The right-sized water heater is like a perfectly-sized shoe: once it’s in use, you don’t give it another thought. But if it’s too big or too small, it can cause you nothing but problems.
A water heater that is too small for your home can leave you suffering through cold showers, while one that is too big heats water you'll never use, which wastes money.
Because of this, selecting a water heater that's the right size is not a minor decision and, should you make the wrong choice, is not an easy replacement. It's important to do your homework before making your purchase and installation because the sizing process can be very specific depending on whether you opt for a heat pump/storage water heater or a tankless model.
Let's take a deeper look at what to consider when choosing the right water heater for your home:
Consider the water heater's first hour rating. The first hour rating determines how many total gallons of hot water the unit can provide each hour. This ratio takes into account the unit’s tank capacity as well as the source and size of its heating unit.
You can find a unit’s first hour rating in the top left-hand corner of its EnergyGuide label. You may also find the first hour rating printed under the title "capacity." This label is required on all storage water heaters, according to the Federal Trade Commission. However, it is not required on units that employ a heat pump, so don’t be surprised if it is missing on some of these units. If you can't find the label on a unit, look for the first hour or capacity rating in the unit’s owner’s manual.
Once you have found the unit’s first hour rating, the natural question becomes: What does this mean to you? As we said above, bigger is not always better. Instead, you should be looking for the properly sized unit to match your needs. To find the proper size, you need to determine your own use.
Your unit’s first hour rating should ideally be within 1-2 gallons of your own peak use, i.e., the time of day when your family uses the most water. This is commonly in the morning when everyone is getting ready to leave for the day, or at night when everyone is winding down for bed. To estimate your home's peak water use, look at the owner’s manuals for your appliances and consider how many could run simultaneously. For example, could you ever use the shower, dishwasher and washing machine at the same time?
This water calculator can also help you figure out how much water you use.
Once you’ve figured out how much water you use at peak periods, you can compare that against the first hour ratings of the water heaters you’re considering. Pick the one that best matches your needs, but remember that buying a unit because it meets your worst-case scenario could mean you’re heating water unnecessarily most of the time. In that case, don’t be afraid to opt for a smaller unit and leave those dishes for another time.
Tankless water heaters have no storage solution, so there's no need to think about a first hour rating. Instead of retaining water inside the system, a tankless water heater simply heats the water as it prepares to flow through the pipe. Because of this, gauging a tankless water heater's ability to satisfy your family's needs becomes a matter of flow rate. The water heater's flow rate is generally printed on the water heater itself but you can also often find it on the manufacturer's website by entering the model type.
To determine the proper flow rate for your needs you’ll need to look at the water usage in your home — where do you need hot water the most? Once you figure out which appliances you need to provide hot water, record the flow rates for each one. Add all of these flow rates together and you’ll find the desired flow rate for your tankless water heater.
After you’ve determined the flow rate you need for your home, you can focus on how quickly your tankless water heater can deliver the hot water you need.
Take your desired final temperature and subtract the incoming water temperature. In most cases this will be 70°F (21°C), as incoming water is usually around 50°F (10°C), and 120°F (49°C) is the common ideal for bathing and other hot water applications. Your water heater salesperson can help you find the right unit to match your temperature needs, but remember that gas-powered water heaters are often more adept at delivering the temperatures you need in greater volumes than their electric counterparts. Keep this in mind, because if faster speeds and increased volume are more important to you, those other options may be nothing more than an ill-fitting shoe.