One unit or two, sized against what your household actually runs at the same time on the coldest morning of the year.
Quick Answer
Most Frisco homes need 180,000 to 199,900 BTU for whole-house service, which delivers roughly 5.5 to 6.9 GPM at a 55 degree F winter rise. Homes above about 4,500 square feet with a roman tub filler usually need two units manifolded together. Frisco Tankless Water Heater Pros sizes every system from measured simultaneous demand. Call (972) 430-9024.
Overview
Whole-house means the tankless system is the only source of hot water for every fixture in the home, which makes sizing the entire ballgame. Undersizing is the number one reason homeowners end up disappointed with tankless, and it almost always traces back to a number chosen from a brochure rather than from the house. The only inputs that matter are what runs simultaneously and how cold the incoming water is.
North Texas gives us a firm design number. Frisco groundwater arrives at roughly 50 to 55 degrees F in January, and a comfortable shower is about 105 degrees, so the design case is a 55 degree rise. At that rise, manufacturer charts put a 199,900 BTU condensing unit near 6.9 GPM. We design to about 5.5 to 6.0 GPM of genuinely usable flow, because setpoint, pressure loss through the exchanger, elevation and aerator behavior all shave the chart number. That covers two showers plus a sink comfortably.
The homes that break a single unit are predictable and there are a lot of them in Frisco. A master bath with a roman tub filler is the usual culprit, because a large tub filler can pull 6 to 12 GPM by itself, more than the entire rest of the house. Add two secondary showers running before school and a laundry cycle and peak demand passes 10 GPM. In the larger floor plans around Starwood, Newman Village and the estate sections off Legacy Drive, two units manifolded on a common controller is not an upsell, it is the correct design.
Two units introduce a gas problem worth knowing about early. Two 199,900 BTU units need roughly 400 cubic feet of gas per hour with both firing. A common residential meter is rated near 250 CFH, so a manifolded system plus a furnace, range and dryer will exceed it. That means a meter upsize request to the gas utility and typically a 1-1/4 or 1-1/2 inch trunk from the meter. We identify that during the assessment rather than after the equipment is on site.
Diagnosis
Symptoms rarely point at a single cause. These are the ones we find most often on North Texas systems.
Cause: Square footage does not consume hot water. Two adults in a 4,000 square foot home have a smaller peak than five people in a 2,600 square foot home with three baths.
Fix: We inventory fixtures, measure actual flow rates, and build a realistic peak-demand scenario from your household's real morning routine.
Cause: A large tub filler can demand 6 to 12 GPM on its own, which exceeds the entire output of a single 199,900 BTU unit at a 55 degree winter rise.
Fix: We either size for the tub with two manifolded units, or set expectations honestly that the tub fills at a slower rate when other fixtures are running.
Cause: Two 199,900 BTU units draw about 400 CFH. A common residential meter rated near 250 CFH cannot supply that alongside a furnace, range and dryer.
Fix: We total connected load, request a meter upsize from the gas utility, and size the trunk from the meter to carry the full bank.
Cause: A large home has long branch runs. Tankless removes the small head start a hot tank provided, so the far master bath waits longer than it used to.
Fix: A dedicated return line with a recirculation pump on a timer or demand control, which in a whole-house design should be planned from the start rather than added later.
Cause: A continuously running loop makes small, frequent calls for heat, which can cycle the burner constantly and wear components faster than normal use.
Fix: A small buffer tank on the loop, or a demand-controlled pump that runs only when someone actually wants hot water, resolves it.
Cause: At 8 to 11 grains per gallon, scale builds on the exchanger and the first symptom in a whole-house application is that the system stops covering two showers the way it used to.
Fix: Isolation valves at install, annual descaling, and scale reduction upstream on high-demand homes where the exchanger works hard every day.
Process
We inventory and measure every hot water fixture including tub fillers, then interview the household about actual overlapping use to build a peak demand figure.
1 hour
Total connected load calculated against the meter rating, developed length measured from the meter, and the required trunk size determined for single or manifolded configuration.
45 minutes
Single unit or manifolded pair, recirculation strategy, buffer tank if the loop requires it, plus an itemized proposal covering gas, venting, condensate and controls.
1 to 3 business days
City of Frisco plumbing and mechanical permits, and a meter upsize request to the gas utility when a manifolded system requires it.
3 business days to 3 weeks
Trunk line run and sized for the full bank, branches to each unit, then the entire new gas system pressure tested and held before connection.
