A second heat exchanger pulls latent heat out of the exhaust. That is what lets a condensing unit vent in plastic pipe and hit the mid-nineties on efficiency.
Quick Answer
A condensing tankless water heater adds a secondary heat exchanger that captures heat from exhaust gas before it leaves the unit, reaching roughly 0.93 to 0.96 UEF and venting in plastic pipe. Frisco Tankless Water Heater Pros installs condensing systems throughout Frisco, Texas, including condensate drainage and neutralization. Call (972) 430-9024 for an assessment.
Overview
In a conventional gas water heater, a large share of the fuel's energy leaves as hot exhaust. A condensing tankless unit adds a second heat exchanger downstream of the primary one, and it does something clever: it cools the flue gas far enough that the water vapor produced by combustion changes phase back to liquid. That phase change releases latent heat, which the incoming cold water absorbs. Efficiency climbs into the mid-nineties, and exhaust temperature drops from the 300 to 450 degree range down to roughly 100 to 140 degrees F.
That temperature drop is the practical payoff, and it is bigger than the efficiency number. Cool exhaust can travel through PVC, CPVC or polypropylene pipe instead of Category III stainless. On a short run that saves a few hundred dollars. On a 40 foot run through a Frisco attic to a roof termination, the vent material difference alone can offset most of the equipment premium. This is why the condensing decision is usually a venting decision wearing an efficiency costume.
The cost of that efficiency is condensate. When flue gas condenses it carries dissolved combustion byproducts, producing liquid with a pH generally between 3 and 5, which is in the neighborhood of vinegar. A condensing unit at full fire generates roughly half a gallon an hour. That fluid cannot go onto a slab, into a landscape bed, or into cast iron drain piping. It needs a routed drain line through a neutralizer cartridge filled with calcium carbonate media that raises the pH before discharge.
In Frisco specifically, condensate routing deserves real thought because so many units live in attics or garages. A condensate line run through an unconditioned attic is a freeze risk, and the February 2021 event proved it is not a theoretical one. A blocked or frozen condensate line will lock the unit out on a safety switch, which reads to the homeowner as a broken water heater. We route condensate with slope, keep it inside conditioned space where possible, and insulate anything that cannot be.
Diagnosis
Symptoms rarely point at a single cause. These are the ones we find most often on North Texas systems.
Cause: Condensate leaves the unit at roughly pH 3 to 5. Dumped into cast iron drain lines or onto a garage slab it corrodes metal, etches concrete and violates code.
Fix: We install an inline neutralizer with calcium carbonate media between the unit and the drain, and we tell you when the media needs replacing, typically annually.
Cause: A line run through an unconditioned Frisco attic can freeze in a hard winter event. Media that has fully spent itself packs and blocks flow. Either way the unit shuts down on a condensate safety switch.
Fix: We route lines with continuous slope inside conditioned space where possible, insulate any unavoidable attic run, and service the neutralizer as part of annual maintenance.
Cause: Manufacturers list specific materials and, in some cases, specific solvent cement. Using the wrong plastic or exceeding the equivalent length budget causes poor draft and nuisance lockouts.
Fix: We install to the manufacturer's listed vent system and stay within the published equivalent length, counting every elbow against the budget.
Cause: The condensing exchanger is not immune to Frisco's 8 to 11 grain per gallon supply. Scale insulates it, reduces the efficiency you paid a premium for, and eventually restricts flow.
Fix: Isolation valves at install plus annual descaling. The efficiency case for condensing evaporates entirely if the exchanger is coated in calcium.
Cause: Cool, wet exhaust is visible as a plume and will stain siding, fog windows and recirculate if terminated under a soffit or too close to an operable opening.
Fix: We locate terminations to the manufacturer's clearances and away from windows, walkways and gathering areas, using concentric kits where a single penetration is preferable.
Process
We map the vent route, count elbows against the equivalent length budget, pick a compliant termination, and locate the nearest approved drain for condensate.
45 minutes
Fixture demand modeled at a 55 degree winter rise, then the gas branch checked against full rated input at the measured developed length from the meter.
30 minutes
City of Frisco plumbing and mechanical permits pulled before installation begins.
1 to 3 business days
Old unit removed, new unit mounted with clearances verified, gas branch upsized if required, and the new gas piping pressure tested.
3 to 5 hours
Listed vent system assembled with correct slope back to the unit, termination set to clearance, condensate line run with continuous fall through a neutralizer to an approved drain.
2 to 4 hours
Full-fire gas pressure test, combustion check, temperature rise measured across both exchangers, condensate flow confirmed at the drain, and isolation valves function tested.
