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Thread: Can a residential variable speed system emulate a dehumidifier?

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    Can a residential variable speed system emulate a dehumidifier?

    Where both the air handler and the compressor are variable speed. Can this system get close to what a dehumidifier would do? That is dry the air without lowering the temperature indoors by much (well a dedicated indoor dehumidifier actually raises indoor temperature).

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    If there is no heat load an air conditioner can do nothing. The dehumidifier on the other hand can remove moisture regardless of the need to cool the building.
    "Is this before or after you fired the parts cannon at it?" - senior tech
    I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
    Unless we change direction we are likely to end up where we are going.

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    Quote Originally Posted by BillDX View Post
    Where both the air handler and the compressor are variable speed. Can this system get close to what a dehumidifier would do? That is dry the air without lowering the temperature indoors by much (well a dedicated indoor dehumidifier actually raises indoor temperature).
    No

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    Quote Originally Posted by R600a View Post
    If there is no heat load an air conditioner can do nothing. The dehumidifier on the other hand can remove moisture regardless of the need to cool the building.
    I'm confused. Isn't a dehumidifier basically a coil that is kept well below the dew point, airflow across the coil is relatively low, water vapor condenses when it comes in contact with the said coil, then water drains.

    If that is accurate, what is stopping a traditional HVAC system (with a variable speed blower) from recreating the same conditions?

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    Quote Originally Posted by BillDX View Post
    I'm confused. Isn't a dehumidifier basically a coil that is kept well below the dew point, airflow across the coil is relatively low, water vapor condenses when it comes in contact with the said coil, then water drains.

    If that is accurate, what is stopping a traditional HVAC system (with a variable speed blower) from recreating the same conditions?
    Running. If it’s hot out not a problem. If cool or cold it doesn’t run!

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    Quote Originally Posted by BillDX View Post
    I'm confused. Isn't a dehumidifier basically a coil that is kept well below the dew point, airflow across the coil is relatively low, water vapor condenses when it comes in contact with the said coil, then water drains.

    If that is accurate, what is stopping a traditional HVAC system (with a variable speed blower) from recreating the same conditions?

    I’m order to not cool the space, just bring the outdoor unit inside the space during dehumidification.
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    A heat source.

    I asked that same question and originally started building myself a large dehumidifier. But eventually I put a 10KW heat kit in the supply duct and switch it and the blower on when the dehumidistat reaches it's set point.

    PHM
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    Quote Originally Posted by BillDX View Post
    . . . what is stopping a traditional HVAC system (with a variable speed blower) from recreating the same conditions?
    PHM
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    The conventional view serves to protect us from the painful job of Thinking

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    Something that I sometimes think about is setting up a 2 stage cooling system so that high stage runs in the standard manner but the low stage runs with the accompanying blower speed adjusted to hold the cooling coil temperature at about 33º F. during low stage cooling.
    PHM
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    The conventional view serves to protect us from the painful job of Thinking

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    If the thermostat is capable of regulating humidity, a variable system can operate in low cooling mode to assist with dehumidifying, but if the room temp is already at temp set point, it will overcool the room… this causes the relative humidity of the cooler air volume to go up… without a heat source, be it Mother Nature, a condenser coil output, or a furnace of some type… it will not be able to adequately regulate humidity…
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    Quote Originally Posted by BillDX View Post
    Where both the air handler and the compressor are variable speed. Can this system get close to what a dehumidifier would do? That is dry the air without lowering the temperature indoors by much (well a dedicated indoor dehumidifier actually raises indoor temperature).
    An excellent question!

    It is a long chapter in the a/c tech manual. Lets see if can do a partial in a couple paragraphs.

    In a dry climate.
    Take air at the desired temperature, %RH, dew point in a typical home like 75^F, 50%RH, 55^F dew point. This is the air that is sucked into the a/c. If the cooling is 55^F which is the dew point of air entering, no moisture will be remove but the home will be the cooled by the 58^F, 90%RH, 55^F dew point air. This may be acceptable in an dry climate where the outdoor dew point may be <50^F. A few occupants adding a lb. of moisture which is removed by fresh outdoor passing through the home. A home needs a fresh air change in 3-4 hours to purge the indoor pollutants and renew oxygen. A couple lbs. of moisture will be removed by the 50^F dew point fresh air passing through the home. 100% of the a/c cooling is sensible cooling. No moister will be removed until the dew point of the air entering the is +55^f dew point.
    No problem in an dry climate.

