Packaged RTU / DX faults
Packaged rooftop and split DX units.
Seven rules for packaged equipment: compressor short-cycling, stage hunting, degraded DX capacity, economizers that never integrate, heat and cool stages overlapping, and fans running with no call. Rooftop units are rarely trended in detail, so these rules are written to work from the few points that usually exist.
How to read it. Six hours of a rooftop unit’s compressor. Each bar is one run. For the first three hours it runs long cycles, about one per hour. Then it starts and stops every few minutes (red): short cycling, which wears the compressor and wastes the energy of every start.
The rules
- RTU-01
Compressor short-cycling
The compressor is starting far more often than a healthy unit should.
Capacity-basedBAS trends - RTU-02
Cooling / heating stage hunting
A stage is flipping on and off many times an hour.
Capacity-basedBAS trends - RTU-03
DX cooling capacity degraded
On hot days with the compressor running the supply air is not getting as cold as the coil should make it.
Capacity-basedBAS trends - RTU-04
Integrated economizer not used
The compressor runs while outdoor air is cool enough for free cooling and the damper is parked at minimum.
Capacity-basedBAS trends - RTU-05
Heat and cool stages on together
A heating stage and a compressor stage are energized at the same time.
Capacity-basedBAS trends - RTU-06
Supply fan running after hours with no call
The fan runs outside the schedule with neither heating nor cooling active.
Capacity-basedBAS trends - RTU-07
High compressor runtime on cool days
On days cool enough that mechanical cooling should barely run, the compressor runs most of the time.
Capacity-basedBAS trends
What is short cycling costing?
Estimate the energy wasted on starts, and remember the wear is the bigger bill.
Enter positive numbers to see an estimate.
Each start costs roughly a couple of minutes of full-load energy before the unit is doing useful work. Extra starts above a sane rate, times that penalty, times the unit size and the affected hours. Energy Agent counts the starts from status trends.
Screening estimate. Energy Agent measures the real fault hours and load from your trends and prices them at your tariff.
See these rules run on your building.
Send twelve months of interval data and a BAS trend export. Energy Agent returns a ranked list of what is wasting money, what each item is worth per year at your tariff, and what to do about it.