45 minutes
We meet the inspector, then show you the neutralizer cartridge, how to tell when the media is spent, and the descaling interval Frisco water requires.
45 minutes
Benefits
Cost
| Factor | Effect on price | Range |
|---|---|---|
| Equipment premium over non-condensing | The secondary heat exchanger and the more complex controls carry a real cost difference at the same BTU rating. | $400 to $900 |
| Vent material and length | Plastic vent is inexpensive per foot, which is the main financial argument for condensing on long runs. | $200 to $900 |
| Condensate drain routing | Cheap if there is a drain within a few feet. Expensive if condensate must travel across an attic or a condensate pump is required. | $150 to $700 |
| Neutralizer and media | Required to raise pH before discharge. Media is a consumable replaced roughly annually. | $90 to $250 plus about $30 per year |
| Gas branch capacity | Independent of condensing versus non-condensing, but a condensing unit's high input rating still has to be fed. | $400 to $1,500 |
| Freeze protection for exposed lines | Insulation or heat trace on condensate and water lines running through unconditioned attic space. | $100 to $400 |
| Concentric termination kit | Combines intake and exhaust in a single penetration, which is cleaner on an elevation but costs more than two separate terminations. | $150 to $400 |
| Permit and inspection | Required by the City of Frisco for water heater replacement. | $100 to $300 |
Typical total: $3,000 to $5,500 installed for most Frisco homes. Ranges, not quotes — a firm number needs eyes on the existing installation.
Comparison
| Vent system | Appliance category | Maximum exhaust temperature | Approximate installed cost per foot |
|---|---|---|---|
| Galvanized Type B vent | Category I atmospheric only | Not suitable for any tankless unit | Legacy material, cannot be reused |
| AL29-4C special gas vent | Category III non-condensing | Approximately 480 degrees F | $30 to $60 |
| 304 stainless single wall | Category III non-condensing | Approximately 480 degrees F | $28 to $55 |
| Schedule 40 PVC | Category IV condensing | Approximately 149 degrees F | $6 to $14 |
| Schedule 40 CPVC | Category IV condensing | Approximately 194 degrees F | $9 to $18 |
| Polypropylene | Category IV condensing | Approximately 230 degrees F | $14 to $28 |
| Concentric PVC kit | Category IV condensing, one penetration | Approximately 149 degrees F | $150 to $400 per kit |
The word condensing describes a phase change, and it happens in one place: on a metal surface colder than the dewpoint of the flue gas passing over it. That is the whole mechanism. A condensing tankless unit is not a better burner, it is a burner followed by a heat exchanger cold enough to rain inside.
Cold inlet water is what makes the surface cold. Water enters the secondary exchanger first, at whatever temperature the street main delivers, and only then passes to the primary exchanger over the burner. In a Frisco January the main delivers 50 to 55F, far below the 127F dewpoint, so the unit condenses hard. In August the main can deliver water in the 80s, so it still condenses, just less of the exhaust does.
The arithmetic is worth doing once. Methane has a higher heating value near 23,875 BTU per pound, so one therm is about 4.19 pounds of fuel, and combustion turns each pound of methane into 2.25 pounds of water. That is roughly 9.4 pounds per therm, and at about 970 BTU per pound to vaporize water, the vapor is carrying near 9,100 BTU. Nine percent of the therm you paid for is the entire prize. The unit does not capture all of it either: 9.4 pounds is about 1.13 gallons, so a 199,000 BTU unit at full fire has roughly 2.25 gallons per hour of vapor available, and real condensate production is closer to half a gallon per hour. It recovers something like a fifth to a quarter of what is there.
Appliance venting categories are not marketing labels. They are a two-by-two grid defined in the fuel gas codes, and every venting rule that applies to your unit descends from which box it lands in. The two questions are whether the vent runs under positive or non-positive static pressure, and whether vent gas temperature is low enough to produce condensate in the vent.
| Category | Vent pressure | Condensate in vent | Typical appliance | Vent material |
|---|---|---|---|---|
| I | Non-positive | No | Atmospheric tank water heater, draft hood furnace | Type B galvanized |
| II | Non-positive | Yes | Rare in residential work | Corrosion-resistant, gravity |
| III | Positive | No | Non-condensing tankless | Category III stainless, sealed |
| IV | Positive | Yes | Condensing tankless, condensing furnace | PVC, CPVC or polypropylene, sealed |
Positive pressure is the half people forget. The blower pushes exhaust through the vent rather than relying on buoyancy, so every joint is a joint that can leak combustion products into the house rather than draw room air into the vent. That is why a Category IV vent is a solvent-welded or gasketed sealed system and why an inspector looks at the joints first. Material listings are equally specific: Schedule 40 PVC to ASTM D1785 is rated near 149F, CPVC to ASTM F441 near 194F, and listed polypropylene evaluated to UL 1738 near 230F. Which of the three you may use is set by the appliance listing, and some manufacturers accept only their own branded polypropylene.