    In a green grass climate.
    We still want a 75^F, 50%RH, 55^F dew point inside the home. Using the same setup of the a/c as the above, the home would be maintained at 75^F, 50%RH, a 55^F dew point if there no occupants or fresh air infiltrating the home. But with 4 occupants and 100 cfm of fresh air infiltration/ventilation. Occupants add .25 lbs. per occupant 1 lb.per hour for 4 occupants. Typical outside air in a green grass is 60-75^F (average 70^F) dew point , 100 cfm of 70^F dew point air is adding 4 lbs. of moisture per hour. What is going to happen? This house needs 5 lbs. of moisture removed per hour to maintain 75^F, 50%RH, a 55^F dew point the home will rise to 75^F, +70%RH, +70^ dew point.
    To fix this problem we adjust the air flow through the cooling coil to remove the +5 lbs. per hour of moisture while cooling the home. Using the example of a 3 ton a/c, slowing the air flow to get a 45^F cooling coil, will supply a 49^F dew point air to the home. With 75^F, 50%RH, a 55^F dew point air entering and a 50^F dew supply we have a 3 lbs. of moisture removed per ton per hour or 9 lbs. per hour for our 3 ton unit. This will remove the 5 lbs. per hour we need. We have enough extra dehumidification we can live with a 50% (half time) cooling cycle and make the 50%RH inside the home.

    OK so far, right. Now we move to evening hours of the day. The moisture from outdoor air is similar to the and worse are rainy days that can have the same 70^F outdoor dew point with no sensible cooling . The only way we can get the a/c to remove 5 lbs. of moisture per hour is to a 50% run time of our 3 ton a/c. If the 3 ton was a 2 speed with a 50% drop in speed, we would be ok at 1.5 ton of sensible-latent load. But we may have mostly low or no sensible cooling, but still have the 5lbs. per hour of latent(moisture) load. The way to get the a/c to remove the 5 lbs. of moisture per hour is to have a 15,000 btu sensible cooling load. We need 5 kwh of reheat for the a/c to remove the moisture. Here is a better way, install a 3-4 lbs per hour dehumidifier that will uses <1 khw that provides 7,000 btus of reheat.

    The means you can use a simple a/c and the right sized dehumidifier, like a Santa F Ultra 70H and maintain 75^F, 50% RH, 55^F dehumidifier at a minimal operating cost. This unit also will provide merv 13 air filter and fresh air when wanted.

    A little confusing, open for comments.

    Regards Teedy Bear
    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"

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    Quote Originally Posted by teddy bear View Post
    uses <1 khw that provides 7,000 btus of reheat
    I'll pretend to understand everything except this part, how can an enclosed system using less than 3412 btus provide 7000 btus of reheat?

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    Quote Originally Posted by BillDX View Post
    I'll pretend to understand everything except this part, how can an enclosed system using less than 3412 btus provide 7000 btus of reheat?
    Because it freed up the latent heat that was trapped in the moisture that it removed.
    "Is this before or after you fired the parts cannon at it?" - senior tech
    I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
    Unless we change direction we are likely to end up where we are going.

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    Got it. Thanks. Now onto some real numbers, let's take an average 2,500ft house with 9ft ceiling first floor and 8ft on second = 21250 cubic feet of air.

    Let's say it's 68F and 80% humidity outside and a target of 68F and 50% humidity inside.

    21250 cubic feet of air at 68F and 80% humidity = 17.55 pints of water
    21250 cubic feet of air at 68F and 50% humidity = 11 pints of water

    So if the house has 4 exchange of air per day that means that ( 17.55 - 11 ) * 4 = 26.2 pints of moisture have to removed every day? (not including moisture from occupants)

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    Quote Originally Posted by BillDX View Post
    Got it. Thanks. Now onto some real numbers, let's take an average 2,500ft house with 9ft ceiling first floor and 8ft on second = 21250 cubic feet of air.

