Page 2 of 2 FirstFirst 12
Results 14 to 22 of 22
  1. #14
    Join Date
    Apr 2003
    Location
    Maryville, Tennessee
    Posts
    432
    The conditions that cause the coil to frost up the fastest, requiring the shortest defrost interval, are misty rain/very hi humidity with outdoor dry bulb temps in the thirties. Any time that the outdoor temp is below 40 the outdoor coil is going to be operating below freezing.

    Cold air holds less moisture at any given relative humidity when compared to warmer air. A coil will frost up faster in 30 degree weather with hi humidity then it will at 10 degree weather with the same RH. In East Tennessee I set the timer to 60. 30 is a waste of energy in my area. I used to set the timer tom 90 and very rarely did it cause a problem. Setting at 60 in my area has never caused a problem for me. YMMV
    -Marty

  2. #15
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,505
    Quote Originally Posted by Kevin Weaver View Post
    Thats the reason manufactures have went with DEMAND DEFROST. now the question is, what temperature do you select for termination?

    I choose 80 degrees.
    Is demand defrost used as part of the HSPF or SEER calculations?

  3. #16
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,364
    I'm a 90 minute guy & I'm in humid SC. It doesn't get that cold normally but we have a lot of damp 30 & 40 degree weather here.
    On a rare occasion I'll set one at 60 but its usually because the customer complains about frost some times & doesn't wait for it to defrost off. If you set them at 30 you'll probably have more complaints about cold air blowing before the strips kick in during defrost.
    I think heat pumps should not have any sequencers on their heat strips so they can come on immediately to offset that initial blast of cold air during defrost.
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

  4. #17
    Join Date
    Apr 2003
    Location
    Maryville, Tennessee
    Posts
    432
    Quote Originally Posted by 54regcab View Post
    Is demand defrost used as part of the HSPF or SEER calculations?
    The type of defrost control is not a factor at all in SEER. HSPF does not account for strip heat use in defrost and assumes that there will be no defrost below 17 degrees. Demand defrost has has performance advantages over time and temp that will not be fully appreciated in HSPF.
    -Marty

  5. #18
    Join Date
    Feb 2004
    Location
    Ky
    Posts
    269
    Short (I hope) essay from a (tired)grunt in the field...
    There are 2 main types of defrost control. Time and Temp, and Demand.
    Time and Temp uses a timer that says 'we've been running for 30/60/90 minutes, is our sensor (that can be anywhere from 28 to 50 degrees) closed?' If the answer is yes, then let's defrost. If we are not defrosting, or cannot force test a defrost, check one of us.
    Demand is a combination of things set into an algorithm designed by the manufacturer. It senses the outdoor coil temp and the outdoor air temp and tries to keep defrost cycles to a minimum as these call for electric heat to come on to temper the cold discharge air.
    You need to be able to check these sensors/thermocouples for the correct resistance.
    Of course, then you have manufacturers (like Tempstar for one), who decide that the original design is not good enough, and tell you to change demand board and sensor to their 'new' time and temp system.
    Good luck, hope we can answer your questions in the future.
    LN

  6. #19
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,505
    Quote Originally Posted by garyed View Post
    I'm a 90 minute guy & I'm in humid SC. It doesn't get that cold normally but we have a lot of damp 30 & 40 degree weather here.
    On a rare occasion I'll set one at 60 but its usually because the customer complains about frost some times & doesn't wait for it to defrost off. If you set them at 30 you'll probably have more complaints about cold air blowing before the strips kick in during defrost.
    I think heat pumps should not have any sequencers on their heat strips so they can come on immediately to offset that initial blast of cold air during defrost.
    +1. I don't see why they just don't use contactors. Sure they may be noiser, but it shouldn't be a problem for most installations. Contactors could use time delays so they don't all kick on at the same time.

  7. #20
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,505
    Quote Originally Posted by m singer View Post
    The type of defrost control is not a factor at all in SEER. HSPF does not account for strip heat use in defrost and assumes that there will be no defrost below 17 degrees. Demand defrost has has performance advantages over time and temp that will not be fully appreciated in HSPF.
    What is bad is the typical consumer doesn't have any idea about this and demand defrost wont show the real world advantage in the HSPF.

  8. #21
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,770
    Quote Originally Posted by 54regcab View Post
    What is bad is the typical consumer doesn't have any idea about this and demand defrost wont show the real world advantage in the HSPF.
    Defrost cycles are taken into consideration in HSPF. And on demand defrost is how some manufacturers get a high HSPF, with low BTU output.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #22
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Quote Originally Posted by beenthere View Post
    Defrost cycles are taken into consideration in HSPF. And on demand defrost is how some manufacturers get a high HSPF, with low BTU output.
    That is what I understood also.
    Remember, Air Conditioning begins with AIR.

Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event