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07-01-2012, 10:35 PM #1Regular Guest
- Join Date
- Jun 2010
- Charlotte, NC
Results of downsizing from 3 to 2 tons
Want to provide a data point, results of advice received here 2 years ago.
A couple of years ago I first came to this site to learn what I should look for as I was replacing my old A/C that died. Learned a lot.
One of the main topics was whether I could downsize from a 3 ton unit to a 2 ton unit in my 40 year old 1800 sf house in Charlotte, NC, since I had better insulated. Opinions varied.
My personal Manual J done with the software on this site said 2 tons should work. Others said I should only downsize to 2.5 tons. But since I wanted two stages, I decided to risk it and go down to 2 tons.
Last summer was not all that hot, but the past two days have tied the all-time record for Charlotte, and the two tons have kept up fine. I only set my A/C at 76--below that and I feel chilled--but it runs on low stage most of the time. Yesterday afternoon, to test the system, I lowered the setting to 75, and that made it kick into high, but it brought the temp down that extra degree in less than an hour with outside temps over 100.
My humidity stays right at 47-48 percent all summer, the unit is quiet and cooling is uniform. When the humidity got down, my son's dust mite allergy went away, no more morning sneezing. Electric bill is been a good bit lower, too.
So I got the right result, thanks to all who helped.
Lessons learned, insulate, airseal attic, and lower size of equipment. My ductwork was not bad, but lowering size of needed equipment will compensate for the often undersized ductwork ya'll say is rampant.
07-01-2012, 11:32 PM #2
Always good to see someone post back results of improving their system. Correct sizing can take some getting used to but it sounds like you still have plenty of reserve capacity in your case. I recently downsized my system upstairs form 3.5 tons to 2 tons. It holds temp, but when it's in the the mid 90's, it can take a few hours to drop it 1F.
Great to see how you son's health has improved as a result and bills have dropped.
2.5 tons would have helped, but as seen by how much your run in low stage, the results would not have been as dramatic.
07-01-2012, 11:48 PM #3
Thank you for reporting back! GREAT information.Make your expertise uniquely valuable.
Make your influence uniquely far-reaching.
07-02-2012, 06:04 AM #4Professional Member*
- Join Date
- Nov 2004
- SW FL
It's Not Rocket Science, But It is SCIENCE with "Some Art".
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
07-02-2012, 08:40 AM #5
I hope that all of the techs on this site read this and take note, homeowners as well but primarily techs.
07-02-2012, 08:40 AM #6
Not ot mention accepting hte fact that 1% design temperatuer in many sourthern cities is much, much lower than some people think. Charlotte for example is 1F cooler dry bulb and 3F cooler wet bulb that where I am. If you took a survey, 99.9% of Americans would assume that North Caroline is hotter than SE Iowa amd NE Missouri. It's not. It stay warmer longer. But you don't size a AC based on cooling degree days.
This house must be really insulated well and sealed. It sounds like your sensible load is pretty low sicne even at 100F it's still able to cool off the house when called.
07-02-2012, 10:44 AM #7
The equipment installation & duct system must also have been done right to make that 2-Ton run on low stage in those outdoor conditions.
How many customers, at existing informed educational levels, could you get to go along with that downsizing even with all the home improvements & a Manual J saying 2-Tons?
Three Cheers for a job well done.
07-02-2012, 11:11 AM #8
Moreover, its adjusting expectations. Ask most customers if they want to be more comfortable 99% of the time, but 1% of the time, it may get a little warmer, but they will still be comfortable and further you can't shut hte systm eoff, then switch it on and have it cool the house down by 8-10F in 30 minutes.
Most would prefer the oversized system and will never, ever believe you that a system that runs longer could use less energy. It just doesn't compute and ever tiem you hit triple digits and high humidity, they are calling because the systme isn't keeping up and running constantly...therefore it must be broken.
Of course nevermind that everyones refrigerator takes nearly a full 24 hours to cool off and storage tank water heaters have a bruner than 1/4 the capacity of the actual instantaneous demand and can take an hour to recover form a long shower.
But we expect those appliances ot work that way.
07-02-2012, 12:26 PM #9Regular Guest
- Join Date
- Jun 2010
- Charlotte, NC
Thanks for the responses.
I checked lifetime runtimes this morning and for A/C, it has run in low stage 1902 hours, and 274 hours in high stage. So 87% of the time has been low stage. I don't know what tonnage that is for the carrier two ton performance series, but I know it less than 2 tons.
I agree most customers don't understand how sizing works, but it ain't just customers.
One local HVAC company (who did their own manual J) refused to do the job when I would not agree to install a 2.5 ton unit instead of the 2 tons I wanted.
Several people on this site insisted I should get a 3 ton two-stage unit, instead of 2 ton two stage unit. But if I had, I would have been oversized at low stage 87% of the time.
If you figure I have had 7 months of cooling season since it was installed, that is 5,040 total hours and it has run 2,176 hours, a little less than half the time, night and day. My highest summer electric bill has been $102 dollars at a rate of 10.2 cent per kWh.
It might not have worked out as well if I had wanted to keep the house at 70, but I really do feel cold if it is lower than 76 at these humidity levels.
I just knew that if I had been ok with three tons for 5 years before improving insulation and airsealing, my load had to be less, and I also had been getting by with a 6500 btu window unit for a couple of weeks after my old one died, which gave me another bit of evidence. But it has worked out even better than I thought.
I do agree it was a good installation, and my ducts were sized ok, hard metal that were not sealed, which I sealed myself later, in a dirty narrow crawl space.
07-02-2012, 12:40 PM #10Professional Member*
- Join Date
- May 2004
- south louisiana
glad to hear that it has all turned out so well.
its a job sometimes just to get the installed tonnage
the house requires. I've had companies refuse to install
correctly sized units for my clients, or to warranty the unit.
it is a pita!
I don't envy you that dirty crawlspace duct sealing..
but you can see/feel the difference it makes.
love the 'high' utility bill. another way your sweat equity and
correctly sized unit continues to pay back.
enjoy! and congrats.
thanks for sharing.The cure of the part should not be attempted without the cure of the whole. ~Plato
07-02-2012, 03:35 PM #11
Low stage is about 1.4-1.5 tons nominally depending on conditions and whether it's at a reduced airflow for dehumidification (if that's an option).
The performance series 2 stage is basically the same unit as the Infinity, but with the communicating board, it might have a slightly different case or fan blade design and no the little Infinity label on the side.
You electric bill is right in line with what I would have guessed. Controlling humidity, controls your comfort, so you are more comfortable at a higher temperature, which increases your savings and allws you to use a smaller unit. It's a domino effect I suppose. By going smaller, you don't need as much capacity since you can operate it in a more efficient manner.
07-06-2012, 03:15 PM #12
This thread should be sticky noted so that it stays on the first page of the AOP forum. This thread is proof to the importance of the benefits of taking the whole house approach. This site should do more on stressing its importance.An answer without a question is meaningless.
Information without understanding is useless.
You can lead a horse to water............
07-06-2012, 03:24 PM #13