Sizing · flow rate and temperature rise
What Size Tankless Water Heater Do You Need?
Not by square footage. By how many fixtures run at once, and how cold the water arrives.
Not by square footage. By how many fixtures run at once, and how cold the water arrives.
Quick answer: A tankless unit is sized on two numbers: the flow rate you need in gallons per minute when the most fixtures you care about are running, and the temperature rise from incoming water to your set point. Square footage and bedroom count have nothing to do with it. Gas whole-house units top out around 199,900 BTU/h and deliver far more flow than electric; a 27 kW electric unit ranges from 2.7 to 6.6 gallons a minute purely on how cold the incoming water is. Undersizing is the single most common cause of a homeowner regretting a tankless unit. Call (843) 718-8177.
Almost every disappointment with a tankless water heater traces back to this page. The unit was sized for the house on paper and not for the way the household actually behaves at seven in the morning.
A tank hides sizing errors: get it wrong and you run out sooner. A tankless unit does not hide them. Ask it for more than it can heat and everybody on the system gets lukewarm water at the same moment, which is a far more memorable failure.
Flow rate is how much hot water is moving. A modern shower head is roughly 1.5 to 2.5 gallons per minute, a kitchen tap about 1.5, a dishwasher or washing machine somewhere between one and two. Add up the fixtures you genuinely expect to run together, and that is your flow rate. Not every fixture in the house — the ones that overlap.
Temperature rise is the gap between the water arriving at your house and the temperature you want. It is the number people forget, and it is the one that changes with the season. The same unit that runs two showers in August may run one in January.
The archived DOE guidance framed the category the same way: “Typically, tankless water heaters provide hot water at a rate of 2–5 gallons per minute. Gas-fired tankless water heaters produce higher flow rates than electric ones.” (U.S. Department of Energy, Energy Saver (archived 30 June 2026 — page since removed)).
The clearest published illustration of temperature rise is the electric sizing table, because the physics is simple: a fixed number of kilowatts buys a fixed amount of heating, and the colder the inlet, the less water you can put through it.
| Incoming water | Flow at 105°F | What that runs |
|---|---|---|
| 37°F | 2.7 gpm | One shower, and nothing else |
| 47°F | 3.2 gpm | One shower |
| 57°F | 3.8 gpm | A shower and a basin |
| 67°F | 4.9 gpm | Two showers, carefully |
| 77°F | 6.6 gpm | Two showers |
Manufacturer figures for the Rheem RTEX-27 and EcoSmart ECO 27, calculated to a 105°F set point. Source: Rheem RTEX electric tankless specification sheet. ⚠️ No utility or agency publishes a Charleston winter incoming water temperature, so we do not publish one either. Ask whoever sizes your unit to measure it in the coldest month rather than assume it.
A whole-house gas unit runs to about 199,900 BTU per hour, which is why it can hold a higher flow through a bigger temperature rise. The comparison that matters is that the 40-gallon tank it replaces was rated around 40,000 BTU/h (A.O. Smith ProLine atmospheric vent specification sheet).
The practical consequence is that on gas, sizing is usually a question of picking the right unit from a short list. On electric, sizing is a negotiation with your electrical panel.
| Unit | Maximum | Minimum | Source |
|---|---|---|---|
| Rinnai RU199i | 199,000 BTU/h | 15,000 BTU/h | Rinnai RU199i specification · 2026 |
| Navien NPE-240A2 | 199,900 BTU/h | 13,300 BTU/h | Navien NPE-240A2 specification · 2026 |
| Rheem RTGH-95 | 199,900 BTU/h | 11,000 BTU/h | Rheem RTGH specification sheet · TK-RTGH rev.1, 10/23 |
| Noritz NRCR111DV | 199,900 BTU/h | 12,800 BTU/h | Noritz Owner’s Guide (NRCR/EZ series) · rev. 10/25 |
The minimum firing rate matters more than people expect. A unit with a high minimum cannot heat a trickle, which is what produces lukewarm water at a slowly-opened tap.
