View Full Version : could this happen
06-14-2007, 03:53 PM
O.K. So I totally understand that Manual J calculations are extremely important to sizing a HVAC system. I also understand that the quality of construction, attic ventilation, quality of windows and doors etc. all have an effect on the
size of equipment needed. Add to the mix that oversized equipment will short cycle and cause unpleasant if not detremental humidity levels.
So hypothetically say I purchase a 7 year old house that was constructed with less than stellar construction techniques, poor windows, not sealed etc. but the system was sized correctly using Manual J. and keeps the house cool during the hottest days of the summer. Now I decide to call one of those numbers from the TV for new windows, the salesman convinces me to replace my old vinyl siding with the new pre-insulated stuff and wraps my house with house wrap while all this is going on. They also seal all of the outlets, switches and junction boxes with expandable foam insulation.
Have I now got a home that is so much better in construction that my old HVAC system is now grossly over sized and my humidity control will be poor?
Do you guys ever have to down size a system because of this?
06-14-2007, 04:03 PM
You would need to re-do the calculation to reflect the updates & see how the new results compare to your current equipment capacities. Insulated siding probably wouldn't result in a major HVAC load improvement (I just did this calculation with my house prior to getting new siding), but the window & infiltration improvements might very well cause your current system to be oversized.
06-14-2007, 04:13 PM
Summer heat gain...biggest areas are windows and ceilings adjacent to hot attics. Walls can fall below infiltration for contributing to heat gain, depending on how leaky house is.
If this 7 year old house has crap for windows and marginal insulation in the attic, improving those two can give you good bang for the buck. Leaky, poorly insulated ducts should also be addressed. These will cause house pressurization problems and reduce your system capacity if not addressed.
06-14-2007, 04:19 PM
I agree with roymcoy. Sidewall insulation doesn't affect the calculations much. It's all in the attic/cap insulation and in air infilatration/exfiltration rate. Oh yeah, and the glass. Low 'e' glass would make a difference, particularly the East, West and South windows. As for having to oversize, it's not usually the case since the equipment doesn't last as long as the modifications to the home. For instance, if your home was 7 years old when you purchased it, the unit is 1/2 way to the retirement home under good conditions. Also you don't suffer the emotional pain of having paid cash for the unit when it was new. So suppose you do the improvements during year 8 and then decide the system isn't performing to your likes. Chances are you'll wait until the beginning of year 9 before you replace it and by then it's anywhere from 2/3 to 3/4 of it's life expectancy anyway. Then you have a new load analysis done with all the mods included and you get a properly sized system. We more often run into systems where they put an addition on the house and some yahoo decided that he could snap duct together therefore he are a certified HVAC installer. Then he just latches onto the existing ducts and runs two or four branch take-offs out to the new addition, which is usually 35 feet from the main trunk. Flex duct is usually used because it comes in 25-foot lengths and so you only need two boxes to get there from the main. Then, since he doesn't own a pair of aviation snips, he just tapes the two pieces of flex together and winds the extra flex up in a coil to make it look pretty. Then wonders what the client is complaining about when they say they don't get any AC in the new addition.
06-14-2007, 04:29 PM
Most of the real improvements you would be making would have a greater impact on the latent load than the sensible. You typcially size to sensible and since that would not have had much change then the A/C will still function without much noticable difference. Plus, with the evap coil getting a bit loaded with dust the capacity of the system is deminishing as the home ages. The improvements just might help the load match the deminished output of the system.
If the original home is bad enough and improvements good enough,you'll be oversized.
In our area of Florida,a home of 1500 sq ft built in the 1970's with R-11 attic insulation,will need a half ton less if they blow in insulation making it R-30.
Change awning type single pane windows to double pane tinted,could reduce the home by half a ton.
We have seen what you describe many times.
06-14-2007, 05:43 PM
I don't think most of the home improvement companies that you see on tv could make that big of a difference in the insulation of your home. Most of those guys seem like "snake oil salesmen". Plus you never mentioned anything about adding insulation to your attic.
06-14-2007, 05:50 PM
Since you claim the system was originally sized correctly for this less than stellar construction, then obviously when you make some various stellar improvements, etc, it will become oversized to some extent.
If the system "short cycles" and the humidity levels are too high, then yes some downsizing remedies will have to take place.
I'm a homeowner in S.Texas where lots and lots of houses are built with oversized ACs. I think you can apply a couple remediation techniques and avoid the more drastic problems of oversizing, especially if your budget could include a variable speed air handler.
There is a Florida Solar Energy Center paper which discussed right-sizing and it gave a lot of attention to AC engineering improvements from the factory in past years, which reduce the magnitude of cycling inefficiencies. So the engineers have made oversizing less of a problem than it used to be. There have been some bonafide knowledgeable pros who consciously oversize compared to Manual J, and still effectively address humidity control, so it appears to be possible (the late David Debien aka Airman1 was one such pro).
If other methods don't do what you want, you have the option of adding a high efficiency dehumidifier and getting your house drier that way. I have one and it really helps in those mild months when every AC is oversized, there is a comfort benefit you can feel. One poster on this board observed that having a dehu is like turning latent load into sensible.
I'm a homeowner and not a pro, but it seems to me your problem is more theoretical than real. I would not work this problem real hard unless some measurement or your 5 senses tell you that a problem exists. Although I do find hypothetical problems interesting!
Hope this helps -- Pstu
06-15-2007, 12:17 AM
There have been some bonafide knowledgeable pros who consciously oversize compared to Manual J, and still effectively address humidity control, so it appears to be possible (the late David Debien aka Airman1 was one such pro).
If I recall correctly, David Debien's approach was to undersize the evaporator coil in relation to the condenser on a split system. If he set a three ton condenser, he might put in a 2.5 ton evaporator, for instance. This was done to lower SHR (thereby raising latent capacity). On a more recent visit to his site, it featured two stage evaporators using liquid line solenoids and two TXV's.
In regard to "oversizing compared to Manual J", the only proper way I can see looking at that is if design conditions exceed the ARI nominal ratings of the candidate equipment. As you've noted in other posts, Trane's performance data is excellent, and you can quickly see that if your design conditions exceed nominal, if you go with nominal you may come up short on sensible capacity on a design day.
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