Tankless Water Heaters: Sizing, Gas vs Electric, and Installation Considerations
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A tankless water heater heats water on demand instead of keeping a tank full of hot water 24 hours a day. The energy savings are real, ranging from 8% to 34% compared to a conventional tank depending on your household usage patterns. But tankless units also cost more upfront, have specific installation requirements (especially gas models), and have flow rate limitations that affect how many fixtures you can run simultaneously. Understanding these tradeoffs before buying saves you from the most common complaint: expecting unlimited hot water and getting a lukewarm trickle when two showers run at once.
How Tankless Water Heaters Work
When you open a hot water tap, cold water flows through the tankless unit and a gas burner or electric heating element heats it to the set temperature as it passes through. There is no stored hot water. The unit produces hot water on the fly, and when the tap closes, the unit shuts off. You only pay to heat water while you are actively using it, which is where the energy savings come from. A conventional tank heater maintains 40 to 80 gallons at temperature around the clock, losing heat through the tank walls even when nobody is using hot water.
The key specification for any tankless unit is flow rate, measured in gallons per minute (GPM) at a given temperature rise. A unit rated at 5 GPM at a 77-degree rise can heat 5 gallons per minute from about 50 degrees (typical groundwater temperature in northern states) to 127 degrees. If the incoming water is warmer, like in southern states where groundwater runs around 70 degrees, the same unit can deliver higher flow at the same output temperature or hotter water at the same flow rate. Geography matters when sizing these units.
Sizing by Flow Rate
Proper sizing starts with adding up the flow rates of the fixtures you expect to run at the same time. A standard shower uses 2 to 2.5 GPM. A kitchen faucet uses about 1.5 GPM. A dishwasher draws 1 to 2 GPM. A washing machine uses roughly 2 GPM. If your household routine involves running a shower and a kitchen faucet simultaneously, you need at least 3.5 to 4 GPM of capacity from the unit.
Next, determine your required temperature rise. Subtract your incoming cold water temperature from your desired hot water temperature. In northern states with 50-degree groundwater and a 120-degree hot water target, the required rise is 70 degrees. In southern states with 70-degree groundwater, the rise is only 50 degrees, and the same unit can support a higher flow rate because it has less heating work to do per gallon.
Gas tankless units typically produce 5 to 11 GPM at standard temperature rises, which is enough for 2 to 3 simultaneous fixtures in most situations. Electric tankless units produce 2 to 5 GPM, which often limits you to 1 to 2 fixtures at a time. If your household regularly runs multiple showers at once, or a shower plus a dishwasher plus a washing machine, a gas unit is the practical choice. Alternatively, some homeowners install multiple smaller electric units at point-of-use locations rather than one central unit.
Gas vs Electric Models
Gas tankless heaters (natural gas or propane) offer higher flow rates, lower operating costs in most service areas, and are the standard choice for whole-house applications. They require adequate gas line capacity (typically 3/4 inch minimum, sometimes 1 inch for high-BTU models), a combustion air supply, and proper venting. Venting options include category III stainless steel vent pipe routed through the roof or a direct-vent configuration that exhausts through an exterior wall.
The hidden cost of gas tankless installation is the gas line. Many homes do not have adequate gas line capacity for a tankless unit. A standard tank water heater might run on a 1/2 inch gas line drawing 40,000 BTU, but a tankless unit drawing 150,000 to 199,000 BTU needs a 3/4 inch line, sometimes all the way back to the gas meter. Upgrading the gas line adds $500 to $2,000 to the installation cost, and this is the expense that catches most homeowners off guard. Have an installer verify your gas line size before committing to a gas tankless unit.
Electric tankless heaters are simpler to install in one sense: no venting, no gas line, no combustion air requirements. But they draw enormous amperage. A whole-house electric tankless unit needs 100 to 150 amps of dedicated electrical capacity, spread across two to four 40-amp circuits wired with 8-gauge or 6-gauge copper wire. Many homes with 100-amp or 150-amp total service cannot accommodate this without a panel upgrade, which adds $1,500 to $3,000 to the project cost.
For point-of-use applications, where you want on-demand hot water at a single fixture (a bathroom sink, a kitchen sink, or a remote shower), smaller electric units drawing 20 to 30 amps are practical, affordable ($150 to $300), and straightforward to install. These supplement a central water heater rather than replacing it.
Installation Costs and Variables
The unit itself costs $500 to $2,000 depending on size, brand, and fuel type. Gas units generally cost more than electric. Major brands include Rinnai, Navien, Noritz, and Rheem for gas; Stiebel Eltron, EcoSmart, and Rheem for electric.
