[tiger08]

Originally Posted by WestieJeff

400 sf per ton is correct.

$2000 per ton is a good estimate for a 14 SEER split unit without knowing any other factors.

Since it is an older unit, they'll be changing to R410a (puron refrigerant) it is highly recommended to change out your lineset (the copper pipes) if at all possible.

I would definitely ask the installer to quote with replacing the lineset (unless completely impossible to do) and make sure and ask if they 1) purge with nitrogen during brazing 2) triple evacuate down with nitrogen down to < 500 microns. That's industry standard, but you want to make sure they do it. A lot of installers are lazy and figure it's cheaper to shortcut the install and just accept the callback if there is a problem.

Angie's List is recommended to find a good contractor.

400 sq ft per ton is incorrect! Only a manual J load calc will determine what the OP really needs. Too much oversizing in this desert.

[lvoc]

Originally Posted by tiger08

400 sq ft per ton is incorrect! Only a manual J load calc will determine what the OP really needs. Too much oversizing in this desert.

While I love calculations and such I would expect that existing units are quite good as a gauge. Why not existing tonnage and duty cycle?

Our larger unit holds 76 at about a 60% duty cycle with an external 100 degrees. Probably won't hold 76 at 110 external though I have not tested it.

We only use the smaller unit for few hours in the afternoon/evening.

I expect I could track duty cycle for a while and get a good handle on how these guys are loaded.

One trickky part is that our units certainly interact. When both are on the duty cycle drops a good bit.

We almost never run both at once though so I think I will ignore that.

[SportyandMisty]

Originally Posted by Coco6163

... The first problem I had was finding a HVAC contractor willing and able to do load calculations on my house to determine the appropriate size of the equipment needed. You can do the google research, but trust me when I tell you that it will lead you to this: unless an accurate load calculation is performed, you risk buying too little or, more likely, too much equipment for your home. I was shocked at how difficult it was to find an HVAC contractor in Las Vegas who both agreed that a load calculation was necessary and was able and willing to do it...

It may be best to get an engineer to do the calcs. I'd contact Comfort Engineering or alternatives. It won't cost that much & you'll get the right result.

Home Page - Comfort Engineering

[tiger08]

Originally Posted by lvoc

While I love calculations and such I would expect that existing units are quite good as a gauge. Why not existing tonnage and duty cycle?

Our larger unit holds 76 at about a 60% duty cycle with an external 100 degrees. Probably won't hold 76 at 110 external though I have not tested it.

We only use the smaller unit for few hours in the afternoon/evening.

I expect I could track duty cycle for a while and get a good handle on how these guys are loaded.

One trickky part is that our units certainly interact. When both are on the duty cycle drops a good bit.

We almost never run both at once though so I think I will ignore that.

Most a/c units in this valley are oversized and most a/c companies wont waste the time to do a manual J and a few dont even know how. Every house is different, even in cookie cutter neighborhoods.
I used to believe bigger is better but the last couple of units I installed in my neighborhood have been downsized and they work great, they run longer and the houses are much more comfortable.
Several of our neighbors have upsized by contractors recommendations and their houses are not comfortable, they short cycle and on a more humid day, they do not feel dry.
Electric motors such as compressors do not like to be cycled off and on constantly, it severly shortens their lifespan and every time the unit has to resart, the amp draw is very high.
A properly sized a/c unit will run constantly on a hot day and even if the house creeps up a couple degrees higher than what the thermostat is set for, it will still be more comfortable than one that keeps going off and on.

With my years of experience, we have very few top notch contractors in this valley, the rest are hacks. Ask any true a/c contractor and they will tell you as I have, manual J load calc.

Also, before installing a new unit, I recommend adding more insulation and then downsize the new unit from there.

[MediocreButArrogant]

I would guess that most homeowners would not be happy with a properly sized system. They would complain their system was undersized because it cooled the home too slowly on the hottest days when they decided to turn the thermostat down a few degrees. The home I'm in is a two-story with a ton of cooling per 300 sq ft. This is what it was built with, and I imagine way oversized.

[ft_chief]

Originally Posted by tiger08

Most a/c units in this valley are oversized and most a/c companies wont waste the time to do a manual J and a few dont even know how. Every house is different, even in cookie cutter neighborhoods.
I used to believe bigger is better but the last couple of units I installed in my neighborhood have been downsized and they work great, they run longer and the houses are much more comfortable.
Several of our neighbors have upsized by contractors recommendations and their houses are not comfortable, they short cycle and on a more humid day, they do not feel dry.
Electric motors such as compressors do not like to be cycled off and on constantly, it severly shortens their lifespan and every time the unit has to resart, the amp draw is very high.
A properly sized a/c unit will run constantly on a hot day and even if the house creeps up a couple degrees higher than what the thermostat is set for, it will still be more comfortable than one that keeps going off and on.

With my years of experience, we have very few top notch contractors in this valley, the rest are hacks. Ask any true a/c contractor and they will tell you as I have, manual J load calc.

Also, before installing a new unit, I recommend adding more insulation and then downsize the new unit from there.

The reason A/C units are oversized in the desert is the low humidity. The longer a unit runs, the more it will dehumidify your house.

[modhatter]

Some really great and informative responses. Thank you.

After reading all the replies, was wondering what some of you home owners have experienced with size of your unit with your square footage.
Anyone have a single level home? What size unit do you have? Does it cool sufficiently?

[ScoopLV]

Originally Posted by ft_chief

The reason A/C units are oversized in the desert is the low humidity. The longer a unit runs, the more it will dehumidify your house.

That doesn't make sense to me. Seeing as the last thing our house needs is more moisture removed...

[WannaliveinGreenville]

Originally Posted by macgeek

Good post, I will add watch out for "really low priced" units, there is a current scam very popular in Las Vegas, that contractors steal units from foreclosed houses, and then re-sell them for a 'discount'

Jonathan

That's awful!

[macgeek]

Originally Posted by modhatter

Some really great and informative responses. Thank you.

After reading all the replies, was wondering what some of you home owners have experienced with size of your unit with your square footage.
Anyone have a single level home? What size unit do you have? Does it cool sufficiently?

My house is 1383 Sq Ft - My A/C is a 4 ton unit - one story

Jonathan