Fault Detection and Diagnostics for Rooftop Air Conditioners

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inspection savings would be $2,027 per RTU. This is a significant initial savings for FDD,
which is usually not considered.
Table 5-1 Planned Preventive Maintenance Costs
Location No. of RTU Total Annual
Costs ($)
Costs per
Inspection ($)
Annual Costs
per RTU ($)
1 4 1,119.00 279.75 279.75
2 5 1,377.00 344.25 275.40
3 6 1,635.00 408.75 272.50
4 7 1,893.00 473.25 270.43
5 8 2,151.00 537.75 268.88
6 9 2,409.00 602.25 267.67
7 10 2,667.00 666.75 266.70
8 11 2,925.00 731.25 265.91
9 12 3,183.00 795.75 265.25
5.2 Operating Cost Savings (OCS)
Many systems are affected by faults that are not detected during preventive
maintenance inspections and that lead to significant system performance degradations.
For example, Proctor and Downey (see Levins, Rice, and Baxter, 1996) investigated
refrigerant charge levels in residential systems in California:
1. One study found that 31% of the units were undercharged and 69% were either
properly charged or overcharged.
2. Another study in 1990-1991 found that about 60% of the units were
undercharged or overcharged, and 40% were properly charged.
3. A third study found 22% undercharged units, 33% overcharged units, and 45%
properly charged units.
According to our initial investigation of California field sites, fifteen of the
twenty-one systems (71%) are significantly impacted by faults: eleven (52%) have
filter/drier restrictions, ten (48%) have refrigerant charge fault, and eight have (38%)
have both low charge and filter/drier restriction faults. As an example, the EER and
cooling capacity of the RTU at the Milpitas site degraded 21% and 30%, respectively.