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  1. #1
    Join Date
    Jun 2011
    Location
    Dallas,TX
    Posts
    1

    2nd story not cooling off

    I know this question has been asked many times, but my situation is slightly different so I thought I'd ask. I am moving into a 2-story duplex, but the only rooms upstairs is a bedroom, bathroom and walk-in closet. There is no attic. The room upstairs is an "A" frame so it's very tall. My roommate has moved in and says it's very hot up there. There's only 1 a/c unit and the thermostat is on the wall of the staircase so it's reading the upstairs temperature. Downstairs is nice and cool, but upstairs not so much. We are closing some of the vents downstairs to see if that helps. Any other thoughts? Thanks!

  2. #2
    First of all, let me start by saying I am not an HVAC professional. I have been troubleshooting the same basic issue in my open foyer house though. I cut my HVAC energy usage by almost 30% and cut down my cycle time from 5 hours to reach a 75 degree set point down to under 2 hours. How do I know all the data, I just installed a whole home energy monitor system where I can see exactly what is going on including the cycle times etc. I would suggest to look at your return duct system first and don't concentrate so much on the vents. If you close too many vents off you will just increase the static pressure of the system and lower CFM considerably. The HVAC guys who designed my house were on drugs. Only 3 returns in the whole house. 2 main ones and one smaller return in the family room. The main return ducts are not even ducts, but are only 18x18 vents screwed in to cover a hole in the drywall. They used a free space in the center drywall studding as return duct. The problem is that the put both 18" main ducts (1st and 2nd floor) in series without sizing them appropriately to even out airflow. It seems the open space also went around an I-beam between floors, also considerably cutting down the open area to the upstairs return. The result was I had 90% of the air pulling in the 1st floor return, which was pulling all my cold air upstairs down to the first floor. I had a very static layer of hot air upstairs since hot air rises. Solution was easy. Go to Home Depot and get the frost king magnetic 8'x15" vent covers. I covered up 90% of the 1st floor vent and then I probably tripled the volume of air being pulled from upstairs (eg hot stagnant air). Temperature difference is now 2 degrees instead of the previous 6 deg. Funny thing is my system as designed works great in the winter without doing anything, most likely as it is already so skewed to pull from the 1st floor, which works well in the winter time so I will totally remove the cover in the winter.

    If you have a better designed system with returns in each room, you have to play a bit more and I would not suggest radically cutting down any single return. A friend of mine had basically the same issue with a system with returns in each room. He played around with cutting off some of the returns in colder rooms downstairs only and also increased the volume of air pulled into the system from the upper floor. He said it made a huge improvement.

  3. #3
    Oh yea, there is more to the story as well. I also wanted to install control in my bedroom, so the system would be able to hold the temperature we wanted properly there. The new Honeywell Prestige HD unit does this very well by using a remote unit that you can hand off control to at night. Overall, the system is more eye candy though than really smart, as you can not memorize that you want to control to the remote in the schedule so I have to manually switch it over each night. I think sometimes these guys are idiots who write software for these systems. Here is the link to the unit http://yourhome.honeywell.com/home/P...ort+System.htm

    I am actually running my system more at night now to control to 75 degrees in our master bedroom, but still using 32% + less A/C energy. I actually calculated my usage figures over one week before the mods and one week after the mods. The second week after the mods was much hotter as well, with one of the days at 99 degF. My usage that day was 56kWh vs. 82kWh on a 94degree day during the "before" week. When I started out with this, I had no idea balancing the returns properly would have that dramatic of an effect.

    I also have a buddy who is in HVAC and when we finished my basement we cut in a properly sized return at floor level. My basement was always 4+ degrees colder than the first floor. Covering up the first floor vent also increased the airflow from the basement into the system and also dropped that temperature difference to 2 degrees or under as well.

  4. #4
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,385
    Cherokee,
    I know exactly what you did to improve your unit efficiency because I have done it myself a few times. It’s the miracle cure. Locating the return air to remove unwanted stagnant air and for it to “pull” conditioned air where you want is key to a well operating system. I've also experienced system efficiency improvements similar to yours.

    I have been trying to convince pros here of this fact for over three years and haven’t found anybody on this forum that is aware of what we found. They nearly all say that return location has a negligible effect on system performance, but we know better don't we?

    I am curious if the pros are going to jump in here to tell that your findings are completely unfounded as they have been saying to me for years now.

  5. #5
    Look at it this way. In my case the I have an open foyer, which is nothing but a very large air duct in the center of the house. I am really not pulling cold air up, but I am stopping pulling even harder downstairs. Physics is going to make cold air sink and hot air rise. All I am doing is helping to evacuate the hottest air first, which will naturally be displaced by the colder air conditioned air, forcing more of the hotter air up and into the HVAC system. I basically had a short circuited system before that was actually helping to stratify the air.

    What most people don't acknowledge is that many houses are simply built poorly. This house was supposed to be a BGE energy wise house meeting the strictest efficiency standards at the time. I could give you a list a mile long of absolutely pathetic engineering issues and workmanship that I have had to fix. The absolute worse thing you could do is throw a "high efficiency" HVAC system on a house with many low tech problems. There is a much larger benefit and payback from fixing the low tech issues first (air infiltration, insulation, solar loading, etc). Every BTU you can stop from getting absorbed into the house, is a 100% payback on not needing to remove. I don't use peoples opinions to solve problems, I use data. Having said all that, I enjoy listening to the pros and learning a bunch on this forum.

  6. #6
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,385
    Quote Originally Posted by cherokee180c View Post
    Look at it this way. In my case the I have an open foyer, which is nothing but a very large air duct in the center of the house. I am really not pulling cold air up, but I am stopping pulling even harder downstairs. Physics is going to make cold air sink and hot air rise. All I am doing is helping to evacuate the hottest air first, which will naturally be displaced by the colder air conditioned air, forcing more of the hotter air up and into the HVAC system. I basically had a short circuited system before that was actually helping to stratify the air.