4 to 10 hours
Units mounted and linked on a common water manifold and cascade controller, isolation valves on each unit, listed venting run, and condensate collected and neutralized.
6 to 14 hours
Return line and pump installed and balanced, staging programmed, gas pressure verified with all units at full fire, delivered temperature confirmed at the furthest fixture, then inspection and walkthrough.
3 to 5 hours
Benefits
Cost
| Factor | Effect on price | Range |
|---|---|---|
| Single unit versus manifolded pair | The largest decision on the project. A second unit brings its own gas, vent, condensate and controller requirements, not just a second price tag. | $3,000 to $5,500 for the second unit |
| Gas trunk sizing | A manifolded pair usually needs 1-1/4 or 1-1/2 inch from the meter, which is a different job than upsizing a branch. | $800 to $3,000 |
| Gas meter upsize | Two units at roughly 400 CFH commonly exceed a standard residential meter. Utility-controlled with real lead time. | $0 to $1,500 plus schedule impact |
| Recirculation system | A dedicated return line, pump and control. On a large Frisco floor plan this is close to mandatory for the system to feel right. | $900 to $2,500 |
| Buffer tank | Prevents short-cycling on a recirculation loop and smooths very short draws across a large fixture count. | $700 to $2,000 |
| Venting for multiple units | Either separate terminations or a manufacturer-listed common vent kit, both of which cost more than a single unit's vent. | $500 to $2,200 |
| Scale reduction | A whole-house system at Frisco hardness moves a lot of water through the exchanger. Pretreatment protects the output you paid for. | $800 to $3,500 |
| Permits and inspections | City of Frisco plumbing and mechanical permitting for the full scope. | $150 to $450 |
Typical total: $3,500 to $5,500 for a single-unit whole-house system; $6,500 to $11,000 for a manifolded pair. Ranges, not quotes — a firm number needs eyes on the existing installation.
Comparison
| Simultaneous fixture scenario | Approximate peak demand | Recommended whole-house system |
|---|---|---|
| One shower plus one lavatory | 3.3 to 4.0 GPM | 150,000 to 160,000 BTU single unit |
| Two showers at once | 3.5 to 5.0 GPM | 180,000 BTU single unit |
| Two showers plus dishwasher and a sink | 5.5 to 6.5 GPM | 199,900 BTU single unit |
| Three showers before school | 5.5 to 7.5 GPM | 199,900 BTU single unit at its limit, or a pair |
| Roman tub filling alone | 6.0 to 12.0 GPM | Two units manifolded |
| Roman tub plus a secondary shower | 8.0 to 14.0 GPM | Two units manifolded, 1-1/2 inch gas trunk |
| Two showers plus laundry plus kitchen | 7.0 to 9.5 GPM | Two units manifolded |
| Estate home, 6+ fixtures with overlapping use | 12.0 GPM and above | Two to three units with cascade control and recirculation |
The most common sizing error is adding up every hot water fixture in the house. A five-bath Frisco home with a roman tub can total well past 25 GPM on paper, which would suggest four units and a commercial gas service. Nobody runs a house that way and no plumbing code sizes anything that way. Water distribution has been sized on probability rather than on totals since Roy Hunter's work in the 1940s, and the fixture unit tables in the plumbing code exist precisely to apply a diversity factor to a list of fixtures.
The right model for a tankless system is a realistic worst case: the single busiest overlapping moment your household actually produces, plus a margin. For most families that moment is a weekday morning, and it is two showers with a lavatory faucet running, not five fixtures wide open. For a household that entertains, the worst case might be an evening with a tub filling and a kitchen sink going. The point is that the peak is a scenario you can name, and if you cannot name it you are guessing.
| Fixture | Total flow | Hot water share at a 105F mix | Duration |
|---|---|---|---|
| Shower, standard head | 1.8 to 2.5 GPM | About 70 percent | 8 to 15 minutes |
| Shower, multi-head or rain | 4.0 to 8.0 GPM | About 70 percent | 8 to 15 minutes |
| Standard tub spout | 4.0 to 6.0 GPM | About 75 percent | 6 to 10 minutes |
| Roman or soaking tub filler | 6.0 to 12.0 GPM | About 75 percent | 8 to 20 minutes |
| Lavatory faucet | 1.0 to 1.5 GPM | About 60 percent | Under 1 minute, intermittent |
| Kitchen faucet | 1.5 to 2.2 GPM | About 60 percent | Intermittent |
| Dishwasher | 1.0 to 1.5 GPM | 100 percent | Short fills, intermittent |
| Clothes washer, warm cycle | 2.0 to 3.0 GPM | About 50 percent | Short fills, intermittent |
The hot water share column is the one that gets left out of most sizing conversations, and it changes the arithmetic materially. A 2.5 GPM shower at a comfortable 105F, mixed from 120F stored water and 55F cold, is drawing roughly 1.7 GPM of hot and 0.8 GPM of cold. Two of those is about 3.4 GPM of hot water, not 5.0. That is the difference between comfortably inside a single 199,000 BTU unit and right at its edge, and it is why we measure fixtures rather than reading rated flows off a box.
