Wise to go with two stage cooling?
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  1. #1

    Wise to go with two stage cooling?

    Due to the added complexity, in a 2500 sq ft house in Northern Virginia, is it wise to go for a two-stage air conditioner? I understand that for heating, two stage has been around longer, and seems to be a no-brainier, but am wondering about cooling....

  2. #2
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    If your cooling season is long enough to justify the added expense I think two stage is the way to go. Good luck

  3. #3
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    Staging in an air conditioner will give you better ability to control humidity. Staging also gives you basically two different air conditioner sizes in a single unit that in effect is better matched to the cooling load of your home more time during the cooling season. The reason for staging is basically the same as for heating....... to closer match the equipment's capacity to the home's needs and by doing so increase the comfort levels in the home.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  4. #4
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    Quote Originally Posted by firecontrol View Post
    Staging in an air conditioner will give you better ability to control humidity. Staging also gives you basically two different air conditioner sizes in a single unit that in effect is better matched to the cooling load of your home more time during the cooling season. The reason for staging is basically the same as for heating....... to closer match the equipment's capacity to the home's needs and by doing so increase the comfort levels in the home.
    X2
    2 stage is real nice in areas where the humidity can be high.

  5. #5
    Thanks! The humidity is dreadful here, for four or five months out of the year. I had heard that two staged cooling was not "mainstream", so was wondering if it is reliable and cost effective.

  6. #6
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    Quote Originally Posted by Expaand View Post
    Due to the added complexity, in a 2500 sq ft house in Northern Virginia, is it wise to go for a two-stage air conditioner? I understand that for heating, two stage has been around longer, and seems to be a no-brainier, but am wondering about cooling....
    A single stage a/c with the air flow adjusted to optimize the moisture removed from the air will maintain <50%RH during moderate to high cooling loads. During the hours of lower/no cooling loads, the indoor %RH will rise above ideal comfort levels. During cool wet weather with low/no cool loads, expect indoor %RH to be above outdoor moisture levels because of the moisture from the occupants.
    A two speed a/c uses the same large cooling coil but slows the compressor/fan to 10-20% to lower capacity. This extends the cooling cycle with reduced air flow and lowers air flow. It takes longer to load the cooling coil with moisture and start draining. Air distribution will be poorer throughout the home. While the longer cooling cycle will remove more moisture, these systems are not able to maintain maintain <50%RH during low/no cooling loads. At the end of any of the cooling cycles, the remaining moisture on the cooling cool evaporates back into the home.
    The next level of a/c complexity is the variable speed a/c which can slow cooling up +50% which increases the moisture removal to the next level. Air flow/compressor capacity are reduce while using the large cooling coil. The air distribution is less but the moisture control is good down to 50% of the cooling load. Same problem at the end of the cooling cycle, the moisture on the large coil/pan evaporates back into the home. Yet during low/no cooling loads, the these expensive, complex systems are unable to maintain <50%RH.
    For the ultimate in comfort, simple maintaince, and reasonable investment, check out a single speed, correctly installed single speed a/c and a whole house dehumidifier (like Ultra-Aire).
    Your set the ideal temperature and %RH. The a/c will maintain the temperature. During high cooling loads, the a/c will maintain <50%RH. As the cooling load declines during cooler hours, the dehumidifier will maintain <50%RH, at fraction of the energy of over-cooling with a/c.
    This also provides <50%RH with a/c off during hours of extended non-occupancy. Special needs like lower levels or basements are maintain <50%RH during the times of the year when the outdoor dew points are +60^F.
    Ideal comfort, reasonable investment, simple service, and minimal operating cost are the benefits of a simple high SEER a/c and a whole house dehumidifier.
    Keep us posted on your choice and the results.
    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"

  7. #7
    Join Date
    Jun 2005
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    Quote Originally Posted by Expaand View Post
    Due to the added complexity, in a 2500 sq ft house in Northern Virginia, is it wise to go for a two-stage air conditioner? I understand that for heating, two stage has been around longer, and seems to be a no-brainier, but am wondering about cooling....
    Data I have indicates around 1000 cooling hours a season in Northern VA., S. FL has 2800 cooling hours; humidity can be rather high at times so that's a tossup.

    If you size properly & use a humidity software control system you may not need a 2-stage system.
    There may not be that much comfort difference between the 2 systems to justify the cost difference...?

    Depends on the price differential & what you want to spend, along with the actual difference it would make concerning your comfort...it's a decision you have to make by discussing it with a number of contractors near you.

  8. #8
    Join Date
    Jan 2009
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    In a 2 story home in a humid but otherwise moderate climate like yours (winter are summers are not extremely hot or cold) I would defintiely go 2 stage. Dual fuel, if you have natural gas is also a good option in that climate as well and fairly pipular it seems in the VA, MD region. I think you have good electric rates.

    Proper sizing is THE MOST IMPROTANT. Better a little small than too big.

    I got to experience what being slightly undersized is like the last few days. I have time of use electric pricing, so I use a feature on my equipment to disable 2nd stage on both of my AC units from 8AM to 9PM on weekdays. As a result, when it's over about 90F, sunny & humid (at or near design conditions), the unit falls behind a little. The result is it's 1-2F warmer (76-77F instead to 74-75F), but humidity is low... really, really low since the units will end up running continously for 16 hours straight. Because the humidity is low, it's still very comfortable and you really don't even notice. I have a lot of windows so tons of solar gain so humidity was 39% downstairs and 42% upstairs by 7PM as the sun started to go down.

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