Gas Water Heating

DEFINITION
CONSIDERATIONS
COMMERCIAL STATUS
IMPLEMENTATION ISSUES
GUIDELINES

    1) System Selection for Combination Systems
    2) Water Heaters


CSI NUMBER

153 110 6000
155 600

DEFINITION:

Gas water heating for domestic hot water can be engineered to provide space heating as well as water heating. Known as a combination( or combo) system, a single appliance can perform two functions.


CONSIDERATIONS:

There are several positive features to a combination water heating/space heating (combo) system. Several high efficiency models are available that allow venting of the water heaters through a wall instead of a chimney. Electric heating is not involved permitting a reduced electric panel size. Less space is needed compared to two separate systems. The air handler can be located to minimize ductwork.

The combo system circulates hot water from the water heater through a heat exchanger in the air handler. A blower will move the heated air through a standard duct system. In the summer, an air conditioner is connected to the exchanger and the system functions similarly, with cool air being pushed through the ductwork.

The use of gas water heaters is generally considered a positive (although not perfect) environmentally-based choice. The pollutant levels created by natural gas combustion are less than other conventional water heating options except solar.

Commercial
Status
Implementation
Issues
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Gas Water Heating Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory
Gas Space/Water Heating Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory
Satisfactory Satisfactory
Satisfactory in most conditions Satisfactory in most conditions
Satisfactory in Limited Conditions Satisfactory in Limited Conditions
Unsatisfactory or Difficult Unsatisfactory or Difficult


COMMERCIAL STATUS

TECHNOLOGY:

Well-developed.

SUPPLIERS:

Plentiful.

COST:

Competitive with other conventional options such as heat pumps and electric water heaters. Rebates are also available from the City of Austin for the installation of these systems.


IMPLEMENTATION ISSUES

FINANCING:

None.

PUBLIC ACCEPTANCE:

Generally accepted; low operating costs desirable; not well-known.

REGULATORY:

Must meet applicable codes.


GUIDELINES

1.0 System Selection for Combination Systems

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

    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.

    Unit Y – 85% recovery efficiency

    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

There are two primary choices in water heaters – demand heaters and storage units.

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.