Gas sizing on a whole-house project runs into a hard limit that pipe sizing cannot solve. The meter itself has a rated capacity, and a common residential diaphragm meter is rated in the region of 250 cubic feet per hour. Natural gas delivers roughly 1,000 to 1,050 BTU per cubic foot, so that meter can pass something like 250,000 BTU per hour total, for every gas appliance in the house at once.
A single 199,900 BTU tankless unit consumes about 200 CFH by itself. That leaves roughly 50 CFH for a furnace, a range, a dryer and anything else. A 100,000 BTU furnace alone is 100 CFH. The sum exceeds the meter, and the reason most single-unit installations work anyway is that the water heater and the furnace rarely sit at full fire simultaneously for long. That is a tolerance, not a design, and it is why cold-morning flame-loss faults are the classic symptom of a gas system that was never actually verified.
| Configuration | Peak demand | Fits a 250 CFH meter? |
|---|---|---|
| One 199,900 BTU tankless alone | About 200 CFH | Yes |
| One tankless plus 100,000 BTU furnace | About 300 CFH | No, relies on the two not peaking together |
| One tankless, furnace, range and dryer | About 360 CFH | No |
| Two 199,900 BTU tankless manifolded | About 400 CFH | No, meter upsize required |
| Two tankless plus furnace, range and dryer | About 560 CFH | No, meter upsize and larger trunk required |
When the total genuinely exceeds the meter, the fix is a utility request rather than a plumbing task. The gas provider owns the meter and the service line, they set the schedule, and the lead time is theirs. It can be quick and it can take weeks. That is a schedule risk on a whole-house project, and it is the single most common reason a two-unit installation slips, so it belongs in the assessment conversation rather than in a phone call after the equipment is already on a truck.
Manifolding two units is not simply plumbing them in parallel. They share a common cold inlet manifold, a common hot outlet manifold, and a controller that decides which unit fires and when. The controller is doing three jobs, and only one of them is capacity.
Staging
The lead unit handles the draw alone until it approaches its capacity, then the second unit is brought in. Below the stage point only one unit is burning gas, which means a two-unit system is not twice as wasteful on a single shower.
Rotation
The controller alternates which unit leads, typically on a runtime or cycle basis. Without rotation the lead unit accumulates all the hours and all the scale, and the two units reach end of life five years apart instead of together.
Turndown management
A 199,900 BTU unit typically has a minimum firing rate around 10,000 to 15,000 BTU and a minimum activation flow near 0.4 to 0.6 GPM. Small draws are the hardest thing for a large burner to serve smoothly. A staged pair keeps one unit modulating in its comfortable range rather than forcing a single large unit to the bottom of its turndown.
Fault fallback
If one unit locks out, the controller keeps the house on the surviving unit at reduced capacity. This is the practical reason a manifolded pair is worth more than its output number suggests, especially in a house where a failure means cold showers for a family of five while a part is on order.
The unflattering side of a two-unit system is worth stating. It is two heat exchangers to descale annually rather than one, two vent penetrations or a listed common vent kit, two condensate connections if the units condense, two gas branches off a trunk that is itself larger, and a controller that is one more thing to fail. The cost is not double, it is more than double once the gas infrastructure is counted. It should be specified because the demand calculation requires it, not because bigger sounds safer.
You can size a system perfectly and still have a household that feels the water heater is inadequate, because the complaint people actually voice is almost never about flow rate. It is about waiting. Capacity is what the unit can deliver. Wait time is a function of how much cooled water is sitting in the pipe between the unit and the tap, and no amount of extra BTU shortens that.
