The answer to your question is to get a properly done heating and cooling load calculation. Most contractors don't want to be bothered, or do a simple calculation based on the area of the house. The result is an over sized AC unit.

Have the contractors showed you the load calculation? A proper calculation requires at a minimum measuring every room and window in the house, entering the insulation values of the attic and walls, and the orientation of the house. In addition the indoor and outdoor design temperatures for summer and winter need to be determined. All this data is then entered into a software program specifically designed to calculate the heating and cooling requirements of the house.

If you are interested you can get lease software online for $50 and do your own calculation. It will give a good approximation.

What are the typical high temperatures during the summer? Was the 3.5 ton unit able to maintain the desired indoor temperature during the hottest part of the day? This would help determine the size.

I get concerned whenever a contractor announces someone needs a 5 ton unit. Most houses do not have the correctly sized duct work to handle the air flow for 5 tons of cooling. It is highly unlikely yours does. If you actually need 5 tons, then you should also investigate increasing the supply and return duct work.

Finally stay away from the big box stores like Home Depot. You are likely to pay high prices and get a poor installation. Search for a factory authorized dealer in your area. The preferred brands are Carrier/Bryant, Trane/American Standard, and Rheem/Rudd.

Hi,
Here's a rough calculator found by googling.

I plugged in 60x40 ranch and it did come back w the 5 ton. Of course it's a rough guide, and for any special cases like unusual amounts of south-facing glass you should get that analyzed. If you want to go that route, I did have an energy analysis done on my California home when I was expanding my south-facing windows -- Cali is pretty strict on how much exposure you can have that direction. It ran about $200 and they added up all the glass and they gave me a huge packet back.

I talked to the guys who did it (the professional firm recommended by the inspector) and he basically said they don't actually look at the house or verify anything, they just take "what was standard building materials of the time" and assume everything is going to be, on average, standard. They add up the glass, factor more for south-facing, and they come up w "yes your furnace is capable of heating your house after you upgrade your windows". Without even knowing what furnace I had.

I never used it, other than to study it for my own learning and to submit it w my permit paperwork, so you can do that if you want to go that way.

Otherwise, a rough calculator (and your own experience of th 3.5T running way too much) seems to indicate 5T is not unreasonable.

We bought ours thru Home Depot, tho of course we got several estimates. The estimates were all over the map, with the high range being basically double the low range for apples to apples. The install job was nice, and we were very happy w it since.

We did add in some returns, since we were upgrading a furnace-only to a furnace / AC. The entire process was really quite nice. They were in and installed in a day, in a very tight closet including some minor ductwork thru the attic.

-mike

Here is a link that might be useful: Rough AC Calculator

The contractor hired by Home Depot didn't show me the load calc but I wasn't there to see it so that isn't really his fault. I understand that he did go through each room of the house and took notes and my wife was impressed by his thoroughness over the others that came so I don't really have a reason to doubt his 55,000 BTU claim.

Temperatures can get to be around 110 for a couple weeks out of the summer. At that point the AC had a difficult time keeping up with the heat.

The Home Depot contractor did say that I would need new ducting for the larger unit and included that in his quote of about $11,000.

He also said that part of the install would be to add a second return and move the first to a different location as it is between two vents and was likely sucking in the cold air being blown out which added to the cooling problem.

When the outdoor temperature is at the design limit, the AC should be sized so that it can maintain the desired indoor temperature even if it has to run continuously to do this. Every area of the country has a published design temperature. The design temperature should not be the highest temperature you may get during the summer. A typical indoor summer temperature is 75 degrees. You should verify the temperatures the contractor used in his software. A few degrees up or down can make a big difference in the size.

I am glad to hear the contractor suggesting larger duct work and another return.

I would also look into adding more insulation in the attic. It may allow you to move down 0.5 or 1 ton in size.

Thanks Mike. I just talked to the second contractor again and he said after more review that a 4 ton would give us marginal results at best. He said that instead of a 5 ton, he would recommend a 3 ton for the front of the house and a 2 or 2.5 ton for the back of the house. He said that while the expense is higher up front (about $2,000), running a smaller unit would save us money in the long run as well as reducing the higher maintenance costs that a 5 ton unit would incur as the unit the Home Depot contractor is suggesting has a variable speed blower. Just what I wanted, more options. :)

Hire an energy rater to look at your house and identify the low-hanging fruit to increase energy efficiency. Choose some improvements and do them before you change your system. You might be able to buy a smaller cooling system and save some money on equipment and save money over they years you are in the house. Check for local programs that might help pay for the energy evaluation.