BTU:

BTU stands for British Thermal Unit which is approximately equal to  1,055  joules.  It takes 1 BTU of energy to increase the temperature of 1 lb. of water 1 degrees Fahrenheit.

Cold Water Sandwich Effect:

This occurs when there is frequent on/off usage of a tankless water heater.  For example,after the first person exits the shower,the water in the pipe will still be hot,and the heating mechanism has turned off at the instant water heater point.  When the second person uses the shower,there will be some residual hot water in the pipe,however,because the on-demand water heater has a slight delay of activating its heating element (3 or more seconds) this short period of time contains colder water.  Once the heating element turns on,the water is again hot.  This represents the “Cold Water Sandwhich Effect.”

Diagram A. ©  When the first person exits the shower,the pipeline still contains warm / hot water,except the section of pipe before contact with the water heater.

Diagram B. © As the second person enters the shower;the pipeline from the heater to the shower still contains mostly warm water,but the water heater doesn’t turn on immediately because it needs to detect water flow first which results in cold water at the heater point.  The water  at the pipe before touching the heater is always cold  and therefore also colored in black.

Diagram C. © After a few seconds,the water heater turns on,and hot water begins to flow,but at this point,you can see the cold water being sandwiched in the middle of the two warmer segments of water.

Condensing:

Condensing tankless water heater technology uses two heating chambers,one is called the pre-heating chamber and the other is the main heating chamber.  Exhaust gas is routed to the pre-heating chamber where it heats the incoming water,instead of being wasted.  This usually results in a much higher heating efficiency rate for condensing water heaters,usually in the 90′s range rather than the 80′s range.

Flow Rate:

Flow Rate is the amount of water volume that comes out of a water fixture;this is measured by the GPM (Gallons Per Minute) unit.

Flue:

Flue is the pipe or duct for directing thankless water heating exhaust gas outdoors.  Often times you will see flue being used to mean flue gas.

Direct Vent (See Venting)

GPM:

Simply stands for Gallons Per Minute.  For example,your shower may have a water flow rate of 1.5 GPM which is the average for a low flowing shower fixture.  Most tankless water heater manufacturers display a GPM flow chart for their products.

** In 1992,shower fixtures made in the USA were mandated by the Federal Energy Policy Act to flow at a maximum rate of  2.2 GPM at 60 PSI.  Shower fixtures that date before 1992 will often have water flow greater than 2.2 GPM.

Non-Direct Vent (See Venting)

Sandwich Effect (See Cold Water Sandwhich Effect)

Temperature Rise:

This is the difference between the temperature set on your tankless water heater and the temperature of the water flowing into it (also known as Ground Water Temperature or Ambient Water Temperature).  For example,you want your instant water heater to produce a temperature of 85 degrees Fahrenheit,but the water flowing into the unit is 40 degrees Fahrenheit.  Here’s an example of a temperature rise calculation:

Set Temperature –Incoming Temperature = Temperature Rise

85F(Set Temperature) –40F(Incoming Temperature) = 45F(Temperature Rise)

The temperature rise number is crucial in determining tankless water heater sizing or tankless water heater ratings.

Thermal Efficiency (TE):

This number is often stated as a percentage.  The higher the percentage, the more efficient the appliance is which results reduced energy consumption and  cost savings.

The first law of thermodynamics states that the energy output can never be greater than the energy input,so the TE formula will normally  be a percentage that is less than 100%.

The formula for TE is:

Q(in) = Energy Input

Q(out) = Energy Output

Nth = Q(in)  / Q(out)

Note: The above formula applies to water heaters,but other products like car engines use a different formula to calculate TE.

Venting:

Direct Venting:

A direct vent is used for natural gas or propane tankless water heaters that are installed indoors,and are used to pull air from outdoors for combustion  and to direct the exhaust gas safely outdoors.  Non-direct venting,in contrast,doesn’t use combustion air from outdoors.

Non-Direct Venting:

In non-direct venting,incoming air needed to fuel a gas tankless water heater  is supplied by the immediate surroundings (garage space not sourced from outdoors as a direct venting set up would be) and is driven by a fan.  The fan is also used to push this waste gas outdoors.