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Here we discuss the operating characteristics of tankless water heaters, also called instant water heaters or "on demand" water heaters. Readers should also see ALTERNATIVE HOT WATER SOURCES to distinguish among these types of hot water producing systems and see Electric Shower Heaters for a discussion of point of use heaters at shower heads.

The original text of this article was provided courtesy of Carson Dunlop - that text has been edited and may not entirely reflect CD's views. Page top sketch of a point of use tankless water heater courtesy of Carson Dunlop. Our OPINION is that readers looking for plenty of hot water heated efficiently should also consider the heater we describe at Indirect-fired Water Heaters.

© Copyright 2009 Daniel Friedman & Carson Dunlop Associates, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

A Guide to Tankless Water Heaters

A tankless water heater saves energy by turning off completely when no hot water is being drawn - there are no standby losses - no tank of hot water needs to be kept hot when hot water is not being used.

Although some instantaneous water heaters can provide up to 9 GPM (and use 200,000 BTUH), smaller units cannot provide very large hot water flow rates in gallons per minute.If the hot water usage rate is expected to be high, a very high BTUH unit (efficient but not necessarily economical) or multiple smaller point of use tankless water heaters may be needed to satisfy all building occupants and uses.

Consumer Reports points out that a tankless water heater may be efficient but not necessarily economical to operate. What they mean is that it is certainly efficient to avoid heating water when no one is using it. But if water usage is high, a high-BTU tankless water heater may consume more total energy than a large but more conventional oil or gas fired hot water tank and burner.

Tankless Water Heater Operation & Characteristics

As the name suggests, tankless water heaters have no tank, and therefore no storage capacity at all. Tankless heaters include a burner, heat exchanger, venting system, and the necessary controls. When the faucets and fixtures in the home are idle, the water heater is dormant.

When a hot water faucet is opened or the dishwasher or clothes washing machine calls for hot water, the tankless heater detects the water flow and ignites the burners. These powerful burners quickly heat the water inside the small diameter heat exchanger. As hot water is drawn out, fresh cold water is drawn in and the cycle continues. A significant advantage of this system is that you can't empty all of the hot water out of the tank because there is no tank - just continuous hot water.

No Stored Water

The other major advantage over conventional water heaters is energy savings. Tankless water heaters have no large reservoir of water that has to be kept hot around the clock in case hot water is needed. One side effect of the lack of storage is that all of the water in the system is cold when the system has not been in operation recently. When a hot water faucet is operated, it may take more time for hot water to be delivered than in a conventional system.

Small Size

Tankless water heaters are much smaller than conventional waters with storage tanks, and are usually wall-mounted. However the burner on a tankless or demand water heater must be much larger than on a conventional heater since it has to instantly heat incoming cold water to its target hot temperature.

Tankless Water Heater Fuel and Venting

Most tankless water heaters are either electric, or they are fueled by natural gas or propane and are vented through a side wall of the house.

As Rheem points out, "The venting must be Category III, Stainless steel, and it applies to the adapter, the vent pipe, all elbows, and terminal vent. This is important because the flue gases and by-products of combustion can cause condensation. Stainless steel will not deteriorate like type B venting under these conditions."

Tankless Water Heater Efficiency

Tankless water heaters are often much more efficient than conventional water heaters, using modulating burners, direct venting and/or condensing combustion systems.

Mixing Valve use on Tankless Water Heaters

Most systems include a mixing (tempering) valve and a means of setting a maximum water temperature to avoid scalding. This tempering valve mixes some cold water with the hot water leaving the unit to reduce the temperature.

In one sense, there should be no need for a tempering valve, since we do not need to keep the reservoir water at 140° to prevent Legionnaires Disease. On the other hand, since we don't know the flow rate of the water through the heater, the tempering valve may be needed since the burner cannot modulate to deliver exactly the temperature we want due to the infinitely variable water flow.

If the flow rate is low, the water moves more slowly through the heater, and picks up more heat from the burner.

Tankless Water Heater Remote Control

Some tankless systems include a remote control, which can be used to monitor the performance of the system, display error codes or change the desired water temperature.

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ALTERNATIVE HOT WATER SOURCES
  Electric Shower Heaters
  High Efficiency Water Heaters
  Indirect-fired Water Heaters
  Instantaneous Water Heaters
  Multiple water heaters in parallel
  Multiple water heaters in series
  Range Boiler Water Heaters
  Side Arm Coil Water Heaters
  Solar Water Heaters
  Tankless Coil for Hot Water
  Tankless Water Heaters
    Guide to Tankless Water Heaters
    Operation & Characteristics
    Problems & Defects
    Hot Water Delivery Speedup
    Installation Requirements & Costs
    Should I Convert to a Tankless Heater?
    Calculate Water Heater Requirements
    Capacities of Tankless Water Heaters
    Life Expectancy & Applications

• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
  
  
• Consumer Reports offers an article on the efficiency versus the economy of tankless water heaters - see http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/water-heaters/tankless-water-heaters/overview/tankless-water-heaters-ov.htm
• Bosch Tankless Water Heaters - http://www.boschhotwater.com/
• Bradford White Everhot Tankless Water Heaters - Bradford White offers this step by step hot water retrofit installation file (large, slow PDF) - www.bradfordwhite.com
• Chromolite Tankless Water Heaters
• Dur-o-Matic point of use electric shower heaters - Duromatic of Puerto Rico Inc - Rio Piedras, Puerto Rico, 787-764-4490
• Eemax Tankless Water Heaters - http://www.eemaxinc.com/
• Marey electric point of use shower heaters: http://www.marey.com/english_/home.htm "Marey electric shower water heaters are installed directly on the shower water pipe. These heaters are commonly used where central hot water is not available."
• Noritz Tankless Water Heaters - http://www.noritz.com/
• Rheem Tankless Water Heaters - http://www.rheemtankless.com/content/
• Rinnai Tankless Water Heaters - http://www.rinnai.us/tankless-water-heaters/
• Stiebel Eltron Tankless Water Heaters - http://www.stiebel-eltron-usa.com/

• Titan Tankless Water Heaters - http://www.titanheater.com/
• U.S. Department of Energy on Tankless Water Heaters - http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820

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