Water Heater Sizing Guide
How to Select the Right Size

AO Smith gas water heaterAO Smith

If you are shopping for a new electric or gas tank-type water heater, you should first get familiar with the water heater sizing guide.

By getting the right tank size, plenty of hot water will be available to your household even during the peak times, but without wasting too much energy and water. The guide is very helpful as it provides expert guidelines, recommendations, and tips from different manufacturers.

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It is essential to know how to pick the right water heater size because water heating is a large energy consumer. It is actually the second-largest energy user in North American homes, and with the rising cost of gas, oil, and electricity, you should be careful when sizing a water heater.

A general water heater sizing rule is this: The heater should provide sufficient hot water at a temperature of min. 120 F at the busiest time of the day.

Keep in mind that an oversized heater means wasted money and energy as you will be heating more than you need. The heat energy will be lost through the tank walls and pipelines, and every time unnecessary quantity of water has to be re-heated.

An undersized heater means problems, while the properly sized water heater will meet all the household needs for hot water.

Water heater sizing guide tips

When you buy a new heater, it is normal to expect that the unit will perform perfectly and deliver hot water whenever needed; day, night, year by year.

There are many different sizes, from the small point of use and medium tankless water heaters to large tank models and boilers for higher demand.

Tankless heaters, for example, are providing hot water on demand, continuously, and their output can be expanded by applying the multi-system and by connecting several units into one system.

Tank-type heating units are different; they are large in size and occupy much more area from the floor space.

The tank-type heater must be adequately sized for your home and family needs and also to meet the demand for various applications, including showers, dishwashing, and washing machines.

Is your water heater undersized?

An undersized heater doesn't provide enough hot water, and it may cause overheated water. It may also result in condensation and pilot flame outage due to drips onto the flame.

Common symptoms of the undersized water heater are:

  • Not enough hot water
  • Heavy condensation
  • Rust chips on the burners and flue tubes
  • Premature tank failure

Improper water heater sizing can create problems that will reduce the heater's life, so undersized units are not eligible for warranty consideration.

On the other side, purchasing a correctly sized tank will minimize burner or other element operation when a small quantity of hot water is used, so there will be less stress on them.

Sizing also means proper:

  • Power in kW to heat water
  • Pipe size to provide required flow and output
  • Amperage
  • Gas pipeline
  • Drain pipeline
  • Vents to handle the total BTU input
  • Recovery rate
  • Electrical fusing
  • Expansion tank
  • The air used from the provided opening size

Important factors to consider

Probably the most important two factors are storage (capacity) and heat input. It is important to have the right water heater size, especially when large quantities of hot water are required in a short period.

On the other side, if the peak usage period is for an extended period (more than two hours), then the heater recovery rate is more important.

Another tip from the experts is to buy a heater with the highest Energy Factor (EF or UEF). EF measures the efficiency of the heater. They are more expensive, but with higher EF, you will purchase Energy Star models, save on energy and get the rebate through government incentives.

With the larger storage tank models, you will get the lower EF, and lower EF gets you the higher operating costs.

When selecting a water heater, it is vital to consider the size limitations like the diameter and height of the unit, as there are models like short, tall, lowboy, tabletop... so it can perfectly fit inside the designated room.

Here are suggestions and major factors that directly influence water usage to investigate and include when selecting a heater:

  • What is the size of your home and family? Determine if your family has a low or high demand for hot water. High demand is if there are more than 2 baths, there are at least two teenagers in a family, and you are using a hot tub, whirlpool bath...
  • How many bathrooms, tubs, dishwashers, washers do you have... any additions in the future?
  • Age of family members, kids, teenagers... family is growing?
  •  What is the usage pattern?
  • Quantity and type of fixtures.

The usage pattern is critical as it gives you the idea of the peak hour demand or when is the busiest hour of usage (i.e., peak time might be in the morning between 6 and 7 before parents go to work and kids to school).

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Calculate how much hot water do you need

Here are the consumption rates as an indication of how much water are required by a specific application:

  • For a shower or bath, you need around 20 gallons.
  • Showers with the energy-efficient shower head - 2.5-3 gal/min.
  • Automatic dishwasher needs around 14 gallons and 1-2.5 GPM flow rate.
  • The clothes washer needs a 1.5-3 GPM flow rate.
  • The washing machine needs 32 gallons to wash the clothes.
  • To shave, you need approximately 2 gallons.

For storage tank-type heaters, if there is a need for more hot water than what your existing conventional unit provides, you may want to consider buying a high recovery unit or split the system and install two units.

Sizing and first hour rating chart

Don't make a mistake like most consumers to buy a water heater based on the size of the storage tank. The peak hour demand capacity or first hour rating is more important. This information can be found on the Energy Guide label or brochure.

The first hour rating is an indicator of how much hot water one unit can provide. For natural gas, for example, the first hour rating ranges from 40 to over 100 gallons.

It is shown in gallons and is the combination of the amount of usable hot water stored in the tank and how many gallons of usable hot water the heater can generate in one hour of operation.

Example: If a 40-gallon natural gas heater stores 31 gallons of usable hot water and delivers 41 gallons of usable hot water in the first hour, its first hour rating is 72 gallons.

High first hour rating allows you to buy a smaller model as it will result in a good performance even during peak time and high demand while minimizing the cycle and standby losses.

Looking at the AO Smith water heater selection guideline for several family sizes, this is what we found:

Family size

2 people
3 people
4 people
5 people
6 people
> 7 people

FHR requirement (gal)

45-55
55-65
65-75
75-85
85-100
>100

Sizing calculators

What are the popular sizes?

The most used tank size (based on the common family size of four to six) and the general rule for water heater sizing are:

  • 30 gallons unit - enough for one bath residential homes or apartments.
  • 40 gallons heater - for two-bath residential homes or two applications.
  • 50 gallons water heater - for three-bath residential homes or three applications. Check out the top gas and electric models

The above is just the general guideline, but if you want to use the electric heater, for example, for a family of four, and you live in a home with two full bathrooms, and using washing machine and dishwasher, you should be within 50 to 80 gallons tank capacity range and around 5.5 kW of power produced by heating elements.

For the same requirements, and if using gas, the recommendation is to use a 50-gallon tank with 40,000 BTU/hr gas input.

When sizing a water heater, consider the following:

  • Electric heaters come with close to 100% recovery efficiency due to its immersion installation.
  • Gas heaters have approximately 75% recovery efficiency, which means that the water absorbs 75% of the total heat produced by the burner in the tank, while 25% is energy waste.
  • Keep also in mind that around 70% of the hot water is drawn before dilution by the incoming cold water. For example, a 50-gallon tank will deliver 35 gallons of usable hot water.

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