The most significant temperature rise achievable at a flow rate is used to evaluate tankless water heaters. To learn how to select the right size of tankless water heater, you must first calculate the flow rate and temperature increase required for its use in your home. It’s crucial to remember that under-sizing your tankless water heater is never a good idea.
Step 1:
Calculate the maximum range of devices you wish to run and the total flow rate of those devices. Then combine their flow rates. This is the flow rate that the requirement water heater should have. Suppose we handle a 0.75 GPM hot water tap simultaneously and a 2.6 GPM shower. The demand water heater’s flow rate would have to be at least 3.26 GPM. Install low-flow water fixtures to lower flow rates.
Step 2:
Evaluate the necessary temperature increase. Subtract the entering water temperature from the required output temperature to increase the temperature. Assume that the entering water temperature is 50°F unless you know differently. Should heat water at roughly 105–115°F for most purposes. In this case, a requirement water heater with a temperature increase of 55° is required.
Step 3:
A typical shower temperature is between 104 and 106 degrees Fahrenheit and utilizes 2.6 liters of water. If the water temperature in your house is 40° and you want to make sufficient hot water to run two baths simultaneously, what heat rise do you need to achieve this?
Answer: Increase the warmth of the incoming water from 40 to 105 degrees Fahrenheit. You must be able to heat 5.2 gallons of water at the very least. As a result, you’ll require a tankless water heater capable of a 60-degree temperature increase at 5.2 GPM.
Flow Rates:
What amount of hot water do you require at any given time? Would you need to operate two showers simultaneously or a bath and multiple sinks? When calculating your total simultaneous water demands, we recommend using 2.5 GPM for a shower and 1.0 GPM for a bathroom as a guideline. This table will clarify your question that how to select the right size of tankless water heater?
| FIXTURE | AVERAGE FLOW GPM | AVERAGE TEMP |
| Tub | 4.0 | 102°F |
| Shower | 2.5 – 3.0 | 104°F |
| Washing Machine | 2.0 | 120°F |
| Dishwasher | 1.5 | 110°F |
| Kitchen Sink | 1.5 | 110°F |
If you’re taking two showers at the same time, the tankless water heater will need to deliver 5 gallons of hot water every minute. You’d need 4.5 GPM from the water heater if you were taking a shower and doing laundry simultaneously. In any case, the unit should be sized to meet or exceed the amount of hot water you require at the same time.
Tankless water heaters should only heat potable (drinking) water, not the intake water.
Other Sizing Notes:
Compared to electric tankless water heaters, gas tankless water heaters can provide a higher temperature rise per GPM. Most demand water heaters are designed to work with a wide range of intake temperatures. With a flow rate of five GPM for gas-fired demand water heaters and 2 GPM for electric water heaters, a 70°F water warming trend is often feasible.