Tankless water heaters, also called on-demand or instantaneous heaters, warm water only while it flows. That single trait separates them from storage tanks: no standby losses, a suitcase-size footprint, and a service life that often reaches 20 years. The trade-off is capacity. A tankless unit can only heat so much water at any given moment, so the sizing decision determines whether the morning shower stays hot while the dishwasher runs. The range of equipment available, including tank-type, tankless, and heat pump models, is compared in this overview of water heater selection and installation for residential applications.
- Calculate your peak hot water flow rate in gallons per minute
- Determine the required temperature rise for your feed water
- Match the unit’s output curve to both numbers
How Tankless Water Heaters Work
A storage tank keeps 40 to 80 gallons hot around the clock, and the burner or element fires periodically just to replace heat leaking through the jacket. A tankless unit removes that cycle entirely. When a tap opens, cold water passes over a high-output burner or heating element and reaches the set temperature within seconds. Nothing sits stored, so nothing loses heat while waiting, which is why tankless equipment cuts water heating bills by roughly 30 percent in most homes.
Within tankless units, two combustion designs matter. Non-condensing models use a single heat exchanger and vent through standard metal pipe. Condensing models pull extra heat from the exhaust, run in the low-90s percentage efficiency range, and need PVC venting plus a condensate drain. The efficiency gain usually pays for the price difference within a few years, and many rebates require condensing equipment.
The same on-demand principle powers combination appliances that deliver domestic hot water and space heating from one burner. These combined hydronic heat and hot water systems save the footprint and standby losses of a second appliance, but they add control and service complexity, so the sizing rules below matter even more when one unit carries two jobs.
Why On-Demand Heating Changes the Sizing Question
A tank absorbs brief surges in demand because stored water is already hot. A tankless heater has no reserve. If simultaneous draws from two showers, a washing machine, and a dishwasher exceed the unit’s heating capacity, the flow controller throttles back and every open tap gets less water at a lower temperature. That is the unit protecting its heat exchanger, not a defect. Tankless sizing therefore starts from peak demand, not from house size or bedroom count.
Peak Demand, Not Average Demand
Most households run several fixtures at once only during morning and evening routines, and those 15-minute windows define the capacity you need. A family of four that showers back to back can require a larger tankless unit than the same family needed with a 60-gallon tank, because the tank recovers between showers while the tankless unit must keep up in real time.
What Tankless Heaters Cost and How Rebates Help
The price gap between tank and tankless is real. A quality storage tank sells for $400 to $900, while a tankless unit runs from about $800 to $2,000 or more before installation, and installation runs higher because gas lines, venting, and sometimes upgraded electrical service are involved. The offset is operating cost. Cutting water heating bills by roughly 30 percent, combined with a lifespan of up to 20 years against 10 to 13 years for a tank, typically puts payback between five and ten years for a family of four.
Incentives shorten that payback. Many utilities, states, and municipalities offer rebates on condensing and high-efficiency units, and some programs cover part of the installation labor. Homeowners in British Columbia can compare local offers in this summary of tankless water heater rebates in Kelowna, and the same pattern of utility and government incentives repeats across North America. Check what is available before you pick a model, because rebate tiers often depend on the unit’s efficiency rating.
Maintenance costs belong in the comparison too. Both types need an annual flush, and tankless units in hard-water areas need descaling every 12 to 24 months, a service that runs $100 to $300 when hired out. Scale buildup is the leading cause of premature heat exchanger failure, so the maintenance line item is part of the real cost of ownership.
Tank Versus Tankless at a Glance
| Factor | Storage tank | Tankless |
|---|---|---|
| First cost | $400 to $900 | $800 to $2,000 or more |
| Expected lifespan | 10 to 13 years | Up to 20 years |
| Standby losses | Continuous | None |
| Energy use | Baseline | Roughly 30 percent lower |
| Footprint | Floor-standing closet | Wall-mounted, suitcase size |
| Peak output | Large but short bursts | Continuous, GPM-limited |
The lifespan gap drives total cost of ownership more than the sticker price does. A tankless unit at 20 years can outlast two storage tanks, and the replacement labor is avoided each time. Against that, tankless repairs cost more, and parts such as heat exchangers and flow sensors are brand-specific, so service availability matters in rural areas.
Step 1: Calculate Your Peak Flow Rate
Flow rate is measured in gallons per minute, or GPM. Because the heater cannot store water, the unit’s GPM rating at your required temperature rise must cover everything that could run at once. Start by listing the fixtures your household might use simultaneously, then measure or estimate each one.
The Bucket Method
Any outlet can be measured with a one-gallon bucket and a stopwatch. Open the faucet or shower fully, time how many seconds it takes to fill the bucket, and apply the formula: flow rate = 60 divided by seconds to fill. A shower that fills the bucket in 12 seconds flows at 5 GPM; a kitchen faucet that takes 20 seconds flows at 3 GPM. Test every outlet that matters and record the numbers, because fixtures vary.
