When It Comes to Air Conditioning Tonnage is No Silver Bullet

Air ConditionerIt was spring of 2008 and I had lived in my 1929 home in York, PA for a little under half a year. This home had not been remodeled since 1952, so it was in dire need of a little updating. Since a lot of the work was wall repair and refinishing, I figured this would be as good a time as any to rough in supply air vents for a new A/C system. But, like most projects I have undertaken, I let the whole thing expand, unchecked, in true “Tim the Toolman” fashion….

First it was roughing in the vents, which led to “well, if I am going to put the vents in, I might as well install the connecting ducts.” When that was complete, naturally my next thought was “I might as well run the main supply trunk, while I’m at it,” followed by “I better get the power wiring run up from the basement.” You can see where this is all going: by the time June rolled around, the system was fully installed.

Okay – I’m not your AVERAGE homeowner.  I actually have several years of experience as an HVAC contractor; otherwise I would not have undertaken such a project.  Still, I learned an important lesson that I might not have as a contractor.  When it comes to an AC system, bigger is definitely not better.

The 2-Ton Bargain I Couldn’t Pass Up…
Prior to ordering the cooling equipment, I decided it would be wise to run a computerized load calculation (often called a Manual J) on the house, which is what I would have done for any of my clients. Even with some wall and attic insulation factored in, my estimate came in a little over 32,000 BTU.

I planned to install a three-ton (36,000 BTU) system, but when I was presented with a sweetheart of a deal from my boss on a “scratch-and-dent” (slightly damaged) two-ton unit, I took it, not wanting to hold up my progress.

Now, some contractors would say I was nuts for putting in a system “too small” for the home but my wallet was talking louder than my professional inclinations at that point.  Besides, it was my own home, so if it didn’t cool that well when it got very hot, it wouldn’t matter all that much. Plus, I had designed the duct system for maximum performance and even distribution, which would help offset some of my capacity issues.

“I can’t believe it’s 78 degrees in here…!”
I’m happy to report that in this case, haste did not make waste – in fact, quite the opposite.  Turns out I shouldn’t have worried about the smaller sized system at all.

Since the system, by virtue of its smaller capacity, had to run longer to reach the thermostat setting, humidity removal was dramatically increased, thus making the air feel cooler than the temperature would indicate. So when it was 94ºF outside, a setting of 78ºF inside actually felt cold because the air was so dry. The true test was the fact that I never had any problems with mold or mildew anywhere in the home, even in the bathroom, which had no vent at all in it, due to its physical location. Microorganisms won’t form when they don’t have moisture for growth.

Longer run cycles also yield a gradual cooling process, which helps maintain a more uniform comfort level, with less temperature variance room-to-room.

There are a couple of other advantages to this: (1) machinery lasts longer when it isn’t turning off and on a lot.  (2) an air conditioning unit consumes less electricity in normal operating mode than it does at start-up, so systems that run longer and cycle on and off at fewer intervals are actually more efficient.

So let’s add them up: more reliability, more comfort, a mold-free, healthier home, lower operational cost…pretty solid reasoning, if you ask me.

I’ve never forgotten the practical experience that this project taught me. There are many contractors who would rather upsize a cooling or heat pump system to cover for lack of real-world experience, or just to satiate the customer’s request–or their own desire–to install a system of greater capacity than is required. I now fall on the side of humidity control every time, even when my motive is questioned. The key ingredient with comfort cooling is humidity removal; sensible cooling (that temperature that registers on thermostat) is a secondary benefit.

Brad began his career with Boyce Heating & Air Conditioning, York, PA, in 1985.  Boyce did residential and light commercial systems, with a special interest in places of worship and civil organizations.  Boyce also worked with Habitat for Humanity,  in urban redevelopment projects in York.  Brad served as general manager of Going Aire in Key Largo, FL, from 2010 to 2011;  he is currently helping Habitat For Humanity with a “green technology” home project in Plantation Key, FL, and with other worthwhile endeavors.

