Gas Water Heating

• The selection of a combination system is based on sizing the combination system to the heat demand of the house, according to the following method:
  • A Manual J (ACCA) calculation, or comparable analysis, is conducted to determine the design heat loss of the building.

  • An air handler and its coil is selected to have a capacity that is equal to or greater than the design heat loss.

  • The water heater output must equal or exceed the rated output of the coil and air handler.

  • Water heaters are rated by their input (in BTUH). Thus, the water heater is selected by dividing the air handler/coil heating capacity by the recovery efficiency of the water heater. To qualify for a rebate from the City of Austin, the minimum recovery efficiency must be 76%. To qualify for a rating in the Green Builder Program, the minimum recovery efficiency is 80% and the Energy Factor must be 0.60 or greater (Recovery Efficiencies and Energy Factors are found in the GAMA directory listed in the Resources Section).

  • The minimum tank size for the system should be 40 gallons.

  • The design heat gain for the building is determined using Manual J.

  • The cooling coil capacity and air conditioner must meet or exceed the design heat gain.

  • Unless special hot water demand such as a Jacuzzi exist in a home, the water heater sized to meet the space heating demand will meet the domestic hot water needs of the household. No extra tank or heat capacity is needed.

  Example: The design heat loss for a house is 26,000 BTUH and the design heat gain is 23,000 BTUH.

  1. The design cooling load requires 2 tons of cooling or 24,000 BTUH.

  2. An air handler is selected that has 30,000 BTUH heating capacity and 24,000 BTUH cooling capacity. (The air handler heating capacity appropriately exceeds the design heat loss.)

  3. Two water heater types are considered:

     Unit X - 76% recovery efficiency

     Unit Y - 85% recovery efficiency

     • Divide the air handler heating capacity (30,000 BTUH) by 0.76 (76%) to get 39,474 BTUH as the required rated input of a water heater from the Unit X series.

     • Divide the air handler heating capacity (30,000 BTUH) by 0.85 (85%) to get 35,294 BTUH as the required rated input of a water heater from the Unit Y series.

  4. Review the models offered in the two series under consideration and select a model that meets or exceeds the required rated inputs calculated in Step C.

2.0 Water Heaters

• 2.1 Demand (or instantaneous) heater.
  • Does not store hot water.

  • Designed to heat water on demand near the point of use.

  • Unit is useful if space is limited for installing a storage tank unit.

  • May not be able to serve simultaneous multiple draws of hot water.

  • A typical medium size unit will raise the incoming water temperatures 60 degrees at a rate of 3.5 gallons per minute (125,000 BTU input).

  • Some units can sense the incoming water temperature and if the incoming water is already adequately heated will not turn on. This is useful for solar interconnections or circulating systems.

  • Demand heaters require a minimum of 15 to 20 PSI, are most often wall mounted, and use vertical venting. Limited horizontal venting can be used.

• 2.2 Storage tank water heating units.
  • Most common.

  • Less costly than demand units.

  • The highest efficiency storage tank units will show recovery efficiencies of 85% and Energy Factors as high as 0.72 (reference GAMA directory - see Resources).

  • The high efficiency units will compare favorably in energy savings to the demand units when the initial costs of the units are taken into account.

  • All gas combustion units require meeting basic safety installation requirements.

See "Heating Contractors" in Yellow Pages

see "Heating Contractors" in Yellow Pages

Real Goods Trading Company
966 Mazzoni St.
Ukiah, CA 95482
(800) 762-7325
demand heaters

City of Austin Environmental and Conservation Services Department Gas Efficiency Coordinator
206 E. 9th St., Suite 17.102
Austin, TX 78701
(512) 499-3500

Southern Union Gas Co.
Marketing Dept.
504 Lavaca St. , 8th Floor
Austin, TX 78701
(512) 477-5981

Publication:

"Consumers' Directory of Certified Efficiency Rating for Residential Heating and Water Heating Equipment"
GAMA Efficiency Certification Program
ETL Testing Laboratories, Inc.
Industrial Park, Route 11
Cortland, NY 13045

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This file last updated on Friday, August 4, 2006