over size/under size
The more I learn, the more I find out how little I know. So I am asking, again, for comments from you all.
Lets say that I can't find a contractor that will/can do a load calc. I have narrowed it down to 2, from 6 interviewed. I am going with an Infinity/Evolution system. Contractor #1 says 3 ton is required to keep my home cool in our hot/humid summers. The other says 2 ton is plenty and 3 is over sized. #1 says if it is really 2 ton required(which he thinks is seriously under sized) the 3 ton, 2 speed comp and VS fan will alleviate some of the over sizing. #2 says no way, 2 speed and vs will not correct over size problems, and may exacerbate the humidity problem.
The current unit is 3 ton and the duct is too small-it will all be replaced.
BTW, I did my own calc(the sponsor here) and got 3 tons. But the more I have read the more I think I can't trust the #s. I think the calc is helpful and I learned a lot, but for proper equipment choice, considering all factors is important and a pro needs to be involved.
Bottom line-I want to go with #1(3 ton) for a variety of reasons. If my real need is 2 ton, will the 3 ton with VS and 2 speed over come at least a portion of over sizing?
Thank you for your comments and opinions!
If you did the calc here, trust it and go with 3 tons
Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced
When there is a choice of slightly undersizing or slightly oversizing, I always lean toward undersizing.
After all, with any calculations a system is designed for the most extreme conditions. Other then on the extreme temperature days, your system is always oversized. Oversizing it even more from the get go makes no sense.
Also, the longer an HVAC system operates with the least amount of cycles is the most efficient way for an HVAC system to operate.
What's the worse that will happen if you undersize? Needing to put on a sweater once in a while when the temps drop to the lowest they will ever drop or maybe needing to close the shades on the windows when the outside temps are the hottest they will ever be. The humidity control alone will make it much more comfortable.
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1.What are the sensible and latent btus from your calc.?
2.What indoor design temp. for cooling?
3.How confident are you with your inputs,sizes,insulation values,etc.?
4.What is the R-value of the existing attic insulation,and can more be added?
To answer dash-
1. Sensible is 32,162
Latent is 3118
2.Indoor design temp is 73
3. I measured carefully and double checked. Insulation is the tricky one, which brings me to-
4. In the load calc I used no insulation for the attic. I have about a 6.5 inch air gap between the ceiling drywall and roof decking. Roof deck is torch down over 3/4 inch ply. There is foil installed right close to the ceiling drywall, but it is from 1957 and I don't think it is very effective. I re-ran the calc with the addition of only R-11(3.5 to 4 inch approx) in the 6 inch air space and the sensible was cut IN HALF to 16,597. That was when I really started questioning the calc #s. (yes, i am consulting an insulator, but it will be difficult/expensive to add insulation)
RoBoTeq- When I ran the calc I did it with and without shading-blinds and trees. The program says to be careful about adding shading from trees and blinds, but the reality is we keep our blinds closed most of the time and we are covered with trees-live oaks that are are green with leaves year round. At any rate, the calc did not change much with and without shading/blinds.
3-Ton 2 Stage Carrier Infinity !
3-TON on High/ LOW Stages provides 35,000/ 25,000 BTU total capacity.
Originally Posted by 7echo
Pages 7, 8, 10, 11
Low stage will provide 21,000 BTUh Sensible which is Mid-range ( 16,597 to 32,162) of your calculated requirements.
Last edited by dan sw fl; 03-09-2008 at 02:44 PM.
It's Not Rocket Science, But It is SCIENCE
with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
Thanks for the link dan.
Will running on 1st stage not be as effective at humidity removal? If I really only need 2 ton will the unit spend almost all its life running on 1st stage, wasting the money spent for a 3 ton unit? Or will a 3 ton never remove humidity as well, even with a VS and 2 stage?
Mfrs. btu ratings are at 80 degrees indoors,the deduct is 835 sensible btus,for each degree below,per 1000 cfms.
Originally Posted by 7echo
So the 3 ton would lose 835 X 7 X 1.2 =7014 sensible btus when cooling to 73 indoors.not likely a two ton will do it .
Are you sure there's no insulation on that foil,or maybe roof deck insulation?
Did your old 3 ton cycle on and off during the days when it was at OD design temp.
Or did it have to run 24/7 to keep it cool.
The unit always cycled and never ran 24/7, but I have found it ok to be set at 74 during the hot days when we are home. At night I crank that stat down to 70 or 69 and it does cycle. The current set up is pushing all the air through a 12" x 12" square duct, if that matters.
dash-Yes, no insulation. A tree fell through the roof on July 4 2004 and I got a good look at what is there. : ) Low slope(2" in 12") with just an air gap.
I like it to be cool, but I know if I can lower the humidity I will feel cooler at a higher temp set.
Thanks for all the replies!
The two stage three ton should push about two tons at first stage. Best of both worlds. Sounds like that duct problem will be remidied, so you're in good shape with the three ton/two stage.
Meet with contractor #2 and show him your heat load calcs. and ask him how he differs with your calculations to come up with 2 tons. Maybe he was assuming attic insulation along with an indoor temp. that was higher than yours. Sounds like they both guessed anyway.
I would wait to see what the insulation contractor comes up with. The timeframe of your life in this house could possibly pay you back many times over if insulation is added. Closed cell foam would be $$$$ but could give you huge savings. Blown in cost less of course but w/ lower R value.
I love the fact that the homeowner knows more about the process of equipment selection than the majority of the pros. in your area. Glad the 2nd contractor isn't going with the same size as he's replacing cookie cutter approach but may be going with the square footage calcs. Good for you for doing your homework.
Most of my (admittedly little) knowledge on HVAC came from reading threads and participating on this site for 9 or 10 months. And I got a few tid bits from one good tech where I work. I also have a resource not everyone has access to and he is going to run a load calc for me with his software. I will be interested to see what he comes up with. I am just smart enough to ask the right questions, and I enjoy learning about this sort of stuff.
Seems like 3 ton is the getting the most votes so far, Again, thanks for all the comments!