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  1. #1

    2 ton evap coil & 1.5 ton condenser.

    I have a Rheem air handler, w/ electric heat, and 2T evaporator coil, that's been heating my two car garage for 5 years. I plan to add a 1.5T, nearly new condensing unit. Both units are 10 SEER. With this combination, will using a TXV instead of a flow-rater (piston), ... provide noticable cooling performance? I was in the trade until 1992, and I'll be doing the work myself. I'm aware that a TXV might require a hard start kit for the condensing unit. Appreciate all input. Thanks

  2. #2
    Join Date
    Apr 2009
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    Well a 10 SEER is a 10 SEER. I don't know if you're going to get much noticeable difference by adding a TXV... You're already prorating the system by oversizing the coil, you'll get a little better dehumidification...

  3. #3
    Join Date
    Aug 2003
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    Quote Originally Posted by muskyslayr View Post
    Well a 10 SEER is a 10 SEER. I don't know if you're going to get much noticeable difference by adding a TXV... You're already prorating the system by oversizing the coil, you'll get a little better dehumidification...
    Better dehumidification with larger coil in relation to smaller compressor pumping capacity...not necessarily. The ability of an evaporator coil to dehumidify rests upon how low the coil surface temperature can be maintained. If a coil is smaller in relation to the compressor's pumping capacity, suction pressure will be lower, hence saturated vapor temperature will be lower, which in turn means a colder coil surface temperature, aka "apparatus dew point".

    So, if a coil is larger than a compressor's pumping capacity, it will tend to run warmer due to higher suction pressure. The compressor can't remove enough refrigerant to result in a lower boiling temperature (phase change occurring at a lower pressure/temperature).

    For the OP...this is not a DIY site. Can't help you much with specifics unless you were registered as a pro.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Join Date
    Jun 2003
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    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by shophound View Post
    Better dehumidification with larger coil in relation to smaller compressor pumping capacity...not necessarily. The ability of an evaporator coil to dehumidify rests upon how low the coil surface temperature can be maintained. If a coil is smaller in relation to the compressor's pumping capacity, suction pressure will be lower, hence saturated vapor temperature will be lower, which in turn means a colder coil surface temperature, aka "apparatus dew point".

    So, if a coil is larger than a compressor's pumping capacity, it will tend to run warmer due to higher suction pressure. The compressor can't remove enough refrigerant to result in a lower boiling temperature (phase change occurring at a lower pressure/temperature).

    For the OP...this is not a DIY site. Can't help you much with specifics unless you were registered as a pro.
    Coil temperature/dehumidification has more to do with the amount of air flowing through a coil than the size of the coil. Slowing the air flow allows any sized coil to get colder and remove more moisture. The lower limit is avoid freezing the coil. The highest efficiency dehumidifcation have large evaporator coils with low air flow. If you are unable to slow air flow than the smaller coil will remove more moisture. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
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    Jan 2009
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    I thought that generally a larger coil has lower percentage of latent capacity since the larger surfance are results in a high coil surface temp with the same capacity.

    I know thsi is the case on my system. In highsight, choosing a oversize coil was a mistake. It gave me 0.25 higher SEER, but I'm running the fan at 350CFM/ton to compensate.

    The othe problem with a larger coil is that it has more surface to wet, so less moisture will find it was to the drain pan. Meaning that more moisture will re-evaporate when the system is idle.

    It woudl be interesting ot compare 2 system side by side with hte sme capacity, air handler, outdoor unit, but a larger coil on one.

  6. #6
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    Quote Originally Posted by teddy bear View Post
    Coil temperature/dehumidification has more to do with the amount of air flowing through a coil than the size of the coil. Slowing the air flow allows any sized coil to get colder and remove more moisture. The lower limit is avoid freezing the coil. The highest efficiency dehumidifcation have large evaporator coils with low air flow. If you are unable to slow air flow than the smaller coil will remove more moisture. Regards TB
    The larger coil will have lower face velocity given hte larger surface area... BUT I think proportionally the capacity of the system is spread across that larger surface area. The net result... at least with my XL16i, was about a 2-5% lower ratio of latent to sensible capacity. Not a big difference. But then again, more misture will remain on the coil when idle and reevaporate.

  7. #7
    Join Date
    Dec 2006
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    Middle Tennessee
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    *

    Quote Originally Posted by motoguy128 View Post
    The othe problem with a larger coil is that it has more surface to wet, so less moisture will find it was to the drain pan.
    good point

    Meaning that more moisture will re-evaporate when the system is idle.
    not really if fan is set to auto

    It woudl be interesting ot compare 2 system side by side with hte sme capacity, air handler, outdoor unit, but a larger coil on one
    yes, it would ne interesting.


    .

  8. #8
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    Jul 2006
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    I used to do that all the time before the vsd came out. Use a txv and suck the moisture out of the air, watch the superheat.
    You sure are cocky for a starving pilgrim.

