Demand Limiting

The "EMS" system will sense the number of electrical pulses generated by the electric metering equipment.  When the number of pulses exceed the demand setpoint their "EMS" systems will start demand limiting.  It should be remembered that the heating and air conditioning equipment operation is determined by the existing zone thermostat.  As the zone temperature rises or falls below the setting of the thermostat, the air conditioning or heating turns on to satisfy the zone temperature.  By simply turning off the heating and air conditioning on a random basis, the electrical demand and resultant demand charges are reduced at the expense of comfort.  Since the "EMS" system is not controlling the zone temperature, the "EMS" system does not know how much hotter or colder the zone temperature is getting because of demand limiting.  This result is increases in both the building humidity and loss of temperature control with resultant loss of comfort.  In other words, this type of demand limiting sacrifices building comfort for reduced utility bills.  Manufactures of "EMS" equipment, who honestly represent their product, will state that duty cycling or conventional demand limiting will reduce the demand charges by approximately ten to fifteen percent, but will cause fluctuations of zone temperatures of approximately five to seven degrees.  In short, demand limiting offered by conventional "EMS" equipment will reduce demand charges by sacrificing comfort.

Sometimes the "EMS" system installer will place a limit sensor in a temperature control zone to measure the amount of temperature rise or fall caused by demand limiting.  When the temperature rises or falls above or below the preset temperature, then they will start to reduce demand limiting.  Since the peak demand periods usually last at least three hours, reducing demand limiting will eventually stop all demand limiting, eliminating any savings.  If the temperature limits are set high enough to allow demand limiting to continue as the temperature of the zones rise or fall, then once again comfort is lost.  In other words, this type of demand limiting either causes uncomfortable temperatures and humidity or stops demand limiting due to elevated temperatures.  If it stops demand limiting why did you purchase their system in the first place?

The EPM "CTC" system will also sense the pulses generated by the electric metering equipment.  Unlike the "EMS" equipment, the "CTC" system will project the electrical demand based upon the history of the demand pulses over the previous five minutes.  The projection is made for the next ten minutes.  The previous five minutes and the next ten minutes make up the fifteen minute metering demand period.  The computer projection of the fifteen minute demand period allows for more precise demand limiting which avoids the false demand limiting created by simply measuring the number of demand pulses in a relatively short period.  This cycles the equipment less and because it is a projection of the demand, it will not miss peak demands when the demand rapidly increases.

When the "CTC" system determines that demand limiting must be performed it calculates the percentage of electrical load that must be removed from the operating heating and air conditioning equipment.  It first measures the temperature in all of the temperature control zones which are cooling or heating.  It compares the actual zone temperatures to the zone temperature setpoints.  It then reduces the electrical load by terminating cooling or heating in those zones that are closest to their temperature setpoints.  During the demand limiting period the temperature control computer continuously seeks out those zones whose actual temperatures are closest to their setpoints.  These zones then have their cooling or heating terminated  and the balance of the zones are allowed to cool or heat.  This eliminates the problems created by "EMS" systems of the zone temperature rising or falling above or below temperatures necessary for humidity and comfort control.  Also, those zones which need the cooling or heating always receive it.  This process is a dynamic process which continues automatically until the demand peak has passed.


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