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How to Size a Whole House Tankless Water Heater

Choosing an electric tankless water heater for your home or business depends on 3 steps.  

When shopping for a tankless water heater you must follow this 3 step process to make sure water heater will work, to meet your hot water needs. Because you are heating hot water on demand, the heater must: 1. heat enough water to meet your demand at one time with out getting cold water; 2. heat the water to your desired temperature for the duration of the application and; 3. and you must have enough power to be able to produce enough hot water for your hot water applications.

The 3 steps to properly size an electric tankless water heater are:

1) Maximum GPM flow rate desired
2) Incoming water temperature & temperature
3) Electrical service of your home

If you have any questions at all email or call us info@iapsales.com or 800 416 1298

Step 1:  Maximum GPM Flow Rate Desired:

Step 1. How many hot water application would you like run at one time? Look at the chart to the right for the applications that call for hot water and much water they use per minute or ( GPM) , base on the applications you plan on running at one time your water heater must heat that much water per minute.

For example 2 showers, an average shower will produce 1.6 -2 GPM of water each minute. So you would need a tankless water heater that can handle 4 GPM of hot water every minute if you plan on running two showers at once )

Once you have gone over step 2 and 3 you may find you will not have enough power to run two or three applications at once. As a Tempra 24 owner in the winter we can only do 1 shower or 2 bathroom sinks at one time  

Typical Hot Water Flow Rates GPM (Gallons per Minute Calculation Chart)

                      Dishwasher 0.5 - 1.0 GPM
                      Bathroom Sink 0.5 - 1.5 GPM
                          Kitchen Sink  1.5 - 2.0 GPM
Utility Sink 1.5 - 2.0 GPM
Washing Machine 2.0 - 2.5 GPM
Shower  1.5 - 2.5 GPM
Body Shower  1.75-2.0 GPM Each
Bath Tub  2.5 - 4.0 GPM
Jacuzzi or Roman Soaker Tub  4.0-5.0 GPM
Step 2: Incoming water temperature & temperature rise

Step 2:  identify the temperature rise needed to produce enough hot water. ( Average Hot Water Temp is 105F-115 F )Look at the color chart of North America to find the average winter incoming water temperature in your region. Then identify how hot you want your water to come out of your shower. To get your temperature rise take your desired hot water temp minus the incoming water temp = temp rise.

Example 55 F entering water 105 F hot water temperature 110-50= 55 F degree temperature rise

Average  Entering  Water Temp Based On Region
Average  Entering Winter
Water Temp
Average  Entering Summer
Water Temp
37 F 50 F
40 F 52 F
50 F 61 F
62 F 75 F
65 F 75 F
Step 2A :  Temp Rise & Max GPM Chart

The following chart illustrates the maximum flow rate that can be achieved based on temperate rise.   step 1 &2
This chart is based on a 240 volt voltage (Not 208 v) and the suggested electrical serviced needed to support the water heater (if you dont have suggest power in show in step 3, step 2A chart not valued)

Temperature Rise 70F Temp Rise 60F Temp Rise 50F Temp Rise 40F Temp Rise 30F Temp Rise
Tempra 36 Plus 3.5 GPM 4.0 GPM 5.0 GPM 6.5 GPM 7.9 GPM
Tempra 29 Plus 3.0 GPM 3.25 GPM 4.0 GPM 5.0 GPM 6.5 GPM
Tempra 24 Plus 2.5 GPM 2.75 GPM 3.25 GPM 4.0 GPM 5.25 GPM
Tempra 20 Plus 2.0 GPM 2.5 GPM 2.75 GPM 3.25 GPM 4.25 GPM
Tempra 15 Plus 1.75 GPM 1.8 GPM 2.0 GPM 2.5 GPM 3.25GPM
Tempra 12 Plus 1.5 GPM 1.7 GPM 1.75 GPM 2.0 GPM 2.5 GPM
Step 3 :  Electrical Service Needed for your Home or Business

Step 3 is the most important of the three steps. The chart below show the amount of power needed to run each model. You must have the suggested power shown below in the chart for the water heater to properly work if not you will have a very expensive wall fixture w/ cold water.
All  electric tankless water heater works entirely on demand, it must transfer a massive amount of energy to heat the water.
If you are unsure of your electrical capacity in your home we
highly recommend
that you consult your electrician to confirm that your home or business has enough power capacity before you make any final decisions regarding model selection.  It is easy to do and could help avoid disappointment or unexpected additional installation costs.

Model Recommended
Maximum Amperage
@ 240 V
Recommended Household
Electrical Service
Recommended Minimum
Wire Gauge
Tempra 36 Plus 3 x 60 AMPS 150 AMPS 300 AMPS 3 x #6 AWG Copper
Tempra 29 Plus 3 x 50 AMPS 120 AMPS 200 AMPS 3 x #8 AWG Copper
Tempra 24 Plus 2 x 60 AMPS 100 AMPS 200 AMPS 2 x #6 AWG Copper
Tempra 20 Plus 2 x 50 AMPS 80 AMPS 125 AMPS 2 x #8 AWG Copper
Tempra 15 Plus 2 x 40 AMPS 60 AMPS 100 AMPS 2 x #8 AWG Copper
Tempra 12 Plus 1 x 60 AMPS 50 AMPS 100 AMPS 1 x #6 AWG Copper
 If you have any questions at all email or call us info@iapsales.com or 800 416 1298

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