Please Help in Phoenix, AZ with sizing new unit

[qstendious]

Please have patience with me. I know nothing more than what I've read and tried to understand about sizing a new electric ac unit for my house. First off, there is a unit on the house but i have no idea how to determine what size it is. The unit has the date of manufacture listed as 1984. I read a little about the correct method for choosing the right size unit which I've gathered this is the Manual J method but I simply don't understand what it all means. I'm sure you guys get this question all the time so i will be as thorough as I can think to be.

So let me tell you about the house. It is 1300 square feet. Built in 1979, is a block home with 8' ceilings throughout, no vaults. Recently I tore out the original fur down hard duct which fed air out several different diffusers along its entire length. I then had an ac guy come out and install all new flex duct so now each room has a diffuser in the ceiling rather than before out the side of the wall. The east wall of the house has only two bedroom windows, each are roughly 4' x 3'. The south wall has only one bedroom window which is about 5' x 6'. The west wall has three windows, two very small bathroom windows, about 1' x 2' each, and a dining room window which is about 3' x 4'. The north wall has two windows, a kitchen window which is about 6' x 2.5' and a living room window which is around 6' x 6'. All of these windows are the original single pane ones which came on the house except the kitchen window. That one is new and it's a double pane picture window. The attic has the original blown in insulation. The block walls that double as interior/exterior are all furred out 1.5" and have fiberglass insulation between the block and sheetrock.

I hope this will be enough information for someone to make an educated estimate on the size unit I will need. Also, if you are not familiar with the climate here in Phoenix Arizona, you can click on the link provided and you will find annual historic weather data there for this region. Any help would be greatly appreciated as it's already getting hot here and I want to get this done as soon as I feel confident about my decision. Thank you very much for reading this long and probably boring post. If I left anything important out which is needed to determine the correct size, please let me know.
Thanks again.

Climate info: http://en.wikipedia.org/wiki/Phoenix%2C_az

[snack pack]

you need a 4 ton unit!!!!!!!! LOL,


seriously, you need to have someone physically come out and do a load calc. anything we give you via. the www. is going to be a futile stab in the dark. there are countless companies that will give you a free estimate, just ask that they do a manual j when you call them out.

on a side note, you seriously need to look at upgrading those single payne windows. that is MURDERING your home efficiency. and BTW that wiki link is way off it's more like 105+* for 90 days straight, and 90* at night. :cool:

[dan sw fl]

The unit has the date of manufacture listed as 1984.

... the Manual J method, but I simply don't understand what it all means.

1300 square feet
Built in 1979, is a block home with 8' ceilings throughout, no vaults.

East wall ... two bedroom windows, each are roughly 4' x 3'
South wall ... one bedroom window which is about 5' x 6'
West wall .. three windows, two very small bathroom windows, about 1' x 2' each, and a dining room window which is about 3' x 4'.
North wall has two windows, a kitchen window which is about 6' x 2.5' and a living room window which is around 6' x 6'.

All ... original single pane ones
The attic has the original blown in insulation.

The block walls that double as interior/exterior are all furred out 1.5" and have fiberglass insulation between the block and sheetrock.

... educated estimate on the size unit I will need.

Any help would be greatly appreciated as it's already getting hot here and I want to get this done as soon as I feel confident about my decision.

108'F Design Temperature

3-Ton, 2-stage with Variable Speed air handler
Air flow total about 1350 CFM.
High Efficiency ...
Electric rate is not cheap in AZ ( $0.14 /kwH ? )

Annual cooling energy use estimate ~ 5,500 kwH (13 SEER) depending on the actual equipment selected
(4,100 KWH ... 19 SEER) ... $800 / year ( which might be ~60% of your current cooling electrical usage)

Note that the cooling usage mgiht be 60% to 80% of your total electric bill
in the AZ summer months.

Higher SEER might save ~$ 200 / year on cooling operating costs.

Ducts in the attic are a significant portion ( ~ 20+%) of the load.
Is R-8 duct insulation now common for Phoenix?

~28,000 Sensible Coooling Load ... Obviously latent load is negligible in AZ
At a high S.H.R. of 0.8 for Phoenix, a 3-ton is adequate.

Replacing the WEST, Dining Room window would be an added benefit.
Adding tint to the South 30 square foot window would be a realistic small energy saver.

[Shophound]

There's no way I'd live in the Phoenix area without radiant barrier roof decking in place at a minimum...spray on if replacing the decking is not feasible.

Also hate to say this, but you may have been better off leaving the fur down ducting in place if it was below the attic. If your a/c guy ran flex up into the attic and you are still running your existing equipment, you may have effectively lost some cooling capacity by relocating the ducts to the attic.

I agree regarding a good load calculation. Dan gave you some numbers, but an on-site evaluation by someone who knows how to do a Manual J well is hard to beat, IMO.

[sammy37]

I know some of you guys would love to throw rocks at me for saying this, but I would rather be slightly oversized in that hot of a climate. I would want something that could pull the house down to the mid 70's on a 115+ degree day.

