Choosing a tankless water heater means matching your home’s hot water demand with the right flow rate, fuel source, and efficiency rating. This guide explains how to choose a tankless water heater by breaking down the key factors every US homeowner needs to consider. Unlike traditional storage tank heaters, tankless models heat water only when you need it, which can reduce standby energy losses by 24–34% according to the U.S. Department of Energy (see Energy.gov’s tankless water heater page).
Key Factors to Consider
Flow Rate (GPM)
The flow rate, measured in gallons per minute (GPM), indicates how much hot water the unit can deliver at a given temperature rise. A typical household may need 3–5 GPM for simultaneous showers, washing dishes, and running a washing machine. Check the manufacturer’s specifications to ensure the unit’s rated flow matches your home’s peak simultaneous demand.
Temperature Rise
Temperature rise is the difference between the incoming ground water temperature and the desired output temperature. In colder US climates, ground water can be as low as 40°F, requiring a larger temperature rise. For example, if you want 120°F water and the incoming water is 50°F, you need a 70°F rise. Higher rises reduce the effective GPM, so always verify the unit’s performance at your local temperature conditions.
Fuel Type: Electric vs. Gas
Tankless water heaters are available in electric and gas (natural gas or propane) models. Electric units are typically more compact and easier to install but may require a significant electrical panel upgrade. Gas models usually offer higher flow rates and are better suited for large homes, but they need proper venting to the outdoors. Both types can achieve high energy efficiency, but gas units often have higher annual operating costs due to fuel prices.
Energy Efficiency Ratings
Look for the Energy Factor (EF) or Uniform Energy Factor (UEF) rating. Higher values mean less energy wasted. Many tankless units have a UEF of 0.82 to 0.96, compared to 0.60–0.70 for standard tank heaters. The EPA’s WaterSense program also offers certification for water-efficient models that save both energy and water.
Sizing Your Tankless Water Heater
Calculating Peak Demand
To size correctly, list all fixtures and appliances that might run simultaneously during the busiest hour. For example:
- Shower: 2.0 GPM
- Kitchen faucet: 1.5 GPM
- Dishwasher: 1.0 GPM
- Washing machine: 2.0 GPM
If a shower and dishwasher run together, total demand is about 3.0 GPM. Add a safety margin (10–20%) and compare with the unit’s GPM at your local temperature rise. Many manufacturers provide online sizing calculators, but consulting a licensed plumber ensures accuracy.
Installation and Maintenance Considerations
Venting and Clearance for Gas Models
Gas tankless water heaters require specific venting materials (typically stainless steel) and must be installed with proper clearance from combustibles. Improper venting can lead to carbon monoxide hazards. Always hire a qualified professional to install gas models; the International Association of Plumbing and Mechanical Officials (IAPMO) provides installation standards that many local codes adopt.
Water Quality and Scale Buildup
Hard water can cause mineral scale to accumulate inside the heat exchanger, reducing efficiency and lifespan. In areas with hard water, consider installing a water softener or using a descaling system. Annual maintenance, including flushing the unit with vinegar or a descaling solution, is recommended by most manufacturers to prevent performance loss.
Below is a quick comparison of typical tankless water heater characteristics:
| Feature | Electric Tankless | Gas Tankless |
|---|---|---|
| Flow Rate (typical) | 1.5–5 GPM | 4–11 GPM |
| Energy Factor Range | 0.95–0.99 UEF | 0.82–0.96 UEF |
| Installation Complexity | Requires heavy-gauge wiring; may need panel upgrade | Requires gas line and venting system |
| Best For | Smaller homes, moderate demand | Larger homes, high simultaneous demand |
Even the best tankless water heater can develop issues over time. If you experience temperature fluctuations or error codes, professional water heater repair can restore performance. Additionally, flushing the unit annually helps prevent clogs from sediment — a task that may be combined with drain cleaning services to keep your whole plumbing system healthy.
Frequently Asked Questions
What size tankless water heater do I need for a family of four?
A family of four typically requires a unit that can deliver 4–6 GPM at a 70°F temperature rise. If you live in a colder region or have multiple bathrooms, consider a higher-flow gas model or two smaller units installed in parallel.
Can a tankless water heater replace an existing tank heater?
Yes, in most cases. However, the installation may require updates to the gas line, venting, or electrical system. It’s best to have a licensed plumber evaluate your current setup. Our guide to pipe repair and installations can help you understand the scope of work.
How long does a tankless water heater last?
With proper maintenance, tankless water heaters can last 20 years or more, compared to 10–15 years for traditional tank units. Regular descaling and filter cleaning are critical to achieving that lifespan.
Do tankless water heaters need regular maintenance?
Yes. Annual flushing to remove mineral scale, checking the air filter (for gas units), and inspecting the venting system are essential. Hard water areas may require more frequent service.
Are tankless water heaters worth the higher upfront cost?
While the initial purchase and installation cost can be 2–3 times higher than a tank heater, the energy savings and longer lifespan often offset the investment over 10–15 years. For households with moderate to high water use, the payback period can be as short as 3–5 years.
What happens if the power goes out with an electric tankless heater?
Electric tankless units will not produce hot water during a power outage because they rely on electricity to heat the water. Gas units may still work if they have a standing pilot or battery-powered ignition, but most modern electronic ignition models also require electricity to operate.
Ready to upgrade your water heating system? Contact the experts at MyTownPlumber for a professional assessment. Schedule your consultation today.