Hot water
Tankless water heater cost
A tankless — on-demand — water heater runs about $3,100 installed where nothing else has to change, and far more where the gas line does. A tankless unit does not have a flow rate: it has a flow rate at a temperature rise, and the one printed on the box is measured at 35°F. This works out yours.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16 · Nothing you type leaves your browser
What runs at the same time?
Not what you own — what could be open at once on the worst morning. A tankless unit is sized on simultaneous flow, which is a completely different question from the daily gallons a tank is sized on.
Shower
2.1 gal/min · A current low-flow head. An older one can be 2.5 or more.
Bath tub filling
4 gal/min · The heaviest single draw in a house, and the one that empties a tank.
Kitchen sink
1.5 gal/min · Higher than a bathroom tap, and often running while someone showers.
Bathroom sink
1 gal/min · Small, but they add up in a house full of people getting ready.
Dishwasher
1.5 gal/min · Most modern machines heat their own water, which takes them out of the peak.
Washing machine
2 gal/min · Only if it is run on a hot cycle, which is increasingly rare.
The common mid-size non-condensing unit. Two fixtures at once in most of the country.
120 is the usual setting and the safe one.
Blank uses 55°F, from your state. Measure it in February for the honest figure.
The branch that feeds the water heater today.
Longer runs carry less. It is the length that matters, not the bends.
Used only to pick a regional labour rate. It stays in your browser.
You need
3.6 gal/min
55°F in, 120°F out = a 65°F rise · 3.6 × 500 × 65 = 117,000 BTU/h to deliver
The Gas, 180,000 BTU covers it, with 4.54 gal/min
That unit can advertise 8.4 gal/min, because the industry measures at a 35°F rise and yours is 65°F. Nothing is wrong with the advertised figure — it is simply not your figure.
Every unit, measured against your water
Advertised flow, then what it gives at a 65°F rise.
| Unit | On the box | At 65°F | Installed |
|---|---|---|---|
| Gas, 140,000 BTU Short of your peak | 6.6 | 3.53 | $2,600 |
| Gas, 180,000 BTU — yours Covers your peak | 8.4 | 4.54 | $3,100 |
| Gas, 199,000 BTU condensing Covers your peak | 10.8 | 5.82 | $4,100 |
| Electric, 18 kW Short of your peak · 94 A | 3.5 | 1.87 | $1,500 |
| Electric, 27 kW Short of your peak · 141 A | 5.2 | 2.81 | $1,900 |
| Electric, 36 kW Covers your peak · 188 A | 6.9 | 3.74 | $2,300 |
Why the number on the box is not your number
Shopping list
- Gas, 180,000 BTU — 4.54 gal/min at your 65°F rise
The box can advertise 8.4 gal/min, measured at a 35°F rise. Same unit, different water.Covers it - Heat you actually need
3.6 gal/min × 500 × 65°F rise117,000 BTU/h - Gas supply — 1/2 in at about 40 ft
Carries about 63,000 BTU/h and the unit wants 180,000. Needs 1 in back to the meter.Upgrade - Descaling, every year
$40 in vinegar and a pump if you do it, $180 if someone comes out. Over twenty years that is $800 or $3,600.every 1 yr
What it costs
National average. Unit and a straightforward installation. Gas line, venting or panel work are separate and they are where the surprises live.
Add $800 over twenty years if you descale it yourself, $3,600 if you call someone — and rather more than that if you skip it in hard water.
Worth knowing
- A tankless unit does not have a flow rate. It has a flow rate AT A TEMPERATURE RISE, because BTU per hour = gal/min × 500 × rise. Your water arrives at 55°F and you want 120°F, so your rise is 65°F — and at that rise the Gas, 180,000 BTU delivers 4.54 gal/min, not the 8.4 it can claim at the 35°F rise the industry advertises against.
- The 1/2 in at about 40 ft carries about 63,000 BTU/h and this unit wants 180,000. The line has to go up to 1 in back to the meter — and the meter itself has to have the capacity spare, which on an older house with a furnace and a range it often does not. This is the cost that turns a cheap tankless swap into an expensive one, and it is invisible until someone looks.
