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Old 08-31-2014, 09:59 AM   #1
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Conflicting super heat and sub cooling


My unit is working fantastic but I want to understand why I'm getting low super heat and sub cooling numbers. It also works so well in heating mode that sometimes I think the resistance heat is on but it's not.

Indoor unit was installed in 1983, Carrier 40AQ. Outdoor unit used to be the round trashcan 38QB series but was replaced in the mid 90's with a 38YKC.

I was hoovering over the tech the entire time but I don't remember if he replaced the indoor piston. He might have said it was OK as is but I just can't remember.

Indoor and outdoor coils were clean but I cleaned them anyway. 3 ton unit and the return air added up from both return registers to about 900 CFM. Most of the return is sealed joist space and wall cavity and I don't know how accurate the 900 figure is due to possible leakage. I removed the 1" pleated filter and it jumped up to about 1000 CFM.

65* wet bulb
85* outside
72 low, 215 high
Sub cool 6*
Super heat 4*
19* delta T

Sub cool target from the 38YKC manual is 15*
Target super heat was 15*

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Old 08-31-2014, 10:30 AM   #2
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Conflicting super heat and sub cooling


how did you concur target? do we understand fully what superheat and subcool mean? Do we know lineset length, mechanical heat, pressure drop per 90, coil fitration, piston or txv? is evap in attic and if so is lineset above evap. oil trap?

what is measured in lab never is the same real world.

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Old 08-31-2014, 10:41 AM   #3
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Conflicting super heat and sub cooling


Indoor unit is in the basement. 35' lineset and it rises and falls about the same distance. Lineset pressures and temperatures measure the exact same at indoor and outdoor units.
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Old 08-31-2014, 10:53 AM   #4
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Conflicting super heat and sub cooling


That super heat chart is for 400 CFM per ton. And you only have 300 CFM per ton. The low super heat and sub cooling is showing you a low air flow condition.
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Old 08-31-2014, 10:56 AM   #5
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Conflicting super heat and sub cooling


superheat is the amount of heat the gaseous refrigerant gains without changing state, remaining a gas.

the lower the superheat the more chance of the vapor changing state along it's travel back to the compressor, where it's put under duress as the compressor takes it from a low temp low pressure vapor to a high temp high pressure gas. from there the vapor condenses as it travels and cools off in the coils, changing state to a liquid as it rejects heat.

i'm interested, just not sure the *variables so no definitive answer possible.

good thread.
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Old 08-31-2014, 10:58 AM   #6
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Conflicting super heat and sub cooling


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That super heat chart is for 400 CFM per ton. And you only have 300 CFM per ton. The low super heat and sub cooling is showing you a low air flow condition.

was just thinking he has no return air to speak of, a variable. good educating, bt.
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Old 08-31-2014, 11:00 AM   #7
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Conflicting super heat and sub cooling


and we have no static pressure to adhere to this equation either....

ducts? evap condition, blower wheel condition, cap mfd which equals blower ability, etc...
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Old 08-31-2014, 11:03 AM   #8
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Conflicting super heat and sub cooling


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That super heat chart is for 400 CFM per ton. And you only have 300 CFM per ton. The low super heat and sub cooling is showing you a low air flow condition.
OK, so I should be happy that it's working well and don't worry about the numbers.
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Old 08-31-2014, 11:08 AM   #9
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Conflicting super heat and sub cooling


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OK, so I should be happy that it's working well and don't worry about the numbers.
how can it be working well with the numbers and data now provided? Ignorance is bliss, but now that is not the case. your system is not working well, it's simply working...for now.
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Old 08-31-2014, 11:15 AM   #10
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Conflicting super heat and sub cooling


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how can it be working well with the numbers and data now provided? Ignorance is bliss, but now that is not the case. your system is not working well, it's simply working...for now.
My system kicks butt. If I would even let you in the front door, what would your solution be?
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Old 08-31-2014, 12:16 PM   #11
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Conflicting super heat and sub cooling


