Arkansas HVACR NewsMagazine September 2024

HVACR NewsMagazine September 2024

Tech News

Controls Design & Setup

When one of these safety controls turns the system off, there will often be a time delay that prevents the equipment from coming right back on. In most of these switches, there will be a gap between the make and break (on and off) points in the switch, which will naturally help to prevent rapid short-cycling. When power is rapidly cycled (turned on and off), the components may go off and then back on quickly if there is no time delay. Here in Florida, this happens often during thunderstorms, but it can also be caused by flipping a breaker on and off rapidly or bumping a condensate switch. These sorts of rapid short-cycling events are hard on motors and controls, and they can even cause scroll compressors to run backward (on rare occasions). The way we control the temperature with most appliances is by running them until they hit the setpoint and then shutting them off. This can result in short run times when the load is low or when the equipment is oversized. The best designs result in the equipment running non-stop when during peak load; this isn't intuitive for most customers, and they will often complain that the system “never shuts off.” You can reassure them that so long as they are staying comfortable, never shutting off is a good thing for system longevity and power consumption. Loss of Power Oversizing/Low Load

The controls play a big part in run time, depending on how they are set up. It is almost never as simple as turning on and off at a set temperature because that would almost certainly result in short-cycling unless the system has variable capacity or the capacity is perfectly matched to the load. In most real world conditions, the controls will need to manage a dead-band or gap between on and off to balance comfort and short cycling.

Controls do this by maintaining an on-off dead-band and maintaining a maximum CPH (cycles per hour), like the Honeywell thermostat shown above. This means that a thermostat with a 3-degree dead-band with a cooling CPH set to 3 and a set-point of 75 degrees would come on at 76 and shut off at 74 while turning on and off a maximum of 3 times per hour. So, those are some of the factors that impact short cycling, but what are some of the issues associated with short cycling? Here is an incomplete list: