Average water temperatures coming into San Diego homes









How to Select the Right Size Tankless Water Heater

There’s no question that tankless water heaters, also known as on-demand water heaters, are gaining in popularity with smart homeowners. A tankless water heater only heats water that is needed and does so very rapidly.

So how do you choose the right size tankless water heater for your San Diego home?

Step One

First, start by estimating the flow rate and temperature rise you’ll need for the application – a whole home vs. just a bathroom for example.

Determine the maximum number of appliances and fixtures you want to run and their total flow rate.

Then, using gallons per minute, add up their flow rates (gallons per minute). This is the desired flow rate you’ll want for the demand water heater.

Tankless Water Heater Flow Rates

How much hot water do you require at any one time? It’s probably unlikely that you will be running two showers at the same time. Maybe a shower and a couple sinks simultaneously would be more realistic. The chart below shows the range of water usage and average water temperatures for various fixtures. We suggest using 2.5 gpm for a shower and 1.0 gpm for a bathroom as a reference point in determining your total simultaneous water needs.

Tub 4.0 GPM 102°F
Shower 2.5 – 3.0 GPM 104°F
Washing Machine 2.0 GPM 120°F
Dishwasher 1.5 GPM 110°F
Kitchen Sink 1.5 GPM 110°F

Here are some other scenarios, where you might have guests using showers simultaneously or a vacation rental where people don’t care how much water they use. For example, if you are running 2 showers at the same time, you will need 5 gallons of hot water per minute from the tankless water heater. If you were running a shower and the washing machine at the same time, you would need 4.5 gallons per minute from the water heater. In either of these situations, you will want to size the unit that meets or exceeds how much hot water you need at the same time.

Step Two

Calculate required temperature rise. To quantify temperature rise, subtract the incoming water temperature from the desired temperature you want from the fixtures. Let’s say your average incoming water temperature is 50°F. Even though your incoming water temperature could be higher, some experts recommend using a lower temperature assumption, thereby ensuring you will not undersize your tankless unit.

For the majority of users, you’ll want your water heated to around 105–115°. On the low end of this example, you’ll need a tankless water heater that produces a temperature rise of 55°.

Here’s another scenario: Average shower temperatures will be between 104–106° and uses 2.6 gpm’s of water. If you assume your water temperature is 50° coming into your home, and you want to ensure enough hot water to run two showers at the same time, what temperature rise would be required?

Here’s the answer: The water temperature coming into your house has to be raised from 40 to 105 degrees. You’ll want to heat at least 5.2 gallons of water per minute.  With this information, spec out a tankless water heater that can produce at least a 60 degree rise in temperature at 5.2 gallons per minute.