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My system kicks butt. If I would even let you in the front door, what would your solution be?
your system sucks ass. ask me to come into your home, pay the corresponding service fee, and I'll take advantage of your ignorance. static pressure, return air flow.

but alas, you don't need me but don't say I didn't say so when your compressor locks up due to slugging due to low air flow due to your system "kicking it's own ass".
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Old 08-31-2014, 12:29 PM   #12
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Conflicting super heat and sub cooling


at 4 degrees superheat your system suspects to be well overcharged but one can only suspect as to why, but low air flow has been brought up...why low air flow? 900 to 1000 return cfm is around half of required. ask me how I know.

superheat is not an answer but a diagnostic tool. by your own doing you've diagnosed your system as inadequate, in poor health.

does sub par mean kick ass in this day and age?

please, you've diagnosed your system already and kudos to you for doing so, but now fix it.
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Old 08-31-2014, 12:36 PM   #13
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Conflicting super heat and sub cooling


so you know, with choked return air your system is starving. results in blower motor overamping as it's a constant torque motor and will do what it is meant to do regardless of what is in front of it or behind it, in this case not enough behind it.

that's just one clause but to make it simple for you it's going to pull higher amps due to lack of return causing more heat on run winding and capacitor causing premature failure and all the while costing you more money than design intention as it runs.

kick ass? don't think so, but now you know.
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Old 08-31-2014, 01:16 PM   #14
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Conflicting super heat and sub cooling


Zapp, a lot of folk think that because their system runs and/or achieves desired set point that it is "kick ass"".

it takes hundreds of educated and monetary triumphs for any comfort system to at bare minimum do what it was designed to do, not talking cooling as an alone subject as you perceive.

Cooling? that's nothing. efficient? that's everything.
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Old 08-31-2014, 02:52 PM   #15
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Conflicting super heat and sub cooling


You are digging yourself a mighty deep hole Doc.


Quote:
Originally Posted by Doc Holliday View Post
ask me to come into your home, pay the corresponding service fee, and I'll take advantage of your ignorance.
Lord help the average customer who knows nothing about HVAC.

Quote:
Originally Posted by Doc Holliday View Post
but alas, you don't need me but don't say I didn't say so when your compressor locks up due to slugging due to low air flow due to your system "kicking it's own ass".
How long do I have to wait for the compressor to lock up Doc? It's been slugging along for 20 years so far.

Quote:
Originally Posted by Doc Holliday View Post
but low air flow has been brought up...why low air flow? 900 to 1000 return cfm is around half of required.
Since when does a 3 ton system need 1800 to 2000 CFM? Locking my front door even tighter.

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Originally Posted by Doc Holliday View Post
superheat is not an answer but a diagnostic tool.
The very reason for my post here.

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Originally Posted by Doc Holliday View Post
please, you've diagnosed your system already and kudos to you for doing so, but now fix it.
Is this what you would do? Tear my walls out and put in larger ducts? Or maybe throw out a perfectly good working system and install a 2 ton unit?

Quote:
Originally Posted by Doc Holliday View Post
so you know, with choked return air your system is starving. results in blower motor overamping as it's a constant torque motor and will do what it is meant to do regardless of what is in front of it or behind it, in this case not enough behind it.
How do you know "in this case not enough behind it"? It could also be a supply problem...you should know this.

Quote:
Originally Posted by Doc Holliday View Post
that's just one clause but to make it simple for you it's going to pull higher amps due to lack of return causing more heat on run winding and capacitor causing premature failure and all the while costing you more money than design intention as it runs.
Another fail....as an HVAC professional you should know that a squirrel cage blower restriction will result in less work and less amp draw on the motor. How long do I have to wait for this motor failure? It just had its 30th birthday last year.

Quote:
Originally Posted by Doc Holliday View Post
Zapp, a lot of folk think that because their system runs and/or achieves desired set point that it is "kick ass"".
Oh it does way more than just achieving set point. I can hear 3 other heat pumps and they run MUCH more than mine. Especially on the cold nights when mine is cycling normally and theirs will run continuously for hours at a time. And my relatively low electric bill backs this up. No smoke blowing here.

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