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Difference between 2-stage and 1-stage A/C and furnace when both have a variable speed fan?

Hi all,

I'm having a really hard time understanding the effective difference in comfort between having a 2-stage A/C or furnace vs. a 1-stage, when both setups have a variable speed fan/air handler. Maybe I'm butchering the nomenclature, but I can't find anything on it.

My impression is that having a 1-stage system with a variable fan is between a 2-stage and a 1-stage with a fixed fan. Is that right? Does it bridge the gap in humidity removal during the summer? I live in the Chicago area and the summers can get muggy, but if there's not a big difference in the humidity control, I see no reason to go for the 2-stage A/C given the price increase (~$1600).

My HVAC guy who gave me the quotes said that there isn't a big difference between the furnace options, but people really comment on the A/C due to the humidity control.

Thanks!

Comments

  • captaincocaptainco Member Posts: 222
    Actually 2-stage A/C doesn't dehumidify as well as a single stage. The A-Coil doesn't get cold enough. I get this from a very smart guy in Texas that teaches airflow and btus with my company.
    2-Stage furnaces are for people that like to wait to get warm, but they do have an advantage if the right tech is working on them and makes some adjustments.
  • John MillsJohn Mills Member Posts: 759
    Agreed. Sometimes the latent heat ratio isn't too good on low so to me, the argument of better dehumidification isn't always valid. I'd rather get a good single stage and use the dehumidify on demand feature of the variable speed blower or if you are really in a moist area, a whole house dehumidifier is wise. No A/C dehumidifies when there is no call for cooling such as 70 and raining out.
  • ChrisJChrisJ Member Posts: 8,467
    edited November 2016
    I'm a bit confused by what has been said here.

    For example if you have a 2 stage 3 ton A\C system that runs the blower at 1200 CFM on high and 800 CFM on low, why would the evap not get "cold enough" for good dehumidifcation?

    You have 67% of the normal air flow and 67% of the normal compressor output so the coil should run at the same temperature. No?

    Most of these also have a dehumification function which forces the fan to run even slower and many thermostats will allow the system to cool up to 3 degrees lower than the setpoint for moisture removal.

    All of the charts I've seen for 2 stage systems showed the same SHR in both high and low stages with slightly improved efficiency in low.


    My opinion at this point is a 2 stage A\C system should have the same humidity removal as a single stage, but should result in far greater comfort and much quieter operation (inside) most of the season. It should also give slightly improved efficiency.



    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    https://goo.gl/photos/ZgpNUTyckkmiEdAf9
    Central air project pictures
    https://goo.gl/photos/4JjnLStEq42sWsQo8
  • captaincocaptainco Member Posts: 222
    Based on field measurements, two stage cooling is not dehumidifying as good as single stage. What you say sounds good it just isn't what is happening.
  • ChrisJChrisJ Member Posts: 8,467
    captainco said:

    Based on field measurements, two stage cooling is not dehumidifying as good as single stage. What you say sounds good it just isn't what is happening.

    So you're saying the manufacturers who show the SHR for both high stage and low stage being identical are publishing false information?

    That's all I've got to go on right now is manufacturer's specifications.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    https://goo.gl/photos/ZgpNUTyckkmiEdAf9
    Central air project pictures
    https://goo.gl/photos/4JjnLStEq42sWsQo8
  • njtommynjtommy Member Posts: 1,058
    I will take a 2 stage system over a single stage any day of the week. My experience is they provide more comfort then a single stage system and sometimes even a bit of money savings( nothing crazy).

    You can and will dehumidify a space running on 1st stage and a lower fan speed. You must dial in your fan speed and make sure you superheat is with in range at the compressor.
  • ChrisJChrisJ Member Posts: 8,467
    edited August 9
    So,

    The results are in.
    My system most certainly dehumidifies better in LOW stage.

    In high stage I typically have an 18 to 20 degree drop across the coil, depending on outdoor conditions.

    In low stage however, 21 to 22 degrees is more common. The evaporator runs cooler and the air has more time around it.

    The results don't lie.





    This 2 stage system squeezes water of the air out just fine.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    https://goo.gl/photos/ZgpNUTyckkmiEdAf9
    Central air project pictures
    https://goo.gl/photos/4JjnLStEq42sWsQo8
  • RichRich Member Posts: 2,212
    captainco said:

    Based on field measurements, two stage cooling is not dehumidifying as good as single stage. What you say sounds good it just isn't what is happening.

    Field measurements of what ? Properly designed systems or systems designed by folks who think they know it all ?
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC 732-751-1560
    Serving most of New Jersey , Eastern Pa .
    Consultation , Design & Installation
    Rich McGrath 732-581-3833
  • RichRich Member Posts: 2,212
    ChrisJ said:

    So,

    The results are in.
    My system most certainly dehumidifies better in LOW stage.

    In high stage I typically have an 18 to 20 degree drop across the coil, depending on outdoor conditions.

    In low stage however, 21 to 22 degrees is more common. The evaporator runs cooler and the air has more time around it.

    The results don't lie.





    This 2 stage system squeezes water of the air out just fine.

    Hard to argue with math and data on a system with right sized ductwork . We must not forget identical systems will perform differently in buildings with different types of envelopes . Something many of these EXPERTS forget to take into consideration , no matter who they work for . There is a reason that these trades got so screwed up , just sayin
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC 732-751-1560
    Serving most of New Jersey , Eastern Pa .
    Consultation , Design & Installation
    Rich McGrath 732-581-3833
  • Harvey RamerHarvey Ramer Member Posts: 1,899
    Dehumidification in a home has everything to do with the amount of time the air is exposed to a condition that causes it to condense.

    If a two stage system is properly set up with the correct sizing and airflow, it will most certainly outperform a single stage system in the same scenario.

    Not talking about efficiency. That's a different animal.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • ChrisJChrisJ Member Posts: 8,467

    Dehumidification in a home has everything to do with the amount of time the air is exposed to a condition that causes it to condense.



    If a two stage system is properly set up with the correct sizing and airflow, it will most certainly outperform a single stage system in the same scenario.



    Not talking about efficiency. That's a different animal.

    According to the numbers on my setup, they claim it's more efficient in low stage as does a study I had read on 2 stage setups a while back.

    Can I prove it? Of course not.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    https://goo.gl/photos/ZgpNUTyckkmiEdAf9
    Central air project pictures
    https://goo.gl/photos/4JjnLStEq42sWsQo8
  • NateEnergySmartNateEnergySmart Member Posts: 1
    Thank you for actually measuring on your system! You ended the thread with it. =)

    Right sized, multiple stage HVAC consistently delivers good dehumidification in our experience. We almost entirely use higher end HVAC. http://energysmartohio.com/case-studies/
  • RichRich Member Posts: 2,212
    By the deafening sound of crickets I would imagine you have your answer Confused .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC 732-751-1560
    Serving most of New Jersey , Eastern Pa .
    Consultation , Design & Installation
    Rich McGrath 732-581-3833
  • GWGW Member Posts: 2,447
    Two stage systems have more run time that one stage

    Ac operation equates to drier air

    Two stage systems are better at dehumidifying
    Gary Wilson

    Wilson Services, Inc

    Northampton, MA
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