I don't think so.

I've got a 2000 square foot home and it has a 3 Ton unit that is sized properly.

Loads are a factor of many many things, and doing the math is the only way to know that the load is correct.

Using square footage doesn't account even for 8 foot or 10 foot ceilings. That alone would alter the volume of air by 20-25%.

Not to mention, R value of insulation, east and west windows. skylights, and building materials.

Robar...that exactly what every installer in my area uses to quickly determine the equipment people need.

This is completely wrong and you end up with the wrong equipment about 80% of the time.

This is the lazy way of doing it to make a quick sale.

Take a look at this free online Manual J calculator to get an idea of all the factors that go into calculating loads: http://loadcalc.net

I'm not affiliated with the site...I just found it and learned to use it. It took me a few days, but it really helps you understand what's the appropriate BTUs for your house for cooling and heating.

This is also called the 500 sq. ft. per ton calculation. This is wrong and leads to oversized equipment.

there is a reason you spend the money for the
software to correctly do a load calc.
rule of thumb sizing causes a LOT of problems.
for homeowners.
hvac companies make more money by selling
larger equipment..with shortened lives due to
excessive stop & starts..utility companies
profit in that it causes more to start up the
unit than to run it for long lengths of time.

its bs OP.

a bit more classy than standing across the
street and holding up fingers to cover house
and using that as guide for sizing..but bs none
the less.

do a search for load calcs, manual j, s, d....

best of luck

+1, the 500sqft per ton is obsolete. Newer homes can do 1,000sqft per ton in southern climates.

SO how do you know whats right?? I have 2 estimates for central a/c. Both for RHeem units. I have a 2 floor colonial with 35 windows. 1st fl - 2300 sq ft 2nd fl 1700 sq ft. I have 2 pretty different estimates. #1- says I need a 3 ton and 4 ton. #2- says I need a 4 ton and either another 4 or 5 ton. He is going to do more calc and get back to me. I have oil baseboard heat. no ducts yet.

The load calculation (Manual J) takes into account a lot of factors like insulation, windows on east/west facing walls, sqft, doors, type of floor, climate, volume of house, doors, appliances, people in house, and so on.

So, a Manual J calculation will spit out your cooling and heating needs in BTUs and this calculation is far more useful than the 400 sqft/ton lazy way of estimating. This is how you know you have the right sized equipment.

I'm sorry I don't mean to b a PIA but if I live on Long Island with a family of 5. 15 windows west 20 windows east. Avg insulation. Im worried b/c I know the size of unit is very important and I got 2 different opinions. All bedrooms (4) and 2 baths upstairs. I like to sleep in cold a/c my DH makes fun of me I'm trying to freeze him like Walt Disney. LOL

Both estimates could be wrong. You are getting estimates of 7 - 9 tons for a 4000 sq ft house. This seems oversized unless you have poor insulation and windows.

In general contractors will tend to oversize in order to play it safe when there is no existing AC.

OK I don't have existing a/c so I guess that could be why. I gues I'm worried because everyone on the forum seems to say getting the correct size unit is very important.

Getting the correct size is very important. This is why you need to ask a lot of questions of the contractor.

You have a big old house, so it is easy to make a lot of incorrect assumptions. I suggest you have the contractor show you the load calculation print out. Make sure he has accurately entered the dimensions of all rooms and windows. Ask what assumptions have been used for insulation and air infiltration. Find out what software he is using. If he can't answer these questions or won't show you the results, then a load calculation was not done.

When you have some data start your own thead with an appropriate title.

Thanks for the great input. I will certainly post results.

I keep reading on forums that a load calc is needed. I have talked to 2 HVAC places that said they would not do a load calc. Finally the 3rd place I called the boss told me he would do one for $450. Maybe in middle Tennessee HVAC people
feel like it's a waste of time. I will keep trying but I understand why people don't get a load calc.

Here I had to get a Load calc in order to get the permit. There was no separate charge for it.

YMMV

This post was edited by saltidawg on Tue, Aug 20, 13 at 17:10

no

I have a 2 1/4 ton unit that was installed with the house i purchased. It is the original unit and is overworked on a day in the 90's in WY. when the unit is set below 77, it will run all day, common feed back on my street. It is a fairly new home (2007) with 1000 sf and 800 sf furnished basement.

New house does not mean efficient house. Builder grade sometimes means shortcuts were taken to finish it quicker. The insulation may be insufficient, attic may not have enough venting, ductwork may be insufficient, or the unit might undercharged. You should not have a problem cooling that sqft with that unit.

So, Craig, don't set it below 77. In Wyoming, isn't it fairly dry?

2007 isn't fairly new. That is 9 years old, very common for problems to develop with HVAC system from year 5 and up. National average life of AC = 15 years.

# I repair AC systems like this all the time... brand doesn't matter one bit.

I have a 2100 sq. ft. home and the 15 year old Lennox heat pump is 2.5 tons. It has never left the interior uncomfortable even at 10* below and 105* outside. It seems to me that running a manualJ estimate would be a waste of time. If the original ducts can be used why not just install another 2.5 ton unit?

