How to correctly Size the Tankless Water Heater by Flow Rate



When seeking the best tankless water heater for your family, think in terms of flow, not capacity. As discussed above, a tankless water heater doesn’t run out of hot water as a storage water heater can. But it may not heat water fast enough to serve multiple fixtures at once.

Quality tankless water heaters like AO Smith or Rheem can provide hot water endlessly but how much water is that? What is its maximum flow rate? It cannot run out of hot water and it can be overloaded though if too much hot water is demanded at the same time.

This is why it is very important to correctly size the water heater before buying one. Tankless water heaters do not hold the water as it is instantly heated as it flows through the heater.

There are two main things to consider when choosing the size and model of the tankless water heater. First, is the amount of water that needs to be heated as it flows through the heater (flow rate). Second, is how hot the tankless water heater needs to heat the water (temperature rise). The difference between the incoming cold water and the outgoing hot water needed rather than the total count of gallons used in a day.

It really depends on how powerful the water heater is. If the demand for hot water exceeds the capacity of the unit, either the flow rate or temperature rise will be reduced. The correct size of the tankless water heater is usually based on the total amount of hot water used during a day. It must be sized based on the flow rate and temperature rise needed.

This sizing decision is made easy with our sizing chart.

Flow Rate Guide (Temperature Rise vs Gallons per Minute)

Temp Rise 110/U/C 310/U/C 510/U/C 140H 240H 340H 540H/P CT-199 910
30° 6.6 8 10 6.6 6.6 8 10 10 14.5
35° 6.6 8 9.3 6.4 6.6 8 10 10 14.5
40° 5.7 7.8 8.1 5.6 6.6 8 9.5 9.5 14.5
45° 5.1 6.9 7.2 5 6.6 7.6 8.4 8.4 13.5
50° 4.6 6.2 6.5 4.5 6.1 6.8 7.6 7.6 12.2
55° 4.2 5.7 5.9 4.1 5.5 6.2 6.9 6.9 11.1
60° 3.8 5.2 5.4 3.7 5.1 5.7 6.3 6.3 10.1
65° 3.5 4.8 5 3.4 4.7 5.3 5.8 5.8 9.4
70° 3.3 4.4 4.7 3.2 4.3 4.9 5.4 5.4 8.7
75° 3.1 4.1 4.3 3 4.1 4.6 5 5 8.1
80° 2.9 3.9 4.1 2.8 3.8 4.3 4.7 4.7 7.6
85° 2.7 3.7 3.8 2.6 3.6 4 4.4 4.4 7.2
90° 2.5 3.5 3.6 2.5 3.4 3.8 4.2 4.2 6.8
95° 2.4 3.3 3.4 2.3 3.2 3.6 4 4 6.4
100° 2.3 3.1 3.3 2.2 3 3.4 3.8 3.8 6.1

First off, determine the temperature of the ground water of the region where the water heater will be installed. This is really important because it requires more power to make 52F ground water hot enough for a shower than it does to heat 62F to the same shower temperature.

Next, add up the flow rates from taps that must run simultaneously. This does not mean to sum up the flow rate for every hot water tap in the house. Most households do not need to run the hot water from all the sources at the same time. The easiest way to determine the flow rate is to add how many showers are taken at the same time. But if there is a constant need to use one of the fixtures while someone is in the shower, then you will certainly add this up to get the maximum flow rate you need.

The best way to measure incoming ground water temperature is to use a thermometer to measure cold water temperature during the coldest season of the year. To simplify the process, use this map to determine whether your installation location is in the Southern Zone, Central Zone or Northern Zone.

Once you have the maximum flow rate for your house, check out the sizing chart to see if the model you are considering can supply that flow rate with the ground water where you are.

Let’s assume that the incoming water temperature is 55 degrees, and, for most users, you’ll want your water heated to around 110°F. In this example, you’d need an on-demand water heater that produces a temperature rise of 55°F at your required flow.

For example, in Nashville, Tennessee, the groundwater is 62F, so the temperature at 24 will supply a hot water flow rate of 3.8 GPM. That is enough to simultaneously run two showers and a lavatory sink or a kitchen sink and a shower.

A tankless water heater is given BTU (British Thermal Unit) input and efficiency ratings. These determine its flow rate, expressed in gallons per minute (GPM).

One BTU is equal to the amount of energy required to raise the temperature of 1 pound of water 1 degree F. The higher a water heater’s BTU rating, the higher the water heater’s flow rate.

Under normal circumstances, it takes about 31,000 BTUs to deliver 1.2 GPM or 190,000 BTUs to deliver 5.7 GPM of hot water. If you live in a cold climate, where the incoming water is very cold, the unit will require more heat to bring the water up to temperature.

Flow rates vary from about 1.2 to 6 GPM. Point-of-use models, such as those that provide hot water to a single bathroom sink, are rated 1.2 GPM. A 2.6-GPM tankless water heater will handle one shower at a time. A 4-GPM tankless water heater one shower and one sink, and a 6-GPM two showers.

Homeowners who are concerned about energy savings also need to pay attention to a unit’s efficiency ratings. These ratings, which range from about 78 percent to the best tankless water heaters at 87 percent. They indicate how much of the fuel is converted to heat by the appliance—the higher the better.


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