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  1. #14
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    So the 2 furnaces are next to each other in the basement/lower level? IF so, on simple fix might be to leak-lag the furnaces and tie the ductwork together and add back draft dampers. I suspect one furnace and the 5 ton AC can carry the whole building. I think one pro on here once said they've never seen a undersized furnace. You'd have to be massively undersized not a ocarry a space over a 24 hour period.

    One thing to remember, the larger the home, comparatively more thermal mass and less surface area to volume ratio. Also being 3 stories, you have a proportionally smaller roof as well compare ot a signle or 2 story home. A 4000 sqft home will never use nearly twice the heating and cooling as a similarly designed 2000 sqft home, and far less than 4X a 1000sqft home. A typcial 1600sqft home might onl need 2 tons and 50k BTU's, while 4 tons and 100k BTU's can carry many 4000 sqft homes.

  2. #15
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    Quote Originally Posted by coolinhouston View Post
    Sorry, I only described how the system was set up. Here are some more details and specific problems:

    1) The building is VERY well insulated. All exterior walls have cellular insulation tightly blown in betweent the wall with a stucco exterior. The attic stays cool (big deal in Houston).

    Does it stay cool because the roof deck is insulated, or a radiant barrier is on the roof decking, or you have reflective roofing? Is the roof shaded all day long?

    2) The weather stripping around all of the doors is solid. I can feel some slightly leaky areas, but that is to be expected with the type of doors typically used in Houston.

    Houston is like Dallas/Fort Worth...soil shifts and doors once plumb can go out of alignment, causing gaps to show up in weatherstripping.

    3) I generally keep the air at 70 on both units at night and 72 during the day. My average electricity bill at these temperatures is relatively modest for those temperatures, the humidity levels in Houston and the square footage. We do run A/C about 10 months out of the year in Houston and my average is about $200 per month. So there isn't a real problem with efficiency.

    Are you running the a/c that cold to keep the humidity controlled? If so, this can be a symptom of an oversized system.

    Problems that I'm looking to correct have a lot to do with comfort and more efficiency. I'm trying to balance changes on equipment with cost to ensure that I have a monetary ROI and comfort ROI. Per a previous post, I'm looking to change the outside units to something more quiet and compact. I think I have that figured out. But in doing this, I want to evaluate rotating the units and the possibility that a larger blower motor to get better temperature balance, consistent comfort and maybe a little cost efficiency (although I don't expect a lot of operating cost savings).

    As other pro members here have already said, start with a heat gain/heat loss calculation for your structure. You may need to pay someone to do this if you want the calc done apart from equipment sales. Either that or you can pay a modest fee for HVAC-Calc available here on this site and do the calculation yourself. Either way, it is important to get good data into any calculation performed.

    1) Interesting, at 72, the airconditioners do not run that much during the day. (I work from home, so I see them). But in the evening and at night, they run almost constantly to keep that 2 degree difference.

    This is due to thermal lag. Sun beats on the building all day long, as well as the air surrounding the building warms up all day long. It takes awhile for that heat to work its way inside. By the time it really gets going the sun is setting or has set, and the air outdoors is still hot. The building interior, being cooler than outdoors, will attract heat well into the night until nighttime conditions cool down. This is called "thermal flywheel"...toward morning, if it is cooler outdoors than inside, heat flow reverses and treks through the building shell to the great outdoors.

    Also, when your thermostat bumps the setpoint up to 72, you delay your a/c coming on to counteract heat gain occurring as the day wears on. Had you left it at 70 you would notice the systems coming on sooner and running more often all day long.



    2) I've adjusted for the imbalance of tonage by keeping the upstairs cooler, earlier than the downstairs. It seems to balance the whole house better. Unfortunately, it's a waste of money because there are only two of us and we don't spend that much time on the top floor.

    Do you have pull-down stairs that go into the attic? Is the attic ventilated? If so, you should consider insulating and weatherstripping the attic access hatch. A product like "Attic Tent" is made for this scenario.

    3) The master bedroom (2nd floor) at night can suddenly turn very warm in the middle of the night. Perplexing, but I suspect that the top floor comes to temperature and shuts off so I don't get that down draft cooling effect. But that doesn't make sense because the thermostat that controls the top floor is right outside the bedroom. Maybe it needs to be moved to where the unit it is connected to is cooling the area - if the unit only cools the 3rd floor, the termostat needs to be on the third floor.

    Are you saying you depend on the third floor system running to keep your second floor master bedroom comfortable at night? And this third floor system is not connected to your second floor master bedroom?
    To Motoguy...Houston residential construction is typically slab-on-grade, so there is no basement. OP's systems are either in a closet or in his attic.
    • 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.

  3. #16
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,702
    Quote Originally Posted by Shophound View Post
    1) Interesting, at 72, the airconditioners do not run that much during the day. (I work from home, so I see them). But in the evening and at night, they run almost constantly to keep that 2 degree difference.

    This is due to thermal lag. Sun beats on the building all day long, as well as the air surrounding the building warms up all day long. It takes awhile for that heat to work its way inside. By the time it really gets going the sun is setting or has set, and the air outdoors is still hot. The building interior, being cooler than outdoors, will attract heat well into the night until nighttime conditions cool down. This is called "thermal flywheel"...toward morning, if it is cooler outdoors than inside, heat flow reverses and treks through the building shell to the great outdoors.

    Also, when your thermostat bumps the setpoint up to 72, you delay your a/c coming on to counteract heat gain occurring as the day wears on. Had you left it at 70 you would notice the systems coming on sooner and running more often all day long.
    Nicely explained.

    DO NOT ABANDON SETBACK STRATEGY or you are likely to have some real latent problems. You probably really need the long pulls.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

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