Right choice of AC unit/type/ton?

London56June 8, 2011

After months of research and hordes of scammy salespeople, I think I've chosen the right contractor and the right unit. According to my printed load calc (thank you, Garden Websters!) I read: apprx 1600 sq ft.with total gain/loss 39,305 = 5 ton unit. Replacing broken 4 ton 15yr old Carrier 3-Phase on the westside of house (on ground, in Phoenix)with Carrier 48ESA600905A,also 3-Phase 13 SEER/11 EER, placed on roof; 2 new 30x14 return air grills in main living areas (old system is 1 small return in hallway); new gas flex and some new duct work/connections in attic. $6505. I've lived in house for 10 years and have never had a comfortable summer. Do y'all think I'm making an OK - or maybe even great choice? Hoping to install day after tomorrow, but would like a blessing! Shout out to Randy, who pointed me to GardenWeb, and all the generous spirits here!

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mike_home

Tigerdunes,

I think you posted the wrong link. Nice shirt :).

London,
I know it gets hot in Phoenix, but I would with a 5-ton unit you have a potential of a meat locker! Most homes don't have a duct large enough to support a 5-ton unit. You may be disappointed.

Why does the unit need to move to the roof? Is there an advantage to do this?

    Bookmark   June 9, 2011 at 10:00AM
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tigerdunes

london,
if you are saying that a load calculation shows 39,305 BTU heat gain, then a 5 ton package system is grossly oversized.

12 KBTUs equal 1 ton of cooling or heating.

I agree about the additional return. So how much return will you have?

The ES mdl is Carrier's low end gaspak package system. I would at least move up to Carrier's Performance series.

I would carefully consider sizing before contracting for a 5 ton system.

IMO

    Bookmark   June 9, 2011 at 10:55AM
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energy_rater_la

I agree that something isn't right
5 tons for 1600sq ft?

you would benefit from installing a smaller unit
and improving the homes thermal and air barriers.
an independent energy audit will point out the deficiencies
of your building envelope.

best of luck.

    Bookmark   June 9, 2011 at 11:44AM
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London56

Sheesh - I've interviewed apprx 7 companies over several months; 1/2 said 5T, about 1/2 said stick w/4T, and 1 memorable guy said a 6T commercial unit was in order.
Would it help to know that the house is 50+ old, brick, not very well insulated attic space, single pane windows with totals from the load calc. of N-1360, S-3685, E/W-15360? The guy was going to order the unit today for install tomorrow (we're dyin') - I will contact him with your concerns, and see what kind of answer I get, meanwhile, I value your opinions - thanks so much for your time!

    Bookmark   June 9, 2011 at 11:53AM
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david_cary

I'm wondering if since it is a dry area, it needs to be 5 tons to get an adequate sensible load.

London - of course lowering the heat gain would payback and allow a smaller unit. The attic space is easy to add insulation too. Solar screens, window overhangs all can dramatically lower the gain. I think you are getting incredulous replies here because people can shoot for 1 ton for 1000 sq ft and you are talking about over 3 times that capacity/energy use. 40kbtus is the gain - doesn't really matter the directional breakdown. Your real issue sounds like you have a lot of E/W windows with minimal overhang which makes you wonder what the builder was thinking 50 years ago.

    Bookmark   June 9, 2011 at 2:50PM
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ionized_gw

The builder was probably thinking evaporative cooling. Cooling with swamp coolers is relatively inexpensive. The cost of water has gone up and the quality has gone down, but it is probably still a relatively cheap way to cool.

    Bookmark   June 9, 2011 at 2:55PM
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ionized_gw

The builder was probably thinking evaporative cooling. Cooling with swamp coolers is relatively inexpensive. The cost of water has gone up and the quality has gone down, but it is probably still a relatively cheap way to cool.

    Bookmark   June 9, 2011 at 3:02PM
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weedmeister

I would step up to a 15SEER unit. The payback of increased efficiency (lower bills) should pay for itself in a few short years since you will be using it a lot in Phoenix.

    Bookmark   June 9, 2011 at 3:27PM
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mike_home

What I don't understand, if the heat gain calculation was 39,305 BTU, then why is the contractor going to install a 60,000 BTU unit? Does he think 48,000 BTU (4 ton) is cutting it too close?

Why bother doing a load calcualtion if you are going to ignore the results? Has the contractor explained the reasoning behind his decision? I know ratings have to be derated for altitude, but I didn't think this was the case for Phoenix.

    Bookmark   June 9, 2011 at 3:39PM
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energy_rater_la

I think I'd hang a bit longer and get it right OP
otherwise you'll be living with a system that is
really oversized.

best of luck.

    Bookmark   June 9, 2011 at 7:15PM
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david_cary

Am I right that if he has 39k of sensible load, then a 4 ton would not cut it?

The way I understand it, is that units can handle about 75% of their load as sensible. In dry areas, nearly all of the load is sensible so you often have to go with bigger units.

    Bookmark   June 10, 2011 at 2:53PM
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tigerdunes

OP does not really make a distinction about the load calc other than to say it is total heat gain. for the Phoenix area/climate, I would think the latent load(humidity) would be low compared to the Deep South/Southeast/Coastal.

IMO

    Bookmark   June 10, 2011 at 4:23PM
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