When choosing a tankless water heater for a family of 4, there are several factors to consider. The size of the unit will depend on the peak demand hot water needs of the household, which is typically determined by the number of bathrooms, fixtures, and appliances that use hot water.
A good rule of thumb is to choose a unit with a flow rate of at least 7.5 gallons per minute (GPM) for a family of 4. This will ensure that you have enough hot water to meet the peak demand of multiple showers, laundry, and other appliances running simultaneously.
However, it is important to note that the flow rate can vary depending on the temperature rise required. For example, if you live in a colder climate, you may need a higher flow rate to achieve the same temperature rise as someone living in a warmer climate.
It is recommended to consult with a licensed plumber or HVAC contractor to determine the appropriate size and type of tankless water heater for your specific needs and household habits. They can help you choose the most efficient and cost-effective option for your home.