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New Central Air AC unit undersized?

Had new Central Air installed. Outside temp averaged about 70 degrees on this day. Previous 3 days were 90s, so indoor temp was 80 when new AC unit was started up.

It took about 7 hours for the temp to drop from 80 to 73 (at 11:30 pm with an outside temp of 67).

After 15 minutes thermostat was up 1 degree to 74. AC kicked on and it took 20 minutes to go from 74 to 73.

20 minutes to cool 1 degree with outside temp at 67. Is this AC unit too weak?

Thanks…

Comments

  • ChrisJChrisJ Posts: 9,626Member
    What size is your house and what size system did they install? What size was the old equipment?

    Can you give us some pictures of their work?
    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
  • newagedawnnewagedawn Posts: 549Member
    rule of thumb 500 sq ft per ton, if the tonnage is right, then did they change the line set? and what refrigerant is being used, pics
    of what was done would help
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • pecmsgpecmsg Posts: 681Member

    rule of thumb 500 sq ft per ton, if the tonnage is right, then did they change the line set? and what refrigerant is being used, pics
    of what was done would help

    So whats the rule of thumb for a boiler???
  • Big Ed_4Big Ed_4 Posts: 1,118Member
    It all depends .... Do you live in a high humidity area or Valley of AZ ? Degrees and percentage of humidity should always be taken in account for comfort . Most people only see the thermometer reading temperature ... Take 80* air with 5% humidity it will be comfortable and 70* with 5% humidity is cold ... The AC system takes out the moisture the longer you let it run. Too big of a system will bring down the temperature but not the percentage of humidity . Which will give you a cold damp environment . In a high humidity region once the homes starts to dries out you may find yourself raising the degree set point higher for comfort ...

    To determine the proper size AC a heat load calculation is needed to be done ... Which figures the Sun shining on the home with shades on windows ...
    I have enough experience to know , that I dont know it all
  • DZoroDZoro Posts: 752Member
    Manufacturers recommendation is to let the system operate for 72 hours before full evaluation of it. Properly sized unit will take that amount of time to remove humidity and then drop temperature. Both need to happen, this can take time with a home that has been subject to hot/humid temperatures.
    A/C is not like a heating system it is much slower when properly sized to properly remove your humidity also.
    Don't turn it off, let it run and do its thing.
    D
  • IronmanIronman Posts: 4,964Member
    An accurate load calc is the only way to know for sure if the sizing is right. That also assumes that the system is properly charged and commissioned.

    As mentioned, give it a couple of days to acclimate.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • GroundUpGroundUp Posts: 482Member

    rule of thumb 500 sq ft per ton, if the tonnage is right, then did they change the line set? and what refrigerant is being used, pics
    of what was done would help

    Are we cooling a tent? Holy crap. New construction here is less than half that
  • ZmanZman Posts: 4,811Member

    rule of thumb 500 sq ft per ton, if the tonnage is right, then did they change the line set? and what refrigerant is being used, pics
    of what was done would help

    As Siggy says, "rules of thumb are great if you are designing thumbs"
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GBartGBart Posts: 753Member

    rule of thumb 500 sq ft per ton, if the tonnage is right, then did they change the line set? and what refrigerant is being used, pics
    of what was done would help

    THE RULE IS DO A LOAD CALCULATION, SQUARE FOOTAGE TELLS YOU NOTHING
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,621Member
    @Big Ed is correct. Humidity plays a huge part. Let it run before you decide. On start up not only is the temperature high but the homes contents are saturated with humidity
  • ChrisJChrisJ Posts: 9,626Member
    Not only does humidity play a huge roll, but everything in that house was hot. Everything took time to cool off, and then even when the system finally shut off all of that stuff, including the walls, flooring etc, were still warmer than normal.


    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
  • ratioratio Posts: 1,929Member
    @regular_guy, just by going off what you described, it doesn't necessarily raise any red flags with me. A long recovery (I'm assuming you had the AC off or set back) is normal in warmer weather with correctly-sized equipment. The only way to have a fast recovery is to significantly oversize the equipment, which drastically decreases efficiency & comfort when it's not working hard, which it will only be doing during recovery.

    As mentioned earlier, let the system run for a week & only then look into how well it's operating. Also, reduce or eliminate the setback/leave it on while you're gone if you can stand to do it, at least until you feel confident that the system is performing adequately.

  • GBartGBart Posts: 753Member
    If the system is properly sized it should run long cycles, you have to remove humidity (latent heat) before you begin to bring down sensible temp. Knowing what they designed it for or if they did a load calculation is helpful, the only way to properly do one is to ACTUALLY measure the home, windows, doors, etc and know the insulation, siding, etc, if no one surveyed the home it wasn't done.

    That said, if the system was designed for a 95 degree day when it is 95 out it should come on and not shut off, these systems are made and designed to MAINTAIN temperature, not to bring a home from 85 to 72 in 12 minutes, set it where you want the temp to be and leave it alone. It's the only way you're going to control humidity as well.
  • GBartGBart Posts: 753Member
    That's way wrong, from Texas up through New England along the cost we have high humidity, long cycles are required to remove humidity, short cycles raise humidity and shorten equipment life.

    You can't just call out a long cycle or 30 minutes, it depends on what the system was designed for and what the current conditions are outside, again if it was designed for a 95 degree day at 72 then on a 95 degree day if the home is 72 it should come on and stay on because you are at equilibrium, you are moving as much heat as the system was designed for. They should never run less than a 10 minute cycle.

