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3799 Main St. #87024
Atlanta, GA 30337
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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.

Videos/Podcasts/Articles

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: air conditioning

DEAR CORBETT: HVAC Equipment Airflow Frustration

Corbett Lunsford

Dear Corbett,

I was going thru the videos in the training portal. Overall, I like your videos. However, every now and then, a portion of one leaves me less than satisfied with the answer. Usually, my disatisfaction is because you are rushing through and just skimming the details and explainations. I realize that your trying to limit the video's length and accept it.

However, I found one video that had something that you brought up, but didn't explain that frustrated me enough to write. The video is in the training portal, 'Mastermind Series Jan 2015' at 30:01 minutes in. Your talking about system airflows and you bring up NCI. You go to say how they add up supply airflows, return airflows, and compare it to what the airflow should be to get leakage. You say that that is wrong but don't explain. You finish by saying that system airflow needs to be measured at the blower. 

So, the question I have is: Are you objecting to comparing either the supply or return airflows to what the system airflow SHOULD be (i.e. not measured) or something else?

Would you agree with the following? 
(Equipment airflow) - (sum of supply register airflow) = (supply side leakage)
(Equipment airflow) - (sum of return register airflow) = (return side leakage)

Equipment airflow to be measured at or near the blower by one of the several methods. I am calling it equipment airflow and not system airflow just to be clear that it is what the fan actually produces.

Finally, are you doing full manual J's or just block load calculations to get ballpark numbers?

Thanks,
TJ

-_-_-

Dear TJ-

Great question, and I'm happy to clarify what I meant! Thanks for letting me know when my broad strokes don't actually answer your question and leave you frustrated- I always want to give a full picture of what home performance testing actually means.

My issue with measuring the supply airflows and return airflows, and inferring duct leakage from that is:

  1. How do you know what airflow the equipment is producing unless you measure the actual airflow at the equipment? You can use a calculation like 400cfm x tons of A/C, but what if it's heating season, 20 degrees outside, and the NEST thermostat won't let you disable the compressor's service disconnect and still run the air handler? Also, what if the installer targeted 350cfm per ton, or 450cfm?
  2. Even if you measure the equipment airflow, you won't be measuring the equipment cabinet air leakage, which is almost always there. In fact, the IECC duct leakage test assumes that 25% of all the duct leakage will be in the cabinet in new construction.
  3. The only way to measure duct leakage is to perform a duct tightness test, or for a quicker and more localized look, perform a pressure pan test during blower door testing.
  4. Measuring airflow is notoriously difficult. What tool and technique are we using? Pitot tube has high user error, passive flow hood (balometer) is not accurate for residential grilles and low flows, and anemometers need to be corrected for net free area of the grilles. Lots to scratch your head about, and it turns out the best way to measure airflow in the 21st century is still a plastic garbage bag.

To answer your final question, if all you need is a ballpark number, a block load is fine, but I always do a full room-by-room load calculation if I'm being paid to do an actual Manual J.

Hope that helps, and keep the questions coming!
Looking Forward,
Corbett

Dear Corbett: Why Don't Homeowners Want It Done Right?

Corbett Lunsford

Dear Homeowners: please read and absorb what the quality contractors come up against with typical clients. This is part of why high quality construction and home improvements are hard to come by...

 --------

Hi Corbett,
Hate to bother you, but you said if I had aquestion to get in touch with you. Sometimes I feel like just throwing my hands up in the air and giving up; why did I read that article about making the heating system airtightback in 1992 when I started my HVAC business? And why six years ago did I bother learning about the whole house approach? There are times I wish I wasn’t disciplined and motivated to do things right.

If a customer just wants a new energy efficient furnace or A/C and is not concerned with all the other issues with the house, should I just give them what they're asking for and take their money? Knowing how easy it would be to do that, I just can’t lower my standards. Even companies that are doing 'whole home performance' now, I’m following behind them fixing some of their mistakes already.

I get people telling me about the problems they are having inside the house. The question I’m always asking the homeowner is, If I see some safety concerns or other issues would you like me to tell you? On a service call, the first thing I have to get right is the problem with the equipment, than I can let them know what I see with the whole house and heating/cooling system generally.

I’m always getting pushback on this: "my heating system is just 7 years old, everything's fine." And here I am, telling them it was not installed correctly, and that we have to start all over again.  Always running into oversized furnaces and air conditioners with undersized ductwork.

Not sure if I’m explaining things correctly to the homeowner as to what I’m trying to accomplish inside the house for them. It always seems to come down to the price. Maybe I have to do a better job in showing value to the customer?

Right now, looking at one job, the call was for high end air purifying filters. The homeowner told me the house is 4000 sq ft but might include the garage and it’s 8 years old. Their 5 year old son has allergy problems, $650.00 utility bill a month, a lot dust in the house. They have 181,000 Btu output for heating and 10 tons of cooling! Told the homeowner the first thing we should do is test the house and HVAC systems first before installing the filters. It apparently costs "too much" to fix the problems. Help?
Thanks,
Sal C.


Hey Sal-
Great question, and I feel your pain!  Sometimes it can seem like our clients actually prefer to hire mediocre contractors to do cheap home improvements that make things steadily worse in the house.  I promise you, though, that it's not true. Yes, I believe it's 100% about education and showing the value, and judiciously using the powerful marketing tools Fear and Sex Appeal to do so.

Keep at it. I'll keep an eye out for your success stories to come. 

Looking Forward, 

Corbett

I Turned My A/C Off All Day at 106 Degrees

Corbett Lunsford

Dear Corbett:

I thought I would send this your way.  I live in the Central Valley of California.  Yesterday it reached 106 degrees.  I did an experiment with my house.  My 1,883 sq./ft. house had a deep energy upgrade over three years ago.  The heating and cooling unit is a 2 stage/2 ton heat pump with no back-up heat.  The second stage of cooling is disconnected so I am cooling with 1.4 tons.  That is one ton for every 1,345 sq./ft.

Yesterday morning I left the house at 7:40 am.  It was 75 degrees in the house and 75 degrees outside.  I closed all of the blinds, turned the HRV off and turned the Heat Pump off.  In other words, I let the house float.  It got up to 106 degrees in Stockton yesterday.  Luci my bride got home at 5:40 in the afternoon and the house was 78 degrees.  She turned the thermostat back on and it brought the temp back to 75 in 30 minutes.

This stuff really works.
Keep up the GREAT work;
Dick Rome


Dear Dick-

You are awesome. I'm not sure I know many other home performance experts who have fixed their own homes- most of our families have to suffer at home while we help other people get comfortable. Thanks so much for sharing the good word!

~Corbett

PODCAST #53 A/C CAN BE SEXY: Kristof Irwin on the Magic of Variable Refrigerant Capacity

Corbett Lunsford

Today we talk with Kristof "Handsome" Irwin, founder of Positive Energy in Austin Texas, about why your air conditioner is probably an idiot (bless its heart) and HVAC heaven that you might not have heard about.

To download this episode or hear more, subscribe in iTunes or visit BuildingPerformancePodcast.com