What Size Water Heater Do I Need? A Fort Collins Sizing Guide

Water heater sizes: why gallons are not the whole answer

Tanks are sold by storage size, such as 40 or 50 gallons, but what tells you how much hot water you get in a busy hour is the first-hour rating. The U.S. Department of Energy (DOE) defines it as the gallons of hot water a heater can supply in one hour starting with a full tank, and says it depends on tank capacity, the heat source (burner or element) and that burner or element’s size.

Same-size tanks can differ: in one older Bradford White spec sheet, 50-gallon atmospheric gas models with 34,000 to 50,000 Btu/h burners carry first-hour ratings of 82 to 93 gallons, more than the tank holds, because the burner keeps heating as you draw water.

For a first guess, This Old House, citing The Home Depot, offers this rule of thumb:

Household	Tank size
1 to 2 people	30 to 40 gallons
2 to 3 people	40 to 50 gallons
3 to 4 people	50 to 60 gallons
5 or more people	75 gallons or more

Then let the first-hour rating decide between, say, a 40-gallon and a 50-gallon water heater. Bigger is not automatically better: DOE notes that a properly sized heater runs more efficiently.

How to size a tank water heater: the peak-hour method

DOE sizes a storage water heater, including a heat pump model with a tank, against your peak-hour demand: the most hot water your home uses in a single hour.

1. Pick your busiest hour: the morning, midday or evening hour when your home uses the most hot water.
2. Count each use in that hour: showers, shaves, hand dishwashing or food prep, dishwasher and clothes washer loads.
3. Multiply by DOE’s average gallons per use from the table below and add the results. The total is your peak-hour demand.
4. Choose a first-hour rating of that amount or higher.

Hot water use	DOE average gallons of hot water per use
Shower	20
Shaving	2
Hand dishwashing or food prep	3
Automatic dishwasher	7
Clothes washer, top-loader	25
Clothes washer, front-loader (H-axis)	15

If showers in your home run long, count a long shower as more than one.

What size tankless water heater do I need?

Size a tankless water heater by flow and temperature rise: add up the gallons per minute of every fixture you run at once, subtract your coldest incoming water temperature from about 120°F, and choose a unit whose flow chart covers that flow at that rise at your altitude setting. DOE explains that tankless units are rated by the maximum rise they can produce at a given flow.

1. Add up the flow you use at once. The EPA puts standard showerheads at 2.5 gpm (WaterSense models 2.0 or less) and standard bathroom faucets at 2.2 gpm (WaterSense models 1.5 or less).
2. Work out the temperature rise. DOE suggests 120°F for most uses and, unless you know otherwise, assumes 50°F incoming water: a 70°F rise. Incoming water is colder in winter, so size for winter. Run a cold tap for a few minutes in January and check it with a kitchen thermometer to get your own number.
3. Read the manufacturer’s flow chart, which shows the gallons per minute the unit can heat at each rise. DOE’s rough rule is a 70°F rise at about 5 gpm for gas units and 2 gpm for electric ones. ENERGY STAR rates tankless flow over a 67°F rise, so check which rise any flow figure assumes.

Still deciding? See our guide to tankless water heater pros and cons and our tankless water heater installation page.

Why altitude matters for gas water heaters in Fort Collins

Gas burners need air, and air is thinner at our elevations. The fuel gas code requires gas flow to be adjusted for altitude above 2,000 feet, and every town we serve sits above 4,600 feet, from Greeley at 4,674 to Estes Park at 7,530 (USGS elevations).

Rinnai uses three altitude bands, and one of its 199,000 Btu/h natural gas models derates like this:

Rinnai elevation band	Input of the 199,000 Btu/h model	Below sea-level rating	Towns at their USGS reference points
2,001 to 5,200 ft	170,000 Btu/h	about 15%	Fort Collins, Loveland, Windsor, Greeley, Longmont, Timnath, Berthoud, Johnstown, Severance, Milliken, LaPorte, Bellvue
5,201 to 7,700 ft	153,000 Btu/h	about 23%	Wellington, Masonville, Livermore, Estes Park
7,701 to 10,200 ft	139,000 Btu/h	about 30%	Any home above 7,700 ft

Less gas input means less hot water at the same rise. The setting depends on your home’s own elevation, not the town’s reference point, so homes near a boundary (Wellington’s point is 5,207 feet) and foothill lots need a site check.

Navien’s NPE-2 natural gas models need a high-altitude conversion kit at 5,400 feet or higher: Livermore and Estes Park, not Fort Collins or Loveland. Rinnai warns that wrong altitude settings can make its heaters run unsafely, damage them and void the warranty, so altitude setup is a job for a licensed plumber.

Gas tank altitude rules vary by model, even within one brand, so the installation manual decides. Electric and heat pump water heaters burn no gas, so the code’s altitude adjustment does not apply to them.

Worked example: a Fort Collins family of four

Take a family of four in a two-bathroom Fort Collins home at about 5,000 feet. On weekday mornings three people shower, one shaves and someone makes breakfast, all within the same hour.

Use in the busiest hour	DOE gallons per use	Times	Gallons
Shower	20	3	60
Shaving	2	1	2
Food prep	3	1	3
Peak-hour demand			65

Tank: the rule of thumb says 50 to 60 gallons; the worksheet says that tank needs a first-hour rating of 65 gallons or more (the label’s medium bin). A top-loading wash in the same hour adds 25 gallons for 90 (the high bin), so moving laundry to the evening matters.

Tankless: suppose two showers run at once on 2.0 gpm WaterSense heads (4.0 gpm), sometimes with a 1.5 gpm bathroom sink (5.5 gpm). Warming one gallon per minute by 70°F takes about 35,000 Btu/h (8.3 pounds per gallon × 60 minutes × 70°F, since one Btu warms one pound of water by 1°F, per the EIA). At 170,000 Btu/h, the derated model above could heat at most about 4.9 gpm at a 70°F rise even if every Btu reached the water, against about 5.7 gpm at sea level.

Two showers at 4.0 gpm sit under that ceiling, with a margin that depends on the unit’s efficiency; two showers plus the sink at 5.5 gpm do not fit at all. In the 5,201 to 7,700 foot band the ceiling drops to about 4.4 gpm, and colder winter water lowers it everywhere. Options include lower-flow fixtures, two units piped together or a tank.

Sizing is the first step of every water heater installation in Fort Collins we quote. The written quote lists the model, its first-hour rating or its flow at your rise, its UEF and the altitude setup, so you can check the sizing yourself. Every job is done by licensed plumbers covered by liability insurance, and we pull the permit and schedule the inspection on every replacement.

Sizing a heat pump water heater

A heat pump water heater with a tank is sized by first-hour rating, like any tank. DOE’s building science training notes that backup electric elements switch on for high demand or when the air is too cold, so sizing for your usual peak keeps the unit in efficient heat pump mode. Recovery also slows in winter, when incoming water is colder and the surrounding air cools, and 120-volt models with small or no backup elements can run out if undersized.

ENERGY STAR says heat pump models typically come in 50, 65 and 80 gallons and advises going up a size when switching from gas. It also wants an indoor space that stays roughly 40°F or warmer, with several hundred cubic feet of air or more around the unit.

If your current tank is failing now, start with water heater replacement in Fort Collins.