The acidity is mostly carbonic acid. Carbon dioxide is a combustion product, it dissolves readily in the water condensing alongside it, and dissolved carbon dioxide forms a weak acid. Traces of nitrogen oxides, and small amounts of sulfurous acid where the gas carries sulfur odorant, push the number lower still. The result is generally pH 3 to 5: weak in strength, persistent in volume, because a busy household produces it daily for twenty years.
A neutralizer is a container of alkaline mineral that condensate flows through slowly enough to react with it. Calcium carbonate chips are the common media, sometimes blended with magnesium oxide for a faster reaction. The acid dissolves a small amount of the mineral, the reaction consumes hydrogen ions, and the discharge is typically pH 6.5 or better. The media is genuinely consumed doing this, which is why it is a replacement item and not a filter you can rinse.
| Property | Detail | Practical consequence |
|---|---|---|
| Common media | Calcium carbonate chips, sometimes with magnesium oxide | Magnesium oxide reacts faster but is consumed faster |
| Untreated condensate pH | Roughly 3 to 5 | Attacks cast iron, mortar and concrete over time |
| Target discharge pH | Generally 6.5 or higher | Acceptable to a public sewer connection |
| Media is consumed | Yes, dissolved by the reaction | Replacement item, not a cleanable filter |
| Typical replacement interval | About annually in residential service | Aligns with the annual descaling visit |
| Failure mode when spent | Media packs down, flow slows, pH rises above target | Unit can trip a condensate switch and stop making hot water |
Look at the media, not the calendar
Fresh calcium carbonate chips are white and irregular. Spent media is rounded, noticeably smaller, and often grey or brown with a film. If the cartridge is clear-bodied you can make this call in five seconds without tools.
Check the level in the chamber
Media dissolves from the top of the bed downward as the condensate passes through. A chamber that was full at installation and is now two thirds full has spent roughly a third of its capacity, and that is a better indicator than elapsed months.
Test the discharge, not the inlet
Aquarium or pool test strips read the range that matters. A strip held under the outlet should read 6.5 or above. If it reads 5, the media is not working regardless of how it looks.
Frisco is slab-on-grade. There are no basements and very rarely a floor drain, which removes the easiest condensate solution available elsewhere in the country. What is left is a lateral run to a laundry standpipe, a lavatory tailpiece, a mop sink, or a run to daylight where local rule permits it. That run has to fall continuously, and gravity is the only thing to rely on if there is any way to arrange it.
| Route | How it behaves | What can go wrong |
|---|---|---|
| Gravity to a laundry standpipe | Nothing to fail, no power, no moving parts | Requires the unit to sit above the standpipe |
| Gravity to a lavatory tailpiece | Simple, short, inside conditioned space | Trap must stay wet or sewer gas enters |
| Gravity to an exterior discharge | No indoor drain needed | Freeze exposure, and local rule may not allow it |
| Condensate pump to a nearby drain | Solves an attic or below-drain location | Pump fails, needs power, needs an overflow switch |
| Long lateral across an attic | Sometimes the only option | Freeze risk, sagging, and a leak lands on a ceiling |
Where a pump is unavoidable its overflow float switch must be wired to shut the appliance down, not left unconnected. Freeze exposure also deserves more weight than it gets. A condensate line is small bore, runs at a trickle rather than under pressure, and holds a thin film of standing liquid most of the time. That is the easiest thing in a Frisco attic to freeze, and it freezes at a milder outdoor temperature than the supply lines beside it because there is no pressure and no flow to help. February 2021 locked out a great many condensing appliances in North Texas by exactly this route, and none of them were broken.
The primary exchanger over the burner is almost always copper, because copper conducts heat roughly eight times better than stainless and its only job is rapid transfer at high temperature. The secondary exchanger cannot be copper. It sits in acidic condensate for its entire working life, and copper does not survive that. So it is stainless, commonly 316L or a ferritic grade such as 439, chosen for acid and chloride resistance rather than conductivity.
The two exchangers therefore fail for entirely different reasons. The primary dies of scale, which everyone is warned about. The secondary dies of chloride pitting, which almost nobody mentions, and the chlorides arrive from the combustion air when a unit draws room air from a garage where pool chemicals, muriatic acid, chlorinated cleaners or solvents are stored. Sealed two-pipe direct venting that draws combustion air from outdoors is genuinely protective here, and it is one of the better arguments for paying for the second pipe.
