water heater sizing

water heater comparison

A properly sized water heater will meet your household's hot water needs while operating more efficiently. Therefore, before purchasing a water heater, make sure it's the correct size.


Sizing a demand (tankless or instantaneous) water heater

Demand water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you'll need for its application (whole house or a remote application, such as just a bathroom) in your home.


First, list the number of hot water devices you expect to use at any one time. Then, add up their flow rates (gallons per minute). This is the desired flow rate you'll want for the demand water heater. For example, let's say you expect to simultaneously run a hot water faucet with a flow rate of 0.75 gallons (2.84 liters) per minute and a shower head with a flow rate of 2.5 gallons (9.46 liters) per minute. The flow rate through the demand water heater would need to be at least 3.25 gallons (12.3 liters) per minute. To reduce flow rates, install low-flow water fixtures.


To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50°F (10°C). For most uses, you'll want your water heated to 120°F (49°C). In this example, you'd need a demand water heater that produces a temperature rise of 70°F (39°C) for most uses. For dishwashers without internal heaters and other such applications, you might want your water heated at 140°F (60°C). In that case, you'll need a temperature rise of 90°F.


Most demand water heaters are rated for a variety of inlet temperatures. Typically, a 70°F (39°C) water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet. Some types of tankless water heaters are thermostatically controlled; they can vary their output temperature according to the water flow rate and inlet temperature.


Sizing a solar water heating system

Sizing your solar water heating system basically involves determining the total collector area and the storage volume you'll need to meet 90–100% of your household's hot water needs during the summer. Solar system contractors use worksheets and computer programs to help determine system requirements and collector sizing.


Collector area

Contractors usually follow a guideline of around 20 square feet (2 square meters) of collector area for each of the first two family members. For every additional person, add 8 square feet (0.7 square meters) if you live in the U.S. Sun Belt area or 12–14 square feet if you live in the northern United States.


Storage volume

A small (50- to 60-gallon) storage tank is usually sufficient for one to two three people. A medium (80-gallon) storage tank works well for three to four people. A large tank is appropriate for four to six people.


For active systems, the size of the solar storage tank increases with the size of the collector – typically 1.5 gallons per square foot of collector. This helps prevent the system from overheating when the demand for hot water is low. In very warm, sunny climates, some experts suggest that the ratio should be increased to as much as 2 gallons of storage to 1 square foot of collector area.


Other calculations

Additional calculations involved in sizing your solar water heating system will include the following:


  • evaluation of your building site's solar resource
  • orientation and tilt of the solar collector

    Sizing storage and heat pump (with tank) water heaters

    To properly size a storage water heater – including a heat pump water heater with a tank – for your home, use the water heater's first hour rating (FHR). The first hour rating is the amount of hot water in gallons the heater can supply per hour (starting with a tank full of hot water). It depends on the tank capacity, source of heat (burner or element), and the size of the burner or element.


    The EnergyGuide Label lists the first hour rating in the top left corner as "Capacity (first hour rating)." The Federal Trade Commission requires an EnergyGuide Label on all new conventional storage water heaters but not on heat pump water heaters. Product literature from a manufacturer may also provide the first hour rating. Look for water heater models with a first hour rating that matches within 1 or 2 gallons of your peak hour demand – the daily peak 1-hour hot water demand for your home.


    Do the following to estimate your peak hour demand:


  • Determine what time of day (morning, noon, evening) you use the most hot water in your home. Keep in mind the number of people living in your home.

  • Use the worksheet below to estimate your maximum usage of hot water during this one hour of the day – this is your peak hour demand. Note: the worksheet does not estimate total daily hot water usage.

    The worksheet example shows a total peak hour demand of 70 gallons. Therefore, this household would need a water heater model with a first hour rating of 68 to 72 gallons.


    Worksheet for estimating peak hour demand/first hour rating
    Use Average gallons of hot water per usage   Times used during 1 hour   Gallons used in 1 hour
    Shower 20 ×   =  
    Bath 20 ×   =  
    Shaving 2 ×   =  
    Hands & face washing 4 ×   =  
    Hair shampoo 4 ×   =  
    Hand dishwashing 4 ×   =  
    Automatic dishwasher 14 ×   =  
    Food preparation 5 ×   =  
    Wringer clothes washer 26 ×   =  
    Automatic clothes washer 32 ×   =  
          Total peak hour demand =  


    3 showers 20 × 3 = 60
    1 shave 2 × 1 = 2
    1 shampoo 4 × 1 = 4
    1 hand dishwashing 4 × 1 = 4
    Peak hour demand       = 70


    Source: Gas Appliance Manufacturer's Association
    *The above worksheet assumes no water conservation measures.


    For sizing combination water and space heating systems – including some heat pump systems, and tankless coil and indirect water heaters – consult a qualified contractor. Don't choose a water heater model based solely on its size.


    You'll also need to consider the following:


  • fuel type and availability
  • energy efficiency
  • costs