To pick the right tankless water heater, you need to match its flow rate to your household’s peak hot water demand and choose between gas and electric models. This guide explains how to pick a tankless water heater that fits your home’s plumbing, budget, and energy goals. Tankless units heat water on demand, saving space and energy compared to traditional storage tanks. But selecting the wrong size or fuel type can lead to lukewarm showers or high installation costs. We’ll walk through every factor so you can make a confident decision for your US home.
Understand How Tankless Water Heaters Work
Unlike storage tank heaters that keep 40–60 gallons hot around the clock, tankless units use a high-power burner or electric element to heat water only when you turn on a tap. This eliminates standby energy loss and can reduce your water heating bills by 8–14%, according to the U.S. Department of Energy. However, the unit’s output is limited by its flow rate (measured in gallons per minute, or GPM) and the temperature rise needed.
Key Factors for Choosing a Tankless Water Heater
1. Calculate Your Peak Hot Water Demand
Your household’s peak demand is the total flow rate when multiple fixtures run simultaneously. For example, a shower uses about 2.0 GPM, a kitchen faucet 1.5 GPM, and a washing machine 2.0 GPM. Add them up: if you run a shower and the dishwasher at the same time, you need a unit that delivers at least 3.5 GPM at your required temperature rise.
- Small household (1–2 people): 3–4 GPM unit
- Medium household (3–4 people): 5–6 GPM unit
- Large household (5+ people): 7–9 GPM unit or two units in parallel
2. Determine the Required Temperature Rise
Temperature rise is the difference between your incoming ground water temperature and your desired hot water temperature. In northern US states, ground water can be 40°F in winter; in southern states, it may be 60°F. To get 120°F water, a northern home needs an 80°F rise, while a southern home needs only 60°F. Most tankless heaters list their maximum GPM at a 35°F, 45°F, or 77°F rise. Always use your local winter ground water temperature for sizing—otherwise, you’ll get cold water in January.
3. Choose Between Gas and Electric
Gas tankless heaters (natural gas or propane) typically produce higher flow rates and can handle larger homes. Electric units are smaller, cheaper upfront, and easier to install if you have sufficient electrical service. However, electric models often require a 120–150 amp dedicated circuit, which may necessitate a panel upgrade costing $1,500–$3,000 (prices vary by region). Gas units require proper venting—usually stainless steel exhaust—and may need a gas line upgrade.
| Feature | Gas Tankless | Electric Tankless |
|---|---|---|
| Flow rate range | 5–11 GPM | 2–8 GPM |
| Upfront cost (unit only) | $800–$2,500 | $400–$1,200 |
| Installation complexity | Moderate to high (venting + gas line) | Moderate (heavy electrical upgrade often needed) |
| Operating cost | Lower per BTU (gas is typically cheaper) | Higher if electricity rates are high |
| Lifespan | 15–20 years | 10–15 years |
4. Check Energy Efficiency Ratings
Look for the Energy Factor (EF) or Uniform Energy Factor (UEF) rating. Most tankless gas units have a UEF of 0.82–0.96, while electric units can reach 0.98–0.99. Higher is better. The Energy Star label ensures the unit meets strict efficiency guidelines set by the EPA. For more details, visit the Energy.gov water heating page.
5. Consider Your Home’s Plumbing and Venting
Gas tankless heaters require Category III or IV stainless steel venting, which cannot share a flue with other appliances. The vent must slope upward and terminate outdoors away from windows. Electric units need a dedicated circuit; check your main panel capacity. If your home has hard water, scale buildup can reduce efficiency and lifespan. Installing a water softener is recommended to protect the heat exchanger. Also, ensure your gas line is sized to deliver the required BTUs—many older homes need a 3/4-inch or 1-inch line.
Installation and Maintenance Considerations
Professional Installation Is Critical
Tankless water heaters must be installed by a licensed plumber to meet local codes and warranty requirements. Improper venting or gas line sizing can lead to carbon monoxide hazards or fire risk. The International Association of Plumbing and Mechanical Officials (IAPMO) provides installation standards; check IAPMO’s official site for code references. A typical installation costs $1,200–$3,500 depending on venting and gas line work.
Routine Maintenance
Annual descaling with a vinegar solution removes mineral deposits. In areas with hard water, install a sediment filter before the unit. Check the air intake and vent for blockages. If you notice fluctuating water temperature or reduced flow, schedule a water heater repair inspection. For other plumbing issues like clogs or leaks, our team also offers drain cleaning and leak detection services.
Frequently Asked Questions
Can a tankless water heater supply my whole house?
Yes, if sized correctly. A whole-house gas tankless unit with 7–9 GPM can serve a 4-bedroom home with simultaneous showers, but you may need two units for very large homes with multiple bathrooms running at once.
How long does a tankless water heater last?
With proper maintenance, a tankless unit lasts 15–20 years for gas and 10–15 years for electric, compared to 8–12 years for a storage tank heater.
Are tankless water heaters worth the higher upfront cost?
They can be, especially if you stay in your home long-term. The energy savings of 8–14% and longer lifespan often offset the higher purchase and installation cost within 5–10 years.
Do I need a permit to install a tankless water heater?
Most US municipalities require a permit for gas line or electrical work. A licensed plumber will handle permits and inspections to ensure code compliance.
What size tankless water heater do I need for a family of four?
Assuming two showers and a dishwasher running simultaneously, you need roughly 5.5–6.5 GPM at a 60–70°F rise. A gas unit rated at 6.5 GPM at a 45°F rise would be a good choice.
Can I install a tankless water heater myself?
DIY installation is not recommended due to gas, venting, and electrical risks. Improper installation can void the warranty and create safety hazards. Hire a licensed professional.
Make the Right Choice for Your Home
Choosing the best tankless water heater comes down to calculating your peak flow, knowing your ground water temperature, and selecting the fuel type that matches your utility costs and home infrastructure. A properly sized unit provides endless hot water and lower energy bills for years. If you need expert advice or a professional installation estimate, contact MyTownPlumber today. Our licensed technicians serve your area with reliable pipe repair, sump pump services, and more.
—MyTownPlumber Editorial Team