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Thread: high humidity

  1. #27
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    It wouldn't run, if it has safeties.
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  2. #28
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    Quote Originally Posted by motoguy128 View Post
    This might be a weird idea, but instead of a whole house dehumidifier, I wonder if you could use a heat pump to defrost the indoor coil on demand? I wonder if you could use the existing coil temperature sensors to do this? Or use the heat pump to add heat to the home when you've reached the setpoint, but you need further dehumidification. What issues would you have running the heat pump at high outside temperatures?
    Adding heat to home with a wet coil is interesting. In the first 10 minutes, 3lbs. of moisture will evaporate back into the home. That will make your home really humid. But your right, that dehumidifier was way to simple. 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"

  3. #29
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    Quote Originally Posted by motoguy128 View Post
    This might be a weird idea, but instead of a whole house dehumidifier, I wonder if you could use a heat pump to defrost the indoor coil on demand? I wonder if you could use the existing coil temperature sensors to do this? Or use the heat pump to add heat to the home when you've reached the setpoint, but you need further dehumidification. What issues would you have running the heat pump at high outside temperatures?
    A heat pump during times of higher outdoor humidity levels and warm temperatures will be in the cooling mode, so it will already be dehumidifying the house.

    During cool, wet weather, should indoor dehumidification be required, your idea of running the heat pump to dehumidify would still require it to be in cooling mode, but with an additional provision to pump hot gas off the compressor through a reheat coil downstream from the normal indoor coil. This would be known as reheat via desuperheating the discharge gas off the compressor. Kind of a complex proposition.

    The fundamentally best way to control high indoor humidity is to attack the sources of humidity entry or generation. Infiltration via leaky structural members, and interior generation via showering, cooking, etc. Dehumidification will still be required at times, but with proper planning and diligence, in many cases the a/c may serve as the sole source of dehumidification, without discomfort issues.
    • 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. #30
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    My main thought was to dehumidify more aggressively with lower airflows, by utilizing the heat pump to defrost the indoor coil as needed... thereby extending it's operating range of airflow and wet bulb temps.

    But as Beenthere mentioned, a typical system isn't designed to operate in heating mode in high ambient outdoor temps.

  5. #31
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    Quote Originally Posted by motoguy128 View Post
    My main thought was to dehumidify more aggressively with lower airflows, by utilizing the heat pump to defrost the indoor coil as needed... thereby extending it's operating range of airflow and wet bulb temps.

    But as Beenthere mentioned, a typical system isn't designed to operate in heating mode in high ambient outdoor temps.
    Not only that, if the coil is frosting, it's running too cold to effectively dehumidify, anyway. Yes, it will remove moisture via freezing on the coil, but airflow will continually diminish to where the rate of moisture extraction can't keep pace with moisture generation within or into the house. The only thing accomplished when the frozen coil is defrosted is that it will re-evaporate moisture into the airstream, defeating the purpose of running the system as a dehumidifier.

    There are better ways to accomplish what you're after without going crazy with re-inventing the wheel.
    • 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.

  6. #32
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    I thought that if you defrost with no airflow, most of the ice, would melt and run off the coil... same as an HP outdoor coil in defrost. Normal indoor temp and humidity levels don't normally support sublimation or evaporation of the water as sometimes happens in cold dry outdoor conditions.

    I'm just trying to find a economical solution to dehumidify further, with the existing equipment without overcooling... and without installing a supplemental whole house dehumidifier.

    I ran the math for using a 5 kw of heat strips with a 3 Ton HP with a COP of 4.0 and compared it to a 1000Watt dehumidifer with a COP of 3.5. I figured a annual increase in energy costs of about $250/year if the dehumidifer ran for 12 hours per day, vs 6 hours for the 3 Ton heat pump with heat strips since it would have roughly twice the latent capacity but uses about 2.5 times more energy (5kw heat strips drops the HP COP to 1.4). Including installation, you could be looking at 8-10 years for a payback. Although my math might be completely screwed up... and the dehumidifiers may be much more effecient than the guess I used.

  7. #33
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    Quote Originally Posted by shophound View Post
    A heat pump during times of higher outdoor humidity levels and warm temperatures will be in the cooling mode, so it will already be dehumidifying the house.