On this page — 9 sections
- 01A tankless unit does not have a flow rate
- 02The same unit in Houston and in Minneapolis
- 03Size it on what runs at once
- 04What a tankless water heater costs installed, and the gas line that decides it
- 05Electric, and the panel it really needs
- 06Descaling, and the cost nobody puts in the comparison
- 07Five ways this goes wrong
- 08How the calculator works it out
- 09The words on a tankless quote
A tankless unit does not have a flow rate
The short version
- It has a flow rate AT A TEMPERATURE RISE. The number on the box is measured at 35°F, which is nobody’s winter.
- Double the rise, halve the flow. That is arithmetic, not a defect.
- Size it on simultaneous flow, not on daily gallons. A tank and a tankless answer different questions.
- The gas line is where the money goes. A half-inch branch will not feed a whole-house unit.
Every tankless water heater is advertised with a number of gallons a minute, and the number is true. It is simply measured at a temperature rise that most of the country does not have.
The physics has one equation in it and it does not negotiate: to raise a gallon of water by a degree takes about 8.33 BTU, so a stream of water needs heat in proportion to both its flow and how far it is being lifted in temperature. Put a fixed burner behind it and the flow is whatever is left over after the rise has taken its share.
BTU per hour = gal/min × 500 × temperature rise in °F
- 500
- 8.33 lb per gallon × 60 minutes × 1 BTU per lb per °F — a conversion, not an estimate
- temperature rise
- target temperature minus the water arriving from the main
- BTU per hour
- heat that reaches the WATER, so output rather than input
Worked through — Two showers and a kitchen sink is 5.7 gal/min. In water arriving at 50°F, lifted to 120°F, that is a 70°F rise: 5.7 × 500 × 70 = 199,500 BTU/h that has to reach the water. The largest residential gas unit made delivers about 189,050.
Tankless water heater manufacturers, Whole-house tankless specification sheets · checked 2026-09-16
The same unit in Houston and in Minneapolis
Rearrange the equation and the shape of the problem appears. For a unit with a fixed heat output, flow equals that output divided by 500 times the rise — a hyperbola, falling away steeply exactly where cold climates live.
The Gas, 180,000 BTU unit gives 7.4 gallons a minute at a 40°F rise and 3.7 at an 80°F one. In south Texas that is two showers; in Minnesota in February it is one shower, and the kitchen tap will be noticed.
This is the single reason tankless installs disappoint, and it is entirely predictable before anyone buys anything. It is also why the complaint is regional: the same model, installed the same way by the same brand, is a success in one half of the country and a grievance in the other.
35°F
The rise on the box
where the headline GPM is measured
~50°F
A southern winter
groundwater around 70°F
~80°F
A northern winter
groundwater around 40°F
½
The flow, at twice the rise
exactly half, every time
Size it on what runs at once
A storage tank is sized on a quantity: how many gallons of hot water the house draws in the busiest hour. A tankless unit has no quantity — it can run all day — so the only thing that can defeat it is a rate.
That makes the sizing question completely different, and it is why this page exists separately from the tank sizing calculator. There the question is how big; here it is how fast.
| Fixture | Flow | Notes |
|---|---|---|
| Shower | 2.1 gpm | A current low-flow head. An older one can be 2.5 or more. |
| Bath tub filling | 4 gpm | The heaviest single draw in a house, and the one that empties a tank. |
| Kitchen sink | 1.5 gpm | Higher than a bathroom tap, and often running while someone showers. |
| Bathroom sink | 1 gpm | Small, but they add up in a house full of people getting ready. |
| Dishwasher | 1.5 gpm | Most modern machines heat their own water, which takes them out of the peak. |
| Washing machine | 2 gpm | Only if it is run on a hot cycle, which is increasingly rare. |
What a tankless water heater costs installed, and the gas line that decides it
A storage heater burns perhaps 40,000 BTU an hour and does it slowly, whenever it likes. A whole-house tankless burns 150,000 to 199,000, and demands all of it the instant somebody opens a tap.
