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  1. #1
    Join Date
    Jul 2012
    Posts
    20

    summer dehumidification, make up air, and room to room grilles

    Now that the Dallas Texas weather is getting better for chores, there are some things that could make my a/c better. Its a 3 ton unit in a poorly insulated 1000SF house but it does well since I improved the return filter area. I may add some insulation but that is problematic/involved, may ask later. The indoor machinery sits on a hollow box serving as an intake plenum.

    1. I've been told that if I like fresher air, I could bring in 50-100CFM from the outside and that this may also help reduce the amount (if any) of air coming in from the crawl space or attic. Will as much as 100CFM of fresh air from the outdoors impact the system in 100 degree weather? It is a small extra load including the humidity. There is a 4" unused dryer duct already making a run near there. I could replace it with new (its full of lint) and route it to the return air box if that is a good idea. an automatic damper would be easy.

    2. The RH is about 50-60% with the a/c on in the summer. Is there any "feels cooler" advantage to it being lower? Is this something a dehumidifer would help, or is one of those be insignificant compared to the dehumidification done by the HVAC system?

    3. There are two returns, a dual 12x24x1 in the dining room and the original 12x24x1 one in a bedroom (presently a catch-all room). Much air flows through the bedroom grille due to less resistance/ the way things were put together, - so the door stays open at least 4" for best airflow. If I put one of those above-door grilles in, the largest that would fit, will this let me close the door without much impediment to the return flow?

    Thanks for your opinions!

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,287
    Quote Originally Posted by Opcom View Post
    Now that the Dallas Texas weather is getting better for chores, there are some things that could make my a/c better. Its a 3 ton unit in a poorly insulated 1000SF house but it does well since I improved the return filter area. I may add some insulation but that is problematic/involved, may ask later. The indoor machinery sits on a hollow box serving as an intake plenum.

    1. I've been told that if I like fresher air, I could bring in 50-100CFM from the outside and that this may also help reduce the amount (if any) of air coming in from the crawl space or attic. Will as much as 100CFM of fresh air from the outdoors impact the system in 100 degree weather?

    2. The RH is about 50-60% with the a/c on in the summer. Is there any "feels cooler" advantage to it being lower?
    Is this something a dehumidifer would help, or is one of those be insignificant compared to the dehumidification done by the HVAC system?

    3. If I put one of those above-door grilles in, the largest that would fit, will this let me close the door without much impediment to the return flow?
    1.
    Q total = 4.5 * CFM * differential enthalpy
    Q sensible = 1.08 * CFM * dT = 1.09 * 100 * (100- 52) =~ 5,000 BTU/Hr or 0.7 ton considering Sensible Heat Ratio ( SHR).

    100 CFM is equivalent to nearly 50% of the capacity of a normal "right sized" unit for a 1,000 sq foot tight, well insulated residence.

    ___

    2. 55% versus 40% would enable one to turn the thermostat up 4' to 7'F.
    http://www.ashrae.org/resources--pub...re/standard-55

    3. Yes, depending on the effective open area of the selected grille.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,439
    Quote Originally Posted by Opcom View Post
    Now that the Dallas Texas weather is getting better for chores, there are some things that could make my a/c better. Its a 3 ton unit in a poorly insulated 1000SF house but it does well since I improved the return filter area. I may add some insulation but that is problematic/involved, may ask later. The indoor machinery sits on a hollow box serving as an intake plenum.

    1. I've been told that if I like fresher air, I could bring in 50-100CFM from the outside and that this may also help reduce the amount (if any) of air coming in from the crawl space or attic. Will as much as 100CFM of fresh air from the outdoors impact the system in 100 degree weather? It is a small extra load including the humidity. There is a 4" unused dryer duct already making a run near there. I could replace it with new (its full of lint) and route it to the return air box if that is a good idea. an automatic damper would be easy.

    2. The RH is about 50-60% with the a/c on in the summer. Is there any "feels cooler" advantage to it being lower? Is this something a dehumidifer would help, or is one of those be insignificant compared to the dehumidification done by the HVAC system?

    3. There are two returns, a dual 12x24x1 in the dining room and the original 12x24x1 one in a bedroom (presently a catch-all room). Much air flows through the bedroom grille due to less resistance/ the way things were put together, - so the door stays open at least 4" for best airflow. If I put one of those above-door grilles in, the largest that would fit, will this let me close the door without much impediment to the return flow?

    Thanks for your opinions!
    A home of your size with 3 bedrooms needs a max of 50 cfm of fresh air when occupied.
    At 100^F, 73^F dew point, you have 3, 200 btus of total cooling. 1,300 btus sensible cooling and 1,800 btus of latent cooling. Consider that your a/c is about 36,000 btus of total cooling capacity. Introducing 50 cfm of fresh air usually reduces air infiltration by about 50% of the mechanically introduced air during an average wind condition. Although during very calm weather, expect no natural infiltration and 50 cfm will be an additional load. You do not need fresh air ventilation when the winds are +10 mph or the home is not occupied.
    My guess is that your a/c would provide the sensible/latent cooling needed during high cooling loads. With a 45^F cooling coil temp, expect, <50%RH during lengthly cooling runs. During cool evenings, the %RH will rise.
    As the cooling loads decline during the shoulder seasons, supplemental dehumidification is needed to to maintain <50%RH.
    I would suggest that you consider a small whole house ventilating dehumidifier as ideal method to duct, filter, circulate and dehumidify fresh air via your a/c ducts throughout your home. Units like the small Ultra-Aire 70H will provide the desire amount of fresh air on a occupancy schedule or with an CO2 controller when the home is occupied and the winds are not blowing.
    Regardless how you procede, you need fan powered fresh air that is not dependent on a cooling or heating load but determined by when occupied and a lack of wind. This suggest a dedicated fan like the fan in a whole house ventilaiting dehumidifier.
    You will have fresh air and 45%RH when occupied.
    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"

