In most all cases, this is the case. But there are other options.
I am describing the zone option and the things that happen differently than individual zoning.
Posting here that they must cycle every 15-20 min to be correct may turn into “An Old Charley Tale” (which superceeds The Old Wives Tale"). We have enough of them to deal with…
Why does a unit run all the time, sometimes? (a point of consideration for others):
The length of run time is controlled by a sensible heat thermostat.
Depending on where you live (New Orleans vs. Phoenix) the equipment must be properly sized to control the Sensible Heat Ratio, not the Temperature. The longer a unit runs, generally the more moisture is taken out of the air.
A thermostat can not measure humidity, it measures sensible temperature. As the unit runs the first thing it does is remove moisture. The temperature will not come down till a corresponding amount of moisture is removed from the IA.
The thermostat then shuts off. The thermostat and the hvac equipment do not speak the same language, they just happen to be able to work together.
If the unit is not big enough, it may never pull down to the sensible set-point temperature. If too big, it will leave behind excessive moisture which may decide to condense on something inside the house that the oversized hvac unit made cooler than the dew-point temperature of the IA (ie. air duct and registers, sheetrock).
- This only applies when equipment and buildings were constructed properly in the first place. If the building leaks air, it lets in moisture and the unit runs more. This is a building problem, not an hvac problem and should not be confused as such.
You must know the equipment performance, not just how long it runs…
Therefore it is safe to say that Home Inspectors can not make this call based on visual observations.
How does one determine equipment performance?
You must know how the unit is operating for the conditions. Is the unit properly charged? Is there proper airflow through the equipment? Does the air duct leak? Is the air duct properly insulated?
What are the properties of air entering and leaving the equipment?
A hygrometer is needed to determine at least two points on a psychrometric chart. From there we can determine everything we need to know about performance;
air temp in
air temp out
sensible heat ratio
amount of total BTU removal (sensible-latent)
weight of the air
evaporator by-pass factor
apparatus dew point
air flow cfm
grains of moisture/lbs of air
BTU’s per pound of air
With this information we can now start to make a SWAG guess at how the unit is operating and why.
As you should be able to comprehend from this, you can not determine anything with Delta-T thermometer measurements or visual observation of the installation. But many of you will continue to wander around in the dark with your Home Inspector School, Old Wives Tales and think you can evaluate HVAC performance.
If you run the equipment in heat and cooling and write in your report that the units appear to be operating adequately your fine… To say anything more without proper testing is a flat out lie.