    During cool, wet weather, should indoor dehumidification be required, your idea of running the heat pump to dehumidify would still require it to be in cooling mode, but with an additional provision to pump hot gas off the compressor through a reheat coil downstream from the normal indoor coil. This would be known as reheat via desuperheating the discharge gas off the compressor. Kind of a complex proposition.

    The fundamentally best way to control high indoor humidity is to attack the sources of humidity entry or generation. Infiltration via leaky structural members, and interior generation via showering, cooking, etc. Dehumidification will still be required at times, but with proper planning and diligence, in many cases the a/c may serve as the sole source of dehumidification, without discomfort issues.
    Everything was going good until I got to the bold print. If you are getting an adequate air change rate (an air change every 4-5 hours) and you have several occupants, discribe planning and diligence for the a/c to provide dehumidification. Let's consider a wet week with 65^F-75^F outdoor weather. Looking forward to the answer. 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"

  8. #34
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    Quote Originally Posted by motoguy128 View Post
    I thought that if you defrost with no airflow, most of the ice, would melt and run off the coil... same as an HP outdoor coil in defrost.

    Did you ever see a vapor come off those outdoor coils during defrost. The a moisture vapor. So if it was inside, some of it would be absorbed by the air, and some of it would condense on the cold duct surfaces.

    Normal indoor temp and humidity levels don't normally support sublimation or evaporation of the water as sometimes happens in cold dry outdoor conditions.
    Did you ever put water into a glass,or pan, and leave it sit over night.
    It doesn't have the same amount of water in it in the morning.

    The hot wet coil will put most of the water on it back into the air.
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  9. #35
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    Quote Originally Posted by beenthere View Post
    Did you ever put water into a glass,or pan, and leave it sit over night.
    It doesn't have the same amount of water in it in the morning.

    The hot wet coil will put most of the water on it back into the air.
    Of course some water will be evaporated, that also happens as well when a dehumidifier shuts off and some moisture remains on the coils. The visible vapor during defrost on a heat pump is partly a function of the dry outdoor air, uneven distribution of ice and high temps used to remove the ice. In a perfect world, you would regulate the output of the compressor to maintain a coil fin surface temp slightly above the frost point.

  10. #36
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    Quote Originally Posted by motoguy128 View Post
    Of course some water will be evaporated, that also happens as well when a dehumidifier shuts off and some moisture remains on the coils. The visible vapor during defrost on a heat pump is partly a function of the dry outdoor air, uneven distribution of ice and high temps used to remove the ice. In a perfect world, you would regulate the output of the compressor to maintain a coil fin surface temp slightly above the frost point.
    Let's go back to defrosting an indoor coil with the heat pump in heating mode. Even if the safeties didn't pop immediately, and you could manage to entirely defrost the coil before the safeties did go, you would increase the air humidity level inside the supply plenum and air handler to at or near saturation. As soon as the defrost timer satisfied and shifted the heat pump back to cooling mode, the indoor blower would re-engage and blow a big slug of moisture back into the house. That would partially defeat all the run time previously needed to attain the humidity level the house enjoyed prior to the defrost.

    In a perfect world, you would regulate the output of the compressor to maintain a coil fin surface temp slightly above the frost point
    Yes, at a sacrifice of performance, since heat transfer in vapor compression refrigeration hinges upon mass flow of refrigerant through the system. That is, if you were speaking of keeping an outdoor coil of a heat pump above freezing when the system is in heating mode. Regarding regulating the output of a compressor, since most compressors are positive displacement pumps, the only ways to effectively regulate output of a compressor is to vary the compressor speed, or vary the amount of suction gas entering the compressor. Sure, you can use hot gas bypass to put a false load on the evaporator, but that isn't so much regulating compressor output as it is redirecting a portion of its output, in terms of volume pumped.
    • 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. #37
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    Quote Originally Posted by motoguy128 View Post
    Of course some water will be evaporated, that also happens as well when a dehumidifier shuts off and some moisture remains on the coils. The visible vapor during defrost on a heat pump is partly a function of the dry outdoor air,
    More like from the outdoor air being cooler then the warm moisture.


    Although we often refer to outdoor winter air as being dry.

    Its RH is well above 40% in many areas. When the outdoor temp is cold enough to require defrost cycles.
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