Ask people who dislike their tankless water heater what went wrong and you will hear the same two complaints: the water goes lukewarm when someone starts the dishwasher, or it takes an uncomfortably long time to get hot in winter. Both are sizing problems, and both are avoidable.
Sizing a tankless unit comes down to two numbers multiplied against each other. The first is flow rate, measured in gallons per minute, which is how much hot water you want moving at once. The second is temperature rise, the number of degrees the unit has to add to your incoming water. Northern Arizona makes the second number larger than most sizing charts assume, which is why generic online calculators tend to undershoot for homes in Cottonwood, Sedona, Camp Verde, and especially the Prescott area.
Why Sizing Beats Capacity
A tank water heater stores hot water. Its limit is volume: once you have drained the tank, you wait. A tankless unit stores nothing and heats on demand, so its limit is not volume at all. It is rate. A tankless heater can supply hot water indefinitely, but only up to the flow it can raise to temperature at that moment.
That distinction is the whole reason sizing matters more with tankless than with tank. A 50-gallon tank that is slightly small still gives you a full shower. A tankless unit that is slightly small gives you a shower that turns lukewarm the instant the washing machine calls for hot water. If you are still deciding between the two technologies, the trade-offs are laid out in our comparison of tankless versus tank water heaters.
Step 1: Add Up Your Peak Flow Rate
Start by deciding what you want to be able to run simultaneously, then add up the flow those fixtures draw. The point is not to plan for every fixture in the house at once, which nobody actually does. It is to identify your realistic worst case, usually a weekday morning.
| Fixture | Typical flow (gpm) | Notes |
|---|---|---|
| Shower | 1.5 – 2.5 | Low-flow heads sit at the bottom of the range; rain heads and body sprays run much higher |
| Bathroom faucet | 0.5 – 1.5 | Small draw, but it adds up across multiple bathrooms |
| Kitchen faucet | 1.0 – 2.2 | Pull-down sprayers land toward the high end |
| Dishwasher | 1.0 – 2.5 | Many newer models heat internally and draw less |
| Clothes washer | 1.5 – 3.0 | Only relevant if you wash in warm or hot |
| Soaking or garden tub | 4.0 – 8.0 | Large tubs are the single biggest driver of oversizing |
A typical morning in a three-bedroom home might be two showers plus a bathroom faucet, which lands somewhere near 4 to 5.5 gpm. A household with a large soaking tub, or one where somebody showers while the washer runs hot, needs materially more.
Measure, do not guess: put a bucket under your shower head, run it wide open for fifteen seconds, and multiply the gallons collected by four. That gives you the actual flow of your actual fixtures, which beats any chart including this one.
Step 2: Find Your Temperature Rise
Temperature rise is simply your target output minus your incoming water temperature. Most households set the output around 120 degrees. The variable that matters is what is coming in.
This is where northern Arizona diverges from the assumptions baked into national sizing charts. Groundwater across the Verde Valley and the Prescott area runs meaningfully colder than in the low desert, and it drops further through the winter. A unit sized against a mild incoming temperature will feel perfectly adequate in August and disappointing in January.
Worked example
Suppose you measure the cold line in February and it reads 55 degrees, and you want 120 degrees at the tap. Your required rise is 65 degrees. Now suppose your peak demand is two showers at 2.0 gpm each, or 4.0 gpm total. The unit you buy has to produce 4.0 gpm at a 65-degree rise, not 4.0 gpm at whatever mild rise the brochure used for its headline number.
The formula behind the spec sheets
If you want to check a manufacturer claim yourself, the relationship is straightforward:
- Gas: gpm = (BTU per hour × efficiency) ÷ (temperature rise × 500)
- Electric: convert kilowatts to BTU by multiplying by 3,412, then use the same formula
- Example: a 199,000 BTU unit at roughly 95 percent efficiency, at a 65-degree rise, works out to about 5.8 gpm
- Same unit, harsher conditions: push the rise to 80 degrees and that number falls to roughly 4.7 gpm
That last line is the entire lesson. The unit did not change. The conditions did.
Altitude and combustion
There is a second elevation effect worth knowing about. Gas-fired appliances lose rated output as air density falls, so manufacturers publish altitude derating for installations above a certain height. In the Prescott area this is a real consideration rather than a footnote, and it is one of the reasons a unit that performs well in Camp Verde may need a different specification a few thousand feet higher. The same principle shows up in cooling equipment, which we covered in our post on SEER ratings and Prescott altitudes.
Rule of thumb: size for a winter morning, not a summer afternoon. Measure your incoming water temperature in the coldest month you can, and size the unit to hit your peak flow at that rise. Everything easier than that condition takes care of itself.
