Gas vs. electric tankless water heaters

The number that decides the size

Tankless units get sized by two numbers: the flow you need at peak, and the temperature rise. The rise is the gap between the water coming in and the water going out. The U.S. Department of Energy's tankless guide says to size on "the maximum temperature rise (i.e., cold inlet to hot outlet) needed and the maximum flow rate," and notes that a house in a cold climate has a higher demand.

The physics fits on one line. One gallon of water weighs about 8.33 pounds, and heating one pound by one degree takes one BTU. That gives:

Gallons per minute ≈ (input BTU per hour × efficiency) ÷ (500 × temperature rise in °F)

The 500 comes from 8.33 pounds per gallon times 60 minutes. For an electric unit, multiply kilowatts by 3,412 to get BTU per hour.

Example flows at different temperature rises

The table below runs the formula for two example units. A 199,000 BTU gas unit is shown at 95% efficiency. A 36 kW electric unit is shown with all of its energy going into the water. These are illustrative numbers from the formula, not any manufacturer's ratings. The inlet column assumes a 120°F setpoint, and those inlet temperatures are examples, not Utah County measurements.

Read the table from top to bottom and you can see the winter problem. The same gas unit loses nearly 40 percent of its flow between a 50°F rise and an 80°F rise. Your manufacturer's flow chart gives the real figures for a specific model.

To estimate demand, add up the fixtures you expect to run at once. The EPA says standard showerheads use 2.5 gallons per minute and WaterSense showerheads use no more than 2.0. Two standard showers at once need about 5 GPM before anyone runs a faucet. Check the flow ratings stamped on your own fixtures.

Gas units

Gas tankless units come in condensing and non-condensing designs.

Condensing. A second heat exchanger pulls heat from the exhaust before it leaves. The unit produces condensate that needs a drain, and DOE's guide notes that condensing units need a way to dispose of it. Ask whether the drain line needs a neutralizer under the manufacturer's instructions.

Both designs draw a lot of gas while running. DOE's guide says the in-house gas line will need to be 3/4 inch in most cases, so a house with a 1/2-inch line to the old tank may need new pipe. Enbridge Gas Utah lists a $300 ThermWise rebate for an ENERGY STAR gas tankless water heater. The rebates guide covers the rules.

Electric units

Electric tankless units skip the gas line and the vent. The tradeoff shows up at the electric panel. A 36 kW unit at 240 volts draws 150 amps, which can exceed what an older panel or service can spare. Before choosing a whole-house electric unit, have an electrician run a load calculation on your panel.

The table shows the other limit. At an 80°F rise, the 36 kW example delivers about 3.1 GPM, about one standard shower. Electric units fit best in a small home with one bathroom, or as a point-of-use heater under a sink far from the main heater.

Provo, Lehi, Spanish Fork, and Springville run their own electric utilities. Rocky Mountain Power's Wattsmart water-heating incentives apply to heat pump water heaters, not tankless units, so check with your own utility before counting on a rebate.

Comparing brands such as Navien and Rinnai

This site doesn't rank brands. Model lines change, and a fair comparison needs the current spec sheet for each model you're considering. Put these items side by side:

• Input BTU and the flow chart. Look up flow at your winter temperature rise, not the headline flow.
• Uniform Energy Factor (UEF). The efficiency rating on the spec sheet. Higher means less fuel per gallon heated.
• Water quality limit. Rinnai's Value Series manual, for example, lists a total hardness limit of 200 mg/L, about 11.7 grains per gallon. Compare that with your city's water report.
• Venting options. Pipe material, maximum length, and where the vent can exit.
• Built-in recirculation. Some models include a pump to cut the wait for hot water at distant fixtures.
• Warranty terms. Read what the manufacturer excludes. Rinnai's SENSEI residential warranty excludes damage from improper maintenance, including scale build-up, and damage caused by poor water quality.

A request through the contact page goes to an independent local contractor.