Ground Water Temperature and Higher Demand Temperatures

Architects who design sustainable buildings know that climate and environment can strongly influence design decisions. This is true for specifying tankless hot water systems. These units heat water from the local water source to a required hot water temperature and the difference between these temperatures influences the size of a unit specified by the design professional. The average ground water temperature varies by climate. Colder climates have colder ground water than a warmer climate. The U.S. Geological Survey publishes ground water temperature data and most manufacturers will provide online or phone support to assist the architect or builder who will need to know the ground water temperature of the building site, before choosing the right tankless hot water heater.

 

Actual usage conditions may require the next size unit or multiple units to meet demand. Colder ground water temperatures or higher set point temperatures may require higher gallons per minute (gpm) capacity or multiple units to properly meet household hot water requirements. Photo courtesy of GE Appliances

 

 

If there are multiple demand fixtures in a residence and if a higher set point temperature is desired, the design will also require more GPM capacity to properly meet household hot water requirements. The common residential temperature average set point is 120 °F. Tankless hot water heaters are the most efficient when there is no greater than 30 degrees differential required for heating water.

Manufacturers will assist the designer with determining the right tankless gas water heater for the project, climate, and design temperature as shown in the chart below.

Cascading Demand

The most common concern by users is whether the volume of water can meet the demand, and if the home user will have enough flow for simultaneous hot water needs such as showers, laundry or running the dishwasher. A typical showerhead uses about 2.5 gallons per minute of total water, hot and cold mixed. Depending on the time of year, the mix could be 50 percent hot and 50 percent cold, making the hot water need about 1.25-1.5 GPM for each shower. This means a home with three showers could need about 3.75-4.75 gallons per minute of hot water, if all showers are used simultaneously. Proper sizing of a tankless heater will match the GPM to the flow rate and the ground water temperature and account for even large temperature differences of as much as 77 degrees.

 

This whole house unit is mounted on an exterior wall and landscaping should be designed to shield the view of the unit, while providing adequate clearance for maintenance. Photo courtesy of GE Appliances

 

By sizing the tankless hot water heater by the number of shower heads and the number of bathrooms, a tankless hot water heater provides a continuous flow of water to as many fixtures, appliances and shower heads as required. Units are designed in tandem to cascade and fire additional heating burners as designed by the professional for each project type. When the unit senses a water flow of about ? gallon, burners in the first demand sequence fire to provide hot water, engaging only a portion of the 180,000 BTU's available for heating. A continuous supply of hot water is available to the homeowner as they turn on the dishwasher, start the laundry, or use additional showers, engaging additional burners to heat the additional demand for hot water. Units can also be clustered to fire even more heating burners as needed for larger homes or commercial installations.