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Tankless water heaters give you endless hot water, as long as you install the right size. You can calculate the correct size on-demand water heater by finding the required flow rate and temperature increase.
Read on for an explanation for all the terms that you need to know and then follow these easy steps to calculate the right size tankless water heater for your house.
Table of Contents
Water Heater Terms Explained
Flow Rate
This is how fast the water moves through the faucet, usually given as gallons per minute or GPM in the empiric system or liter per minute in the metric system. Each type of household water fixture has a different flow rate, for example, the sink tap generally runs slower than the showerhead.
You can find the flow rate of each appliance in the product description or manual. Alternatively, you can refer to a list of estimate flow rates for the most common household appliances.
Incoming Water Temperature
This refers to the temperature of the water as it enters the water heater, usually, this is the groundwater temperature of the area that you live in. Bradley Corp has a handy map showing the different temperatures in the United States that you can use as a reference point if you are unsure.
Required Temperature Rise
This refers to how many degrees, Fahrenheit or Celsius, the water heater needs to increase the incoming water to get to the desired temperature. For example, if the incoming water temperature is 55°F and the desired temperature for a dishwasher is 120°F, you need a device that can deliver a 65°F temperature increase.
Tankless Water Heater Size
When speaking about the size of a tankless water heater, it refers to its performance or capacity rather than its actual dimensions. There is no storage with this type so the actual dimensions of the unit do not reflect on its performance.
Guide to Calculations
Calculating the right size for your household is easier than it may seem. You can come to a conclusion quite quickly and you don’t need that much information, either.
Step 1: Find the Total Flow Rate
Make a list of all the water fixtures that you want to connect to a single water heater. Next to each item, note down their flow rate.
With the list complete, add all the flow rates together to get the total desired flow rate. This total number is how much the unit must be able to handle.
Step 2: List the Desired Temperatures
For your list of connected appliances, note down the desired warm temperature for each. The most important one is the household appliances that needs the hottest water, usually a shower or dishwasher but it could also be a hot tub.
Step 3: Find the Incoming Water Temperature
Check out an online resource like the council area website or a map showing the groundwater temperature in your area. Find where your house is located and the corresponding temperature.
Step 4: Calculate the Required Temperature Increase
Take the highest desired water temperature and subtract the incoming temperature from this. This gives you how many degrees is required for the tankless heater.
Example Calculation
You want to connect 2 showers and a sink. These have a flow rate of 2.8 GPM and 1.5 GPM, respectively so the total flow rate is 6.3 gallons per minute.
The house is in Florida with an incoming water temperature of 72°F. The highest temperature requirement is for the shower at 105°F so the required temperature increase is only 33°F.
So, at the very least your chosen unit should be able to increase the water temperature by 33°F with a flow rate of 6.3 gallons per minute.
It is always better to get the data that applies to your own area and fixtures because this gives you the most accurate final number. However, if you are okay with a more general sizing, you can plug in standard numbers into an online water heater sizing calculator.
Don’t Forget the Dimensions of Your Tankless Water Heater
Yes, in this case, the size usually refers to the capacity of the device but you cannot forget about the actual dimensions of the main unit, either. Always check the product description for the length, width, and height so you can be sure it fits where you want it to fit.
This is especially true for point-of-use appliances like under the kitchen sink because space is more limited here. The same goes for a whole-house device that needs to fit in the ventilated closet.
What If You Can’t Find the Right Size?
If you completed these calculations for a single whole-house unit, you might come to the conclusion that you set a very high requirement. This might mean that you need to invest in a more expensive unit – if you can find one that is big enough in the first place.
The alternative is installing point-of-use units so that there fewer fixtures connected to a single unit which in turn lowers the requirements in your calculation. This lets you opt for more affordable and smaller size tankless water heaters.
Think about which tankless size option makes the most sense for your home and your water use. For example, a single person or couple living in an apartment might make do with just one unit while a family of 4 might need to install point-of-use units in the kitchen and another unit for the bathrooms.
How you use water in the house can also influence your sizing decision. You can still choose a smaller heating device if you are okay with running household appliances separately.
For example, you can install a unit with a smaller GPM if you only run the dishwasher and shower separately. However, this is less practical so consider if this limitation is worth what you are saving on the purchasing cost.