Step 3: Match the Numbers to a Unit
With a flow target and a rise target in hand, you are looking for a unit whose published output meets both at the same time. Manufacturer tables list flow at several temperature rises for exactly this reason. Read across to your rise, not to the headline maximum.
| Household profile | Typical peak demand | General sizing direction |
|---|---|---|
| One to two people, one bathroom | 2 – 3 gpm | Smaller gas unit is usually sufficient |
| Three-bedroom home, two baths | 4 – 5.5 gpm | Mid-to-upper residential gas range at our temperature rise |
| Larger home, three or more baths | 6 – 8 gpm | High-output single unit, or two units in parallel |
| Home with a large soaking tub | 7 gpm and up | Size to the tub fill rate, or the tub will always disappoint |
| Single remote fixture or casita | 1 – 2 gpm | Point-of-use unit, electric is often practical here |
Sizing also has to account for what the house can actually supply. Gas line diameter, meter capacity, venting path, and available electrical service all constrain which units are installable without additional work. That assessment is part of any competent water heater installation quote, and it is a good question to ask any contractor who gives you a number over the phone.
Gas vs. Electric Tankless in Arizona Homes
The two technologies are not interchangeable at whole-house scale, and the temperature rise in our area widens the gap.
Gas tankless
Gas units carry far more heating capacity per unit, which is what lets them hold a usable flow rate at a high temperature rise. They need proper venting and adequate gas supply, and in some homes the existing gas line has to be upsized. For whole-house duty in northern Arizona, gas is usually the answer.
Electric tankless
Electric units are compact, need no venting, and install almost anywhere there is service to feed them. The limitation is raw capacity. Whole-home electric models require very large dedicated circuits, and even then their output drops sharply as the rise increases. Where electric shines is point-of-use: a guest bath far from the water heater, a workshop sink, or a casita.
Hybrid approaches
Some homes do best with a properly sized main unit plus a small point-of-use heater at a distant fixture. That solves the long wait for hot water at the far end of the house without oversizing the main unit to compensate for pipe run.
How Hard Water Changes the Sizing Math
Sizing is not a one-time calculation in a hard-water area, because the unit does not stay at its rated performance indefinitely. Dissolved calcium and magnesium precipitate onto the heat exchanger as scale, and scale is an insulator. As it accumulates, heat transfer falls and the unit delivers less usable flow at the same rise.
Practically, this means a tankless unit sized with zero margin will start behaving like an undersized unit within a few years. Local hardness varies by community, and our post on how hard the water is in Cottonwood, Sedona, and Prescott gets into the specifics.
- Allow headroom. Size slightly above your calculated peak so scale accumulation does not immediately show up as a comfort problem.
- Descale on schedule. Tankless heat exchangers need periodic flushing, and hard water shortens the interval.
- Consider pre-treatment. Softening or conditioning ahead of the unit slows scaling at the source. Our guide to whole-home water filtration covers the options.
- Install isolation valves. Service valves at the unit make future descaling straightforward rather than a small project each time.
- Watch for early symptoms. Falling output, fluctuating temperature, or error codes often mean scale before they mean failure.
Hardness is worth understanding at a system level rather than a single-appliance level, since the same minerals affect fixtures and supply lines throughout the house. Our overview of hard water in the Verde Valley covers the broader picture.
Common Sizing Mistakes
- Using the headline maximum flow. That figure assumes a mild temperature rise no Arizona winter will give you.
- Sizing off a national online calculator. Most default to incoming temperatures well above ours.
- Forgetting the tub. A large soaking tub sets the requirement single-handedly.
- Ignoring gas supply. The right unit on an undersized gas line will never reach its rated output.
- Leaving no margin for scale. In hard water, today’s exact fit is tomorrow’s shortfall.
- Assuming pressure problems are sizing problems. Weak flow at a fixture can be a separate issue entirely, as our post on low water pressure at home explains.
How Nichols Can Help
Nichols Plumbing, Electrical, & HVAC is a family-owned, triple-licensed contractor based in Cottonwood, serving the Verde Valley, Prescott, Sedona, and Camp Verde areas. We install and service tankless and tank water heaters across all major brands, including Rheem, A.O. Smith, and Bradford White, as part of our broader plumbing services.
Sizing is the part of a tankless project worth getting right on paper before anything is ordered. We measure incoming water temperature, work through your realistic peak demand, check gas and venting capacity, and factor in local hardness rather than sizing off a generic chart. If an existing unit is underperforming, that is often a descaling or supply issue rather than a replacement, and plumbing repair may be the shorter path. Our maintenance plans keep descaling intervals from slipping.
Every job is backed by our 100% satisfaction guarantee, and we offer a free second opinion if you already have a quote in hand. You can reach our team or read more about our family-owned company any time.
The short version: add up the fixtures you want running at once, measure your coldest incoming water temperature, and buy the unit that meets both numbers together. Then leave a little headroom for hard water. That is the whole method.
Not Sure Which Size Fits Your Home?
Nichols will measure your incoming water temperature, work through your real peak demand, and check what your gas and venting can actually support before recommending a unit.
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