When it comes to choosing a tankless water heater, size is one of the most important considerations when selecting the right unit for your home. If your water heater is too small, it won’t be able to produce enough hot water to meet the demands of your household. For instance, if two showers are running at the same time and your water heater isn’t large enough, one of your family members could get a very unpleasant blast of cold water.  

On the other hand, if you buy a water heater that’s larger than what you need, you’ll pay more up front than necessary. However, when you’re choosing a tankless water heater, make sure to plan for your long-term needs. If you’re going to add more rooms to your home or you expect to have a larger family in the future, it’s best to select a water heater that will serve your current needs as well as those you anticipate having over the next several years. 

To choose the right size on-demand water heater for your home, you’ll need to determine two things: the flow rate and the temperature rise you’ll need. 

Flow Rate (GPM)

The flow rate, which is measured in gallons per minute (GPM), describes how much hot water you’ll typically need at any given time, and it’s the first factor to consider when sizing a tankless water heater. For example, a 5.3 GPM tankless heater can heat 5.3 gallons of water per minute, which is about enough to provide hot water for a sink and a shower. To figure out the correct capacity for your needs, add together all the fixtures you expect to be using simultaneously and how much hot water each fixture will use. This total will give you the desired flow rate you’ll want for your tankless water heater.

For instance, if you know one person is going to be running a hot water faucet with a flow rate of 0.75 GPM while two other family members are taking showers (with each shower head having a flow rate of 1.2 GPM), you would add those numbers together and determine you need a water heater with a flow rate of around 5.3 GPM, which is the smallest unit available. It’s best to round up so you don’t end up with a water heater that’s too small for your needs.

Temperature Rise 

In addition to flow rate, you’ll need to know the difference between the temperature of the incoming cold water (the groundwater temperature) and the desired temperature of the water at the faucets. This is called the temperature rise, and it refers to how much your heater will have to raise the temperature of your water.

To determine your temperature rise needs, subtract the incoming water temperature from your desired temperature. For example, if the water coming into your house is at 70 degrees and you want to heat it up to 120 degrees, you’ll need a tankless water heater that can add 50 degrees. As the difference between the groundwater and the desired fixture water temperature grows, the water heater won’t be able to supply as many fixtures at once.

Groundwater temperatures in San Diego average 65 degrees, but when temperatures drop in the winter, your water heater might need to add more heat to achieve the required temperature rise. For most purposes, you’ll want your water heated to around 105–120 degrees, so San Diegans would need tankless water heaters that produce a temperature rise of approximately 55 degrees. 

Gas vs. Electric Tankless Water Heaters

A gas tankless water heater can produce a larger temperature rise per GPM than an electric unit can. Typically, a gas water heater can raise the water temperature by 70 degrees at a flow rate of 5 gallons per minute, while the rate is 2 gallons per minute for an electric water tankless heater. 

If you aren’t sure which size tankless water heater is right for your family’s needs, seek advice from the experts. If you’re looking for the premier tankless water heater specialists in San Diego, call on Arnett’s Water Systems. We are available 24 hours a day, 7 days a week. Whether you need highly trained professionals to install a whole-house filtration system or a tankless gas water heater, you can trust us to provide nothing less than exceptional service. Give us a call today at (619) 223-1209 to find out how we can help you make the choice that’s right for you.