Two numbers decide the size
A tankless heater is sized by flow rate in gallons per minute (GPM) and temperature rise, the number of degrees it must add to incoming water. Add up the fixtures you run at the same moment, then subtract your coldest incoming water temperature from the hot temperature you want.
Pick a unit whose rated output at that temperature rise covers your peak flow. A heater that looks big on the box can fall short in January, because every model delivers less flow as the rise grows.
Step one: add up what runs at the same time
| Fixture | Typical flow (GPM) | Likely to run at once? |
|---|---|---|
| Shower | About 2.0 to 2.5 | Yes, often two at once on busy mornings. |
| Bathroom faucet | About 1.0 to 1.5 | Sometimes. |
| Kitchen faucet | About 1.5 to 2.2 | Often, during a meal. |
| Dishwasher | About 1.0 to 1.5 | At night, with a shower. |
| Washing machine | About 1.5 to 3.0 | Weekends, when everyone is home. |
Count the busiest realistic moment, not every fixture in the house. A family of five with two showers and a dishwasher running has a very different peak than a couple.
Not sure what your peak flow is?
Call (855) 575-2890 and walk through your fixtures with a plumber.
Step two: find the temperature rise
Incoming water is coldest in winter, and Brigham City's January low averages 18.1 F. Size for that month, not July. Your plumber can tell you the real incoming temperature for your line, or measure it.
Here is a made-up example to show the math, not a recommendation. If incoming water is 40 F and you want 120 F at the tap, the rise is 80 degrees. Two showers at 2.5 GPM plus a 1.5 GPM kitchen faucet need 6.5 GPM, and the unit must deliver that at an 80 degree rise.
Reading a spec sheet without getting lost
- Rated flow at a stated temperature rise. Compare units at your rise, not the highest number on the box.
- Maximum gas input, which tells your plumber what the gas line must supply.
- Minimum flow to fire, since a very slow trickle at a tap may not turn the burner on.
- Venting type and clearances, which decide where the unit can hang.
- Service access for flushing the heat exchanger.
If a spec sheet leaves out the flow at a given rise, ask for it. A unit that cannot show you that number is hard to size honestly.
Where sizing meets your gas line or panel
A larger tankless heater burns more gas per minute, and the gas piping to it must be big enough to supply it. That is a plumber's check, not a guess. An older house may need a larger gas line.
The vent matters as well. Many tankless units use special venting, and it must be installed to the maker's directions. Electric models draw heavy current and need panel capacity, which means a licensed electrician.
Hard water changes the long-term picture
Scale coats the heat exchanger and slows heat transfer, so a unit that was sized well on day one loses margin as it ages. Flushing the unit on a schedule keeps scale from building. A water softener upstream reduces the scale that forms in the first place.
Ask for a unit whose manual makes flushing easy, with service valves you can reach. Skipping flushes is the quiet reason many tankless units fail early.
Sizing mistakes that cost comfort
Average instead of peak
The heater has to meet the peak, not the average. Peak is the 7 a.m. rush.
Summer numbers in a winter town
Incoming water is much colder in January. A unit sized for July runs lukewarm in winter.
Bigger is not always better
A larger unit needs more gas supply and venting, which can add work. The right size is the smallest that meets your peak.
Long pipe runs overlooked
Water that must travel far to a fixture wastes time. A recirculation setup or a second unit may suit a large house.
Questions to bring to a pro
- What is my peak flow, and how did you count it?
- What is my real incoming water temperature in January?
- Is my existing gas line big enough, and what would change?
- Where does it vent, and who has verified the route?
- How do I flush it, and where are the service valves?
A licensed plumber answers these directly. Our tankless water heater service covers the sizing, supply, and venting questions together. Crews serve Willard, about 7 miles south, and Perry, about 4 miles south.
If you ever smell gas
Tankless units are gas appliances. If you smell gas anywhere in the house, leave immediately, then call 911 or your gas utility from outside. Do not try to find the source yourself.
Enbridge Gas Utah lists a 24-hour emergency line at 1-800-767-1689. Save it in your phone now, and tell everyone in the house what the plan is. Any gas work behind a new heater, including the line sizing, belongs to a licensed professional.
Brigham City note: winter lows and hard water
NOAA climate normals show Brigham City's average January low at 18.1 F, and the city sees about 38.3 inches of snow a year. Cold winters mean colder incoming water, so size the heater for January, not July, or showers will run lukewarm exactly when you need them hot.
The Brigham City Water Division lists hardness at 17 to 20 grains per gallon. That scale builds on a tankless heat exchanger and cuts into performance. Plan for flushing, and ask Public Works for the current hardness figure before you choose a unit.
Source type: NOAA climate normals as reported for Brigham City and the city water page
Frequently Asked Questions
It depends on the peak, not the headcount. Add the flow for fixtures that run together, then check the unit's rating at your winter temperature rise. A plumber can do this on site in a short visit.
Yes, if the unit's flow at your temperature rise covers both. Two showers at about 2.5 GPM each need around 5 GPM, plus anything else running. In winter many units deliver less.
It affects care more than size. At 17 to 20 grains per gallon, scale builds on the heat exchanger and costs margin over time. Plan on service valves, regular flushing, and a softener to protect the unit.
Not always. These units draw more gas than a tank heater at full fire. A plumber must check the line size, the length, and what else is on it. Permit rules vary, so ask the Brigham City building office at (435) 734-6604.
It depends on how you use hot water. Tankless gives continuous hot water and takes less floor space, but it costs more to buy and install, needs maintenance, and depends on your gas or electric supply.
Sources and standards
- Plumbing codes in the UPC and IPC families set the rules for fixtures, venting and drains. Ask the Brigham City building office at (435) 734-6604 which edition and permit rules apply to your job.
- The Utah Division of Professional Licensing offers a public Verify a License tool for checking any plumber's license class.
- The Brigham City Water Division explains hardness, pressure testing and water quality on its water page. Call Public Works at (435) 734-6615 with questions.
- Enbridge Gas Utah says to leave the building first if you smell gas, then call 911 or its 24-hour emergency line from outside.
Written by Brigham City Pro Plumbing. Last updated October 29, 2026.