10 Responses »

  1. Great article! Love the tips about decreased humidity leading to mold prevention!

  2. It is refreshing to see comments relating to “Bigger is not better in HVAC”. As an energy efficiency consultant and auditor, one of the big problems I face with new home builders is educating them about “right sizing” HVAC systems. Many HVAC contractors will use a Manual J program but then they play with the inputs to add “SAFETY FACTORS” – this insures that the one day when summer temps are 102 the client does not call and complain that he cannot maintain 68 degrees at 3 PM. Doing this leads to short cycling the compressor, poor energy efficiency, humidity and moisture problems – not good for the equipment, the home or the home owner. This project also addressed the other big problem – proper ducting. Manual D gives you a design, then you have to install, fully seal and insulate the ductwork properly.

  3. Enjoyed the life learning lesson and story. Doing the home survey correctly is a critical point. Also, correcting any defects or making necessary modifications to increase a units efficiency is paramount. I would like to know the sq footage of the home and the seer rating of the unit as well. Here in Arizona the high temperatures are the equation breaker for a lower BTU unit.

  4. Thanks for the valuable info, as a building inspector, many time a client will ask me, “what do you think”
    many times this is after the fact and job completed, this gives me some info other than up sizing the unit,
    I’m also into environmental inspections which includes mold, since yours is an after the fact study, I’ll more
    that likely use the info repeatedly. Thanks again

  5. In my professional work, I’ve checked dozens of loads line-by-line and rarely see one that comes close to being correct. Maybe one out of 20. Just because the dealer does a load doesn’t mean squat.

    I live in SE Arizona. When I bought my current home (previously owned), I contacted the HVAC dealer who did the install to see if he ran a load. He showed me his summary report — 3.5 tons more or less. He installed 4 ton condenser with 90kbtu furnace. I noticed he had used MJ7. Bad news. Right off the bat I knew my system was *at least* a ton too large.

    The following year, I replaced the still-shiny AC and furnace with a smaller heat pump. My careful MJ8 load came to 2.2 tons (2589 sf). The following summer, using “cycle timing” method, I verified true load to be1.8 tons, at the design temp + 3 degrees. This wasn’t terribly surprising since it’s well known that MJ8 has fudge factor of at least 10% (MJ7 was much worse). Apparently, the dealer used default selections throughout, and ignored shading from overhangs and large covered patio on south facade.

    It gets worse. The homes on either side of me (same orientation) are 400 sf smaller than mine, with fewer windows. Same builder, same specs, same HVAC dealer. Yet these homes have 5 ton condensers. I asked original owner about this. He said the distributor apparently ran out of stock so the builder asked permission to install 4 tons, assuring him it would be fine. Had this not happened, my 1.8 ton home would have had a 5 ton condenser. Sheesh! At least here in AZ, we don’t have to worry about moisture issues.

    But this raises an interesting question. If the dealer and builder knew 4 tons was adequate, why in the heck did they install 5 tons on homes that were 400 sf smaller than mine? I did some investigation. As it turns out, the previous owner, feeling cheated, requested a credit for the difference in price. The dealer claims he actually had to pay slightly more for the 4 ton condenser. From what I gather, the distributor was running a special on 5 ton condensers at the time — you know, “one-size-fits-all” brings down cost! At the height of the building boom, they were shipping 5 ton condensers to AZ by the boxcar. Who needs to bother with a careful load when there’s a sale on 5 ton condensers!

    • No excuse….

      • This happens all the time. When I had my own plumber check out my new condo & suggest ways to reduce the noise from our hot water heater, I learned the fan was strong enough to push exhaust 50 ft to the exterior wall. Problem is my hot water heater is sitting right inside the exterior wall so absolute waste. Maybe I should replace it.

  6. Enjoyed reading this post! Am very familiar with York, PA. Head up that way often to visit my husband’s family. A little warmer where you are now, I’d say ;-).

  7. When we built a house many years ago, I was dully impressed with all the energy calculations related to window coverage … but I really didn’t understand a thing. Maybe it would be helpful if there were online calculators for homeowners to run these calculations themselves. They’ve got a lot more invested & would likely do a better job of factoring in all relevant information like the covered patio mentioned. It would certainly keep contractors focused on doing a legitimate estimate vs buying the product on sale this month.

  8. Thanks for sharing your experience and the lessons you learned when it comes to getting an AC system.

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