FDRs
(Fan Delay Relays)

GOAL:

To make students aware of fan delay relays and how they equate into energy savings.

OBJECTIVE:

The student will be able to explain the function of fan delay relays and why they are used.

LESSON / INFORMATION:

Many people do not understand why their indoor fan continues to run for up to a minute after the outdoor section of the air condition unit has stopped running. An understanding of fan delay relays (FDRs) enlightens many homeowners.

One advantage of the indoor unit continuing to run is that it is giving free air conditioning. Systems using bleed type thermostatic expansion valves (TXVs) or fixed metering devices allow refrigerant to continue to flow after the outside compressor has stopped running. The flow is because of the pressure difference between the liquid and suction gas lines. A system running a 70 pounds per square inch gauge (psig) suction pressure and a 210 psig liquid pressure will continue to refrigerate for up to 30 seconds after the outdoor unit has stopped. The fan delay relay is a tuned relay that keeps the indoor blower motor running to take advantage of this continued refrigeration and of the coil that is still cold after the refrigeration cycle has stopped. Most relays operate flow for about 60 seconds and then shut off.

All equipment manufacturers have taken advantage of this and rate their units with and without these relays. One major manufacturer has this feature built into the computer board on all its heaters and branded heaters. The technician does not have to worry about additional wiring on installation, and feels it is still an advantage over the 100% shut off systems due to temperature differences between coil surface and air in the house.

ACTIVITY 1:

1. Check off all the items that are TRUE about FDRs.

_____a. offers the homeowner "free" air conditioning

_____b. causes a strain on the outside unit

_____c. stands for fan delay relays

_____d. allows the indoor blower to run longer than the outdoor unit

_____e. can cause an increase in your utility bill.

INFORMATION CHECK

1. Explain what an FDR is to a prospective air conditioning customer.

2. Explain the relationship between FDRs and energy savings.

TEACHER'S NOTES

ACTIVITY 1:

The following are true and should be checked: a, c, and d.

ANSWERS TO INFORMATION CHECK:

1. An FDR is a "fan delay relay." An FDR refers to the time that an indoor fan unit continues to run after the outside section of the air conditioning unit has stopped.

2. The FDR is an energy saver as it takes advantage of the refrigerant flowing in the lines after the outside compressor has stopped. This allows approximately 30 seconds of "free" air conditioning per cycle.

Comments or questions to: TechAsmt@LA.GOV

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