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Lower speed on Bryant AC Handler FB4ANA036 Condenser #561AJ036-A

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D107
D107 Member Posts: 1,849
in A-C, Heat Pumps & Refrigeration
I have a 1982-installed 3-ton AC in the attic, which we will be spray-foaming. Anticipating a much lower cooling load, is it a big job to lower speed on the unit--is that on the handler or condenser or both?

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  • newagedawn
    newagedawn Member Posts: 586
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    you would have to ajust the blower in the air handler, but that equipment needs acertain amount of cfm, youd have to check the output with an anemometer and ajust accordingly with the manufactures specs, personally i think it would be fine the way it is, your unit wont run as much to cool your house, saving energy , but AAB makes a cheap anemometer for like 40 bucks. if you want to check it
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
    D107
  • D107
    D107 Member Posts: 1,849
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    There's a great article in Green Building Advisor that talks about how much less cooling load there is in a tightly insulated home. The new number they're working with is 1000sq ft per ton. We'll have to see how it works after the R38 attic spray foam added to the already dense pack cellulosed walls. http://www.greenbuildingadvisor.com/blogs/dept/building-science/ac-sizing-benchmark-high-performance-homes
  • newagedawn
    newagedawn Member Posts: 586
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    interesting article, i doubt youd get that but let us know what happens, id like to know
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,524
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    Normal blower speed should provide 400-450 cfm/ ton of cooling.

    If you lower the blower speed you will drop the evaporator pressure and run the possibility of freezing the coil. 250cfm /ton is used in labs and for process cooling but only with special controls to keep the coil from freezing (hot gas bypass)

    also dropping the blower speed will reduce the cooling efficiency. The lower suction pressure will cause the compressor to operate with an increase compression ratio
    D107
  • JUGHNE
    JUGHNE Member Posts: 11,061
    edited May 2017
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    Pictures give a good visual concerning air flow.

    Removing the humidity is the key to comfort.
    D107
  • D107
    D107 Member Posts: 1,849
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    Thanks Ed. So I guess there's no substitute for a properly sized AC system. When I had a blower door test done back in 2005--before any insulation--the result was .9ACH. After insulating the walls (2006) and attic I would hope to achieve .3 or .4ACH. I'll have to see how well this 3 ton unit does in removing humidity. AC load is about 1250 sq ft.
  • ChrisJ
    ChrisJ Member Posts: 15,677
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    I think that's a good approach.
    If you find your humidity is staying too high, drop the blower speed.

    My plans (and we know how plans always go) are to run 400CFM per ton, and drop down to 340CFM per ton if humidity is above 45%.

    That's my plan, for now. The AHU has a dehumidify mode which works with the thermostat to lower the blower speed. If I didn't have that option, I'd probably plan on always running 350-ish per ton. I'd rather have a lower efficiency system with a dry house.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    D107

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