That is four or five times the gas through the same pipe, and pipes have a capacity that falls with length. The half-inch branch that comfortably fed a tank for twenty years usually cannot feed its replacement.
| Pipe | 20 ft run | 40 ft run | 60 ft run | 80 ft run |
|---|---|---|---|---|
| 1/2 in | 92,000 | 63,000 | 50,000 | 43,000 |
| 3/4 in | 190,000 | 130,000 | 105,000 | 90,000 |
| 1 in | 360,000 | 245,000 | 195,000 | 170,000 |
BTU per hour carried, natural gas. A whole-house unit wants 150,000 to 199,000.
National Fire Protection Association, NFPA 54, National Fuel Gas Code — pipe sizing tables · checked 2026-09-16
Electric, and the panel it really needs
Electric tankless units look like the simple option — no venting, no gas, a small box on the wall. The catch is that electricity is a poor way to deliver a lot of heat quickly, and whole-house flow needs a lot of heat quickly.
A Electric, 27 kW unit draws about 112 amps of continuous load at 240 volts once the code’s 125 per cent factor is applied. On a 200 amp service that is more than half of everything the house has, before the oven, the dryer, the air conditioning or an electric car.
Which is why these are excellent at the point of use — a single sink, a garage, a workshop, where the flow is small and the run is short — and frequently a service upgrade in disguise when asked to run a house.
| Gas tankless | Electric tankless | |
|---|---|---|
| What it needs | 3/4 or 1 in gas, and meter capacity | 100–150 A of service |
| Venting | Stainless or PVC, depending on the unit | None |
| Whole house in the north | Workable with the largest units | Rarely realistic |
| Point of use | Overkill | Where it genuinely shines |
| Running cost | Follows the gas price | Follows the electricity price, usually higher per BTU |
| Install cost | Gas and venting are the expensive parts | The panel is the expensive part |
Descaling, and the cost nobody puts in the comparison
A tankless heat exchanger is a narrow passage with a burner on it, so scale builds where the water is hottest. In hard water, skipping the annual descale is how a fifteen-year unit becomes an eight-year one.
It is not difficult — isolation valves, a small pump, a bucket of vinegar, an hour — and it is the thing that separates a unit that lasts twenty years from one that fails in eight. The isolation valve kit costs very little at installation and is miserable to add afterwards, so it goes in on day one or it effectively never goes in.
At installation, so the next twenty years are easy
- Isolation valves with service ports on both sides. They cost little now and are a plumbing job later.
- A gas line sized for the unit's full input over its actual run, not for the tank that was there.
- Venting to the manufacturer's spec — condensing units in PVC, non-condensing in stainless, never mixed.
- A condensate drain if it is a condensing unit, and somewhere legal for the condensate to go.
- The unit set to the temperature you actually want, not the factory default.
- A water softener or scale filter if the water is hard, which is cheaper than the exchanger it protects.
- The descale interval written down somewhere you will find it in a year.
Still deciding between the two? The water heater sizing calculator runs a storage tank, a heat pump and a tankless unit against the same household, with the twenty-year energy cost of each.
Tank or tankless?Five ways this goes wrong
01Sizing from the number on the box
That figure is measured at a 35°F rise. In a cold climate the real flow is close to half of it, which turns two showers into one.
Instead — Divide the unit's heat output by 500 times YOUR rise. That is the number that matters, and it is the one this page computes.
02Using daily hot water use to size it
That is how a tank is sized. A tankless unit never runs out of quantity — only of rate — so daily gallons tell you nothing about whether it will keep up.
Instead — Add the gallons per minute of the fixtures that could run at the same moment.
03Assuming the existing gas line will do
A whole-house unit wants four or five times the gas a tank did, and a half-inch branch over a normal run carries well under half of it. The meter may be short too.
Instead — Check the pipe size and the run before buying anything, and ask the utility about the meter.
