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Air Conditioning FAQ's for Homeowners


How often do I have to change my air filters?

This is a loaded question and I cannot give a specific answer. It all depends on the quality, filter, home conditions, environmental conditions, the amount of people living in the home, if it gets turned on and of frequently or just maintains the setpoint temperature for the summer. You would have to get on a check and save schedule on your own. Check it monthly and change as needed and note the frequency. As a rule of thumb change it quarterly or semiannually dependent on your home situation.

How often do I have to have my air conditioning serviced?

It's called an annual check-up for a reason. Have it done pre-season before he rush, some companies have early bird specials to get them done and keep the tech's busy during slow times. Note: If it's a heat pump twice a year may be better due to the different operating specifics and OD air temperatures are different.

If the old unit cooled the house well is it OK to replace with one of the same size?

After 50 years active in the HVAC industry I have not installed a heat or cooling system without a Manual-J heat Loss/heat gain calculation to proper size the equipment to maximize operation cost and increased comfort levels for over years 20 . There could have been upgrades to the home like weatherization or additions to the home.

What is meant by saving operation and maintenance cost?


Most heating and air conditioning units are oversized. This causes the unit to turn on and off too frequently. When they start they draw more electric than it takes to run. The frequent starting will also add to part failure.

I have heard not sometimes the a/c could was not able to maintain the thermostat setting on very hot days when installed after a manual-J calculation.


Usually one of these is happening:
  • Manual J was based on optimistic assumptions instead of accurate information
  • A quick online "Rule of Thumb" Manual-J loss was done. The easier it is more than likely it is less accurate
  • The current outdoor temperature is greater than then what was chosen by the Manual-J calculation
  • The ductwork is leaking, restricted, lack of return air, or poorly designed
  • The house has high solar gain not included in Manual-J
  • The AC isn't delivering its rated capacity because of install issues, low refrigerant, dirty coil/filter, weak airflow, or wrong blower setting.
  • The thermostat is placed in a spot that does not measure an accurate room temperature or the system is short cycling due to being oversized

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    What can I look for?

    Check these in order:

    • Filter and airflow
      • Clean or change air filter?
    • strongx air coming from every supply vent?
      • Return grille not blocked?
    • Thermostat and runtime
      • Set to cool, fan on auto Does it run almost nonstop on hot days? If it cycles off quickly, it may be short cycling or oversized
    • Temperature split
      • Measure return-air temp and supply-air temp You want roughly a 16–22°F drop Much lower suggests airflow/refrigerant issues
    • Duct leaks / losses
      • Are ducts in attic, crawlspace, or garage? Any obvious disconnected, crushed, or leaky ducts? Weak airflow in far rooms = likely duct problem
    • Outdoor unit
      • Is the condenser coil clean? Is the fan running? Is the unit icing, noisy, or constantly shutting down?
    • House shell load
      • Hot rooms with big sun exposure? Attic very hot? Old windows or poor insulation? Air leaks around doors, attic hatch, recessed lights?
    • Humidity
      • If temp is close but it feels clammy, the issue may be latent load or oversized AC

      These are best if done by an A/C tech.

      • The equipment was sized for sensible load, but latent humidity load is too high. Were people added into the load?
      • Duct losses in attic due to low R-value
      • Supply/return imbalance starves airflow
      • Outdoor design conditions are harsher than expected

      Is The Operation less effective when the A/C is installed in the attic?


      Could be depending on your attic space and home location. Some locations like the south the attic temperature could be as high as 150f to 160f by late afternoon. This would be much different in the north. Let's look at scenarios.

      • The higher the attic temperature the warmer the air is coming out the air vents.
      • Is there proper amount of ventilation in the attic?
      • Adding a power attic fan will help cool the attic to a lower temperature. If you do this verify they have a thermostat and shut down in case of fire.
      • If the R-value of the ductwork is a low R-value like R-6. Today you can get greater than R-10.
      • I the existing main trunk ductwork insulated either outside or inside
      • Is there an insulated mechanical room built around the unit and ductwork?
      • Is there spray foam insulation on the roof trusses?
      • The idea is to loose as little air temperature as possible the entire length of the duct system.

      Do basement installations have drawbacks?

      A big problem is moisture collection especially on uninsulated metal ductwork.
      Another downside could be in a two story home would be the volume of cool air out the supply air vent, while heating could be better due to lighter air. If this problem exists it could get worse after a new unit is installed if a variable speed or two stage unit is installed when the fan slows down.

      Benefits of basement installations?

      Temperature stability: Basements stay cooler than outdoors, which can help some equipment run in a more stable environment. Basement temperatures could be 90f less than attic temperatures
      More protected location: It's sheltered from weather, sun, and debris.
      Easier maintenance in some homes: If the basement is accessible, technicians may have a straightforward place to work.

      What is meant by saving operation cost?

      When units are oversized they short cycle and they may cool the home but after the unit shut off the humidity levels are too high and it feels warm quickly. The unit turns back on quickly causing premature failures and increased elecrical cost. Proper sizing will use less electrical costs and lower maintenance but the comfort level will be much higher.

      Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website, as a professional should review it.