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3799 Main St. #87024
Atlanta, GA 30337


Advanced residential construction and home improvement consulting and owner's advocacy in Atlanta, using the latest building performance diagnostic and modeling techniques and tools. Airtightness, insulation, HVAC, ventilation, moisture, and air quality and EMF consulting for homeowners and building professionals alike.


Home performance articles and stories from the field with internationally respected building forensics guru Corbett Lunsford at the Building Performance Workshop. Hear new episodes of the Building Performance Podcast, see new videos from the Home Performance YouTube channel, and learn all about how diagnostic testing (more than an 'Energy Audit') can make home improvement and new home construction a proven process!

Filtering by Tag: hvac basics

Dear Corbett: How Long Should My A/C Ideally Run?

Corbett Lunsford

Hi Corbett,
How long should the air conditioner run in a given day?  The upstairs zone was running for 16 hours yesterday, and 10.5 and 11.25 each of the days before.  Is that reasonable amount of time for the A/C to run given the recent summer temperatures?  We are still trying to get the developer in to fix the ductwork, and figured this might be an symptom of the problem.

Hey Graham!
Great question- your air conditioner is actually designed to run continuously when it's hotter than 89 degrees F outside.
The A/C's job is to both COOL and DRY the air, and if it's too big, it doesn't run long enough to wring the humidity out of the air. This leaves you with a muggy house, where you keep lowering the temperature to try to get comfortable.
So don't be concerned when your air conditioner runs for long periods on hot summer days- that means everything's working the way it's supposed to!
Looking Forward,

How to Understand Heating, Cooling and Ventilation (HVAC)

Corbett Lunsford

To keep most people in first world countries comfortable, you need at least two things: an enclosure to hang onto heated or cooled air, and an engine to make the air that way. ALWAYS FIRST: create an enclosure at is relatively airtight and insulated. Now for the engine: the HVAC- air conditioner, furnace, heat pump, etc.


The question is: how much heating or cooling should I install in this home so it’s both very comfortable and cost-effective?

Great question! There are very precise calculations for this (spelled out in ASHRAE Fundamentals and in Manual J from the Air Conditioning Contractors of America. You take detailed information about weaknesses in the home’s enclosure (window size and orientation, how much air leakage exists, etc) and put it into a computer.

Doing an HVAC load calculation by hand isn’t good enough in most homes because you need an hour-by-hour look at the effect of the sun and weather on the home. This calculation gives you a very specific amount of heating and cooling that a home needs. You don’t need to add anything to this, because the calculations were written by engineers, who are very cautious and conservative people-they’ve added all the safety margins already to make sure you’ll be comfortable.

Next question: how are we going to deliver the warm or cool air evenly everywhere in my home? Wow, that’s a great question! Guess what? There’s a calculation for that!

Again, ASHRAE Fundamentals or Manual D will tell you exactly how the ducts should be designed and installed so that every single room feels comfortable and refreshing, which is possible for every home, in every place, new and old alike.

Now that we know exactly how much heating and cooling each home needs, we have to buy an engine that can make it. Here’s where most people make a big mistake: they buy an engine that’s too big. These people may think that they need a little extra power for when the weather is really crazy. In fact, many of my clients have twice as much machinery as their home actually needs. That’s like building an escalator and installing a V8 engine to run it- it takes you where you need to be super fast, but it’s also uncomfortable, really loud, and slightly nauseating. So you pick an air conditioner, furnace, or heat pump that fits the home like a tailored suit.


Last step: we test the tightness of the duct system. A duct system is plumbing for air- you do not want it leaking. You might think if the ducts are all inside the envelope and they leak air here or there, it’s not such a big deal. It IS a big deal, and here’s why:

If we spent time and energy calculating exactly how much heating and cooling this home needs, exactly how the duct system needs to be designed and installed, and picked out the perfect heat pump or furnace, all of that gets flushed down the toilet if the conditioned air doesn’t actually go to the rooms where you need it!

Proof Is Possible, and careful planning and performance testing are the keys. Your home won’t need rescuing if you do it right the first time.