Sizing a Water Heater: Finding the Right Size for Your Home

When it comes to sizing a water heater, it’s crucial to get it right. A properly sized tank ensures your household has enough hot water during peak usage times, without wasting energy or water.

By following a reliable guide, you can make an informed decision with expert tips and recommendations from various manufacturers, ensuring the perfect fit for your home’s needs.

The Importance of Selecting the Right Water Heater Size

It is essential to know how to pick the right water heater size, as water heating is a major energy consumer. In fact, it is the second-largest energy user in North American homes, with roughly 18% of overall consumption in Btu. With the rising costs of gas, oil, and electricity, it’s important to be cautious when sizing a water heater.

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

Keep in mind that an oversized heater leads to wasted money and energy, as you’ll be heating more water than needed. Heat energy will be lost through the tank walls and pipelines, and any unnecessary quantity of water must be reheated.

An undersized heater causes problems, while the properly sized unit will meet all of your household’s hot water needs.

Why an Improperly Sized Water Heater Can Be Trouble

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

Key Considerations for Proper Water Heater Sizing

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:

• Size of Your Home and Family: Determine if your household has a low or high demand for hot water. High demand applies if you have more than two bathrooms, at least two teenagers, or use a hot tub or whirlpool bath.
• Number of Bathrooms and Appliances: Consider how many bathrooms, tubs, dishwashers, and washing machines you have, as well as any planned additions.
• Age of Family Members: Assess the ages of your family members, such as kids or teenagers, and whether the family is growing.
• Fixtures: Evaluate the quantity and type of fixtures in your home.
• Incoming (Ground) Water Temperature: The groundwater temperature varies by location, elevation, and climate. Use a local map to determine the average temperature in your area.
• Location: Water temperatures tend to be colder in northern regions compared to southern ones, influencing heater performance.
• Usage Pattern: Understanding your usage pattern is crucial for sizing the unit. For example, peak usage might occur between 6 and 7 AM when parents are preparing for work and kids are getting ready for school.

Related: Sizing a tankless water heater

Family Size 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 only. 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	FHR requirement (gal)
2 people	45-55
3 people	55-65
4 people	65-75
5 people	75-85
6 people	85-100
> 7 people	>100

Table of Average Consumption and Peak Hour Demand

This calculation is based on the family size of 4:

Application	Average consumption (gallons)	Peak time usage  (in one hour period)	Used hot water (gal.)
Shower	20	4	80
Automatic Dishwasher	14
Clothes washer	32
Hand dishwashing	4	1	4
Shave	2	1	2
Food preparation	4	1	4
House cleaning	6
Hand washing	2
Hot water demand in the peak time (gal.)			90

Sizing Calculators

Sizing calculators help determine the right water heater size by considering factors like household size, water usage, and the number of fixtures. They use your input on daily hot water needs, peak usage times, and local water temperature to recommend an appropriately sized unit, preventing energy waste and ensuring sufficient hot water.

Most manufacturers, such as those listed below, offer these calculators on their websites, tailored to their specific products. They help you select the optimal size, power input, and efficiency, making the decision process easier and more accurate.

• Bradford White sizing calculator
• State sizing calculator

What Are the Popular Sizes?

Here are general guidelines to help determine tank sizes based on family size:

• 30 gallons: for one-bath homes or apartments
• 40 gallons: for two-bath homes or two applications
• 50 gallons: for three-bath homes or three applications

For a family of four with two full bathrooms and a washing machine and dishwasher, consider a 50-80 gallon tank for electric models or 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.