    Let's say it's 68F and 80% humidity outside and a target of 68F and 50% humidity inside.

    21250 cubic feet of air at 68F and 80% humidity = 17.55 pints of water
    21250 cubic feet of air at 68F and 50% humidity = 11 pints of water

    So if the house has 4 exchange of air per day that means that ( 17.55 - 11 ) * 4 = 26.2 pints of moisture have to removed every day? (not including moisture from occupants)
    Why would you run your A/C when its 68°F inside. Add to the fact you would probably freeze the unit up if you tried!

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    Quote Originally Posted by BillDX View Post
    I'll pretend to understand everything except this part, how can an enclosed system using less than 3412 btus provide 7000 btus of reheat?
    It is a good question. Here is my answer.

    The dehumidifier is a closed system. The compressor and fan uses 1 kwh. The heat from i kwh used by motors generates the 3412 btus of heat. The cooling system condenses 5 lbs of moisture at 1,050 btus per lb. or 5,250. This is a total of 8,664 btus. The actual total btus depends on the efficiency of the dehumidifier. The Santa Fe Ultra units range from 4.75 to 9 lbs of condensation per kwh.
    The reheat could be used by the a/c to remove additional moisture. It all works out. The a/c starts to short cycle and the indoor %RH rises until the dehumidistat is activated. The dehumidifier runs, removing moisture and adding reheat. When the a/c is setup to remove moisture while running to remove the reheat from the dehumidifier. Finally, as the outdoor temperature continue to decline but the outdoor dew point is above the desired indoor dew point, the a/c stops cooling. Temperature may continue to decline, the reheat of the dehumidifier continues to warm the cooling building with reheat at a COP of +3-4. Warming the space also lowers the indoor 2%RH per degree of warming. It is like magic. Just kidding.

    Make sense?

    Regards Teddy Bear
    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"

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  23. #16
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    Quote Originally Posted by teddy bear View Post
    Make sense?
    Surprisingly it does. Thanks.

    Does my math above look right about sizing a dehumidifier? Or is there more magic to it?

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    Quote Originally Posted by BillDX View Post
    Surprisingly it does. Thanks.

    Does my math above look right about sizing a dehumidifier? Or is there more magic to it?
    Again how long will that unit run at 68° outside. Run the stove, turn on all the lights, maybe a few hair driers and you might get 15 minuets run time. Not enough.

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    Quote Originally Posted by pecmsg View Post
    Again how long will that unit run at 68° outside. Run the stove, turn on all the lights, maybe a few hair driers and you might get 15 minuets run time. Not enough.
    My math attempt is for sizing a dedicated indoor dehumidifier. Which I assume would run as close to 100% if I size it according to my estimate.

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  27. #19
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    Quote Originally Posted by BillDX View Post
    Got it. Thanks. Now onto some real numbers, let's take an average 2,500ft house with 9ft ceiling first floor and 8ft on second = 21250 cubic feet of air.

    Let's say it's 68F and 80% humidity outside and a target of 68F and 50% humidity inside.

    21250 cubic feet of air at 68F and 80% humidity = 17.55 pints of water
    21250 cubic feet of air at 68F and 50% humidity = 11 pints of water

    So if the house has 4 exchange of air per day that means that ( 17.55 - 11 ) * 4 = 26.2 pints of moisture have to removed every day? (not including moisture from occupants)
    Ideally, a home needs a fresh air change in 3-4 hours. The air change in 6 hours is a little low. Plus figure about .25 lb of moisture per hour per occupant. This may get you up to 5 pints per hour. Also during times of the average 7 mph, expect additional infiltration.

    You get the ideal. A 70 pint per day dehumidifier may get extra capacity you could need.

    All in all, Ok.

    Keep us posted.
    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"

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    Thanks. I was just making sure that applying a common sense calculation was all it took to get a proper estimate, provided all additional sources of humidity are added in to the total.

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