  9. #9
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    Quote Originally Posted by teddy bear View Post
    Coil temperature/dehumidification has more to do with the amount of air flowing through a coil than the size of the coil. Slowing the air flow allows any sized coil to get colder and remove more moisture. The lower limit is avoid freezing the coil. The highest efficiency dehumidifcation have large evaporator coils with low air flow. If you are unable to slow air flow than the smaller coil will remove more moisture. Regards TB
    I was speaking in terms of central a/c vs. a dedicated or stand alone dehumidifier. Very often whenever a larger evap coil is mated to a smaller compressor, no thought is given toward adjusting airflow. Wefixumgood A/C and Heat says "Oh, that bigger evap coil should suck more moisture from the air". You hear this all the time in mixed humid climates like mine and hot humid climates like Houston, Texas. The coil is upsized, the compressor is not, the airflow is not slowed. The result is a net loss of latent capacity. Or...indoor air handler/furnace is replaced, larger evap coil installed, blower motor set for highest speed (since many installers are conditioned to do that to overcome what amounts to crappy flex duct installation methods) but no match effort applied to the outdoor section. It is left at the smaller capacity. Same deal. Worst of all is doing all of that with fixed orifice metering. Might as well yank out the piston and let raw liquid refrigerant roar through the evap coil. It would kill the compressor quickly, but oh well.

    Couple that to a widescale ignorant or apathetic understanding of building science via more than one trade; the need to dehumidify is never really mitigated, only bandaged. Homes in green grass climates, as you say, will always need some form of dehumidification. It becomes a matter of how intelligently does one wish to go about managing humidity. Cobble together a system and hope for the best? Or actually study all the dynamics in place and make informed design and construction decisions?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
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    Quote Originally Posted by motoguy128 View Post
    I thought that generally a larger coil has lower percentage of latent capacity since the larger surfance are results in a high coil surface temp with the same capacity.

    I know thsi is the case on my system. In highsight, choosing a oversize coil was a mistake. It gave me 0.25 higher SEER, but I'm running the fan at 350CFM/ton to compensate.

    The othe problem with a larger coil is that it has more surface to wet, so less moisture will find it was to the drain pan. Meaning that more moisture will re-evaporate when the system is idle.

    It woudl be interesting ot compare 2 system side by side with hte sme capacity, air handler, outdoor unit, but a larger coil on one.

    Here's the deal. For the sake of this illustration, the airflow will always be the same, regardless of coil size. I say this because your post bears out what often happens in the field. A larger indoor coil is mated to a smaller outdoor section, with no change in airflow. Net result? Increased sensible capacity, reduced latent capacity.

    To illustrate this matter, consider the following:


    • The pumping capacity of a reciprocating compressor increases with an increase in suction pressure (if all other factors remain constant)
    • To maintain a given suction pressure, an evaporator must be selected that can vaporize as great a weight of refrigerant as the compressor can handle
    • The compressor must be selected so that when the system is operating at full load, the compressor will maintain a suction pressure corresponding to the highest evaporator temperature needed

    On the final bullet point, a load calculation on the structure could best determine what that optimal evaporator temperature might be, for the given expected design conditions, both sensible and latent, that the structure might expect to see for its location. If the system is mismatched and it comes up short on latent, and then airflow is reduced to increase latent, it then might come up a bit short on sensible under design days or conditions where design day limits are exceeded.

    Until the industry reaches a point where it can make truly variable capacity refrigeration affordable to a large percentage of residences, we're stuck with sizing a system for a weighted average of worst expected climatic conditions. Even with variable capacity, it is still sized for worst expected, but has much better ability to ramp up or down according to actual need, resulting in greater comfort, humidity removal, and energy efficiency.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    My point is that I am able to remove as much moisture with my 13.5 SEER a/c as I could with my old 8 SEER. I slowed the old a/c to the point of fresh-up when operated below 74^F. The 13.5 SEER unit will go a couple degress cooler. I agree that the amount of moisture retention on the coil is higher on the larger coil which is a minor problem. How can any system work well without fine tooling the air flow to max the moisture removal? I suggest maxing the moisture removal of systems all of the time. If you are able to achieve <50%RH during high cooling loads, raise the tstat a couple degrees like +78^F. The other limitation on how low the coil temperature can be is duct/grill sweating. Keep the air temperature/duct temp above the point were condensation forms on the supply ducts. The idea making the system more complex to allowing shifting to high latent removal only when the humidity excedes 50%RH seems comples and wasteful. I agree with the idea that in all green grass sclimates, there are many days that the operating time of any a/c is not long enough or there is no cooling load, yet moisture must be removed. Most a/c contractors suggest avoiding fresh air ventilation because of the inability a/c to remove the moisture during low/no cooling load conditions. A whole house dehu is a simple solution to this problem is also able to manage the fresh air function. Also the dehu is able to maintain,50%Rh without any a/c operation when the home is unoccupied. 78^F, <50%RH is very comfortable and efficient. Enough fresh air to purge indoor pollutants during a time when a home gets little natural fresh air is a must for a healthy, comfortable home. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  12. #12
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    While pound for pound of air moved, a 4.6 sq ft surface area coil at an air flow of 280CFM(21 pounds of air a minute, 1,260 pounds an hour) may be able to remove as much moisture as a 3.4 sq ft surface area coil at 350CFM(26 pounds of air a minute, 1,560 pounds an hour). Roughly a 20% air flow difference.

    The air flow may be too little with the larger coil to meet the homes infiltration rate.
    And not keep the humidity as low as the smaller coil, with the higher air flow volume.
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  13. #13
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    Quote Originally Posted by muskyslayr View Post
    Well a 10 SEER is a 10 SEER.

    This isn't true either.

    Different brands have evap coils with different sq ft of surface area at the same tonnage.
    And some also use a different sized piston for the same ton age.
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