Tankless water heater (C) Carson Dunlop AssociatesTankless Water Heater Repair Tips
Demand Water Heaters or Instantaneous Water Heaters or
Endless Hot Water Systems

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Tankless Water Heater Repair Advice: this article explains concerns, problems, & special characteristics of tankless water heaters, also called instant water heaters or "on demand" water heaters.

Page top sketch of a point of use tankless water heater courtesy of Carson Dunlop Associates.

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Tankless Water Heater Problems & Defects

Tankless water heater (C) Daniel Friedman Pro PHCC BrandArticle Contents

Tankless Water Heater Maintenance Costs

Conventional water heaters are relatively inexpensive due to their simplicity while tankless water heaters are more expensive to purchase and install. In 2009 in Toronto, Ontario a high capacity instantaneous water heater installation could cost $4000. CDN.

Their complexity also means that maintenance and repairs can be more expensive. Dedicated isolating valves help simplify draining and other regular maintenance.

Fuel Supply for Tankless Water Heaters

The water heater must have a continuous fuel supply. Malfunctioning burners, sensors or controls will cause poor operation or may result in the system not working at all, meaning no hot water.

Tankless Water Heater Scale Build-up Will Reduce Hot Water Flow

The small diameter of the heat exchangers means that these units are susceptible to clogging with scale, especially in areas with hard water. In hard water areas, annual de-scaling is recommended.

As with tank-type water heaters, some manufacturers recommend regular flushing - a step that removes sediment and debris from the heater, not scale.

Tankless Water Heater Dirty Filter Will Reduce Hot Water Flow

If the heater is equipped with a water filter, usually an in-line screen that can be removed, inspected, and cleaned. The demand water heater filter screen should be checked and cleaned monthly, or performance will suffer.

Tankless Water Heater Hot Water Supply May Not be Constant

Consumer Reports reports that some tankless water heaters do not heat continuously and constantly. If a particular model has this problem users will find that in the shower the water runs in cold bursts from time to time.

Watch out: In our own tankless water heater installation experience in the U.S. & Mexico we have confirmed that there can be hot water constancy & flow regulation problems, particularly in low-pressure tankless water heater installations.

Depending on exactly how hot and cold water pipes are routed to the tankless water heater and to various plumbing fixtures in a building, low pressure tankless water heater units are sensitive to reductions in water flow rate when additional plumbing fixtures are in operation in the same building.

The result can be complete cessation of hot water (the heater turns off on a too-low-flow-rate condition) or difficulty balancing the proper hot water temperature by attempts to mix cold water in at the shower or sink where hot water is in use.

We suspect that for this reason, after about 2004, Bosch stopped selling low pressure tankless water heaters in the U.S. However as recently as in 2012 these units continue to be sold and installed in other countries including Mexico where some homes depend on low water pressure delivered from a (not very high) rooftop water storage tank.

These low water pressure problems can be addressed by

  1. Careful use of cold water mix-in at the plumbing fixtures in use
  2. Setting down the hot water temperature at the tankless water heater and reducing the mix-in of cold water at the plumbing fixture
  3. Installation of a water pressure boosting pump and pressure tank
  4. Installing a dedicated cold water input line from the water storage tank to the tankless water heater cold-input fitting
  5. Replacing the low-pressure tankless water heater with a high pressure unit combined with installation of a water pressure boosting pump and tank system - the most expensive solution to these complaints.

Tankless Water Heater Cold Water Sandwich Effect Explained

Depending on how you use your demand hot water heater, an unpleasant jolt of cold water often occurs when water is turned on and off fairly quickly. This can happen at a kitchen sink when rinsing dishes or at the bathroom basin while washing and shaving, for example.

Rinnai™, a maker of tankless water heaters explains the cold water sandwich effect as follows:

"The term “cold water sandwich effect” is a term that is used to describe the introduction of cold water into the hot water supply line during frequent on/off operation of an instantaneous water heater. The cold water sandwich effect, when present, appears as a momentary drop in hot water temperature as it is discharged from a hot water supply outlet (i.e. shower, tub, or faucet). This phenomenon is present in the operation of all instantaneous, tankless style, water heaters, but is minimized with the high tech design of Rinnai water heaters.

The technology built into the Rinnai water heaters is designed to minimize the cold water sandwich effect. Rinnai water heaters are microprocessor controlled and when water flow through them ceases, they remain in a “ready to fire state” for approximately 1 minute. If water flow through a Rinnai water heater begins within the first minute following water flow stoppage, the water heater will fire back up within 1 to 2 seconds.

This minimizes the cold water sandwich effect that would otherwise be experienced with a low tech tankless water heater. It should be noted that the cold water sandwich effect cannot be removed completely from tankless style water heaters. The safety standards developed to insure the safe operation of water heaters require a delay in the ignition sequence of all gas water heaters.

While the cold water sandwich effect cannot be completely eliminated from standard plumbing systems, it can be eliminated from plumbing systems that have a supply and return hot water circulating system. Rinnai has developed 2 methods to eliminate the cold water sandwich effect in residential hot water circulating systems.

The first (and preferred) method to remove the cold water sandwich from circulating systems utilizes a small electric tank water heater (powered up) that is used with a dual purpose. The small water heater acts as a mixing tank to eliminate the cold water sandwich effect from the Rinnai water heater and it uses its electric heating element to offset the heat losses from the hot water circulating system.

The second method to remove the cold water sandwich from circulating systems utilizes a small 2 to 6 gallon storage tank installed on the hot water outlet of the Rinnai water heater. This tank acts as a mixing tank to blend the cold water sandwich with hot water and eliminating its effect at fixtures."

Tankless Water Heater Maximum Hot Water Flow Rate & incoming water temperature

The hot water flow rate is not only dependent on the heating capacity of the water heater and the output water temperature, but also on the inlet water temperature.

Homes in northern climates draw water from colder water supplies, and since it takes longer to heat up colder water, tankless water heaters installed in these homes will have lower hot water flow rates. The design temperatures for many water heaters is 60 deg. F. but in northern climates (Toronto, Ontario, or Two Harbors MN), the incoming water temperature may be 50 deg. F. or even as low as 40 deg. f.

Portions of this article were provided courtesy of Carson Dunlop - that text has been edited and may not entirely reflect CD's views.


Continue reading at TANKLESS WATER HEATER SIZE REQUIREMENTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ALTERNATIVE HOT WATER SOURCES to distinguish among these types of hot water producing systems.


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