Swapping the appliance is the easy part. The gas line, the flue the furnace was sharing, and the house pressure are what make a conversion succeed or fail.
Quick Answer
Converting a storage tank to tankless in Frisco typically requires upsizing the gas branch, replacing the atmospheric flue with a listed vent system, resizing any furnace flue the tank was sharing, and mounting the new unit. Frisco Tankless Water Heater Pros handles the full conversion including City of Frisco permitting and inspection. Call (972) 430-9024.
Overview
A tank-to-tankless conversion is not a like-for-like swap and anyone describing it that way has not looked at the gas system. A 40 or 50 gallon atmospheric tank has roughly a 40,000 BTU burner and vents by gravity through a galvanized flue. The tankless unit replacing it has a 160,000 to 199,900 BTU burner, a combustion blower, and a completely different vent category. The appliance is the smallest part of the project. Everything it connects to changes.
Frisco makes this predictable, which helps. The housing stock is young and consistent, and the builder pattern is familiar: a half-inch gas branch sized for the tank, a B-vent to the roof, an attic platform or garage wall location, and a water heater that was fine right up until it was not. That means we can usually tell you what the conversion involves during the assessment with real confidence, instead of discovering it mid-job.
The item that gets missed most often is the orphaned flue. In many Frisco homes the water heater and the furnace share a common B-vent, sized under NFPA 54 common venting tables for both appliances together. Remove the water heater and the furnace is left venting alone into a flue that is now oversized for it. Exhaust cools on the way up, condenses inside the chimney, and over time corrodes it. It is a combustion safety issue and it is invisible from the water heater. We recalculate and reline or resize as part of the conversion.
The last piece is pressure and thermal expansion, which behaves differently after a conversion. Frisco street pressure frequently runs high enough to require a pressure reducing valve, and code caps fixture pressure at 80 psi. High static pressure pushes flow past a tankless unit's design range and shortens component life. The upside is that a tankless system holds under a gallon of water instead of 50, so thermal expansion becomes a much smaller factor. We check static pressure and the existing expansion control as part of every conversion rather than assuming the old setup still applies.
Diagnosis
Symptoms rarely point at a single cause. These are the ones we find most often on North Texas systems.
Cause: The builder ran half-inch pipe because that is all the tank needed. Half-inch carries about 95,000 BTU at 30 feet and 81,000 BTU at 40 feet at a half-inch water column drop, nowhere near 199,900.
Fix: We run the load calculation, then upsize the branch to 3/4 or 1 inch based on measured developed length from the meter, or pull a dedicated line.
Cause: The tank and furnace shared a common vent sized for both. With the tank gone, the furnace vents alone into an oversized flue, exhaust cools and condenses, and the chimney corrodes from the inside.
Fix: We rerun the common vent calculation and reline or resize the furnace flue. This is the single most commonly omitted item on cheap conversion quotes.
Cause: Galvanized B-vent is designed for atmospheric appliances with gravity draft and unsealed joints. A tankless unit vents under positive pressure and, if condensing, produces acidic condensate that destroys galvanized steel.
Fix: We install the manufacturer's listed vent system, PVC or polypropylene for condensing and Category III stainless for non-condensing, with correct terminations.
Cause: A tank on an attic platform gets touched once a decade. A tankless unit needs annual descaling at Frisco water hardness, and attic service quietly stops happening.
Fix: We evaluate relocating to a garage wall or utility closet during the conversion, when the gas, water and vent are all open anyway and the marginal cost is lowest.
Cause: Frisco street pressure can run above the 80 psi that code allows at fixtures. High pressure drives flow past the unit's design range and accelerates wear on internal components.
Fix: We measure static pressure during the assessment and install or adjust a pressure reducing valve when the reading calls for it.
Cause: Without service valves, descaling requires cutting into the piping, so it never gets done. At 8 to 11 grains per gallon, that turns a 20 year appliance into a short-lived one.
Fix: A full isolation valve kit with hose bibs on both ports goes on every conversion. It adds very little during the job and it is what makes the service life claim real.
Process
We document the existing tank, measure the gas branch and developed length, determine whether the flue is shared with the furnace, test static water pressure, and evaluate relocation options.
60 minutes
A written scope covering the unit, gas work, new venting, furnace flue correction if applicable, condensate handling, pressure control, permit and labor. No open allowances that get revised mid-job.
Same day
City of Frisco plumbing and mechanical permits pulled. Texas requires a licensed plumber for water heater replacement and the work is inspected.
