Being a homeowner is not easy. You're a house cleaner, a contractor, and a designer. Depending on how brave you are, an electrician, a plumber, and a carpenter. It takes a lot to maintain a home. It takes more to improve and make a home more efficient.
Since you made your way here, you've decided to stretch your engineering skills on sizing a water heater.
The different system types are sized according to how they operate. Let's start with how each system works regardless of its electric or natural gas.
In a boiler or tank system, the tank fills up and then heats the water. As hot water is being used the bottom fills up with cold water. The building is limited by how much hot water is in the tank.
Therefore, when comparing boilers you'll see them categorized by gallons.
A tankless system heats the water as it passes through heating coils through the inlet pipe. There is no tank. It is essentially endless hot water.
The limitation is the flow rate of the water. The flow rate is the limit because if the water moved through the heating coils too fast then it wouldn’t reach the desired temperature.
For this reason, when comparing systems look for gallons per minute (GPM).
To size a tank or water storage system you will need to know two main things:
In other words, you have to determine the highest demand the system will experience during one hour. For example, in a home of 4 people, you may have two showers, a sink, and a dishwasher running at the same time. You need to determine how many gallons are available in the first hour, with a full tank.
You don't want to hear screams coming from a shockingly cold shower.
This is known as the water heater's first-hour rating. Also known as the capacity.
The EnergyGuide label lists the first-hour rating in the top left corner.
Once determined what you typically run during the highest use hour, add all the gallons per usage. Simple.
Let's do an example!
You've determined you use the most hot water in the morning with your 4-person household. There are usually two showers running before work and school. The dishwasher and washing machine are loaded. Lastly, the kitchen faucet will be running for breakfast preparation.
By using the table below add up the gallons of hot water per usage.
For this household, a 75 or even an 80-gallon water heater will work great. No one will be left shivering in a cold shower. Chores can continue at any time. However, as the tank size increases, so does the price tag.
By doing this exercise, it is now easier to see some other options. If the family is willing to change the time they do their chores, a smaller size may be possible.
At a minimum, a 40-gallon water heater will be enough for two showers. It will also be enough to run the dishwasher and washing machine at the same time if no one is showering.
Gallons of Hot Water Used
To size a tankless system you will need to know three main things:
Different from a storage system, you are trying to determine the highest hot water flow rate demand. The flow rate demand is dependent on two things: the fixtures that are running and the water temperature rise.
The flow rate of your fixtures also varies.
It depends on their age, older fixtures tend to have higher flow rates than newer fixtures. The type and manufacture of the fixture vary.
A typical shower runs at around 104°F. Ground temperature is important because it will determine how much energy the system needs. It takes more energy to heat the water from 40°F than 50°F.
Use the map below from epa.gov to estimate your groundwater temperature.
The rating of a tankless water heater will tell you the GPM at a certain temperature rise. If you determine your GPM but require a higher temperature rise, then opt for a higher size.
Let's do an example!
We’ll use the same household as the previous example.
You've determined you use the most hot water in the morning with your 4-person household. There are usually two showers running before work and school. The dishwasher and washing machine are loaded. Lastly, the kitchen faucet will be running for breakfast preparation.
By doing this breakdown of the gallon-per-minute flow rate for each fixture, we know we need 11gpm. The next key thing to determine is where this family is located.
Tankless water heaters typically come with a legend of what their unit is capable of in different climate zones.
If the family is located in San Antonio, Texas, they’ll require a temperature rise of about 32°F. They can purchase one large system that has a gpm of 12 to handle all of their needs. They can choose to get a smaller system if they decided to wash their clothes and dishes at different times.
If they are located in Fargo, North Dakota they’ll need a 62°F temperature rise! If the family purchases the same 12 GPM system, they will be forced to coordinate their hot water usage. They can choose to get a second tankless water heater to make up the difference.
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