Difference is in technologies. If you had a heat pump and replace with another, the technology was similar back then. If you are replacing a super old furnace with new, the blower and efficiency can be much different requiring ductwork modifications to get the required CFMs. If you are replacing a furnace with similar technology as current then the ductwork might be ok, depends if the previous installer addressed the ductwork or not and if there were problems or not.

i have a home in central Florida that is 1750 sq ft , what size is best to replace my unit

How many windows? where do the windows face? How well is the home insulated?

With just the square footage alone I would go with a 2 speed, 3 Ton AC.

In first speed it will deliver about 2 tons of cooling. It will use first speed upwards of 90% of the time. (with a proper controller)

In the heat of the day 3-6pm - 3-7pm on hot days it will ramp to 2nd speed giving you full 3 ton capacity.

You will have to replace the WHOLE system to do this properly. A heat load could tell you 4 ton. Put in the 3 ton.

If it doesn't cool enough... reinsulate. The attic, replace the duct system to no less than R8 duct.

If you want the system to be efficient and run well the whole system needs to be designed to do the job. A 3 ton 2 speed AC is cheaper to operate than a 4 ton AC.

Operational costs decide in terms of $$ how efficient the AC system is.

Then because you put in the 2 speed AC system, you get better dehumidification as a bonus. For a location like Florida it's money well spent.

I live near Richmond VA. I have dual zone systems. Both are 20 yo. We've been told the systems needs to be replaced. But they want to replace them with the same size systems. When we first got them they cost us about $12k total. Now its over twice that amount total. We're trying to find a way to save money. My home is a 1700sq. ft. But lets go with 1800 sq.ft. to make it simple. It's a 2 story stucco built in 1937. fair insulation, 8.4 ceiling height. 20 windows. 1/2 facing east. The sun rises nearly at the left rear corner of my home. Currently the units are 4 ton each. 1 4 ton should be sufficient for entire house. So why isn't it divided and have 2 2 ton units, 2 2.5s, or even 2 3 ton units? My thinking is even at 1000sq.ft. for each floor a 2-3 ton unit for each floor should be fine. Don't you divide the btus between floors? Not double it? I understand there's a new method to determine system size, I just feel like I was misinformed the first time. Now I want the facts so I don't over pay again.

Great question. This is a great place for you to Start a Thread to get help.

OR - add on to nearly decade old thread.

You need to ignore these bogus, arcane methods and run Manual J loads based on your house location, orientation, insulation, glazing and the design conditions. [free websites available]

Also there may be different sets of issues related to your ductwork original design, going with 2T may sound good but causes far room air throw issues, etc. Sounds like a lot of conditioning for only 1800 sq.ft house but I do not know. Sometimes adding attic insulation can help...

I have 2 4 ton units. The math says just 1 is enough. 1 is upstairs, the other downstairs. Instead of doubling the units they should've been divided. I think 1 3 ton downstairs and 1 2.5 upstairs is plenty. We never set the thermostats higher than 65° in the winter and 68 in the summer. We like it a bit cool. We also use the fireplace when it's really cold. We have these little space heaters too. In an emergency we have a kerosene heater. We have a maintenance plan with them so they come out every 6 months or more if we want. But the techs never once said anything about rust forming on the coils. On top of that they're supposed to clean the units. The unit in the attic had nearly an inch of mold on the coils. The last guy that came out was honest and he took pictures and showed me. I believe if they would've done their job, my attic system wouldn't be filled with mold and rusted to the point that it could blow out. I'm handicap. I've had 12 surgeries on my back. Every single tech that came out was told. Because of their laziness and not doing their job properly the coils are going to go out. I found out from another service that there was steps they could've taken to prevent the rust and the mold isn't that difficult to clean. At the very least they should've told me about it before it became an issue. But now they want to replace them with the same size system and the other service said I didn't need 2 4 ton sytems. He said 2 2 tons or 2.5 tons was plenty. I asked about the 3 ton and a 2 or 2.5 upstairs, he said that would be more than sufficient. My house is 1700 sq.ft. the back porch adds the 100 sq.ft for 1800 sq.ft. when we asked them why they gave us 2 4 tons they have no answer. When we asked to go to smaller units to save money they said I should replace what I have with the same system. I explained the math and 1 4 ton was plenty for the whole house and they gave me bs about how old it is and they didn't even offer to come out to check to see what I actually needed. The other guy offered. So I guess were just going to have the test done and then get estimates. Before we went to 2 heat pumps, we had 1 oil furnace that heated the entire house. We only had to fill the 375 gal. Tank once a winter then another 100 gal. Most the time we didn't use it all. The oil furnace was ancient so it died. We had 3 10,000 btu window air conditioners and 1 5500 in our bedroom window. So we figured we'd get a heat pump. We thought we needed what they said. But we didn't. I only have about 900 sq ft down stairs and 900 upstairs. Actually it's closer to 1000sq.ft. downstairs and 800sq.ft upstairs. We keep the house a littler cooler year round. So we're always comfortable. Only time the emergency heat came on is when the system downstairs broke down. Now because of its age and inefficiency, it costs more to heat and cool the house. Like I said, we think we're just going to get the test done to see what size system we need and use a different company because we don't trust this one any more.