  4. #4
    Join Date
    Jul 2012
    Location
    Western KY
    Posts
    1,221
    3 tons on 1000 sqft is way more then you need. Doubt adding 50cfm of outside air would hurt the cooling at all. Don't know what the humidity is like in Dallas but here in KY that set up would cool, way to fast, and leave the humidity. Dehumidification with 330 sqft per ton is not going to happen. The system will not run long enough. I'd add on to the house and get the sqft per Ton up to 500-550.

  5. #5
    Join Date
    Dec 2005
    Posts
    1,918
    Quote Originally Posted by Opcom View Post
    Now that the Dallas Texas weather is getting better for chores, there are some things that could make my a/c better. Its a 3 ton unit in a poorly insulated 1000SF house but it does well since I improved the return filter area. I may add some insulation but that is problematic/involved, may ask later. The indoor machinery sits on a hollow box serving as an intake plenum.

    1. I've been told that if I like fresher air, I could bring in 50-100CFM from the outside and that this may also help reduce the amount (if any) of air coming in from the crawl space or attic. Will as much as 100CFM of fresh air from the outdoors impact the system in 100 degree weather? It is a small extra load including the humidity. There is a 4" unused dryer duct already making a run near there. I could replace it with new (its full of lint) and route it to the return air box if that is a good idea. an automatic damper would be easy.

    2. The RH is about 50-60% with the a/c on in the summer. Is there any "feels cooler" advantage to it being lower? Is this something a dehumidifer would help, or is one of those be insignificant compared to the dehumidification done by the HVAC system?

    3. There are two returns, a dual 12x24x1 in the dining room and the original 12x24x1 one in a bedroom (presently a catch-all room). Much air flows through the bedroom grille due to less resistance/ the way things were put together, - so the door stays open at least 4" for best airflow. If I put one of those above-door grilles in, the largest that would fit, will this let me close the door without much impediment to the return flow?

    Thanks for your opinions!
    Your house is already leaking like a sieve, why bother bring fresh air in the return box.

  6. #6
    Join Date
    Jul 2012
    Posts
    20
    100CFM = 50% of my tonnage? Ok I will reconsider that. Less is more.

    I'll look into this "Ultra-Aire 70H". I'll ask my a/c contractor about it, likely need a professional to install something like that properly. I like it cold, so anything that makes it feel cooler might be worthwhile.

    Ok, well looks like the only thing going in my favor is the over-door grille. And maybe continuing to improve the return. Ultimately I'd like to avoid returning through a bedroom but I've lived with it for this long.

    The place has its problems but was only $14K back in 1989. It was made in the 1940's when the people who bought these little houses opened the windows for a/c in the summer. Too bad about crime, prevents that today.

    Thanks!

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,085
    If the bedroom door needs to be open at least 4" for best air flow. A 7 foot high door being open 4" gives you a free area of 2.33 sq ft of free area. You would need a large grille above the door to match that.
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  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by dan sw fl View Post
    1.
    Q total = 4.5 * CFM * differential enthalpy
    Q sensible = 1.08 * CFM * dT = 1.09 * 100 * (100- 52) =~ 5,000 BTU/Hr or 0.7 ton considering Sensible Heat Ratio ( SHR).

    100 CFM is equivalent to nearly 50% of the capacity of a normal "right sized" unit for a 1,000 sq foot tight, well insulated residence.

    .
    The coil or supply air might be 52F, but indoor temperature would be 75F, so the impact on sensible capacity would be 1/2 your calculation. Also consider that adding 100CFM would pressurize the home slightly and reduce normal infiltration. So the impact would probably be closer to 2000-3000 BTU's.

    Corret me if I'm looking at this the wrong way.

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by Opcom View Post
    The place has its problems but was only $14K back in 1989. It was made in the 1940's when the people who bought these little houses opened the windows for a/c in the summer. Too bad about crime, prevents that today.

    Thanks!
    They make bars to go on the windows.

    In 1940's AC was relatively new, an expensive luxury, a still primary only found in commerical buildings and retail stores. IF you store had AC, it was a popular place to shop. Most older homes have a lot of windows, better shading designed into the strucures, higher ceilings, sleepping proches, and other features to cope with summer heat. Even just plaster walls themselves have the advantage of greater mass to help a structure stay cooler later in the day. They are also more tolerate to very high humidity and don't redily support mold growth. I've seen some old home that had horrible roof leaks and water damage going on for years that look better than a new construction thats had a water leak for just a couple weeks.

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