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03-28-2007, 12:35 PM #1
Phoenix, AZ - Need Unbiased Opinions!
I wish I had found this site a few months ago. Agg! I'm at the point of getting bids and have cash-in-hand. I've *HATED* the A/C in my house from DAY ONE, because it's never cooled the house very well. Lost of issues - I'll try to be succinct (a challenge for me!).
- 2,222 S.F. 2x4 frame stucco single level home
- Split floor plan - see attached attempt to draw it
- We are original owners - newly constructed tract house in 1992
- 5-ton SEER 10 Trane split system
- Single return
- Vaulted ceilings - 12-14 feet
- Dual pane low-E windows
- Huge western exposure - large windows
- All bedrooms on southern side of house
- Master bedroom on south west corner of house
- Phoenix, AZ
- We plan on living in this house for the next 20-30+ years.
- Master B.R. is hottest room in house.
- A/C system takes forever to bring temp down - runs for HOURS w/ minimal temp change in HOT weather
- A/C has seemed inadequate since day one
- 2 contractors have commented about duct design problems:
- 12" duct to kitchen comes off back of plenum, putting most of the A/C on the north side of the house;
- two 8" ducts that supply great room are too small;
- two registers for great room are too small;
- supply air plenum too small.
- system not "pressurizing"
CONCESSIONS TO ISSUES
- House WILL maintain 30ºF below ambient
- We probably "abuse" the system - we keep the house at 80ºF until 8pm, then ask it to do 74º.
- We are on a "time of use" electricity plan - power gets MUCH cheaper @ 8PM until 12Noon the next day.
- We have tried "pre-cooling" in the AM hours. House won't hold a "pre-cool" - not enough to hold the cold it seems.
I've had 3 contractors visit so far. 1 won't return my calls - guess he doesn't want the job - small company - never got a proposal from him. 2nd contractor - medium sized company - recommends Trane 5-ton XL 19i or XL 16i, and says the problems with the A/C are due to the ducting, too much air going to the north (kitchen) side of the house due to 12" duct coming off the back of the plenum, inadequate ducting to the Great Room. His bid seems competative. 3rd contractor returned his info to their saleman who didn't know anything about A/C - his job was to close the deal I guess. They're a big outfit. They're bid was in between the XL19i and the XL 16i, but is for a XB 13! One more due out - small company that has been servicing my A/C for years - father/son operation, and both will come.
- I don't know if a true load calculation was done by any of them or not. What does the number look like that I should be able to get if they DID do a load calc?
- Any thoughts as to whether or not a 5-ton unit is the right size? I'm thinking 6 tons of A/C would leave me happier. Would it be a mistake to go a tad over??
- I was thinking that a dual system would be better for my house. I'd really like to be able to cool the bedrooms FAST, for sleeping - like from 80ºF to 74º in an hour. But there's no need for this capacity for the rest of the house, plus there's no need to cool the rest of the house to 74ºF all night.
- Both contractors want to replace my registers. Mine don't have the "doors" - they just have the up/down and right/left louvers. Is this worth the expense?
- I like the idea of two XL 19i units - the SEER is great, and contractor 2 suggested one 3-ton for the bedrooms and a 2.5-ton for the rest of the house. How can I determine the return on investment? Should the contractor be able to do this calculation for me?
- Is there any advantage to looking at scroll vs. reciprocating compressors?
- Should I be concerned about R-22 vs. the newer refrigerant (Puron-type stuff - I forget the numeric designation)?
Any and all advice, suggestions etc. gratefully welcomed! We plan on staying in this house, and I don't want to make any mistakes. I want to be DONE with the A/C issue for a long time. Please help!
03-28-2007, 01:00 PM #2
1) If a Manual J load calculation was not done, there is no way to know exactly how to size the equipment and ductwork. With a Manual J and Manual D, followed by a proper installation, every room in the house should have adequate cooling for what is called a "design day" for your area.
2) Do not oversize. Heat load calculation, proper duct size, proper duct and equipment install.
3) 80 to 74 is a lot to ask a system to do during the heat of a Phoenix summer day. If you get an adequately sized system it will only be sized to maintain an indoor design temperature and humidity level. It will not be sized for set up or set back situations. As it is you're burning up a lot of money by trying to pull 80 down to 74...doing it in an hour would require equipment to be grossly oversized. You will not like the real world results.
4) Curved blade registers for wall mount supplies, also good for ceiling but a diffuser would be better....who uses diffusers in residential? I've seen very few. Would depend on architecture of room, however.