The arithmetic is unforgiving and easy. Three-quarter inch type L copper holds about 0.025 gallons per foot. Half-inch type L holds about 0.012. Three-quarter inch PEX holds about 0.018. A far primary bath sixty feet away, fed by fifty feet of three-quarter inch trunk and ten feet of half-inch branch, is holding roughly 1.37 gallons of water that has gone cold since the last draw. At a 1.7 GPM hot draw that is about 48 seconds before hot water arrives, and the unit's own few seconds of ignition and ramp are added on top.
| Pipe | Gallons per foot | 60 ft holds | Wait at 1.7 GPM |
|---|---|---|---|
| 1/2 inch type L copper | 0.012 | 0.73 gal | About 26 seconds |
| 3/4 inch type L copper | 0.025 | 1.51 gal | About 53 seconds |
| 1 inch type L copper | 0.043 | 2.57 gal | About 91 seconds |
| 1/2 inch PEX | 0.009 | 0.55 gal | About 19 seconds |
| 3/4 inch PEX | 0.018 | 1.10 gal | About 39 seconds |
| 3/4 inch CPVC | 0.021 | 1.25 gal | About 44 seconds |
That table explains something counterintuitive. Oversized distribution piping makes a whole-house tankless system feel worse, not better. A one-inch trunk run to a bathroom that draws 1.7 GPM holds more than three times the water of a half-inch branch and takes more than three times as long to purge. On a large single-storey Frisco floor plan with long horizontal runs, this is why the layout matters at least as much as the equipment, and why a home-run manifold system in smaller-diameter PEX often outperforms a generously sized trunk and branch.
Want this priced for your specific Frisco home? Talk to a tankless specialist at (972) 430-9024.
For most homes, yes. A 199,900 BTU condensing unit produces roughly 6.9 GPM at a 55 degree winter rise on the manufacturer chart, and we design to about 5.5 to 6.0 GPM of usable flow, which covers two showers plus a sink comfortably. Where a single unit stops working is a large roman tub filler, which can demand 6 to 12 GPM by itself, or a household with three genuinely simultaneous showers. Those homes need two units manifolded on a common controller.
When measured peak demand exceeds about 6.5 GPM at a 55 degree rise. In practice that means a master bath with a large tub filler, four or more full baths with overlapping morning use, or an estate floor plan where fixtures are grouped in separate wings. Manifolding also buys redundancy, since a fault in one unit leaves the house at half capacity rather than none. The trade is gas infrastructure: two units need roughly 400 CFH and usually a larger trunk and meter.
In a home over about 3,500 square feet, almost always. Long branch runs hold a lot of standing water, and tankless removes the small head start a hot storage tank used to provide, so the far master bath actually waits longer than it did before. A dedicated return line with a demand-controlled or timer-controlled pump brings that wait down to a few seconds. Designing it during the installation costs far less than retrofitting a return line through finished walls later.
A single 199,900 BTU unit usually fits within an existing residential meter once the branch is upsized. A manifolded pair drawing roughly 400 CFH frequently does not, since a common residential meter is rated near 250 CFH and the furnace, range and dryer are already on it. That means a meter upsize request to the gas utility, which has real lead time, plus a 1-1/4 or 1-1/2 inch trunk. We run the connected load calculation during the assessment so this lands in the schedule early.
Many large Frisco homes are running two 50 gallon tanks precisely because one could not keep up. That configuration gives you 100 gallons of pre-heated water, standby loss on both units around the clock, and 100 gallons of potential flood if either splits. A manifolded tankless pair produces 12 to 13 GPM continuously, has no standby loss, holds under two gallons total, and returns the floor space. It costs more up front and needs annual descaling at Frisco water hardness.
It is expensive to fix, which is why we spend the time on sizing. An undersized unit will produce hot water at a reduced flow rate rather than failing outright, so the symptom is a shower that goes lukewarm when the dishwasher starts. Correcting it means either a larger unit, which usually means new gas sizing and possibly new venting, or adding a second unit and a manifold. Both are far more expensive after the fact than getting the calculation right the first time.
Related
Installation
Sized against your actual fixture count and North Texas inlet water temperature, not a rule of thumb printed on a box.
Installation
The BTU rating on the box is meaningless if the pipe feeding it cannot deliver the gas. We size the fuel first.
Components
A tankless unit heats instantly. The 80 feet of pipe between it and your primary bath does not. That gap is a distribution problem, and recirculation is how you solve it.
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.
Conversion
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.
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.
Troubleshooting · 11 min read
Your tankless unit did not get worse in December. The water got colder, and flow rate and temperature rise trade against each other. Here is the arithmetic, and the three real causes people mistake for it.
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.
Troubleshooting · 11 min read
An error code tells you which sensor tripped, not what went wrong. Here is the translation from code to root cause, and the honest line between homeowner troubleshooting and a service call.
Coverage
We take whole-house 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