Installation costs are where tankless gets expensive. Gas installation typically runs $1,000 to $3,500 on top of the unit cost. The major variables include venting (a new vent run costs $200 to $800 depending on routing complexity), gas line upgrades ($500 to $2,000 if the existing line is undersized), condensate drainage for condensing models (which produce acidic condensate water that needs a drain line, adding $100 to $300), and removal and disposal of the old tank heater.
Electric whole-house installation runs $500 to $1,500 for labor, but add a panel upgrade if your home cannot supply the required amperage. Total installed cost for a gas tankless unit ranges from $1,500 to $5,500. For an electric whole-house unit, $1,000 to $3,500 is typical. Compare this to $800 to $1,500 for a conventional tank water heater fully installed.
The energy savings, typically $100 to $200 per year depending on your usage and local fuel costs, take 5 to 15 years to offset the higher initial cost. The payback period depends heavily on whether your installation requires gas line or electrical upgrades.
Realistic Expectations
The cold-water sandwich is a common experience with tankless units. When you turn on hot water shortly after someone else finished using it, there is a slug of hot water already sitting in the pipe, followed by a burst of cold water (the pipe cooled between uses), followed by freshly heated water from the unit. This is a characteristic of how tankless systems work, not a malfunction. A recirculation pump or a small buffer tank can mitigate this if it bothers you, though both add cost and complexity.
Minimum flow activation is another quirk to understand. Tankless units need a minimum flow rate (typically 0.5 GPM) to trigger the burner or heating element. If you crack a faucet to a slow trickle, the unit may not activate and you get cold water. This is rarely an issue with normal use, but it can be noticeable with very low-flow fixtures or when you are trying to get just a small amount of warm water.
Maintenance matters more with tankless units than with tanks. Gas tankless heaters need annual flushing with white vinegar to remove mineral scale buildup, especially in hard-water areas where calcium and lime deposits accumulate inside the heat exchanger. Scale reduces efficiency and can eventually damage the heat exchanger, which is the most expensive component to replace. An inline isolation valve kit ($50 to $100 installed during the initial setup) makes annual flushing a 20-minute DIY job. Without isolation valves, flushing requires disconnecting water lines, which is more involved. Electric units also benefit from flushing but are generally less prone to scale damage.
Tankless units last 15 to 20 years with proper maintenance, compared to 8 to 12 years for conventional tank heaters. This longer lifespan partially offsets the higher upfront cost and is a significant factor in total cost of ownership calculations.
Heat Pump Water Heaters: An Alternative Worth Considering
Before committing to tankless, consider a heat pump (hybrid) water heater. These units cost less to install than tankless (the installation is similar to a standard tank replacement), use 60% to 70% less energy than a conventional electric tank, and provide a large supply of stored hot water so flow rate limitations are not a concern.
The tradeoffs are size (the heat pump unit sits on top of a standard tank, so the overall height is taller), ambient cooling (the heat pump extracts heat from surrounding air, which is a benefit in summer but can cool the space in winter), and recovery rate (slower to reheat after heavy draws compared to gas). For many homes, especially those with electric water heating, a heat pump water heater delivers better economics than tankless when installation costs are factored in.
Frequently Asked Questions
Will a Tankless Water Heater Save Me Money?
On operating costs, yes. Expect 8% to 34% energy savings compared to a tank heater. On total cost of ownership, it depends on installation complexity. If your home needs gas line or electrical upgrades, the higher installation cost can push the payback period to 10 or more years. If the installation is straightforward (adequate gas line, suitable vent location), the payback is typically 5 to 8 years.
Can I Install a Tankless Unit Where My Tank Heater Is?
Sometimes. The gas line and vent route from the existing tank heater may not work for a tankless unit. Tank heaters use atmospheric venting (natural draft up a chimney flue). Most tankless units require sealed combustion venting (stainless steel ducted to the exterior), which may need a completely different route. The gas line may also need upsizing. An installer should evaluate your specific situation before you commit to a purchase.
What About a Hybrid (Heat Pump) Water Heater Instead?
Heat pump water heaters are a strong alternative. They cost less to install than tankless (similar to a tank replacement), use 60% to 70% less energy than a conventional electric tank, and provide a large supply of stored hot water. The tradeoff is that they need space (the heat pump unit sits on top of a tank), they cool the surrounding air (helpful in summer, less so in winter in a heated space), and they are slower to recover than gas. For many homes, a heat pump water heater is the better economic choice.