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Thread: 20 Degree split

  1. #14
    Join Date
    Apr 2007
    Posts
    6,285
    I may get blasted for this but I don't think it get's to technical....this was a reply to a test we had to take for work that had a question with the answer being 20* split....

    "Cooling coils are rated with what is known as sensible heat ratio. This basically tells you the amount of heat the coil will remove which can be verified with a dry bulb thermometer.

    Cooling coils are responsible for removing more than just sensible heat. They also remove latent heat which would have to be measured with a wet bulb thermometer. When you know the SHR you also the know the latent heat ratio or latent load.

    example: SHR for a cooling coil is .75.

    That means that 75% of the capacity of the coil is sensible heat and 25% is latent heat.

    You may ask why this is important. Typically when your verifying capacity on a HVAC system you would need three measurement the dry bulb and wet bulb and airflow so you could plot the enthalpy change (total heat removal) so capacity can be calculated.

    Sometimes you might hear you should have a 20 degree delta T or temperature difference for the system to be operating correctly. This can lead to problems. Cooling coils remove the moisture first...the higher the moisture content the less SHR and more latent heat is removed. So if you walk up to an A/C system on a hot/humid day use a dry bulb thermometer or what you would think as a normal thermometer you will only be detecting 1 of the two heats removed. Since A/C's remove the latent heat first that dry bulb temp. difference across the cooling coil will be lower than what you would expect because of the large latent load.

    How can this lead to problems:
    You walk up and take DB delta T (temp. across cooling coil) and you get 15*F difference. The person looking at this system may think something is wrong with the system when in fact it is operating normally it just has a large latent load. Which can be verified with a hygrometer or wet bulb thermometer. If the person checking the A/C checks just dry bulb delta T and see 15* he may start trying to fix a problem that isn't there.

    This can lead to overcharged systems, setting the fan speed to low, wasted time...ect.
    Please don't think in terms of absolute when thinking air conditioning way to many variables to put up one number and call it good.

    I hope this helps anyone that might be looking for that magic 20* delta T"
    Last edited by BigJon3475; 07-16-2008 at 01:19 AM. Reason: Clarifying for this forum.

  2. #15
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    Thumbs down Why don't you help him

    Making stupid statements like that is not going to help him. You and him need to read the rules.
    He needs to get is count up to 15 then apply pro status and then we will help[/SIZE]
    Do it right the first time.

  3. #16
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    You are good

    Thandar56, But not that good read the rules IDIOT
    Do it right the first time.

  4. #17
    Join Date
    Apr 2002
    Location
    Omaha, NE
    Posts
    1,561
    Quote Originally Posted by 21degrees View Post
    Thandar56, But not that good read the rules IDIOT
    Hmmm.......sounds like your response is a clear violation of Rule #1

    1. Who Is Welcome Here?

    Anyone in the HVAC industry and home or building owners with HVAC/R concerns and other polite and fun people that are willing to contribute.

    Feel free to ask any question relating to HVAC, or other topics of general interest except DIY.

    At the same time, be polite and respectful in your answers. We welcome questions so please don't be critical of someone's question, or other member's answers.

    Any reply that we feel violates this rule will be edited or deleted.

    All members must treat other members with respect. Anyone who posts defamatory, insulting or otherwise disrespectful comments towards others in the HVAC industry will find their posts deleted and possibly their posting privileges suspended or revoked.

    Any member that becomes belligerent, argumentative, posts smart-ass comments or requires too much attention of the moderators will lose their posting privileges.

  5. #18
    Join Date
    Jul 2007
    Location
    Memphis,Tn
    Posts
    8

    20 Degree split

    Sorry, for any rules violation that may have occured. I was sent on this call becaused the unit had frozen up I was sure it was a dirty A coil or a leak some where in which I had to find. I had a nitrogen bottle, a electronic leak detector and some bubble soap so I had the ability to find the leak but it would take time. I found the leak after it had defrosted at the low side Schrader valve, I replaced both low side and high side and began to charge.
    I always check the temp at the supply register before leaving I was not satifyed so I took a reading at the return and my split was only 12 degrees. Thanks to all who may have given me me advice as to what may have caused this I will talk to our senior tech an see what he thinks.

  6. #19
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    Oh must be Typo

    I was not trying to insult him just trying to help him out. I hire people like this day in and day out and some times gets the best of me. You tell them and tell them but they never change. Oh by the way Did I miss you post to him about no DIYs and step by step information.
    Do it right the first time.