Typical Fixture Flow Rates
| Fixture | Typical flow rate (GPM) |
|---|---|
| Showerhead | 1.5 to 2.5 |
| Bathroom faucet | 1.0 to 1.5 |
| Kitchen faucet | 1.5 to 2.5 |
| Washing machine | 1.5 to 3.0 |
| Dishwasher | 1.0 to 2.0 |
| Bathtub | 4.0 to 6.0 |
If you cannot measure, use these ranges as estimates, then add up the fixtures you would actually run together. A household running two showers, the washing machine, and the dishwasher at once might total 9 to 11 GPM. That sum, not the average daily use, is the number the heater must satisfy.
The same flow math applies when you upgrade an existing tank, because the new heater has to cover the same simultaneous draws the old tank absorbed from storage. The practical differences in that swap, from gas line capacity to venting changes, are covered in this guide to advanced water heater replacement with tankless and heat pump units.
Step 2: Determine Your Required Temperature Rise
Temperature rise is the difference between the feed water temperature entering the house and the output temperature you want at the tap. The formula is simple: required rise = output temperature minus feed water temperature. A home with 50-degree feed water that wants 110-degree water needs a 60-degree rise. The same home in a colder region with 40-degree feed water needs 70 degrees of rise, and that difference decides whether a given unit can keep up.
Comfort targets are fairly standard. Most households set the heater between 105 and 115 degrees Fahrenheit. For showers, 105 degrees sits at the upper end of the comfortable range, and kitchen sinks usually run near 110 degrees. Set the output higher than needed and the unit works harder for no benefit; set it too low and the dishwasher or laundry underperforms.
Feed Water Temperature Depends on Where You Live
Groundwater temperature follows climate. In southern Florida, feed water can reach 77 degrees, so a modest rise does most of the work. In Alaska, North Dakota, Minnesota, Wisconsin, and other northern states, groundwater can sit at 37 degrees, forcing the heater to produce a much larger rise. Measure yours with a thermometer at the cold inlet during the coldest month, or consult a regional groundwater chart.
Typical Groundwater Temperatures by Region
| Region | Typical feed water temp (F) |
|---|---|
| Southern Florida | 72 to 77 |
| Gulf Coast | 65 to 72 |
| Mid-Atlantic | 50 to 60 |
| Midwest | 45 to 55 |
| Pacific Northwest | 45 to 55 |
| Northern Plains | 37 to 45 |
| Alaska | 37 to 42 |
The two numbers, peak GPM and required rise, are the same inputs every manufacturer uses to rate its units, and the same method appears in guides to instantaneous hot water systems, which explain how to read capacity curves. Compare any two models by their output at your rise, not by the maximum GPM printed on the box.
Homes with a hot water recirculation loop present a special case. Tankless units can serve a loop, but the pump must be sized so it does not defeat the on-demand design, and some installers recommend a small buffer tank or a recirculation-ready model.
Step 3: Put It All Together
With peak GPM and required rise in hand, look for a unit whose output curve at your rise meets or exceeds your peak flow. Manufacturers publish capacity tables, for example 5.5 GPM at a 70-degree rise and 4.0 GPM at a 90-degree rise. If your peak demand is 9 GPM at a 65-degree rise, you need a larger unit than the 5.5 GPM sticker suggests. Undersizing throttles flow on the coldest mornings; oversizing adds upfront cost but no meaningful operating penalty.
Confirm the installation requirements before you buy. Gas units need a properly sized gas line and dedicated venting; electric units may require a 120-amp or 150-amp service upgrade. Clearance, condensate drainage, and local code rules all apply, and the full process is covered in this guide to water heater installation, sizing, and code compliance, which walks through the steps in order.
Checklist Before You Order
- Peak GPM calculated from measured fixture flows
- Required rise based on winter feed water temperature
- Unit output curve that clears both numbers
- Gas line, venting, or electrical service confirmed
- Rebate paperwork checked against the model number
- Service access and condensate drain planned
Warranty terms are worth comparing too. Most tankless units carry 10 to 15 years on the heat exchanger and shorter coverage on parts, and registration deadlines are strict, so register the unit the week it is installed. Keep the manual and maintenance schedule accessible, because descaling on schedule is the difference between a 10-year unit and a 20-year unit in hard-water areas.
Two numbers, peak flow rate and temperature rise, settle the tankless sizing question for any household. Measure the fixtures, check the groundwater chart, and read the output curve before you spend. If you are weighing whether one appliance can handle space heating as well, the trade-offs of tankless water heaters for space heating are worth reviewing before the equipment goes in.