04Fitting whole-house electric on a 200 A service
About 112 amps of continuous load leaves very little for the rest of the house, and the first hot summer evening finds the limit.
Instead — Price the service upgrade as part of the decision, or use electric where it belongs — at a single point of use.
05Skipping the isolation valves
Without service ports, descaling means cutting into pipework. So it does not get done, scale builds in the exchanger, and a twenty-year unit fails in eight.
Instead — Valves with ports at installation, and an hour with a pump and vinegar once a year.
How the calculator works it out
- 01Add up what runs at onceNot what you own — what could be open on the worst morning. Two showers and a kitchen sink is about 5.7 gallons a minute.
- 02Measure the incoming waterRun a tap cold for a minute and take its temperature, ideally in February. The winter figure is the one that sizes the unit.
- 03Subtract to get the riseTarget minus incoming. 120°F out of 50°F water is a 70°F rise, and that number divides everything else.
- 04Convert to heatGallons per minute × 500 × rise gives BTU per hour that must reach the water. That is output, not input.
- 05Check the unit at YOUR riseDivide its output by 500 times your rise. That is its real flow, and it is roughly half the number on the box in a cold climate.
- 06Check what feeds itGas line capacity over its actual run, or the amps against the service. This is where the cost of a tankless install is actually decided.
The flow equation is physics: 8.33 pounds per gallon, sixty minutes, one BTU per pound per degree. Heat outputs and efficiencies come from manufacturer specification sheets, gas pipe capacities from the national fuel gas code’s sizing tables, and groundwater temperatures from the same climate data the climate zone table uses. Everything is on the sources page, dated.
Two assumptions worth stating. Efficiency is applied to the input, so the figure compared against your demand is heat reaching the water rather than gas entering the unit — which is why a 199,000 BTU condensing unit beats a 199,000 BTU non-condensing one. And the groundwater figure from your state is an annual average: measure your own tap in February if the answer is close, because February is the month that decides it.
Nothing you type is sent anywhere — the whole calculation runs in your browser.
The words on a tankless quote
Temperature rise is the one that matters. Everything else on this list is downstream of it.
- Temperature rise
- Target temperature minus the temperature of the water arriving from the main. It is the whole sizing problem: everything a tankless unit can do is divided by this number.
- GPM at rise
- The only honest way to state a tankless unit's capacity. Flow equals output divided by 500 times the rise, so a unit's flow rate is meaningless until the rise is named alongside it.
- Simultaneous flow
- The total gallons per minute of every fixture that could be open at once. It is what sizes a tankless heater, and it is a different question from the daily gallons that size a tank.
- Activation flow
- The minimum flow that lights the burner, typically a third to half a gallon a minute. Below it the unit does nothing, which is why a trickling tap runs cold.
- Cold water sandwich
- A slug of cool water between two hot ones, felt when a tap is turned off and on again. It is the water that sat in the exchanger while the burner was off, and it is inherent rather than a fault.
- Condensing unit
- A tankless heater with a second heat exchanger that pulls heat out of the flue gas. It reaches about 95 per cent efficiency and vents in plastic, because the exhaust is cool enough.
- Input against output
- Input is the gas the unit burns; output is the heat that reaches the water. The difference is efficiency, and it is output that does the work — a 199,000 BTU input unit at 82 per cent delivers less than one at 95.
- Gas line capacity
- How much gas a pipe of a given diameter carries over a given run. It falls with length, which is why a half-inch branch that feeds a tank heater usually cannot feed a tankless one.
- Continuous load
- Electrical load that runs for three hours or more, which the code requires to be sized at 125 per cent. It is why a 27 kW heater is a 112 amp calculation rather than a 90 amp one.
- Descaling
- Circulating a mild acid through the heat exchanger to dissolve the scale that builds where the water is hottest. Annual in hard water, and the maintenance nobody includes when comparing prices with a tank.
- Recirculation
- A pump that keeps hot water moving in the pipes so it arrives at the tap immediately. It removes the wait, and it removes a good part of the efficiency advantage that justified the unit.
