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Size Your Water Heater - Atlas Plumbing Your Dallas PlumberA general rule of thumb for most homes says a properly sized water heater will meet your household's hot water needs while operating more efficiently.

That said, you really need to evaluate that demand before purchasing a water heater, to make sure it's the correct size. For a commercial or institutional facility like a school, hotel, kitchen or office complex please consider there are many more factors that may determine the type of water heating solution you need in a given situation.

Atlas Plumbing, your favorite Dallas Plumber, is here to make sure that you have all the information you need to accurately determine what is the right size water heater for your home or business, institution or commercial facility. Save Money and More with ENERGY STAR Qualified Whole-Home Gas Tankless Water Heaters as your first consideration.

Here in the Dallas market, sizing combination, water and space heating systems -- including some heat pump systems, and tankless coil and indirect water heaters all matter. Remember that Atlas can help you by setting an appointment to consult a qualified contractor keeping specifications in mind.

Discussion Points:

  • Tankless or demand-type water heaters
  • Solar water heating system
  • Storage and heat pump (with tank) water heaters

Sizing Tankless & Demand-Type Water Heaters

Tankless or demand-type water heaters are usually rated by the maximum temperature rise possible at a given flow rate. When you need to size a demand water heater, flow rate is must be determined. Next the temperature rise you will need for it's application. That information would include the whole house, office, or commercial location like the bathroom in the home.

To start, let's make a list of the number of hot water devices you expect to use at any one time. Now add up their flow rates (gallons per minute). The result will be the desired flow rate you'll want for the demand water heater. Here is an example:

  • If the expectation is 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, then the flow rate through the demand water heater would need to be at least 3.25 gallons (12.3 liters) per minute. If you need to reduce flow rates, you can install all low-flow water fixtures.
  • Next in order to determine temperature rise, subtract the incoming water temperature from the desired output temperature. If this is unknown, let's assume that the incoming water temperature is 50ºF (10ºC). In most cases, standard water heated to 120ºF (49ºC) is normal. In this example, you'd need a demand water heater that produces a temperature rise of 70ºF (39ºC) for most uses. An exception might be in this case for a dishwasher without internal heaters and other such applications, that water may need to be heated to 140ºF (60ºC). In which case, a temperature rise of 90ºF (50ºC) will be needed.
  • Most demand water heaters are rated for a variety of inlet temperatures coming into the tank. Typically, a 70ºF (39ºC) 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. A faster flow rate or cooler inlet temperature might 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 if they need to.

Sizing a Solar Water Heating System

The proper sizing your solar water heating system basically involves several things. First to determine 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. Most typical Solar system contractors use worksheets and computer programs to help determine system requirements and collector sizing. If you need specifics about your system, just give Atlas Plumbing a call and we can discuss your options.

Collector Area

General contractors can usually follow a guideline of around 20 square feet (2 square meters) of collector area for each of the first two family members. Additions need to be made for more persons.  By adding 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, will usually be recommended.

Storage Volume

Here are some general guidelines. 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.

In a very active system, the size of the solar storage tank needs to increase with the size of the collector. This is typically around 1.5 gallons per square foot of collector. Using this guideline, prevention of the system from overheating takes place when 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. Hot summers in Texas may need an adjustment, but each system may vary, so let our qualified staff here at Atlas examine the correct options here, and make recommendation.

Other Calculations

Additional calculations involved in sizing your solar water heating system may include evaluating your building site's solar resource and determining the proper orientation and tilt of the solar collector. Each system may vary, along with current City Codes in your area. Atlas can visit the manufacturer recommendations on the solar water heaters specs page for more on these calculations.

Sizing Storage and Heat Pump (with Tank) Water Heaters

Another must is to properly size a storage water heater for your home. This will include a heat pump water heater with a tank use that has the water heater's first hour rating. The first hour rating is the number of gallons of hot water the heater can supply per hour (starting with a tank full of hot water). Be mindful this totally depends on the tank capacity, source of heat (burner or element), and the size of the burner or element. All three must be considered.

The EnergyGuide label provided on the tank by the manufacturer lists the first hour rating in the top left corner as "Capacity (first hour rating)." The Federal Trade Commission requires that this EnergyGuide label must be 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.

How To Estimate Your Peak Hour Demand:

Several factors play a part here. First, determine what time of day (morning, noon, evening) you use the most hot water in your home. For most it's in the morning getting ready for work or school for the kids. Remember to keep in mind total numbers of people that occupy the home every day.

A guideline worksheet can estimate your maximum usage of hot water during this one hour of the day—this is your peak hour demand. Note: usually this worksheet does not estimate total daily hot water usage. Find the link here:

The worksheet example shows a total peak hour demand of 36 gallons. Therefore, this household would need a water heater model with a first hour rating of 34 to 38 gallons.worksheet for Estimating Peak Hour Demand/First Hour Rating *

Use: Average gallons of hot water per usage Times used during 1 hour Equals Gallons used in 1 hour

  • Shower     10     ×           =
  • Shaving (.05 gallon per minute) 2 × =
  • Hand dishwashing or food prep (2 gallons per minute) 4   × =
  • Automatic dishwasher 6    × =
  • Clothes washer  7 × =   Total Peak Hour Demand     =      


  • 3 showers     10     ×     3     =     30
  • 1 shave     2     ×     1     =     2
  • 1 hand dishwashing     4     ×     1     =     4
  • Peak Hour Demand                       =     36

Adapted from information from the Federal Energy Management Program Energy Cost Calculator see the link above *The above worksheet is based on standard usage with no water conservation measures.

Check it out today, call us here at Atlas Plumbing Your Dallas Tankless Water Heater specialist (972) 243-8051 and set up an appointment to speak with our qualified staff about your commercial location for a new Commercial Tankless Water Heater.

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