Return Air placement problem?

Discussion in 'HVAC Heating & Cooling' started by lkrav, Aug 4, 2008.

  1. lkrav

    lkrav New Member

    Messages:
    1
    My situation: I have a 1 & ½ story house (2,478 sf) with an entry and a living room that have 20’ ceilings. The second floor has half walls that look down into the living room. I think my two units are 4 ton (dn) and 2.5 ton (up). There are only two return air vents/intakes in the entire house – one for each unit – and they are both mounted in the ceiling of the upstairs hallway. Brand new coils in the big unit (old ones were leaking) and a brand new compressor in the big unit (old one died last year). Outside coils/fins are as clean.

    The living room with the high ceiling faces south & west, with the west wall having a ton of windows. (By now you’re probably getting the idea this house was poorly designed?... Me too.)

    Even with the thermostat set at 78, our units are running literally all day long, without a break. Granted, it’s Africa-hot here in Dallas right now, but my units couldn’t keep the house any cooler than about 82 yesterday.

    My thoughts…
    - We have one high arched window that is uncovered on that west wall – that’s a no-brainer that I should’ve taken care of long ago.
    - Attic radiant barrier… Thinking that maybe I should study the cost & DIY possibilities of buying the rolled stuff and covering the underside of the attic.
    - But, my main concern is my return air vent situation. It seems to me that this design of having both of the intakes mounted in the upstairs ceiling is a bad one. Wouldn’t the big unit be able to operate more efficiently if it were able to pull air in from the downstairs area (say, an 8’ ceiling in my dining room, for instance) ?

    Any feedback or advice anyone can offer will be most appreciated.

    Thanks,
    Luke
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,251
    Location:
    Cave Creek, Arizona
    return

    Your problem is that you do not really have two floors, you have one big open area about 20' high. Heat rises and cool air descends. This creates a logistic problem over a 12 month period. In the summer the AC air works best if it is injected at the lowest level and then rises as it warms up. In the winter, the ideal is to "suck" the cooler air out of the lower level send the warm air in high so it will disperse its heat as it descends, is the exact opposite of the AC needs. But, if the units run constantly without satisfying the thermostat setting, they are too small for the house's heat loss. Otherwise at least one would turn off if any area of the house were getting cool.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    Personally, I find it more comfortable to inject cold air high and let it fall down - this is not the most efficient, especially with high ceilings. Reduces the cold feet, hot head situation. If you pull the heated air out from where it normally goes - high up, you can create a problem whereby it gets sucked from the supply back to the return without actually conditioning much. If you have a natural flow that works to prevent that - good placement and directional vents, it's more comfortable.

    When heating and a/c share the same ducts, you have a disconnect for in the winter, the natural flow of your heat is up, so if you inject it high, you end up with the same hot head, cold feet symptom, but you're less comfortable, so I find it more comfortable to inject heat low and let it flow up.

    In my house, I pull the return from high in both seasons, but I have the advantage of radiant floors, that prevents the cold feet situation...but, I don't have the hot head situation because I suck it out from high and recirculate it.

    Placement and flow of the ductwork is often a compromise, and may need to change between cooling and heating seasons to obtain the best comfort levels - it is usually a compromise.

    The idea is to force the conditioned air through the space so, eventually, there is little stratification.
  4. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    hj....have to pick a bone with you on this one. I really think heat works best if the heat source...be it radiators or forced air registers...works best LOW. Then the heat rises to more evenly heat the room. Heat injected high stays high. Just the opposite for the cooling season...inject it high and let it settle downward. Cool air injected low just gives you cold feet!
  5. sixlashes

    sixlashes Plumber in Previous Life

    Messages:
    83
    Location:
    Pensacola. FL
    You did not explain if the high air temps were isolated to some rooms or the entire house. I will assume it is the whole house. If this is the case, the return grill placement is not the culprit. You can better balance the air if they were more strategically placed, but that does not seem to be the issue. As long as there is a return path for the volume of air the systems are discharging (for both returns), that is not your larger problem.

    Based on your square footage and equipment capacity, you have one or more issues to resolve. I do not enough information to calculate your heat gain, but 78.000 Btus of cooling capacity seems like a truckload for your house.

    I assume when you replaced your faulty equipment the contractor checked the output of the system to ensure it is cooling properly. Have you had the other system checked as well? If the problem is not the cooling output of the equipment, you need to look at ductwork and windows. If your ductwork is in the attic, which it probably is, it can very well be the problem. I would inspect all supplies and returns for leaks. The superheated air in your attic can easily rob you of enough cooling capacity to cause your problem. If any of the ductwork is metal, ensure it is insulated. You did not mention if you have a humidity issue as well. This would also reinforce the ductwork as the culprit.

    All of your west-facing windows are a huge heat gain source in the late afternoon/early evenings. Since bahama shutters would not fit in for your area, I would look at a reflective film for your west facing windows. It is not an easy task to complete, but would substantially reduce the overall heat gain. It would also reduce the large temperature swings throughout the day, ie. huge gains during the late afternoon/evening.

    I am not a fan of a radiant heat film in the attic. I think your money is better spent on sealing ALL air leaks in your supply and return ducts. Once that is done, blowing in more insulation to raise the R value would be less in cost and hassle. That being said, it would also be a good idea to ensure you do not have paths for the superheated attic air to migrate into your living space.

    I have a similar physical layout in my house. To reduce stratification in the tall room, I installed a ceiling fan dropped down on a 6' rod. This thoroughly mixes the air in the large room. Since you have such high room temps, this will not solve your stated problem, but may help to fine tune your situation once you resolve the larger issue.
  6. tedfrk

    tedfrk New Member

    Messages:
    16
    Location:
    New Jersey
    just a thought do you have room for an attic fan they can help reduce alot of heat.
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