Air Conditioning Questions? (heat, insulate, color, ceiling)

[Ultrarunner]

There are many variables and I have 25 years experience with Hospital and Commercial systems...

A 30 degree split is just about optimal for my Hospital location...

If tripple digits are forecast the only effective thing I can do is to lock on the system nonstop... in other words not turn it off or go to a warmer after hour setting... this allows all the interior surfaces to cool which helps.

In residential... there are many things that can mitigate... oversize eves, shade trees, trellises, foam insulation, quality insulated and reflective windows... to more thermal mass to slow down temp changes.

If I wanted to maximize limited cooling I would look at all of the above...

[Zippyman]

An oversized unit is a bad idea in the desert, and I've yet to encounter an "undersized" unit so far here - for a new-build 1300 sq-ft house in the Phoenix metro, even a two-ton might be too much. My parent's mid-eighties build @ 1132 sq/ft has a 2.5 ton unit & it frequently short-cycles in 100+ weather - with a little better windows & a few more inches of attic insulation, 2 tons would cool it just fine & cost less to run all year. If you're set-up to cool *beyond* design standards at 108 degrees, you're paying too much for cooling at 99 degrees (which happens a lot more often here than 110 degrees +.

Worst case, (if you were undersized) the half-dozen days a year when it gets to 115 outside, your indoor temps might climb to 80, instead of 77.. Not that big a deal & certainly not worth oversizing a unit for. If you like 68 degrees as a set-point, oversizing might be a good idea.

Your a/c should run all day long on a 108 or above day & if it doesn't, it's too big & costing you money on cooler days.

[btuhack]

Originally Posted by Zippyman

If you like 68 degrees as a set-point, oversizing might be a good idea.

More like a necessity. Capacity is measured at an 80° indoor temp and below that derates the equipment. 105° outside with 68° inside leaves a 5 ton system with 2.7 tons capacity assuming zero duct loss. 1/2 ton is lost derating the condenser above it's 95° rating temp and 1.8 tons is lost due to the low indoor setpoint. At 72° you're at 3.3 tons and 75° provides 75% of 5 tons...3.75 tons. The good news is you still get to pay for 5 tons worth of electricity : /

[Harpaint]

When we got a new air conditioner in a HOT location, the installer told me to save electricity by hosing the unit down when I used it. That was 20 years ago, and I've done it on all units I've had. It REALLY cools the house down faster. I hose it really well in the morning or whenever I turn it on, and then again later,

[Zippyman]

Originally Posted by btuhack

More like a necessity. Capacity is measured at an 80° indoor temp and below that derates the equipment. 105° outside with 68° inside leaves a 5 ton system with 2.7 tons capacity assuming zero duct loss. 1/2 ton is lost derating the condenser above it's 95° rating temp and 1.8 tons is lost due to the low indoor setpoint. At 72° you're at 3.3 tons and 75° provides 75% of 5 tons...3.75 tons. The good news is you still get to pay for 5 tons worth of electricity : /

Pretty sure your "de-rating" percentages are off, at least with the equipment I've done the math for & in my climate (Phoenix). In any case, oversizing is already the rule here & it isn't even that unusual to see a 4-ton unit on a 900 sq-ft house. Which is awesome if you turn your air off entirely all day & only click it on at 6 pm (and you want a 70 degree house in an hour). In any case, taking a walk through a neighborhood here on a 110 degree day & you'll see (hear) lots /most / nearly all your neighbor's a/c units cycling off for five minutes & on for five minutes.. Which is really great for the future if you believe in global warming & you think average temps here are going to be 130. Otherwise you might as well light a couple dollar bills on fire every day it isn't 130 out.

[john3232]

I'm pretty sure the AC units in my properties are the original units.

sq. ft.
1490 3.5 ton package
1601 4.0 ton package
1402 3.5 ton package
1236. 3.5 split
2100 5.0 split
2400 3.5 and 3 ton split
2063 5.0 split


The 1490 3.5 ton package and the 1402 3.5 ton package units will need to be replaced in the near future.

The tonnage seems a bit high given the size of the homes.

If I'm not in AZ and a unit needs to be replaced I plan on using https://newacunit.com/shop/

If the tech sent out to inspect the AC to be replaced and recommends say a .5 ton smaller unit would it be advisable to go that route?

Or should I stick with the size the builder installed?

Thanks

[btuhack]

Originally Posted by Zippyman

Pretty sure your "de-rating" percentages are off, at least with the equipment I've done the math for & in my climate (Phoenix).

Turns out my numbers were a little off.

The derated number for a 5 ton system at 68/105 is 2.2 tons as per published mfg data, not 2.7 tons. That of course is for new equipment prior to fouling and inefficiencies developed over time. My apologies.

[btuhack]

Originally Posted by john3232

I'm pretty sure the AC units in my properties are the original units.

sq. ft.
1490 3.5 ton package
1601 4.0 ton package
1402 3.5 ton package
1236. 3.5 split
2100 5.0 split
2400 3.5 and 3 ton split
2063 5.0 split


The 1490 3.5 ton package and the 1402 3.5 ton package units will need to be replaced in the near future.

The tonnage seems a bit high given the size of the homes.

If I'm not in AZ and a unit needs to be replaced I plan on using https://newacunit.com/shop/

If the tech sent out to inspect the AC to be replaced and recommends say a .5 ton smaller unit would it be advisable to go that route?

Or should I stick with the size the builder installed?

Thanks

Sizing methods have changed. Sizing by Sq/ft is bush league by todays standards.

That said, if these homes are rentals and you're paying for the project but not the utility bill, why pay more for a proper load calc while risking what's been a good thing just to save someone else money?

[john3232]

Originally Posted by btuhack

Sizing methods have changed. Sizing by Sq/ft is bush league by todays standards.

That said, if these homes are rentals and you're paying for the project but not the utility bill, why pay more for a proper load calc while risking what's been a good thing just to save someone else money?

What would you recommend I ask the tech to ensure I'm getting the correct size?

[btuhack]

Originally Posted by john3232

What would you recommend I ask the tech to ensure I'm getting the correct size?

You'll need a complete and accurate "Manual J" calculation. It's a service not usually done for free but the energy savings would recoup the cost.