The unflattering truth is that a condensing unit adds a second exchanger, a condensate trap, a drain line, a neutralizer and a condensate safety switch, and every one of those can stop your hot water. A non-condensing unit has none of them. The complexity buys about twelve points of efficiency and cheap venting, and whether that trade is worth it in your house is a question about vent length and drain availability.
Want this priced for your specific Frisco home? Talk to a tankless specialist at (972) 430-9024.
Frequently yes, but the reason is usually venting rather than fuel savings. The efficiency gain from roughly 0.83 to 0.95 UEF saves a typical Frisco household on the order of 25 to 45 dollars a year on gas, which by itself takes a long time to repay a $400 to $900 equipment premium. Where condensing wins decisively is on a long vent run, because plastic pipe costs a fraction of Category III stainless per foot. On a 40 foot run the vent savings alone can exceed the equipment premium.
To an approved drain, through a neutralizer, never onto a slab or into landscaping. A condensing unit at full fire produces about half a gallon of liquid per hour at roughly pH 3 to 5, acidic enough to corrode cast iron drain piping and etch concrete. We route it with continuous slope through a cartridge filled with calcium carbonate media that raises the pH, then to a compliant connection. If gravity does not reach a drain, we add a small condensate pump.
Yes, when the manufacturer lists it, because the second heat exchanger removes so much heat from the exhaust that flue temperature falls to roughly 100 to 140 degrees F. That is well inside PVC's service range. Some manufacturers require CPVC or polypropylene instead, and the listing is what governs, not general practice. We install the specific system the unit is listed for and stay within its published equivalent vent length, counting each elbow against the total.
Slightly, and it is predictable. They need the same annual descaling that Frisco's 8 to 11 grain per gallon water requires of any tankless unit, plus a neutralizer media change roughly once a year and an occasional check that the condensate trap and line are clear. That is maybe fifteen extra minutes during an annual service visit. What actually causes trouble is skipping it: a packed neutralizer or a blocked condensate line will shut the unit down on a safety switch and look like a failure.
It can, and February 2021 demonstrated exactly how. A condensate line run across an unconditioned attic or down an exterior wall is exposed to the same air that froze pipes across North Texas that week. When the line freezes, the unit locks out on its condensate safety and stops making hot water. We route condensate inside conditioned space wherever the layout allows, keep continuous slope so nothing stands in the line, and insulate or heat trace any run that must go through unconditioned space.
Usually yes, and that is one of its advantages. Condensing units are typically installed as sealed combustion appliances with a two-pipe direct vent, drawing combustion air from outside rather than from the room. That removes the make-up air problem that would otherwise disqualify a small closet, since the indoor-air method would require roughly 10,000 cubic feet of space for a 199,900 BTU appliance. You still need service clearance and a route for condensate, which is what actually decides most closet installations.
Related
System Type
Fewer parts, no condensate drain and a lower price. The trade is stainless venting and about ten points of efficiency.
Installation
The BTU rating on the box is meaningless if the pipe feeding it cannot deliver the gas. We size the fuel first.
Components
Tankless units vent under positive pressure. That single fact rules out every B-vent flue in Frisco and decides which unit can go where.
System Type
Attic, garage wall or utility closet. Each location has a different set of code requirements, and the right one is usually not where the old tank sat.
Maintenance
Frisco water runs 8-11 grains per gallon. Calcium goes exactly where it does the most damage: the hottest surface in your heat exchanger.
Installation
Properly sized, permitted and vented on-demand hot water for Frisco homes — from Starwood estates to Frisco Lakes patio homes.
Background reading
Longer explanations of the tradeoffs behind this work, written for homeowners rather than trade readers.
Buying Guide · 11 min read
The efficiency gap is about ten points of UEF. The venting cost gap can be a thousand dollars in the other direction. Which one wins depends entirely on where the unit is going.
Comfort & Efficiency · 11 min read
A recirculation pump can cut a 90-second wait to five seconds. Run it wrong and it reintroduces exactly the standby loss the tankless unit was bought to eliminate. Here is how to get the first without the second.
Ownership · 11 min read
Twenty years is the number on the brochure. Frisco's water decides whether you get it. Here is what actually kills these units, which repairs make sense at which ages, and where the line is.
Coverage
We take condensing tankless water heater installation calls throughout Collin County and Denton County. These are a few of the areas covered.
See every Frisco neighborhood and surrounding city we cover.
Call (972) 430-9024 and speak to someone who works on tankless systems every day. Open 24 hours a day, 7 days a week.
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Frisco, TX Tankless Pros