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Fan Delay Relay

Efficiency is a ratio of btus transferred to power (watts) used. Since the evaporator coil is still cold at the end of the cooling cycle (when the compressor has cycled off), circulating room air (which is warmer than the coil) will cause btus to be transferred from the air to the refrigerant in the coil. This results in an increase in the number of btus transferred while not affecting the power used, since the compressor is off.

This increases the SEER rating of the equipment.

For example (fictitious numbers), if a piece of equipment transfers 20,000 btu while consuming 2,000 watts, the efficiency rating is 10 (20,000/2,000). If the fan runs at the end of the cycle, an additional 6,000 btus are trasnferred and the total number of btu is 26,000. The efficiency rating has now increased to 13 (26,000/2,000).

BTW, since the evaporator coil is below the dewpoint temperature, the cold coil will remove humidity, not add moisture to the air. Even if the evaporator coil is the same temperature as the air moving over it (assuming the coil is dry) no moisture will be added to the air.


  • greg_28
    greg_28 Member Posts: 22

    I have read that bleed-type TXV systems can have increased efficiencies from fan off delay relays. Would non-bleed txv systems benefit as well. I would imagine so, but is there some possible harm to system if air handler continued to operate for a few minutes after compressor stops running?
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Fan Off Delay

    The main purpose for the fan=off-delay on air conditioning equipment is to make use of the cool coil at the end of the cooling cycle. This is a technique used by equipment manufactuers to increase the SEER rating of the system. More btus transfered to the air = higher efficiency ratings.

    There is no danger of damage to the system as the operating fan is simply moving air through the duct system.
  • don_157
    don_157 Member Posts: 2
    off delay

    for the fan in my opinion is ok for a furnace but,when it
    comes to ac I say get rid of them.

    Why put the moisture back into the home,after trying so
    hard to take it out.

    Please enlighten me how a cold coil can increase efficency
    after the compressor have stop.

  • don_156
    don_156 Member Posts: 87

    thats the word of the day assume.Its odd that you said that
    professor but, I read higher humidity at the supply vents
    verse what going into the return.

    I know when it get out into the room that it mixes with drier air,however its always higher coming out of the vents and I can only assume it from water on the coil and what set in the pan.

    By the way, I purchase your heatpump book and it so much easier to understand then alot of other ac books out there.

    Thanks again Professor for sharing your time.

  • greg_28
    greg_28 Member Posts: 22
    Fan Off Delay Relay

    Thanks for your help. I'm off to buy one. As a test I placed the fan switch in a constant run setting and noticed that the delta T had only dropped from 20 degrees to 10 degrees, 4 minutes after compressor shut off. That's roughly 8000 cubic feet of cooler air (with my 2000cfm A/H) into my house - almost for free.

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Higher humidity

    Very astute!

    Here's the deal about high humidity on the supply side of the evaporatro coil.

    As we all know, the evaporator coil provides dehumidifaction as well as cooling. Since the coil is at a temperature that is lower than the dewpoint temperature of the air, moisture in the air condenses out of the air when it comes in contact with the cool coil. Then, the water drips off the coil and, hopefully, down a drain line.

    As hikgh velocity air is pushed (or pulled) through the evaporator coil some of the moisture is carried from the coil along with the cooled air. This raises the humidity of the air cming off the coil to over 90%!

    As the air travels through the supply duct and enters the occupied space, the air mixes with the room temperature air and heats up. This increase in temperature increases the air's ability to hold moisture and the relative humidity drops faster than a 90-year-old lady's jaw at a Chippendales show.

    Now, back to the fan-off-dealy issue. Since the compressor is off and the coil is dry, the humidity level of the air coming off the cooler coil will typically be no higher than the humidity of the air returning to the coil. Cool, huh?

    And, by the way Dan, thanks for your kind words about my book. I always try to make this stuff easy to read and, more importantly, understand. I will keep you all posted as more cool books come out. Have you checked out the Residential COnstruction Academy: HVAC book?
  • Frank_20
    Frank_20 Member Posts: 5
    dry coil

    Professor,I understand everything your saying except where you say "compressor off and coil is dry". Won't the coil still be wet because it is still cool and still de humidifying.What am I missing?
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Dry Coil

    Once the compressor cycles off, the temperature of the refrigerant in the evaporator coil will rise relatively quickly. This will raise the coil to a temperature that is above the dewpoint temperature of the returm air. The time that it takes for this to happen will vary depending onthe dewpoint temperature of ther eturn air but, for the most part, the coil will be substantially drier once the compressor cyhcles off than when the compressor is operating.

    Hope this helps.
  • Frank_20
    Frank_20 Member Posts: 5
    Dry Coil

    Yes sir that does help. Temperature of coil will come up above dew point temp. fairly quickly but coil will remain cooler than air temp for short time after compressor shuts down. Is this correct? Did not realize having a fan off delay would be much benefit with ac. Been in heating business a long time,doing ac for last 3 to 4 years. Picked up a lot in short time I have been reading this site. Thank You.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    My pleasure

    My pleasure Frank.

    Let me give you some cool stuff to think about.

    The energy effciency ratio of the system is directly to the coefficient of performance of the system. The COP is basically the number of BTU/lb absorbed into the system in the evaporator (Net refrigeration effect, NRE) divided by the number of btu/lb absorbed into the system in the suction line as well as the heat generated in the compressor (heat of compression, HOC).

    The EER is the COP x 3.413.

    So, if we can add a few btus to the NRE without affecting the HOC, we can increase the COP ever so slightly. But that's all we need, beccause an increase in COP of only 0.3 will increase the EER by a full point!

    Check out this example.

    Let's say we have an air conditioning system that has an NRE of 65 btu/lb and a HOC of 20 btu/lb. This system has a COP of 3.25 and an EER of 11.1. If we can increase the NRE by only a few btu/lb to 70, the COP inreases to 3.5 and the EER increases to 11.9.

    So, yes. The fan off delay, although the result is only a relatively small increase in btu tansfer, the overall results change drastically. Cool, huh?

    This is a sample of the neat stuff that will be showing up in my new book as well as in the new seminar I am presently putting together. I'll keep all of you guys posted about availability, dates, etc.
  • Frank_20
    Frank_20 Member Posts: 5

    Thats a lot to think about. Thank You Sir.
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