1 to 3 business days
Tank drained, disconnected and hauled away, old flue connections removed, and the platform or floor area prepared for the new mounting or freed for reuse.
1 to 2 hours
Gas branch upsized to carry full rated input, new gas pressure tested, listed vent system installed to a compliant termination, and the furnace flue resized or relined where the tank was sharing it.
4 to 7 hours
Unit mounted with clearances verified, water connections made, isolation valve kit installed on both ports, condensate routed through a neutralizer if condensing, and pressure control corrected if the reading required it.
2 to 4 hours
Full-fire gas pressure verified, combustion checked on both the new unit and the furnace on the corrected flue, every fixture tested, inspection completed, and the descaling procedure demonstrated.
1 to 2 hours
Benefits
Cost
| Factor | Effect on price | Range |
|---|---|---|
| Gas branch upsizing | Nearly universal on conversions, since builder branches were sized for a 40,000 BTU tank. Cost tracks developed length from the meter. | $400 to $1,500 |
| New vent system | The old B-vent cannot be reused. Condensing units vent in plastic, non-condensing in Category III stainless at a much higher cost per foot. | $300 to $1,600 |
| Orphaned furnace flue correction | Required whenever the removed tank was sharing a common vent. Frequently omitted from lower quotes, which is how you spot them. | $250 to $900 |
| Condensate drain and neutralizer | Applies to condensing units only. Cheap near a drain, expensive across an attic or with a condensate pump. | $150 to $700 |
| Relocation from attic to garage | Cheapest to do during the conversion when gas, water and vent are already open. Improves service access and removes overhead water. | $500 to $1,400 |
| Pressure reducing valve | Installed or adjusted when measured static pressure exceeds what code allows at fixtures. | $250 to $650 |
| Isolation valve kit | Non-negotiable on a conversion. Without it, the annual descaling Frisco water requires will not happen. | $120 to $300 |
| Permit and inspection | Required by the City of Frisco for water heater replacement. | $100 to $300 |
Typical total: $2,900 to $5,800 for a complete conversion in Frisco. Ranges, not quotes — a firm number needs eyes on the existing installation.
Comparison
| Consideration | Replace with another 50 gallon tank | Convert to tankless |
|---|---|---|
| Up-front installed cost | $1,400 to $2,600 | $2,900 to $5,800 |
| Expected service life | 8 to 12 years | 20 years with annual descaling |
| Purchases needed over 20 years | Two units | One unit |
| Efficiency (UEF) | 0.58 to 0.70 | 0.81 to 0.96 depending on type |
| Standby loss | Continuous, occupied or not | None |
| Hot water availability | 50 gallons, then 45 to 60 minute recovery | Continuous within the flow rating |
| Worst-case failure | Tank splits, releases 50 gallons | Unit shuts down, holds under a gallon |
| Space occupied | Roughly 9 to 12 square feet of floor or platform | Roughly 2 square feet of wall |
| Required maintenance | Anode rod inspection, periodic tank flush | Annual descaling at Frisco water hardness |
In a large share of Frisco homes the storage water heater and the furnace share a single vent to the roof, joined at a common connector. That flue was not sized for either appliance individually. It was sized under the common venting tables in NFPA 54 for the two of them together, which means its diameter reflects the combined exhaust volume of a draft hood water heater and a fan-assisted furnace. Take the water heater out and the arithmetic that justified the diameter no longer holds.
The failure that follows is slow and invisible from the water heater. Cooled exhaust condenses on the inside of the vent, that condensate is acidic, and it attacks the galvanized steel or the masonry liner from the inside. Over several heating seasons the liner degrades, joints open, and the furnace's combustion products start finding their way into the building rather than out of it. Nothing about that failure points back at the water heater conversion that caused it, which is exactly why it goes unnoticed.
There is a second, quicker mechanism as well. A draft hood water heater firing into a shared flue was helping establish and maintain draft for the whole system. Remove it, and on a cold start with a cold chimney the furnace can struggle to establish draft at all, particularly on a tall flue in an otherwise well-sealed house. That presents as furnace short-cycling or spillage at the draft diverter, in the same winter the new water heater was installed, and it gets blamed on the furnace.
