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Sizing a tankless water heater needs two numbers: the flow rate you need in gallons per minute, and the temperature rise from your incoming water to about 120 degrees. Add up the fixtures you run at once for the flow, then subtract your winter groundwater temperature from 120 for the rise. Size for January, not July. Our tankless water heater service sizes and fits them across the Triad.
Key Takeaways
- Two numbers decide it: flow rate in GPM and temperature rise in degrees.
- Triad winter groundwater sits far colder than summer, and that is the number to size against.
- Most 2 to 3 bathroom homes here need 7 to 9 GPM at a 60 to 70 degree rise.
- Undersizing is the number one complaint about tankless, and it is a sizing error, not a product fault.
- Electric tankless rarely suits a whole house in this climate without a large service upgrade.
Step One: Work Out Your Flow Rate
A tank holds a reserve. A tankless unit has none, so the only question is how much hot water your house demands at the same moment.
Walk through a realistic worst case. Not every tap in the house, but what actually happens on a weekday morning in your home.
| Fixture | Typical Flow | Notes |
|---|---|---|
| Shower | 1.5 to 2.5 GPM | Older heads run higher |
| Bathroom faucet | 0.5 to 1.0 GPM | Low draw |
| Kitchen faucet | 1.0 to 2.2 GPM | Pot filling spikes this |
| Dishwasher | 1.0 to 2.5 GPM | Short bursts |
| Washing machine | 1.5 to 3.0 GPM | Hot cycles only |
| Tub filler | 4.0 GPM plus | The big one |
Two showers at 2.5 plus a kitchen tap at 1.5 is 6.5 GPM. A tub filler running while someone showers is easily 6.5 on its own. Add what genuinely overlaps and that is your target flow.
Step Two: Work Out Your Temperature Rise
This is where most online calculators quietly mislead Triad homeowners, because they assume a national average incoming temperature.
Temperature rise is simply your output target minus your incoming water temperature. Output for a house is usually 120 degrees.
In the Piedmont, incoming water is comfortably warm in late summer and genuinely cold in January. Size for the January number. A unit that keeps up in August and disappoints in February was sized against the wrong season.
Homes on private wells in Stokesdale, Wallburg and Advance often see colder incoming water still, because the supply comes from deeper and is not sitting in surface mains.
What That Means in Practice
| Home | Peak Demand | Flow Needed | Unit Size to Look At |
|---|---|---|---|
| 1 bathroom, 1 to 2 people | One shower | 2.5 GPM | Small gas or point of use |
| 2 bathrooms, 3 to 4 people | Shower plus sink | 4 to 5 GPM | Mid range gas condensing |
| 3 bathrooms, 4 to 5 people | Two showers plus laundry | 7 to 9 GPM | Large gas condensing |
| 3 plus bathrooms with a tub filler | Tub plus shower | 9 GPM plus | Two units in parallel |
Those flow figures assume a winter rise. In summer the same unit will deliver noticeably more, which is exactly why summer numbers flatter a unit that is too small.
Gas or Electric?
For whole-house use in this climate, gas is almost always the answer. The reason is electrical capacity rather than preference.
A whole-house electric tankless unit capable of a real winter temperature rise draws an enormous amount of current, often needing multiple double-pole breakers and sometimes a service upgrade. That cost frequently exceeds the saving over a gas unit.
Electric tankless does make sense in one place: point of use. A small unit under a remote sink, a garage utility sink or a guest bath a long way from the main heater works well and installs easily.
What Sizing Will Not Fix
Two complaints get blamed on size when they are something else entirely.
- The cold water sandwich. A short burst of cold between two hot draws, caused by the unit cycling. A small buffer tank or a recirculation loop solves it, not a bigger unit.
- The wait for hot water at a far tap. That is pipe length, not the heater. Recirculation fixes it.
Hard water is the third thing worth planning around. Scale in the heat exchanger reduces output over time, so a unit that was correctly sized can start behaving like an undersized one after a few unserviced years. A hard water test tells you whether you need a softener first.
Tankless Sizing Questions
What size tankless water heater do I need for a family of four?
In a two to three bathroom Triad home, plan on roughly 7 to 9 GPM at a winter temperature rise. That is a large condensing gas unit. If your peak overlap is only a shower and a sink rather than two showers, you can size down from there.
Can one tankless water heater run two showers?
Yes, if it is sized for it. Two showers at 2.5 GPM each needs at least 5 GPM of real capacity at your winter rise, before you add anything else running at the same time. Many mid-range units cannot do that in January even though they manage it in July.
Is a 199,000 BTU tankless enough for a whole house?
For most homes here, yes. A 199,000 BTU condensing unit is the common whole-house choice. What matters as much is whether your gas line and meter can actually deliver that load, which has to be checked before ordering.
Do I need two tankless water heaters?
Only for genuinely high simultaneous demand, such as a large home with a tub filler and multiple showers in use together. Two units in parallel share the load and give redundancy. For most three bathroom homes one correctly sized unit is enough.
Why does my tankless water heater run cold in winter?
Because the incoming water is colder, so the unit has to work harder for the same output and its flow capacity drops. If it manages in summer and struggles in winter, it was sized on summer numbers. Scale in the heat exchanger makes the same symptom worse.
Get the Sizing Right Before You Buy the Unit
The expensive mistake is buying the unit first. A tankless heater that is too small cannot be fixed with settings, and returning a fitted unit is rarely straightforward.
Call All Star Plumbing at (336) 462-1080 and we will measure your incoming water temperature, work through your real peak demand, check whether your gas supply can feed the unit, and quote the whole job before anything is ordered. A real person answers 24 hours a day, and if we cannot fix it you do not pay. You can also book a tankless consultation online.
Still weighing the two types up, start with our tankless water heater vs tank comparison. For sizing a conventional tank instead, read what size water heater you need. Efficiency ratings for both are published on ENERGY STAR water heaters, and the Department of Energy covers water heating energy use in more detail.
Family owned in High Point since 2007, licensed, bonded and insured, certified on Navien, Rinnai, AO Smith, Rheem, Bradford White and State.
All Star Plumbing
4003 North Main Street
High Point, NC 27265
Phone: (336) 462-1080
Hours: Open 24 hours, 7 days a week
NC Plumbing License #15130
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