Sizing a tankless water heater for a Utah County winter

A tankless water heater has a fixed amount of heat to give. Spread it over cold water and you get fewer gallons per minute. Spread it over warmer water and you get more. Sizing comes down to knowing your coldest incoming water and the most hot water you'll draw at once.

The U.S. Department of Energy's tankless guide puts it this way: size on "the maximum temperature rise (i.e., cold inlet to hot outlet) needed and the maximum flow rate." It adds that a cold-climate house has a higher demand.

Step 1: Find your winter temperature rise

NOAA's 1991-2020 normals for the Provo BYU station put the normal January low at 23.8°F and the January average at 32.5°F. Air temperature doesn't tell you the water temperature, though. Measure your own.

To measure it:

1. Pick a cold morning in January or early February.
2. Open a cold tap near where the water enters the house and let it run for a few minutes.
3. Hold a kitchen or aquarium thermometer in the stream until the reading settles.

If the tap reads 45°F and you want 120°F, your temperature rise is 75°F. That example is arithmetic, not a measurement of Utah County water. Use your own reading.

If you're planning before winter, ask each contractor what inlet temperature they assumed and why. A bid that doesn't state one leaves you guessing.

Step 2: Add up peak flow

List the fixtures that might run at the same time on a busy morning, then add their flow rates. The EPA says standard showerheads use 2.5 gallons per minute and WaterSense showerheads use no more than 2.0. Faucets, dishwashers, and washing machines vary, so check the rating on the fixture or in its manual.

Size for the peak you have. Sizing for three showers that won't run at once adds cost. Sizing for one when two run each morning leaves someone with a lukewarm shower.

Step 3: Check the flow at your rise

Manufacturers publish a flow chart for each model showing gallons per minute at different temperature rises. Find your winter rise on the chart and read the flow. That number has to meet your peak.

You can sanity-check a chart with the physics. One gallon of water weighs about 8.33 pounds, and one BTU heats one pound by one degree. That gives:

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

Worked example for a 199,000 BTU gas unit at 95% efficiency and a 75°F rise: 199,000 × 0.95 ÷ (500 × 75) = about 5.0 GPM. That covers two standard showers and little else.

For an electric unit, convert kilowatts to BTU per hour by multiplying by 3,412. A 36 kW unit makes about 122,800 BTU per hour, or about 3.3 GPM at a 75°F rise. That same unit draws 150 amps at 240 volts, so check your panel before you plan around it.

These figures are illustrations from the formula. Real units vary, and the manufacturer's chart governs.

Step 4: Decide what to do if one unit falls short

If your winter peak exceeds what one unit delivers, you have two practical options:

• Cut the peak. WaterSense showerheads, or staggering showers and laundry, lower the demand.
• Keep a tank. For a large household, a high-efficiency tank or a heat pump water heater can make more sense than a tankless unit. Rocky Mountain Power's Wattsmart incentives cover heat pump water heaters for its customers.

Hard water changes the math over time

Scale on the heat exchanger slows heat transfer and restricts flow. A unit sized with no margin can fall short as scale builds. Rinnai's Value Series manual lists a total hardness limit of 200 mg/L, about 11.7 grains per gallon, while Provo reports 10 to 15 grains per gallon. The flushing guide covers how to keep scale down.

A sizing checklist for your bid

• Your measured or assumed winter inlet temperature
• Your target outlet temperature
• The temperature rise those two give
• Your peak flow in GPM, fixture by fixture
• The model's flow at that rise, from its flow chart
• Gas line size and length, or electric panel capacity

Using the checklist

Ask every contractor to fill in the same lines. If one bid assumes a warmer inlet temperature or a smaller peak than another, the numbers will show it, and you can ask each contractor to explain the difference. Keep the filled-in lists with the bids so you can come back to them when you decide.