5) If you want to zone the house via two systems that can be done...not sure on ROI for size of your dwelling.
6) Scrolls are IMO the way to go. Recips are good, sturdy compressors, no doubt, but I believe higher efficiency is found in the scroll technology.
7) 410A is here to stay...22 won't be found in new equipment come 2010, and from 2015 on it gets tougher for existing supplies. 2020, no more R22. Since you're speaking about a long term committment to this house...take a good look at 410A.
Installation is CRUCIAL!!!! You can have the best equipment but a crappy install will kill everything. You should also consider additional remedies alongside your HVAC issues...insulation and infiltration. Your house may not be all that old but it would be worth your time to get an energy audit done."In this house we obey the laws of thermodynamics!"
- Homer Simpson
03-28-2007, 01:09 PM #3
Only take the bids that do a Heat Load calc. Sounds to me that the concensus is your ductwork is messed up.
You can't have your cake and eat it too....meaning:
You can't expect a huge return on your investment when you need new duct design and you're talking about having 2 seperate units. comfort costs $$. Have your insulation checked...you mentioned it doesn't hold the temp well in the "pre cooling"
Expecting a 6 degree swing in an hour in Arizona might be asking a bit much!Before you criticize someone, walk a mile in their shoes...that way you are a mile from them and have their shoes
03-28-2007, 01:10 PM #4
First, fix the things that need it the most.
Add solar screens to all windows on west end of house.
If possible, add more attic insulation - cellulose has best payback. R40 to R50 is a good goal.
Make sure duct work is sealed and insulated to at least R6.
After that is done, have a load calculation done and figure out what the right answer for the HVAC is.
03-28-2007, 01:21 PM #5
Thanks for your thoughts. One of the contractors (the 2nd bid) wants to put some 4-way registers in the ceiling of our great room - 4 of 'em evenly spaced.
Is that like a diffuser, or do you mean a diffuser like the pic below?
As far as reasonable expectations... If it's 115ºF out and says 30% humidity (which is pretty high when it's that hot), should the system be able to maintain say 74ºF, or am I in La-La land in wanting that? My current system MIGHT be able to keep the house at 80ºF on such a day, and it will take it hours and hours and hours to drop the temp down.
An energy audit sounds like a good idea. Didn't know what to call it. Is that something I can just look in the phone book for, or is there some other catch-phrase that I'd have better luck searching on? In other words, who does energy audits - my power company, an A/C firm, an insulation company? Yeah, I'm clueless on this...
What exactly will the output from a "Load Calculation" look like? Can somebody post an example of the output?
03-28-2007, 02:23 PM #6
I think that site has links for contractors throughout the country that can perform energy audits.
The first picture is a four way curved blade supply register. The second is a ceiling diffuser. Selection of either depends on architecture of room...do you already have four supply registers in the great room that the four-way would directly replace?
Are you saying that when Phoenix reaches 115 degrees, the outdoor humidity level is 30%? That would make your outdoor conditions more unbearable at that state (85 degree wet bulb!) than my local sultry conditions of 102 degrees at 35% relative humidity (78 degree wet bulb).
I just glanced at some weather data for your area from last summer and I got three separate days, one from each month, June-August. Highs each time were about 105 to 106 and humidity levels were around 15. This is what I'd expect to see in a desert region vs. 115/30.
That being said, you could size the system for 105 degrees outdoors, and keep the inside at 75 degrees on that type of day. Beyond 105 degrees you will see the interior temperature creep up...about three degrees or so. In your area sensible heat capacity of a system is crucial; you would also have to choose equipment larger than the nominal tonnage to meet that load, since equipment is rated at outdoor and indoor conditions less extreme than yours. All of this is what a genuine HVAC professional contractor should know. The ones that don't IMO hold quesitonable pro status."In this house we obey the laws of thermodynamics!"
- Homer Simpson
03-28-2007, 02:43 PM #7
LOL - thanks for the reply. Yeah, 105ºF and 15% humidity is more typical, but make no mistake, we get some absolutely unbearable weather here - and sometimes for days on end. Back in 1989, we hit 122ºF. Sure, we had single-digit humidity, but man - it was miserable. And every year, we have periods of 110+ºF days. A few years ago, we had something like 30 days in a row over 110ºF. Where we REALLY get killed is when the monsoons come in late June/early July. The humidity shoots up, and the temps stay hot.
I want to be comfortable on those miserable days. I don't want to suffer quietly in my bedroom until midnight, with fans blowing hot air on me. I want to be able to get to sleep and be comfortable. It's not unusual to have it still be 110ºF at 10pm here. Happens every year. It's not unusual to have 90+ºF low temps - happens every year.
