The two numbers that set the size
Flow rate is measured in gallons per minute (GPM). Add up the hot water fixtures that may run at the same moment, and that total is your peak flow. A typical shower runs about 2.0 GPM, a kitchen faucet around 1.5 and a bathroom sink about 1.0.
Temperature rise is the difference between the cold water entering the heater and the hot water you want, often about 120 F. The colder the incoming water, the bigger the rise. A heater that gives 5 GPM in July may give far less in January, so size for the cold season. Plumbers handling tankless water heater work start from that cold-season number.
Worked example: what each household scenario needs
This example assumes 50 F incoming water in winter and a 120 F target, which is a 70 F rise. Output needed is GPM times 500 times the rise, in BTU per hour. The 500 comes from the weight of a gallon of water and 60 minutes.
| Scenario | Fixtures running | Peak GPM | Winter output needed | Summer output needed (70 F water, 50 F rise) |
|---|---|---|---|---|
| One shower | 1 shower | 2.0 | 70,000 BTU/hr | 50,000 BTU/hr |
| Shower and sink | 1 shower, 1 kitchen faucet | 3.5 | 122,500 BTU/hr | 87,500 BTU/hr |
| Two showers | 2 showers | 4.0 | 140,000 BTU/hr | 100,000 BTU/hr |
| Busy morning | 2 showers, 1 dishwasher | 5.5 | 192,500 BTU/hr | 137,500 BTU/hr |
Output is the heat delivered to the water. Gas input ratings are higher because no burner is 100 percent efficient. Summer figures use a 70 F assumption for illustration only.
How to size your own heater in five steps
- 1
List your simultaneous fixtures
Think of your busiest hour, such as two people showering while the dishwasher runs.
- 2
Add the GPM
Use the flow rate on the fixture's label or a quick bucket test, since older fixtures often flow more than new ones.
- 3
Measure the incoming temperature
On a cold morning, run a cold tap for two minutes and check it with a kitchen thermometer.
- 4
Find the rise
Subtract the cold temperature from your target, for example 120 F minus 50 F is 70 F.
- 5
Multiply
GPM times 500 times the rise gives BTU per hour. Pick a unit whose rated output meets that number at your rise, not the headline GPM on the box.
Winter and water chemistry in Mauldin
The National Weather Service station at Greenville-Spartanburg records an average January low of 32.1 F and 16 January days with lows at or below 32 F. Air temperature is not water temperature, but cold snaps chill buried lines and exposed pipes, so winter is when your incoming water is coldest and your rise is largest. Size for January, not for April.
Greenville Water's finished water is very soft, with total hardness near 5.7 mg/L in its 2026 monthly reports. That means scale is less likely to choke a heat exchanger, so you do not need to oversize the unit as a hedge against buildup. Flushing sediment out still helps, but sizing does not change.
Source type: NWS Greenville-Spartanburg January normals and Greenville Water monthly water quality report.
What a sized unit can actually deliver
Take a gas unit with about 180,000 BTU per hour of usable output. At a 70 F rise it delivers roughly 5.1 GPM. At a 50 F rise it delivers about 7.2 GPM. That is the same machine in two seasons, and why the box rating, usually quoted at a modest rise, can mislead.
If your busy-morning demand is 5.5 GPM, that unit would fall slightly short in January. Options are one larger unit, two units, or a policy of not running the dishwasher during showers. A plumber doing water heater installation can compare these with you.
Gas versus electric: the honest trade-off
The same 3.5 GPM at a 70 F rise needs about 36 kW of electric power, which is roughly 150 amps at 240 volts. Most houses do not have a spare circuit that large, so whole-house electric tankless often needs a service upgrade. Gas units need adequate gas line capacity and venting, which also may add cost.
Ask for a load calculation before you commit to either fuel. An undersized unit shows up as lukewarm water whenever two fixtures run together.
What changes the installed price
Typical U.S. installed prices for a whole-house tankless unit range from about $2,500 to $6,000 or more. Your real number comes from a call and a look at the house.
For a clear price before work starts, call (855) 575-2890. Permit rules vary, so ask the Mauldin Building and Zoning office, and note that the city says a permit is required before you add, alter or replace any plumbing system.
When to call a plumber and when you can do the math yourself
The sizing math is easy for a homeowner. The install is not.
- Do the fixture list and the bucket test yourself, since it costs nothing.
- Call a plumber for gas line sizing, venting and combustion air.
- Call an electrician or plumber for any 240 volt circuit that is not already in place.
- Get the cold-weather inlet temperature measured, not guessed.
- If you ever smell gas, leave the house and call the gas utility or 911 before calling a plumber.
Households near Simpsonville and Greer use the same sizing method, since the math depends only on flow and rise.
Tankless sizing questions from Mauldin homeowners
No. Size depends on flow and temperature rise, not on water chemistry. Greenville Water's low hardness means less scale to worry about, but your January inlet temperature and peak GPM still decide the unit.
Many homes land between 4 and 8 GPM at peak. A couple with one shower at a time may need only 3 or 4. A family running two showers and a dishwasher needs 5 or more. Count every fixture that could run together, not just the ones that usually do.
It is the number of degrees the heater must add. If water enters at 50 F and you want 120 F, the rise is 70 F.
Often yes at 2.0 GPM each, if the unit is rated for 4 GPM at your winter rise. Check that rating, not the summer number.
The unit has hit its limit. The flow exceeds what it can heat at the rise needed, so the outlet temperature drops. A larger unit, lower-flow fixtures or staggering use fixes it.
A margin of 10 to 20 percent is reasonable, since flow, inlet temperature and gas supply all vary. It is not a case for doubling the size.