On-Demand Water Heaters

In contrast, an on-demand unit has a heating device that is activated by the flow of water when a hot water valve is opened.  Once activated, the heater delivers a constant supply of hot water.  By only heating water when there is a demand for it, the on-demand water heater eliminates stand-by heat loss associated with conventional hot water systems.

On-demand water heaters come in a variety of sizes for different applications, and have been used extensively for diverse functions in Europe and Japan.  In North America, on-demand water heaters are predominately utilized as a water heater supplying the whole house.

Determining the correct sizing for an on-demand water heater is crucial and to do so correctly, the peak hot water demand the unit will need to accommodate must be established.  On-demand water heaters are rated according to the number of gallons of water per minute that can be raised to the desired temperature.

Advantages

• more energy efficiency than conventional hot water heaters in that they do not have to maintain a supply of hot water in a tank; they are fully modulating and heat water only as needed.
  
  
• are much smaller than conventional units, and can easily be wall mounted to conserve space
  
  
• good thermal efficiency, consume up to 30% less energy than conventional hot water heaters
  
  
• hot water is delivered continuously and endlessly, limited only by flow rate
  
• have, on average, 20 year life expectancy

Disadvantages

• usually cannot provide large amounts of hot water at the same rate as a conventional hot water tank; we recommend that water conservation measures be undertaken when operating an on demand system.
  
  
• owners need to change their habits to arrange non simultaneous high rate usages of hot water (ie: may not be able to run a washing machine, dishwasher, and shower concurrently).
   
• cost, on average, 3 times more than a conventional hot water heater
  
  
• demand units that can accommodate higher flow rates may require larger capacity piping to accommodate increased load