What size water heater do I need?

by Joyce

I am thinking between tank water heater and tankless unit. But with an abundance of info online I am confused.

Please suggest how to select the right water heater. I don't know how to size a water heater. Is it the tank capacity, water flow, or efficiency that will determine the appropriate unit size?

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Jan 14, 2020
Sizing a tankless water heater
by: PacificTankless.com

Old but still relevant question. What to take into consideration when choosing a water heater. For a tank water heater that would be the capacity, for a tankless water heater that would be the flow. The efficiency of the water heater will determine how much energy you will spend to heat the water. You can calculate the price of the unit, installation, spent energy, etc., but the super basic rule, aside from habits, is for tank water heaters you account for the number of residents, for tankless water heaters the number of full bathrooms. A regular shower head flow is about 2 gal per minute. Depending on the area, for a 2 bathroom house, 6.5 to 8 gpm should be more than sufficient.

May 17, 2010
How to size a water heater
by: Shridar

As you probably know, the first thing what you should do is to answer the following questions:

- What is the size of your family and for how many application you will use hot water? Determine is your family low demand or high demand.

- Do you have teenagers living with you as they use more hot water for shower and cloth washing?

- Do you have a whirlpool bath or tub? If the capacity of your tab is 75 gallons for example, your water heater should provide enough hot water which is 100 % or more of the tub capacity.

Here is the recommendation when sizing the tank type water heater; the heater's output requirement relative to the family size:

For 2-people family size you are looking at 45-55 gallon size water heater.

For 4 people you will need from 65 to 75 gallons water heater.

For the family size of over 6 people you need 85-gallon tank size and up.

Keep in mind that the size of the storage tank is not the only criteria when buying the water heater. On the EnergyGuide label you can find another, more important figure which is the first-hour rating. Estimate your household peak hour demand and find the water heater with the first-hour rating close to that number.

If you are looking for the gas water heater, first-hour rating is higher that electric, so what can happened, you can meet your needs with a unit with the smaller storage tank than an electric.

Next criteria you will use to determine what is the right water heater for you is the EF, energy factor. With the higher EF, water heater is more efficient.

And finally, the cost. What is the price for a new water heater, installation cost, maintenance cost and operating costs.

To size a tankless water heater you have to determine the flow rate and the temperature rise. Temperature rise is the difference between outgoing and incoming water temperature.

As you can see when sizing a water heater, you should not select and buy a water heater solely based on the capacity of its tank or flow rate.

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