So... That's some of the challenge here.
As far as existing registers for the great room, there are two std. sized registers, each fed by an 8" round flexduct. They're both mounted on the wall about 10 feet off the floor. The one contractor that seemed to do the best job suggested putting 4 ceiling-mount registers - probably the our way curved blade supply register, adjustable type, I think. He wants to feed each of those with an 8" round flexduct. He thinks the biggest problem is the supply air plenum being too small, and the next biggest problem is the 12" round flexduct coming directly off the back of the plenum - which is routed across the top of the great room to supply the kitchen/dining/laundry. He says most of the A/C is going to those rooms thru the 12" duct, which should NEVER have been connected to the back of the plenum, because it keeps the system from pressurizing.
I'll check out the link to the energy audit. Thanks very much for that!
03-28-2007, 02:51 PM #8
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03-28-2007, 03:22 PM #9
We've already got the solar screens on all sun struck windows. Insulation in the attic was R-38 when installed in 1992 - cellulose. It was blown with a water mist or something I remember being told, which is supposed to reduce settling and loss of R- value. I don't know what the R- value today would be tho, with it having had 15 years to settle.
I think most of the duct work will end up being re-done, based on problems with the initial installation. But that's a good point - I think the current flexducts are like maybe 1" or 2". I've written down your R-6 recommendation - I'll spec that in my proposal requests.
03-28-2007, 03:50 PM #10
This is heresy, but since you want the AC to have fast pull-down capability... you can only get that by sizing bigger than Manual J says. Manual J has a goal of just barely maintaining setpoint during peak cooling needs (while you want to move from 80F to 74F with some speed). Therefore this sounds like one case where "super sizing" might give you what you want.
Humidity is evidently not an issue, quite unlike my hot-humid S.Texas where I am a homeowner. That knocks out one usual argument against oversizing. There is a small energy penalty due to cycling, but that will be minimized if you go with 2-stage technology. Your time-of-day (TOD) electric rate is a very powerful argument in favor of doing things your way.
I would not say this but for your experience with the TOD electric rates, and your low humidity, and willingness to rebuild the duct system.
Hope this helps -- Pstu
03-28-2007, 04:08 PM #11"In this house we obey the laws of thermodynamics!"
- Homer Simpson
03-28-2007, 04:21 PM #12
LOL - heresy! I love it.
Well, FINALLY someone "in the know" that doesn't say I'm crazy! Of course, the next question is HOW oversized. I mean, there's no point in going way overboard. But leaving things at exactly what the book says will leave me wanting, I think.
I'll give you a for-instance... I installed a 1/2-ton window shaker a coupla years ago in the master B.R. I don't remember the exact dimensions of the room, but it's like 18'x20' maybe w/ 12' foot ceilings. SouthWest exposure to room also. The window shaker with the central A/C is a bit less performance than I'd like. It's barely adequate on the hottest days. I'd double the window shaker to 1-ton and I bet I'd like it more. Heck - maybe even 18,000 BTUs and I'd still say it's good. I have a good intuitive sense of what I'd like in a room the size of a bedroom since installing a 1-ton casement unit in my 200 s.f. converted single-stall garage room that I built. I insulated that as much as humanly possible in a 2x4 - I think I ended up with R-23 on the one wall, and the other is the same, since it's got stucco and foamboard on it. Anyhow, with 12,000 BTUs of A/C for this 200 s.f room that has a 20 c.f. fridge in it, it cools off quickly - about like I want my master bedroom to cool. I can take the room from unairconditioned all day, have the unit kick on at 2pm, and by 3pm, it's cool enough to stay in. At 110ºF, I can pre-cool the room to the mid-70s and open the garage door about 2 inches and smoke a cigar and maintain mid-to upper-70's.
Anyhow, I'm running at the mouth... I like the heresy. I'm just not sure how MUCH heresy I should be thinking about. Oh - and bear in mind, this would be JUST for our 3 bedrooms. The rest of the house would be properly sized...
03-28-2007, 04:24 PM #13
No problem, shophound. I actually found an outfit to do one - at least I *THINK* that's what they're going to do. For $295, he'll be out there for 2-3 hrs and will use a "blower door" to "depressurize the house" and will be checking for air leaks using infarred gizmos and for A/C "pressures", by which I think he means the ability of the a/c system to pressurize the house, and for leaks in the ducting and for insulation "hot spots". Then they'll make recommendations and offer up companies to do the work, if we want.
Does that sound like an energy audit??