You turn the tap on, get hot water, then a slug of cold, then hot again. It is a common complaint and it has a boring explanation.
When you last ran the tap, the pipe between the heater and the tap was left full of hot water. It cools. When you open the tap again, that cooled water comes out first — warm — then the water that was sitting in the heat exchanger, then a gap while the burner lights and comes up to temperature, then properly hot water. The gap is the cold slice.
It is worst on short, frequent draws: rinsing a plate, washing hands, a shower turned off to soap and back on again.
A unit with a low minimum firing rate and a good modulating burner. A buffer tank, on units that offer one. And most effectively, recirculation, because the pipe is not full of cold water to begin with.
Recirculation does not require a return line. Rinnai: “No dedicated return line, no problem. A thermal by-pass valve added to the cold water line helps ensure recirculation is enabled … This technology allows for the simple installation of a thermal bypass valve at the fixture farthest away from the water heater. Once installed, the unit temporarily uses the cold water line for recirculation.” (Rinnai — recirculation accessories).
It does require control, or it costs you the efficiency you bought the unit for. Stiebel Eltron: “To cut down on energy consumption, an aquastat can be installed … and cut power to the pump when the return of the loop is less than 5° to 10° below the set point. A timer is sometimes used as well to disable the recirculation loop during times of low use, such as the middle of the night.” (Stiebel Eltron — Tempra in a recirculation loop).
⚠️ Written in our own words deliberately. No manufacturer publishes a definition of this term — we looked across seven of them — so anyone quoting one at you has made it up.
Bathroom count is a proxy, and a poor one. The real inputs are the flow rate you need when the most fixtures you care about run together, and the temperature rise from the incoming water to your set point.
Two showers at once is the test most three-bathroom households actually apply, which is roughly 3 to 5 gallons a minute of hot water. On gas that is well inside a single whole-house unit. On electric it depends entirely on the season: a 27 kW unit delivers 6.6 gallons a minute at a 77°F inlet and 2.7 at 37°F, on the manufacturer’s own table.
A correctly sized gas unit, yes. An electric unit, sometimes — and often not in winter.
Two showers is roughly 3 to 5 gallons per minute. A whole-house gas unit at around 199,900 BTU per hour handles that comfortably at Lowcountry inlet temperatures. A 27 kW electric unit manages it at a 67 to 77°F inlet and falls to 2.7 gallons a minute — one shower — at 37°F. This is exactly why sizing has to use a measured winter inlet temperature rather than an annual average.
Not from duration. It will run one shower indefinitely. It can absolutely fail on simultaneous demand, which is a different thing and the one that produces complaints.
Exceed the unit’s flow rate and it does not run out — it delivers everything at a lower temperature, so everyone gets lukewarm water at the same time. That failure mode is why sizing for how the household behaves, rather than for the house on paper, is the whole job.
The pipe between the heater and the tap holds water that was heated on your last draw and has since cooled. On a short second draw you get that lukewarm water, then a brief cold gap while the burner lights and reaches temperature, then hot water.
It is worst on short, frequent draws. What reduces it is a unit with a low minimum firing rate, a buffer tank where the model offers one, and recirculation, which keeps the pipe warm so there is no cold slug to begin with. Recirculation does not require a dedicated return line — a thermal bypass valve at the furthest fixture will do it — but it does need a timer or an aquastat, or it spends the efficiency you paid for.
No, and this is the most common misunderstanding about them. It heats water instantly; it does not deliver it any faster than the tank did.
The wait at the tap is the time it takes to push the cold water already sitting in the pipe out of the way, and that is a function of pipe length and diameter, not of the heater. If you currently wait ninety seconds at the far bathroom, you will still wait ninety seconds. Recirculation is what fixes that, and it is far cheaper to fit at installation than to add afterwards.
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Southeastern Renovation contracts the job as the general contractor. The plumbing and gas work is performed by licensed plumbing subcontractors who pull their own trade permits.
Part of Southeastern Renovation's plumbing division. Charleston's water chemistry is covered in more depth by the water treatment division.