    What most people don't acknowledge is that many houses are simply built poorly. This house was supposed to be a BGE energy wise house meeting the strictest efficiency standards at the time. I could give you a list a mile long of absolutely pathetic engineering issues and workmanship that I have had to fix. The absolute worse thing you could do is throw a "high efficiency" HVAC system on a house with many low tech problems. There is a much larger benefit and payback from fixing the low tech issues first (air infiltration, insulation, solar loading, etc). Every BTU you can stop from getting absorbed into the house, is a 100% payback on not needing to remove. I don't use peoples opinions to solve problems, I use data. Having said all that, I enjoy listening to the pros and learning a bunch on this forum.
    I find it very strange that only people that stumble across the importance of return location are the ones who know about it. Probably because it isn’t taught in traditional HVAC circles, such as by the HVAC Scientists.

    You’re right, it is simple physics, but it still flies over most of their heads – something that I think is an absolute crime.

  7. #7
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    Brian, have you changed your mind or finally just learned ?

    cherokee180c wrote
    I covered up 90% of the 1st floor vent and then I probably tripled the volume of air being pulled from upstairs (eg hot stagnant air). Temperature difference is now 2 degrees instead of the previous 6 deg. Funny thing is my system as designed works great in the winter without doing anything, most likely as it is already so skewed to pull from the 1st floor, which works well in the winter time so I will totally remove the cover in the winter.
    So this person has a majority of the return from high area in both summer and winter. Did you not advocate for high-low, the" you can never have too many returns" approach? Seems to me your position has changed
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  8. #8
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    PS Brian

    What many of "us" have been trying to share with you is:
    a brilliant return duct location will NOT make up for a lack of supply air.

    I happen to agree with a single, high, ducted (no building cavities) return with "jump" or transfer ducts that may be ONE stud space so that the air has a path back to the unit and the difference in zone pressures is less than 2 pascals.

    Remember, there are usually no balance dampers on the return so the air will simply take the path of least resistance.
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  9. #9
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    Brian GC, what you're simply running against is those who are uneducated. As you've stated, you STUMBLED across a solution that worked for you. Well, a true pro can't afford to just stumble across a solution. He needs a scientific solution that works every time. That begins with a blower door test to establish the air infiltration/exfiltration rate. That rate is then used for a Manual 'J' calculation, that takes all the physics of the structure into consideration. Then a Manual 'D' design is made for low static and adequate air supply to each room in the home, again, based on the shapes within the structure. Manual 'T' is used to determine exactly which register/diffuser should be used on each outlet, as the actual CFM delivered from each supply can vary with the design of the register/diffuser.

    If all of that work is done (rarely and it yes, it costs) then the system should be properly sized and will work properly. Issues that can be more readily solved with zone controls would also be revealed in the design process. When the supply side of the system is properly designed and installed, the actual location of returns is far less critical. However, return location should not be ignored. It does have an effect on the airflow through the occupied space and therefore deserves as much attention as the supply side. But all-in-all, the location, sizing and airflow characteristics of the supply outlets trumps the location of the returns, as long as the returns are adequately sized for the system.

    So once again, this isn't a carp shot nor is it a guessing game. When a company takes the time to actually design and install properly, using the established science available, the results are stellar. When the science is ignored, well, garbage in, garbage out, right?

  10. #10
    Join Date
    Jun 2011
    Posts
    11

    please delete this post

    please delete this post

  11. #11
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,385
    Quote Originally Posted by genduct View Post
    So this person has a majority of the return from high area in both summer and winter. Did you not advocate for high-low, the" you can never have too many returns" approach? Seems to me your position has changed
    The OP says he switches the returns depending on the season. He pulls the majority of return from upstairs in the summer and pulls the majority of the air from downstairs in the winter. This is exactly what I advocate.

    It is true you cannot have too much return, but I also say the returns must be in the correct area of the house to help remove the “bad” air and to pull/push the “good” air to the right areas (eg pulling cold air up, warm air down).

    Rather than directing your questions at me, would you tell the OP his fix is not a legitimate fix? It reduced 66% of the delta T and shortened the run-time substantially.

  12. #12
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,385
    Quote Originally Posted by skippedover View Post
    Brian GC, what you're simply running against is those who are uneducated. As you've stated, you STUMBLED across a solution that worked for you. Well, a true pro can't afford to just stumble across a solution. He needs a scientific solution that works every time.
    “Stumbled” is just a word. It could also be phrased as “I used the laws of physics to experiment with return location and discovered that positioning them high in the summer and low in the winter will yield the results the OP found”.

    An average “pro” probably designed his house in the first place. That didn’t prove to be very effective. And as far as I know, the HVAC scientists and industry as a whole does not push this approach.

  13. #13
    Quote Originally Posted by genduct View Post
    cherokee180c wrote


    So this person has a majority of the return from high area in both summer and winter. Did you not advocate for high-low, the" you can never have too many returns" approach? Seems to me your position has changed
    Please show me where I said you can never have too many returns. I chastized the original designer of my house for not sizing the upper and lower holes in my walls (to call it ductwork is a travesty) which is in series properly to provide even flow rate into the sytem at both floor levels. As you pointed out, air will take the path of least resistance, hense my system was short circuited in summer pulling approximately 90% of the return air from the already cooled first floor, ie leaving all the hot air stratefied upstairs. The issue in the winter is simple, since hot air rises, I want most of the return air pulled from the first floor, there is no need to cover up anything, since the original design already provides the airflow pattern highly biased to the first floor return.

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