The gas line is the other item that gets underestimated, and unlike the flue it is easy to verify. NFPA 54 publishes capacity tables for Schedule 40 metallic pipe carrying 0.60 specific gravity natural gas at a 0.5 inch water column pressure drop, indexed by nominal pipe size and developed length. Developed length means the run from the meter to the appliance including the equivalent length of the fittings, not the straight-line distance across the yard.
| Developed length | 1/2 inch | 3/4 inch | 1 inch | 1-1/4 inch |
|---|---|---|---|---|
| 10 ft | 172 | 360 | 678 | 1,390 |
| 20 ft | 118 | 247 | 466 | 957 |
| 30 ft | 95 | 199 | 374 | 768 |
| 40 ft | 81 | 170 | 320 | 657 |
| 50 ft | 72 | 151 | 284 | 583 |
| 60 ft | 65 | 137 | 257 | 528 |
| 80 ft | 56 | 117 | 220 | 452 |
| 100 ft | 50 | 104 | 195 | 400 |
Now use it. At roughly 1,000 BTU per cubic foot, a 199,900 BTU tankless unit needs about 200 CFH. Read across the 40 foot row: half inch gives 81, three-quarter inch gives 170, one inch gives 320. Half inch is not close, three-quarter inch falls short by fifteen percent, and one inch is the answer. Run the same reading at 20 feet and three-quarter inch carries 247, which is comfortably adequate. The distance from your meter to your water heater is doing more to set the price of your conversion than the model you choose.
It also explains why the builder's original pipe was correct at the time and is now wrong. A 40,000 BTU tank needs 40 CFH. Half-inch pipe carries 81 CFH at 40 feet, comfortably double what the tank required. The builder did nothing wrong. The appliance simply grew by a factor of five, and no amount of arguing that the old line has always worked changes what the table says.
A storage tank arrives with a set of accessories that exist because it is a pressure vessel holding fifty gallons of hot water. A tankless unit is not a pressure vessel and does not hold fifty gallons, so several of those accessories become either unnecessary or differently specified. This causes confusion at inspection and it is worth taking one at a time.
| Item | What it did on the tank | What happens after conversion |
|---|---|---|
| Temperature and pressure relief valve | Protected 50 gallons of stored hot water | Leaves with the tank; the tankless unit's relief requirement is set by its own instructions and is generally pressure-only |
| T and P discharge pipe | Carried a discharge to a safe location | Often reusable for the new relief if routing and size still comply |
| Drain pan and pan drain | Caught a tank leak or a T and P discharge | Still required by most manufacturers where a leak would damage the structure |
| Thermal expansion tank | Absorbed roughly half a gallon of expansion | Frequently no longer needed; the expansion volume drops by two orders of magnitude |
| Gas shutoff and sediment trap | Served a 40,000 BTU appliance | Reused only if the new branch size and the trap location still comply |
| 120V receptacle | Not present, the tank needed no power | New requirement; garage receptacles need GFCI protection under NEC 210.8(A)(2) |
The expansion tank line deserves its arithmetic. Water heated from 55F to 120F expands by roughly one percent by volume. On fifty gallons that is about half a gallon, which in a closed system with a check valve or a pressure reducing valve has nowhere to go and drives pressure up sharply, so an expansion tank was genuinely necessary. A tankless unit holds well under a gallon at any instant, so the same one percent is a few thousandths of a gallon. The problem does not scale down, it essentially disappears.
It does not disappear entirely in every case. If the conversion includes a recirculation loop with a buffer tank, that buffer restores a stored volume and the expansion question comes back with it. And the pressure question is separate from the expansion question: a closed system with high street pressure is still a closed system with high street pressure, and code caps pressure at fixtures at 80 psi regardless of what appliance is making the hot water.
Conversions get sold on energy savings, and the energy savings are real but modest. It is better to look at the number squarely than to be disappointed by it in year three. The comparison below assumes a household delivering roughly 165 therms of hot water annually, an existing tank around 0.61 UEF, and a condensing tankless unit around 0.95 UEF.
| Factor | Tank replaced twice | One tankless conversion |
|---|---|---|
| Gas input for hot water per year | About 270 therms | About 174 therms |
| Annual gas difference | Baseline | About 96 therms less |
| Annual dollar difference | Baseline | Roughly $100 to $130 less |
| Twenty-year fuel difference | Baseline | Roughly $2,000 to $2,600 less |
| Up-front cost | $1,400 to $2,600, then again at year 10 | $2,900 to $5,800 once |
| Equipment purchases in 20 years | Two | One |
| Annual maintenance | Anode inspection, periodic flush | Descaling, roughly $150 to $250 per year |
| Twenty-year maintenance total | Low | Roughly $3,000 to $5,000 |
| Net financial position | Roughly comparable | Roughly comparable |
That last row is the honest conclusion and it is not the one the industry usually offers. Once you count the descaling that Frisco water at 8 to 11 grains per gallon genuinely requires, the fuel savings and the avoided second tank purchase are largely spent on maintenance. Over twenty years the two paths land close enough together that anyone presenting a conversion as a clear financial win is either omitting the maintenance or assuming a household that uses far more hot water than most.