    Variable speed equipment is another story.
  • DanInNapervilleDanInNaperville Posts: 24Member
    It's a tough call. Most of us don't live in a place where the outdoor temperature and humidity never change, so the A/C will only be "correctly" sized for a few hours each year. Guidance from companies and the DOE is designed to provide the highest possible "sticker" efficiency sized for the day after a fresh install.
    Specifying units that can't cool a house on hot days even right after a service that cleaned the condenser coils, optimized the freon charge, and replaced the air filter (much less at late summer when the system efficiency is down 15% or more) just pisses people off. No matter how many ASHRAE or DOE guidance charts you wave at them.
  • ChrisJChrisJ Posts: 9,626Member
    > @DanInNaperville said:
    > It's a tough call. Most of us don't live in a place where the outdoor temperature and humidity never change, so the A/C will only be "correctly" sized for a few hours each year. Guidance from companies and the DOE is designed to provide the highest possible "sticker" efficiency sized for the day after a fresh install.
    > Specifying units that can't cool a house on hot days even right after a service that cleaned the condenser coils, optimized the freon charge, and replaced the air filter (much less at late summer when the system efficiency is down 15% or more) just pisses people off. No matter how many ASHRAE or DOE guidance charts you wave at them.

    Did you just say Freon?
    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
  • DanInNapervilleDanInNaperville Posts: 24Member
    I did indeed! I suppose I should have typed out "refrigerant charge" or I could have typed "R22 (freon) or RS-44b or R404 or R407c or R410a" but I got lazy and typed freon instead.
    Sorry if it caused any confusion.
  • ChrisJChrisJ Posts: 9,626Member
    > @DanInNaperville said:
    > I did indeed! I suppose I should have typed out "refrigerant charge" or I could have typed "R22 (freon) or RS-44b or R404 or R407c or R410a" but I got lazy and typed freon instead.
    > Sorry if it caused any confusion.

    Freon is a Dupont brand name. To me, when someone says Freon it's a dead give away....
    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
  • TheKeymasterTheKeymaster Posts: 37Member
    I've said freon before but I'm a novice in HVAC. I'm an expert in everything else though. ;) I also call copies Xeroxs.
  • GBartGBart Posts: 753Member

    It's a tough call. Most of us don't live in a place where the outdoor temperature and humidity never change, so the A/C will only be "correctly" sized for a few hours each year. Guidance from companies and the DOE is designed to provide the highest possible "sticker" efficiency sized for the day after a fresh install.
    Specifying units that can't cool a house on hot days even right after a service that cleaned the condenser coils, optimized the freon charge, and replaced the air filter (much less at late summer when the system efficiency is down 15% or more) just pisses people off. No matter how many ASHRAE or DOE guidance charts you wave at them.

    Umm, units are sized for a given day, what is the expected maximum heat and humidity for that area, over sizing equipment in some areas is against code and energy mandates, it wastes energy. For efficiency go with variable speed or variable refrigerant VRV systems, American systems are 30 years behind the times in most cases.

    In areas with high humidity if you over size you will not be able to control humidity and you might create a health hazard, short cycling increases humidity, what should happen in those areas is equipment is used that has a hot gas bypass capability where as the unit is reaching set point if the humidity is still above 50% the bypass comes on to further dry the air without more cooling, take a psychrometric chart and pick out a 70F 60%RH point and lower the dry bulb and watch what happens to RH.
  • GBartGBart Posts: 753Member
    Good reading--- from attached article "Oversized air conditioners don't run for a long time because they satisfy the cooling load quickly and then shut off. Properly sized air conditioners run longer, so if you want your air conditioner to dehumidify your home as well as cool it, don't let the HVAC contractor oversize it."...."When an air conditioner is oversized, it starts up and shuts down a lot more because it runs for only a short time to meet the thermostat setpoint. Then a few minutes later, it comes on again and runs for a short time. Over the course of a day, an oversized air conditioner can have a lot more start-ups and shut-downs than a properly sized air conditioner. That means you'll probably be repairing it more often and replacing it sooner."

    https://www.energyvanguard.com/blog/55157/Why-an-Oversized-Air-Conditioner-Is-a-Bad-Idea
  • GBartGBart Posts: 753Member
    and---
    8 Mistakes
    People often buy new heating and air conditioning systems because they need to heat or cool their home immediately!
    It’s the biggest mistake of all! They buy a system without good information and they shop for the lowest dollar installation…which leads to a 50-100-200% higher cost over time. What they don’t know is the…

    “8 Common Mistakes of Heating and Air Conditioning Contractors”

    Short Video about The Top 7 Technician in America–Click Here
    This content is copyrighted



    Heating and Air Mistake #1 video: Wrong Size Equipment Installed–Click Here
    The contractor puts in the wrong size of equipment. What this means is the contractor does not match the size of the equipment to the load of the individual house, or determine the actual amount of heating and cooling that it needs. You want to get the highest performance (or miles per gallon) and to do that means you need the right size equipment. When you put in a system that is too large (very common) you are going to pay for it in operating expenses. It takes more “fuel” to operate the larger system, plus you don’t have as much comfort. It’s the reverse of what you would expect with a larger system.

    People generally think that “bigger is better.” It’s not true. Bigger is NOT better.

    Too large of equipment is a) initially more expensive, b) increases operation-expense, c) turns on-and-off, d) comes on full force because it is so powerful, e) can’t create stable conditions, e) coils can’t get saturated to do their job, f) equipment can’t handle humidity and moisture problems properly because it can’t get to a steady state of operation g) comfort is decreased, h) the equipment can’t last as long, i) the equipment demands more fuel to operate, j) the temperature of the house goes up and down, k) costs of energy increase, l) cost of repairs increase, m) frequency of repairs increase, n) new-installation-frequency increases.

    http://www.comfortmasterheating.com/8-mistakes-when-it-comes-to-heating-and-air-conditioning-systems8-mistakes/
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