drain set in concrete. The cost of the cartridge is small against a corroded drain.
A neutralizer is a plastic cartridge filled with calcium carbonate media (limestone or marble chips). Condensate flows through it and dissolves a little of the media, raising the pH. The media is consumed and needs replacing on the maker's schedule.
The route
CONDENSING HEATER
+-----------------+
| |
| internal trap? | check the manual: if the
| (per maker) | heater has one, add NONE
+--------+--------+
| condensate outlet, 1/2 in
|
+-+-+ external trap ONLY if the
| U | maker calls for one,
+-+-+ filled before start-up
|
slope 1/8 in per ft, toward the drain
------------------------------+
|
+-------------------------+
| NEUTRALIZER, media |
| flow arrow as marked |
+-------------------------+
|
| open end held above
| the rim: AIR GAP
----+----
FLOOR DRAIN or
condensate pump
Before you start
Have on hand: the heater's installation instructions, PVC or CPVC pipe and fittings sized to the condensate outlet (or the maker's tubing), solvent cement and primer for that material, a neutralizer kit if one is going in, pipe straps, pH test strips, and a condensate pump with an overflow safety switch if there is no drain below the outlet.
Read the maker's instructions for three things first: whether the heater has an internal condensate trap, the outlet size, and whether a neutralizer or a condensate blockage switch is required. These decide steps 1, 3 and 4.
Hazards on this job. Condensate and neutralizer media: gloves and eye protection when handling either, and flush skin with water if condensate gets on it. Solvent cement and primer: ventilate the space, keep ignition sources away while the vapor clears, and follow the product's safety data sheet. An unprimed trap on start-up lets flue gas, including carbon monoxide, into the room: wear a personal CO monitor during start-up and stop the heater if it alarms.
The procedure
Size the line to the condensate outlet, never smaller. If the outlet is 1/2 in, the line is at least 1/2 in; if the maker calls for 3/4 in from the outlet, that is the size. Reason: a reduced line slows drainage and fills in winter when flow is highest. Many installers go one size up on long runs because the flow is small and the line easily plugs with sediment.
Keep continuous slope toward the drain, at least 1/8 in per foot. That is 1 percent, the slope the adopted mechanical code typically states for condensate lines; follow it or steeper. No dips, no flat runs, no uphill loops. Reason: condensate moves by gravity only, and a low spot holds water that freezes in a cold space or grows slime that plugs the line. Strap it so it cannot sag.
Trap it once, as the maker says, and no more. If the heater has an internal trap, do not add another: two traps in series trap a pocket of air between them, the line air-locks, condensate backs into the heater, and the blockage switch locks it out. If the heater has no internal trap and the maker calls for one, install a single trap close to the outlet. Reason: the trap's water seal is what keeps flue gas from coming out of the drain line, and a sealed trap is the only thing between a positive-pressure vent and the room.
Install the neutralizer in the orientation marked on it. Most mount horizontally with the inlet and outlet at the marked ends, flow arrow toward the drain, and with access to replace the media. Reason: installed backwards or on end, the condensate can short-circuit across the top of the media and leave as acidic as it entered. Mount it after the trap and before the drain.
End the line at an air gap. The open end of the condensate line sits above the flood rim of the floor drain, standpipe, laundry tub or pump reservoir, not sealed into it. Reason: a direct connection lets sewer gas and a backed-up drain flow back up the condensate line into the heater, and the codes treat condensate as an indirect waste that must not connect directly.
Keep the whole line inside heated space. A condensate line routed outside, through an unheated garage wall or into a crawlspace freezes in the first hard cold, the heater's blockage switch shuts it down, and the customer has no hot water on the coldest morning of the year. Where there is no indoor drain, use a pump (below), not an outdoor discharge. Where a short exposed section cannot be avoided, the maker's approved heat tracing is the fallback, not the plan.
Prime the trap before start-up. Pour water into the condensate system, through the vent connection port or the trap fill point the maker shows, until it runs out of the drain end. Reason: on a new install the trap is dry, and the first minutes of firing push flue gas out of the drain line into the room.
Start the heater and watch the line. Run hot water long enough for the burner to fire at a high rate. Within a few minutes condensate should drip steadily from the end of the line at the air gap. Watch the trap and fittings for leaks and the CO monitor for any reading at the drain end.
Finish test
The line passes when all of these are observed: condensate flows freely from the line end at the air gap during a long burn; no water stands in any part of the line once the burner stops (it should drain back to the trap seal only); no leaks at any fitting; the CO monitor reads zero at the drain end with the burner firing; and, where a neutralizer is installed, a pH strip in the water leaving it reads higher than a strip in water taken before it, at or above the maker's target for the media.
The condensate pump, when it is needed
A pump is needed when there is no drain at least the line's fall below the condensate outlet, for example a heater on a slab where the nearest drain is a laundry standpipe above the outlet. Choose one rated for acidic condensate or install the neutralizer before it, and wire its overflow safety switch to shut the heater off if the maker provides terminals for that. The pump discharge runs to the drain's air gap like any condensate line. Plug the pump into a receptacle where nobody will unplug it to run something else; a pump that loses power with the switch not wired in floods the floor.
Worked example: a basement tankless with a long run
A condensing tankless heater on a basement wall. Condensate outlet: 1/2 in, 9 in above the floor (illustrative, read off this unit). Manual: internal trap present, neutralizer required. The floor drain is 14 ft away, its rim at floor level.
Fall budget. Slope: 14 ft x 1/8 in per ft = 1 3/4 in. Neutralizer: about 2 in of drop between its inlet and outlet as mounted (illustrative, from the kit). Air gap: the line end held 1 in above the drain rim. Total needed: 1 3/4 + 2 + 1 = 4 3/4 in. Available: 9 in. Margin: 9 - 4 3/4 = 4 1/4 in, so the plumber gave the run more than the minimum slope.
Install. No external trap, because the heater has one. 3/4 in PVC from a 1/2 in to 3/4 in adapter at the outlet, one size up for the long run, cemented with the basement door open and the vapor cleared before start-up, strapped every few feet. Neutralizer horizontal on the wall near the drain, arrow toward it. Line end held 1 in above the drain rim with a strap.
Start-up. Trap primed through the maker's fill point until water ran out the line end. Long hot draw: condensate dripping steadily within a few minutes, no leaks, CO monitor zero at the drain. pH strips: water taken at the heater side of the neutralizer read about 4; water leaving it read near 6, which met the media maker's stated target.
What changes the answer. Move the same heater to a slab-on-grade garage where the only drain is a laundry standpipe higher than the outlet, and gravity cannot work. That job gets a condensate pump below the outlet, the neutralizer ahead of it, and the pump's overflow switch wired to the heater's blockage terminals, with the discharge run to the standpipe's air gap.
What doing it wrong looked like on the last callback in that neighborhood. An installer added an external trap below a heater with an internal one and ran the line out through the rim joist to the yard. It air-locked within a week and, after a repair, froze in January. Both failures showed as the same blockage error code and no hot water.
References
- IPC 2021 Section 314 and IMC 2021 Section 307, condensate disposal, as adopted
- UPC 2021 Section 814, condensate wastes, as adopted
- Heater and neutralizer manufacturers' installation instructions, part of the listing
- See related: How to Install a Tankless Water Heater
- See related: How to Cut, Prime and Glue a PVC DWV Joint