- Point of use
- A small tankless unit serving one fixture rather than the house. It sidesteps both the flow problem and the pipe run, and it is where small electric units genuinely make sense.
Common questions
- What size tankless water heater do I need?
- Work it in gallons per minute at your temperature rise, not in gallons of storage. Add up the fixtures that could run at once — a shower is about 2.1 gal/min, a kitchen sink 1.5 — then subtract your incoming water temperature from your target to get the rise. A Gas, 180,000 BTU delivers about 4.2 gal/min at a 70°F rise and 7.4 at a 40°F one. Same unit, same house, different state.
- Why does my tankless water heater say 7 GPM but not keep up?
- Because that figure is measured at a 35°F temperature rise and yours is probably much larger. The physics is fixed: flow equals heat output divided by 500 times the rise. Double the rise and you halve the flow. Nothing is broken — the advertised number was never yours.
- How do I calculate temperature rise?
- Target temperature minus the temperature of the water coming into the house. Most people set 120°F. Groundwater runs around 70°F in southern Florida and around 40°F in the northern Midwest in winter, so the same target is a 50°F rise in one place and an 80°F rise in the other. Measure it yourself in February rather than trusting a map.
- Can my existing gas line feed a tankless water heater?
- Usually not without work. A half-inch branch carries roughly 63,000 BTU/h over a forty-foot run, and a whole-house tankless wants 150,000 to 199,000. It normally means three-quarter or one inch pipe back to the meter, and the meter needs spare capacity too. This is the single most common reason a tankless quote comes in far above the price of the unit.
- How many amps does an electric tankless water heater need?
- A Electric, 27 kW unit is about 112 amps of continuous load at 240 volts once the code's 125 per cent factor is applied — more than half of a 200 amp service before the oven, the dryer or the air conditioning. Whole-house electric tankless is frequently a service upgrade with a water heater attached, and the panel work should be priced before the decision, not after.
- Is a tankless water heater worth it?
- It depends much more on your water and your gas line than on the unit. Endless hot water and a cabinet's worth of floor space back are real. Against that: the install often costs more than the appliance, hard water demands an annual descale, and in a cold climate the flow you actually get may be one shower rather than two. The comparison page runs both against a tank on your own house.
- Do tankless water heaters really save money?
- On energy, modestly — there are no standby losses, which is worth perhaps ten to twenty per cent of the water heating bill. On total cost of ownership, often not, because the install premium and the annual descaling eat the saving. The honest reasons to buy one are endless hot water, the space, and a twenty-year life instead of twelve.
- What is the cold water sandwich?
- Turn a tap off and on again and you get hot, then a slug of cool, then hot. The cool part is the water that was sitting in the heat exchanger while the burner was off. It is inherent to how the unit works rather than a fault, and a small buffer tank or a recirculation loop is what removes it.
- How often does a tankless water heater need descaling?
- About once a year in hard water. A tankless heat exchanger is a narrow passage with a burner on it, so scale builds where the water is hottest. In hard water, skipping the annual descale is how a fifteen-year unit becomes an eight-year one. It costs around $40 in materials if you do it yourself with a pump and a bucket of vinegar, or roughly $180 if someone comes out — and over twenty years that is a real line in the comparison against a tank.
- Can two tankless units run in parallel?
- Yes, and in a large house with a cold supply it is often the right answer — two smaller units banked together, firing in sequence, give the flow without demanding a single enormous gas line. It also gives redundancy, which a single unit does not.
- Does a tankless water heater work in cold climates?
- It works; it just delivers less. The unit is producing a fixed amount of heat and cold groundwater makes each gallon need more of it, so the flow falls in exact proportion to the rise. In the north it means sizing up, accepting fewer simultaneous fixtures, or two units — none of which is a defect, and all of which should be decided before the purchase.
- What is the minimum flow to activate a tankless heater?
- Typically 0.3 to 0.4 gallons a minute. Below that the burner does not light at all, which is why a barely-open bathroom tap can run cold indefinitely while the shower next door is fine.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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