Which way to go to cool the house?

Discussion in 'HVAC' started by EdNerd, Jul 24, 2018.

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  1. Jul 24, 2018 #1

    EdNerd

    EdNerd

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    I've attached a PDF image of a diagram of my house, which is in SW Arizona. The problem is the very front room, which is actually an enclosed garage. The house was like this when I bought it almost 20 years ago.

    The AC unit is located at the very back of the house - it's a single package unit with gas heat setting on the ground. It's a 4-plus-ton unit, about 15 years old. It works great to cool the house - except the front room. The previous owners had run 12-inch diameter flex duct through the attic into the front room (total about 35 feet long), and the AC fan just can't push the air that far. Finally, realizing I had a duct in the attic full of cold air that wasn't getting into the rest of the house, I disconnected the duct and blocked off the tee from the main trunk.

    The room is now blocked off with heavy curtains; otherwise, the uncooled air in that room migrates back into the rest of the house. As shown in the diagram, this hot air first hits the living room. The dining room and living room (which are actually one undivided room) are fed cool air by the one last vent in the end of the AC duct. And it's hardly enough to cool that off (the same vent also has to cool off the dining room). The hot air in the front room is one issue. Other factors are: the house was built in 1954 and is not well insulated, and in the living room is a large (4' high x 6' wide) single-pane window facing east. (Yes, the window has to go, but it won't solve the poor AC problem.)

    I'm now at a place where I can address this hot room and make it livable. The question is which option to take?
    -- A mini-split for just that room (234 sq ft)
    I'm sure this would be the cheapest option, both the purchase and the installation cost.
    -- A small second AC for the living room and the front room (total about 580 sq ft)
    A split unit with the air handler in the attic and the outflow split to cover both rooms. The advantages here are: this helps the main AC cool the undivided dining/living room; I can pipe in natural gas from the fireplace for heat.

    What other options and / or issues are there that I am not seeing?
    How would you advise in this situation?

    Ed
     

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  2. Jul 24, 2018 #2

    EdNerd

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    Another possible consideration is to rework the existing ducts to properly feed all the rooms. The main question here is: can the fan push from the back of the house all the way forward? This could be resolved by replacing the existing AC with a split system, and putting the air handler in a central location in the attic.

    Decisions, decisions .....
    Any advice is greatly appreciated.

    Ed
     
  3. Jul 24, 2018 #3

    joecaption

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    What's your location?
    Did they add a return air duct in the converted garage?
    How much insulation is in the attic?
    How is your roof vented?
    These questions may seem unrelated to you but may have something to do why your present system was unable to cool that space.
    Often times when people do a garage conversion they leave in the old ext. door which is more or less air tight instead of replacing it with an interier door with a gap at the bottom.
    The supply duct will act like blowing into a straw with your finger over the tip without a return duct or at least a gap under the door.
     
  4. Jul 25, 2018 #4

    Snoonyb

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    As mentioned, the return air is a big part of the problem, it, like you, has to suck to blow, and if the return is insufficient, to far away or obstructed.

    You can change the window to double or triple glazed, but most of your heat gain will be from the west.
     
  5. Jul 27, 2018 #5

    slownsteady

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    Please explain how this is factual, in a general way, without regard to one's location.
     
  6. Jul 27, 2018 #6

    Snoonyb

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    Solar radiation and it's duration are directly proportionate to the asmyth angle of the sun.

    The greatest length of solar exposure is from noon to dusk, therefore the greatest amount of heat gain.
     
  7. Jul 27, 2018 #7

    slownsteady

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    Still not sure that is applicable on the truly right coast
     
  8. Jul 27, 2018 #8

    Snoonyb

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    What time is sunrise and sunset, and do the math.

    The cool of the night is burned through by noon, and the usual hottest part of the day is around 2 or 3pm, the heat gain from dark color roofing and siding, stucco in the OP's case, is then reradiated into the dwelling envelope.
     
  9. Jul 28, 2018 #9

    EdNerd

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    I am located in Yuma, AZ - at the junction of the CA, AZ, and Mexico borders.

    The enclosed garage is well insulated in walls and ceiling; the rest of the house is not as well done. The enclosure is three walls (common with the living room, an east wall and a south wall) and open to the rest of the house at the hallway on the fourth side (see diagram); no return needed at this time. (I would put one in if we decided to wall that room off.)

    In this 3-sided condition, I could put my hand up by the air duct and feel coolness - like holding your hand over a bag of ice. But that was all.

    Ed
     
  10. Jul 28, 2018 #10

    Snoonyb

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    As explained, you do not have enough volume in the return air system, either by restriction or by inefficient return duct sizing in other parts of the dwelling.
     
  11. Jul 29, 2018 #11

    EdNerd

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    This >> "inefficient return duct sizing in other parts of the dwelling"!
    They way you put that made me think of a project I've been planning but have never gotten around to doing.

    The return into the AC unit, a package with gas heat sitting on the ground, is the bottom of the closet in the back bedroom. It's all of 10 inches high and 26 inches wide. It was sized for the original 3 1/2 ton unit on the house without the additional enclosed room. Granted, the enclosure only added about 235 sq feet (room plus entry), but it added it to the very front of the house.

    I've been wanting to enlarge that closet bottom return to about 25 inches high, and move the air filter inside the house. (The return grill is in the main hallway, but the filter is outside at the unit.)

    How do I calculate how big is the right size?
    Ed
     
  12. Jul 29, 2018 #12

    Snoonyb

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    From your description it does not appear that you have the ability to add another return duct, and increasing the return register size, without increasing the duct size, won't change the volume.

    Reduce the supply to the balance of the dwelling, either by duct dampers or adjustable registers which will force an increased flow and return too the added room.
     
  13. Jul 29, 2018 #13

    EdNerd

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    I can increase the duct size by raising the bottom of the closet floor. Or I could add a return through the attic, next to a supply duct.

    Or I could shift focus to adding separate cooling for just this room.

    Ed
     
  14. Jul 29, 2018 #14

    WyrTwister

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  15. Jul 29, 2018 #15

    Snoonyb

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    So, you are excluding the, reduce the supply to other rooms and attempting to force the return air to pull from that area of the house. Which, I surmise, that the supply is maxed out and comfortable as is. which means your system is undersized.

    Adding a return air duct in the attic is fine, except that, it needs to be near the floor to have proper circulation, which means a channel built out from the wall, or opening the wall.
     
  16. Sep 20, 2018 #16

    EdInKentucky

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    EdNerd,

    I think you should just install a mini-split system or window unit into the space that's not being cooled well.
    This will buy you time to get bids and designs to really do things right .. almost like starting over.

    Later, when the large outdoor unit fails in a big way ... take a good look at installing two separate HVAC systems to properly cool the living space, with new (and very well insulated) attic ductwork, improved ventilation of the attic, maybe improved wall/ceiling insulation, and good vapor barriers.

    Ed
     
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