Incoming water temperature for water heater sizing

Why it matters so much

Every degree colder the incoming water is, the more heat the water heater has to add. For tankless, the effect is direct: two showers need 112,500 BTU/hr with 60°F water but 162,500 BTU/hr with 40°F water, 44% more. For a tank, blended fixtures draw more from the heater when the cold side is colder: the same busiest hour needs an FHR of 54 gallons at 60°F but 58 at 40°F.

It also matters because published ratings assume 58°F supply water. A tank’s First Hour Rating and a tankless unit’s maximum GPM are both measured at 58°F, so in colder water both deliver less than advertised.

How to measure it

1. Pick a cold tap close to where the water line enters the house, before any softener or filter tank if possible.
2. Run it for several minutes, until the temperature stops dropping. The first water has been sitting in warm pipes.
3. Hold a probe thermometer in the stream and read it once it settles.
4. Repeat in the coldest month. Mains water is usually coldest in late winter, weeks after the coldest air, because the ground cools slowly.

A tankless unit that shows inlet temperature on its display or app is another quick source once installed.

Estimated mains water temperature by city

No public agency publishes a table of tap-water temperatures by city, and the groundwater maps you’ll find online rarely say where their numbers come from. So these estimates are calculated: NOAA’s 1991–2020 climate normals for each city’s airport station, run through the mains-temperature correlation developed at the National Renewable Energy Laboratory and used in the EnergyPlus building simulation program.

Treat these as estimates. The correlation’s authors report errors of about 4°F on average across their test sites and note that mains temperature varies by ±5°F across a single water network. Its worst fit was Duluth, Minnesota, off by about 13°F. Cities that draw from mountain reservoirs, snowmelt or deep wells can differ more. A measurement beats any table.

How the estimate is calculated

mains(day) = (Tavg + 6) + ratio × (ΔT ÷ 2) × sin(0.986 × (day − 15 − lag) − 90) ratio = 0.4 + 0.01 × (Tavg − 44) lag = 35 − 1.0 × (Tavg − 44) days Tavg = annual mean air temperature, °F ΔT = warmest monthly mean − coldest monthly mean, °F

The average column is Tavg + 6; the coldest column is the low point of the curve. The published paper’s parameter table prints the lag coefficient as −0.01, but its text and EnergyPlus both use −1.0, which is what we use. Station data and the calculation are listed on the methodology page.

Using it in the calculator

Enter your coldest-month value as the incoming water temperature in the calculator. The calculator starts at 50°F, which is on the cold side for much of the country: warmer than Duluth’s estimated winter low of about 36°F, far colder than Honolulu’s 81°F. For well water, use roughly the local annual average air temperature.

Sources

1. Burch & Christensen, Towards Development of an Algorithm for Mains Water Temperature (NREL), ENERGY STAR (hosted).
2. U.S. Climate Normals, 1991–2020, NOAA National Centers for Environmental Information.
3. 10 CFR Part 430, Subpart B, Appendix E: Uniform test method for measuring the energy consumption of water heaters, eCFR.