[qstendious]

Thanks for all the replies, I greatly appreciate them. I had a feeling that you couldn't just describe the situation without having someone actually come out and perform a test to get the answer. The reason I took the hard duct out was purely aesthetic. It was below the attic and it made my hallway only 7' high and ran all the way out into the middle of the living room and kitchen. It was a hideous looking thing. I guess I'll start calling around to find someone who will do the manual J test. Thanks again

[mchild]

Thanks for all the replies, I greatly appreciate them. I had a feeling that you couldn't just describe the situation without having someone actually come out and perform a test to get the answer. The reason I took the hard duct out was purely aesthetic. It was below the attic and it made my hallway only 7' high and ran all the way out into the middle of the living room and kitchen. It was a hideous looking thing. I guess I'll start calling around to find someone who will do the manual J test. Thanks again

Another option is to download the program from this site and do your own (HVAC Calc tab near top of page). Very reasonably priced and then you can spend some time doing "what ifs" to see what would be the impact of making some changes to the building envelop that will save you money everyday.

[qstendious]

I just talked to a friend of mine who told me how to determine the size of the existing unit. He said you have to look at the model number and find a two digit number which will be something like 36 or 42 and then divide that number by 12. If this is correct the existing unit reads 42 in the model number which would make it a 3.5 ton. Now my question is if it is a 3.5 ton, is it safe to just get new a 3.5 ton being that all the ducts have been changed to flex?

[dan sw fl]

... how to determine the size of the existing unit.
If this is correct the existing unit reads 42 in the model number which would make it a 3.5 ton.

Now my question is if it is a 3.5 ton, is it safe to just get new a 3.5 ton being that all the ducts have been changed to flex?

Does the current unit maintain 80'F when it is 108'F outrisde?

Do you keep the inside temperateure at 75, 80 or 85'F?

What is the actual cooling performance?
(total air flow rate and dT, room temp/% R.H., O.A. temp) ...
Is that equal to 2.5 or 3.0 ton specs?

Manual J 8th edition was completed based on windows
and S.F. information provided. I could have sent the Manul J results but your profile is not complete.

[qstendious]

Dan, I don't use the current unit. I turned it off towards the end of last summer because it stopped working right. It started blowing out semi-cold air and it seemed to run almost continuous. The unit up there right now is 24 years old and is a 9.5 seer so it was costing me a fortune to run it. Plus, it was very noisy. So I haven't even turned it on in many months.

I don't know how to determine the actual cooling performance. But not only does the current unit blow semi cold air, it also seems to push very little air volume. I think something might also be wrong with the blower motor. As far as temp goes, ideally I would like to keep the inside temp between 75-78 degrees. And I would like the new unit to be able to do this with no problem, even if it is 115 degrees outside.

I updated my profile (added an email address) so you can send the results if you want. I greatly appreciate your help with this.

[cmajerus]

you need a 4 ton unit!!!!!!!! LOL,


seriously, you need to have someone physically come out and do a load calc. anything we give you via. the www. is going to be a futile stab in the dark. there are countless companies that will give you a free estimate, just ask that they do a manual j when you call them out.

on a side note, you seriously need to look at upgrading those single payne windows. that is MURDERING your home efficiency. and BTW that wiki link is way off it's more like 105+* for 90 days straight, and 90* at night. :cool:

yeah but its a DRY heat right?:eek: Makes it so much better!!

I don't know how you guys can work out there during the day!

I did some work on the flightline at Luke AFB in the early 1990's and we worked from 10pm to 6 am so the crete could set up with out evaporating all the moisture out before setting up( and to keep us hydrated of course!)

[bell3156]

once you find the requird btu's this needs to be matched to the sensible btu performance at the design temp. on 1300' a 3 ton would be pushing it here in phoenix, I have a 3 ton on 1280' the unit will not go below 82 when it is above 115deg.. thank god there are less than 20 days when it gets that hot.
Also a concerne is the unit should have been selected before the duct work was designed, unless you were using rules of thumb for cfm, if you were you would have to put in a 3.5 ton to get the proper airflow with your ducts ( unless it is a vs)

[qstendious]

Bell, I didn't really have anything to do with the duct design. I just hired a guy to come out and put in the flex duct. I guess I should have done all my research before I even started doing any of this. What are the rules of thumb when it comes to duct design? What is a vs? What is a design temp? I'm so lost...

[mayguy]

Are you planning on updating the windows and adding more attic insuation anytime soon? If you are, do let the dealer know when they do the Manual-J, so they can put that into system, that way you don't have an oversized system.

[snack pack]

yeah but its a DRY heat right?:eek: Makes it so much better!!

I don't know how you guys can work out there during the day!

I did some work on the flightline at Luke AFB in the early 1990's and we worked from 10pm to 6 am so the crete could set up with out evaporating all the moisture out before setting up( and to keep us hydrated of course!)

you get used to it pretty fast, it's much nicer than digging through snow to get to a unit. the hard part is keeping tools cool enough to handle when gloves are outta the question.

[cmajerus]

you get used to it pretty fast, it's much nicer than digging through snow to get to a unit. the hard part is keeping tools cool enough to handle when gloves are outta the question.

I'll bet, I was stationed at Nellis in Vegas, so I know what ya mean, but it seemed to be 50 degrees warmer in Phoenix!:D