The reasons to convert are real, they are just not primarily financial. Hot water that does not run out changes how a household with teenagers actually functions. Removing fifty gallons of water from an attic platform above finished bedroom ceilings removes a specific, expensive and reasonably likely failure. Getting nine to twelve square feet of garage or closet back is worth something. And a twenty-year appliance means one disruptive installation over that period instead of two.
Want this priced for your specific Frisco home? Talk to a tankless specialist at (972) 430-9024.
Most complete conversions land between $2,900 and $5,800. The variables are the gas branch, the vent path, and whether the old tank was sharing a flue with the furnace. A garage-located tank on an already adequate gas line with a short sidewall vent run sits at the low end. An attic tank on a half-inch branch sixty feet from the meter, sharing a flue with the furnace, sits at the top. We give a firm number after the assessment rather than a phone estimate.
On gas savings alone, longer than most marketing suggests. A household delivering roughly 165 therms of hot water annually burns about 270 therms through a 0.61 UEF tank and about 174 through a 0.95 UEF tankless unit, saving on the order of $100 to $130 a year at North Texas gas prices. Against a premium of $1,500 to $3,000 that is a long horizon. The stronger arguments are the doubled service life, the removal of 50 gallons from above your ceilings, and hot water that does not run out.
In many Frisco homes the water heater and furnace share one common B-vent, sized for both appliances together. When the tank comes out, the furnace is left venting alone into a flue that is now too large for it. The exhaust cools on the way up, condenses inside the chimney, and corrodes it over time. It is a combustion safety issue that has nothing to do with the water heater you just installed, which is exactly why it gets skipped. We recalculate and correct it as part of the conversion.
Yes, and it is the most common conversion scenario in Frisco since the housing stock is slab-on-grade with no basements. The real question is whether to keep the unit in the attic or relocate it. Annual descaling is required at Frisco water hardness, and attic service is unpleasant enough that it tends to lapse. Moving the unit to a garage wall during the conversion typically adds $500 to $1,400, shortens the vent run, and removes a water appliance from above your bedrooms.
Often not, and this is one of the genuine simplifications a conversion brings. Thermal expansion is proportional to the volume of water being heated, and a tankless unit holds under a gallon instead of fifty. The pressure question does not disappear though. Frisco street pressure can exceed what code allows at fixtures, and high static pressure pushes flow past the unit's design range and wears components. We measure static pressure during the assessment and address it with a pressure reducing valve when the reading calls for it.
For a conversion where the gas branch is already adequate and the flue was not shared with the furnace, water is off for most of one working day and hot water is back that evening. Most Frisco conversions involve upsizing the gas line, correcting an orphaned furnace flue, or relocating the unit, which pushes the job into a second day. We tell you which category yours falls into during the assessment so you can plan around it. Call (972) 430-9024 to schedule.
Related
Installation
Properly sized, permitted and vented on-demand hot water for Frisco homes — from Starwood estates to Frisco Lakes patio homes.
Components
A tankless unit can demand five times the gas of the tank it replaced. The pipe that fed the tank almost never carries it, and the failure is intermittent rather than obvious.
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.
Installation
Sized against your actual fixture count and North Texas inlet water temperature, not a rule of thumb printed on a box.
Components
A check valve or pressure reducing valve at the meter turns your plumbing into a closed system. Heated water has to expand somewhere, and without a tank it expands against your fixtures.
Installation
Replacing a tankless unit that has reached the end of its life is a different job than a first-time install. We treat it that way.
Background reading
Longer explanations of the tradeoffs behind this work, written for homeowners rather than trade readers.
Cost & Value · 13 min read
Two identical units installed a mile apart in Frisco can differ by two thousand dollars. Almost none of that gap is the equipment. Here is where the money actually goes.
Buying Guide · 10 min read
Tankless is not automatically the right answer. Here is the comparison with the inconvenient parts left in, including the three situations where a storage tank is still the better buy.
Installation · 11 min read
The half-inch branch that fed your 40,000 BTU tank will not feed a 199,000 BTU tankless unit. This is the line item that turns a $3,000 quote into a $4,500 one, and here is how to see it coming.
Coverage
We take tank to tankless conversion 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