  7. #20
    Join Date
    Dec 2006
    Location
    Dallas
    Posts
    189
    Those Who Can, Do; Those Who Can't, Teach HVAC. Those who can't teach HVAC are coaches.

  8. #21
    Join Date
    Apr 2007
    Posts
    6,285
    Quote Originally Posted by gonekuku View Post
    Those Who Can, Do; Those Who Can't, Teach HVAC. Those who can't teach HVAC are coaches.

    Do what?

  9. #22
    Join Date
    Apr 2007
    Posts
    6,285
    Quote Originally Posted by BigJon3475 View Post
    I may get blasted for this but I don't think it get's to technical....this was a reply to a test we had to take for work that had a question with the answer being 20* split....

    "Cooling coils are rated with what is known as sensible heat ratio. This basically tells you the amount of heat the coil will remove which can be verified with a dry bulb thermometer.

    Cooling coils are responsible for removing more than just sensible heat. They also remove latent heat which would have to be measured with a wet bulb thermometer. When you know the SHR you also the know the latent heat ratio or latent load.

    example: SHR for a cooling coil is .75.

    That means that 75% of the capacity of the coil is sensible heat and 25% is latent heat.

    You may ask why this is important. Typically when your verifying capacity on a HVAC system you would need three measurement the dry bulb and wet bulb and airflow so you could plot the enthalpy change (total heat removal) so capacity can be calculated.

    Sometimes you might hear you should have a 20 degree delta T or temperature difference for the system to be operating correctly. This can lead to problems. Cooling coils remove the moisture first...the higher the moisture content the less SHR and more latent heat is removed. So if you walk up to an A/C system on a hot/humid day use a dry bulb thermometer or what you would think as a normal thermometer you will only be detecting 1 of the two heats removed. Since A/C's remove the latent heat first that dry bulb temp. difference across the cooling coil will be lower than what you would expect because of the large latent load.

    How can this lead to problems:
    You walk up and take DB delta T (temp. across cooling coil) and you get 15*F difference. The person looking at this system may think something is wrong with the system when in fact it is operating normally it just has a large latent load. Which can be verified with a hygrometer or wet bulb thermometer. If the person checking the A/C checks just dry bulb delta T and see 15* he may start trying to fix a problem that isn't there.

    This can lead to overcharged systems, setting the fan speed to low, wasted time...ect.
    Please don't think in terms of absolute when thinking air conditioning way to many variables to put up one number and call it good.

    I hope this helps anyone that might be looking for that magic 20* delta T"

    Is this a decent reply that the average Joe would understand?

  10. #23
    Join Date
    Apr 2007
    Posts
    6,285
    Quote Originally Posted by gonekuku View Post
    Those Who Can, Do; Those Who Can't, Teach HVAC. Those who can't teach HVAC are coaches.

    NM I think I took your response offensively like teachers can't do the job....I see the other side now. Those who can't do the job anymore teach newer batches?

  11. #24
    Join Date
    Jul 2007
    Location
    Memphis,Tn
    Posts
    8

    20 Degree split

    21 Degrees
    I hire people like this day in and day out and some times gets the best of me.
    Not to be personal but with an attitude such as yours it only make sense that you would not be able to keep anybody. Beside if everybody was as smart as you, then you would not have a JOB. You did Know everything when you started and still don't, so lighten up.

  12. #25
    Join Date
    Apr 2007
    Posts
    6,285
    Quote Originally Posted by Anthony49 View Post
    21 Degrees
    I hire people like this day in and day out and some times gets the best of me.
    Not to be personal but with an attitude such as yours it only make sense that you would not be able to keep anybody. Beside if everybody was as smart as you, then you would not have a JOB. You did Know everything when you started and still don't, so lighten up.
    He means good. He is trying to preserve the board. No harm in that. Lot's of people that really wish to totally ignore the rules....sometimes they aren't 100% clear....in terms of peoples opinions of course.

  13. #26
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,123
    Quote Originally Posted by Anthony49 View Post
    I always check the temp at the supply register before leaving I was not satisfied so I took a reading at the return and my split was only 12 degrees.
    Temps at the supply and return registers are not reliable. The return could be drawing basement, or attic air, depending where the air handler is located. Which will throw your readings off. Along with attic temps being able to influence the supply temp also.

    Charging till the vapor line is 68.5PSIG or some other arbitrary pressure isn't proper, or close to getting the charge right.

    Study up on superheat and subcooliing.

    After you get your post count up to 15, apply for pro membership.
    Once accepted, you can ask questions in the pro tech forum. Which is where we can explain in detail.
    Contractor locator map

    How-to-apply-for-Professional

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

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