How much water does a shower actually use?
The EPA says standard showerheads use 2.5 gallons per minute (gpm). WaterSense labeled showerheads must stay at or below 2.0 gpm. Showering accounts for nearly 17 percent of indoor residential water use, about 40 gallons per day for an average family.
A tankless heater is sized in gpm, so these two numbers are the starting point. One 2.5 gpm shower asks for 2.5 gpm of hot-mixed water. Switch to a 2.0 gpm head and the demand drops by a fifth.
Add up what runs at the same time
Size the heater for the worst moment, not the average one. This table shows the arithmetic with common flows.
| Scenario | Fixtures running | Total flow needed |
|---|---|---|
| One WaterSense shower | One 2.0 gpm head | 2.0 gpm |
| One standard shower | One 2.5 gpm head | 2.5 gpm |
| Two showers at once | Two 2.0 gpm heads | 4.0 gpm |
| Two standard showers | Two 2.5 gpm heads | 5.0 gpm |
| Shower plus a sink | 2.5 gpm head and a 1.5 gpm bathroom faucet | 4.0 gpm |
Those totals are flow after mixing hot and cold, so the heater delivers somewhat less hot water than the total. Your plumber will convert that using your target temperature.
Temperature rise is the hidden variable
A tankless unit's gpm rating depends on how much it must heat the water. Temperature rise is the target temperature minus the incoming water temperature. For example, water arriving at 40 F and heated to 105 F is a 65 degree rise. This is an illustration, not a Zionsville measurement.
A bigger rise means a lower flow from the same unit. The box often lists a high flow at a modest rise. Compare units at your rise, not at the best-case number printed in large type.
Want a licensed plumber to look at it?
Call the 24/7 line and describe what you are seeing.
How to size a tankless for your showers
You can do the first four steps before anyone visits. The last one is for the plumber.
List the showerheads
Check the flow stamped on each head or the box. Multi-head setups and rain heads add up fast.
Decide what runs together
Two showers at once, a shower with the dishwasher, or laundry during a morning rush. Pick the heaviest realistic case.
Add the flows
Use the table above and write down the peak gpm.
Think winter
Incoming water is coldest in winter, so a unit that works in July may struggle in January. Size for the cold season.
Let the plumber check the gas and vent
A unit with a large flow rating also needs a gas line and vent built for it.
Shower problems tankless can cause, and fixes
Most complaints come from sizing or fixture mismatches, not from the heater itself.
- Shower runs lukewarm The heater is undersized for the rise, or two fixtures are drawing at once. Lower the flow or stagger use.
- Hot, cold, hot The unit shuts off at very low flow and re-lights. A very low-flow showerhead or a half-open valve can cause it.
- Wait before hot arrives Cold water in the pipe between heater and shower must clear first. A recirculation loop shortens it.
- Scale buildup In hard water, mineral deposits coat the heat exchanger. A scheduled descaling flush keeps the output up.
Tankless or a tank for shower-heavy households
Lifespan favors tankless, which DOE puts at about 20 years against 10 to 15 for a tank. Daily habits decide the rest.
Tankless suits
- Long showers back to back
- One or two bathrooms in use at a time
- Small closets where a tank crowds the space
- Owners planning a long stay
A tank suits
- Several showers starting at once
- Low gas capacity at the meter
- Homes where a vent route is difficult
- Short-term ownership
Scalding and safety checks
A tankless unit can deliver water hot enough to scald in seconds if its temperature is set high. Check the setting on the display, and keep it lower around children and older adults. Run the shower for a minute and test with your hand before stepping in.
Gas units depend on correct venting. A gas smell means everyone leaves, and the call to 911 or the gas utility happens outside. Battery-operated or battery-backup CO alarms, per the CPSC, belong on each floor and near bedrooms.
Call a plumber or DIY
You can swap a showerhead, read the flow stamp and run the arithmetic yourself. Choosing the unit, sizing the gas line, venting it and connecting the water all belong to a licensed plumber.
Before hiring, look the plumber up in the state's license verification tool. If you already own a tankless and the shower has gone cold, our water heater repair page covers common faults like scale and a failed flame sensor.
What this means in Zionsville
NWS Indianapolis lists a normal January average low of 20.9 F for 1991 to 2020. That is air temperature, not water temperature, and Indianapolis is the nearest long-term station rather than Zionsville itself. It does show the season when your heater works hardest, so size for January, not for a July shower.
Permits in town run through the Planning & Building Department on Oak Street. A tankless install that changes gas piping or venting may call for one, and that is a question for the Town, not a guess from us.
Hardness matters to a shower unit because scale coats the heat exchanger. Your water bill names your utility, and the utility's annual water quality report lists hardness. We have not confirmed the hardness of Zionsville water, so rely on that report, not a neighbor's guess.
Serving nearby towns
Showers in Whitestown and Boone County get the same sizing math. Not sure we reach your address? See the areas we serve.
Frequently asked questions
Add up the flow of every fixture that could run at once. A 2.0 gpm WaterSense showerhead needs 2.0 gpm, and two of them need 4.0 gpm. Then match the unit's rating at your temperature rise, not at the best-case figure on the box.
Yes if its output at your winter temperature rise covers both. Two 2.0 gpm heads need 4.0 gpm, and two 2.5 gpm heads need 5.0 gpm. If it cannot, you may notice both showers cooling.
Common causes are a flow below the unit's minimum, a clogged inlet screen, scale on the heat exchanger or a second fixture opening. A plumber can check flow, error codes and scale.
Yes. Incoming water is coldest in winter, which makes the temperature rise biggest. NWS Indianapolis lists a January normal low of 20.9 F, so a unit that suits July may fall short in January.
Most do, but a very low flow can fall under the unit's minimum activation flow. Check the heater's minimum flow in the manual before buying a new head.
DOE puts tankless at about 20 years, and storage tanks at 10 to 15. Descaling in hard water helps a unit reach that range.
Sources
Figures and guidelines on this page are taken from these public sources. Local rules and utility details can change, so check the source before you rely on a number.
- Standard showerheads use 2.5 gpm, WaterSense 2.0 gpm or less, showers nearly 17 percent of indoor use, about 40 gallons per day (Federal agency)
- DOE lifespans: tankless about 20 years, storage tanks 10 to 15 (Federal agency)
- Indianapolis 1991-2020 normal January average low 20.9 F (Federal agency)
- Zionsville Planning & Building Department, 1100 W Oak St, building permit review (Local government)
- CPSC CO alarm advice (Federal agency)
- State online license verification (Agency)