Hot Water on Demand: Natural Gas Tankless Hot Water Heaters Fit your Energy Budget

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Sponsored by GE Appliances
Celeste Novak, AIA, LEED AP

Ventilation

Tankless gas water heaters come in two types. Outdoor models can be hung directly on the outside of a house and self vented with no additional ventilation required. Indoor models require venting and placement adjacent to an exterior wall. Venting must be 36 inches above grade and use the manufacturer's specified pipe. Many utilize direct vent, concentric pipe so only one vent hole is needed. Exhaust gases pass through the center of the pipe while the air needed for combustion is pulled in from the outer ring. This method does not require the use of "make up" air from the homes ventilation system.

Venting should be as direct as possible and for maximum efficiency, ventilation runs should be no longer than 21continuous feet. Each elbow reduces the length of the ventilation piping. For example, a 90 degree elbow equals six feet of ventilation run and a 45 degree elbow equals three feet of ventilation run. Ventilation piping can go as long as 41 feet but will longer runs will lessen the efficiency of the heater.

Designers should consider the following intake/exhaust guidelines for ventilation:

  • Do not combine vent components from different manufacturers' since the tankless hot water heater is certified and listed with the vent system.
  • The vent system must vent directly to the outside of the building and use outside air for combustion.
  • Every vent connection must be accessible for inspection, cleaning and replacement.
  • Support horizontal vent runs every four feet and support all vertical vent runs every six feet or in accordance to local codes.
  • Do not reduce the vent diameter.
  • Venting should be as direct as possible avoiding long runs and a minimum number of pipe fittings.
  • Do not connect the venting system with an existing vent or chimney or any other vent pipe of other appliances or water heaters.
  • Provide an inlet screen to minimize blockage from debris.

 

Installed in an unheated attic space, the professional will include an
automatic emergency drain valve in case of power failures.

Photo courtesy of GE Appliances

 

 

Because a concentric, direct vent system is used, condensation will occur. Some manufacturers include a condensate collar and drain attached to the top of the tankless heater where the vent pipe will be attached. A flexible tube with a loop will hang from the heater and drop into either a floor drain or stand pipe provided by the plumber.

Proper placement of the ventilation piping is a key to the location and installation of tankless hot water heaters. In new installations, the designer will locate the heater along an outside wall or near a cluster of appliances to reduce plumbing runs as well as the delivery time for the hot water. However, when used in renovations, the designer must consider the plumbing already in the house to determine where the unit should be placed. Should the designer locate the unit where the original hot water storage tank was installed? Will this reduce the amount of plumbing required to add the unit? How close can the unit be placed near an exterior wall in order to reduce ventilation piping? Is it better to provide multiple units for several areas of a home? Can long plumbing runs be avoided by separating baths on the second floor of a residence from those on the first floor and by providing separate units to those areas? Professionals should also take into consideration any local codes which apply to installations in different parts of a residence.

Efficient Delivery - System Comparisons

The basic components of a traditional hot water storage system are a burner, a thermostat and a storage tank that heats water to a constant temperature and which is attached to plumbing connected to the appliance or faucet. Cold water replaces hot water as needed and the entire tank is heated to an average temperature - between 120 °F to 140 °F. The higher temperature is recommended for optimal dishwasher use and laundry use and to prevent bacterial growth in the tank. For use in a shower this hot water is tempered by cold at the faucet in the ratio of one part hot water to four parts cold water. In a hot water storage tank, as hot water is drawn out by the user, at a typical flow rate of 2.5 gallons/minute, approximately 40 percent of the tank is filled with cold water which must be over-heated to maintain an average temperature in the hot water storage tank. The tank must be constantly heated to maintain an even temperature. Since the tank holds both cold and hot water, this temperature in the tank must be set to a higher rating than required at the faucet.

As a water storage tank ages, given that 85 percent of the United States has hard water, mineral deposits can reduce the area of the tank used for water storage. In fact, an older tank may not be able to supply the amount of hot water as originally designed as its capacity is reduced by deposits.

A natural gas tankless water heater system is very different in its construction and ability to deliver hot water. Only the water used is heated, and to the exact temperature requested - no water is stored and the heating burners are off when not in use. The components of a tankless hot water system are a powerful burner, a blower for exhaust and sophisticated electronics to ensure it delivers a continous flow of hot water.to existing or new plumbing. A tankless water heater system can provide water for the whole house with a single unit or the professional can cascade a couple of units together to prevent flow rates from diminshing. They can even be zoned placing two units in different locations to take advantage of shorter plumbing lines. Tankless hot water heaters can provide hot water simultaneously to many faucets, fixtures and appliances because multiple units can be cascaded to prevent flow rates from diminishing. Demand-activated technology turns the heating system on when a hot-water tap is opened and off when the tap is closed.

Tankless hot water heaters are not instant hot water, and the time the hot water takes to reach the faucet can be similar to the time it would take for hot water to travel from a traditional storage system. The location of the units can reduce the amount of cold water that is run through the plumbing lines before hot water is delivered to the source. Most fixtures drain cold water while waiting for hot water to be delivered to the source. To save water, tankless hot water heaters can be placed close to the demand for the hot water, near tubs, showers, laundry areas and kitchens because they can be located in small areas throughout the residence.

 

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Originally published in Architectural Record
Originally published in September 2008

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