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A tankless water heater is a wall-mounted unit that heats water directly, without storing it in a tank.
After meticulous selection, if a tankless water heater is installed and maintained properly, it can meet the hot water needs of a house and save a lot of energy and money.
One additional criterion that also decides the efficient operation of a tankless water heater is the size of the heater. Choosing the right size is as important a selecting the right model of a tankless water heater.
How to Size a Tankless Water Heater
The capacity or flow rate (measured in gallons per minute) is decided by the size of the tankless water heater that is being installed.
Another factor that needs to be considered is the temperature rise required for the water flowing through the unit.
The flow rate and temperature rise are the two parameters that decide the size of the tankless water heater. Let us take a look at the definition of these parameters and how they are affected by the size of the heater.
1. Flow rate
Flow rate is defined as the amount of water flowing through a pipe per minute.
It is measured in gpm (gallons per minute).
To determine whether the tankless water heater will be able to meet the flow rate demands of the house, measure the individual flow rates of all the hot water fixtures that may be used simultaneously.
Now add these flow rates and the resultant flow rate is the one you want for the tankless water heater. Water fixtures with low flow rates can be installed to reduce the total flow rate.
For example, consider you have a hot water faucet with a flow rate of 0.75 gallons and a shower head with a flow rate of 2.6 gallons that are regularly used.
In order to operate these two fixtures at the same time, your tankless water heater must have an output flow rate of at least 3.26 gallons per minute.
2. Temperature rise
After calculating the flow rate, the next procedure is to determine the temperature rise.
If the temperature of the incoming water is not known, assume it as 50ºF. For most applications, the desired water temperature will be 120ºF.
This means that your tankless water heater should be able to produce a temperature rise of 70ºF.
At regions with warmer climates, the tankless water heater may not necessarily have to give out water at 120ºF.
Also, the temperature of the incoming water may be higher than 50ºF. In such cases, the tankless water heater need only produce a temperature rise of about 50ºF.
Let us consider an example of having to run two hot water shower heads simultaneously. On average, showerheads are run at about 106ºF. Their flow rates are usually around 2.6 gallons per minute.
In order to run two shower heads with these requirements at the same time, the tankless water heater should be able to create a temperature rise of at least 50ºF, considering the incoming water temperature is 50ºF.
Apart from a 50ºF rise in temperature, the tankless water should also be able to give out hot water at the rate of 5.2 gallons per minute.
3. Sizing pipelines
Other than the size of the water heater, the sizing of the plumbing lines is also critical for the delivery of hot water to the necessary fixtures.
The length and diameter of each pipe connecting the water heater to the individual points of use must be carefully sized so that only the needed amount of water is heated.
For example, for a 50 ft. long pipe whose diameter is ½ in., if it held 0.46 gallons of water at 1gpm, it would take 28 seconds to push all the cold water out when the tap is turned on.
However, if the same 50 ft. long pipe was only 3/8 in. in diameter, it would hold only 0.25 gallons of water. At 1gpm, it would only take 15 seconds to push the cold water out and the hot water to come out through the tap.
4. Gas vs. Electric Tankless Water Heater
Before purchasing a tankless water heater, it is important to note that a gas tankless water heater is capable of producing a higher temperature rise than an electric type, for the same output flow rate.
For example, a typical gas tankless water heater can output 5 gallons of water per minute for a temperature rise of 70ºF. An electric tankless water heater, for the same temperature rise, can give out only 2 gallons of water every one minute.
Tankless water heaters with different temperature rises for the same flow rates are available.
Purchasing a heater with the appropriate flow rate and temperature rise is essential for meeting the hot water needs of a home.
At places with lower temperatures, a tankless water heater with high rating should be installed. If the size of the heater is too small, it will not be able to meet the hot water demands of the house.
5. Other Attributes
Sizing a tankless water heater carefully alone does not ensure hot water as required by the household. There are two other factors that need to be considered along with the temperature rise and flow rate of a heater.
- Inlet temperature
- The distance of the hot water tap
The inlet temperature of the source water and the distance of the hot water tap from the heater also contribute to the temperature of the water at the faucet.
If the inlet water is too cold, the temperature of the water at the most distant faucet may be lower than the desired temperature.
For a tankless water heater that is meant for an entire house, try to install it at the most central location, so that its distance from the hot water faucets is not too far.
Calculating the right output flow rate and temperature rise are the two basic steps in sizing a tankless water heater.
Installing a heater that satisfies these two factors is essential if you do not want to run out of hot water in the house.
It is important to avoid buying a smaller tankless water heater in order to save money.
You may end with a heater that does not meet the hot water requirements of your house. Sizing a tankless water heater appropriately may be initially expensive but it will help you save money on utility bills.