[rwaldo]

Hello,
I've had 5 dealers (all representing different name brands) come to my house to give me an estimate on a HP. Only one tried to figure out what size the unit should be (by asking me the square footage of the house). Since it's a fairly new house (6 years old) they probably figured they'd just replace it with what exists today but the problem is that the air handler is a 4 ton and the outside unit is a 3.5 ton. Not sure why the builder did it that way - from what I've read on this site maybe there are jumpers in it that can accomdate the mis-match. But anyway, 4 dealers quoted me on 3.5 ton units and one quoted me on a 4 ton unit. When I asked the guy that quoted me on the 4 ton why he was quoting 4 tons (the one that asked me how many square feet) he said "every time we've done a Manual J calculation it comes out to about 1 ton per 500 square feet and since your house is 1780 square feet, and you have vaulted cielings that need an extra .5 tons - you should have a 4 ton unit." So then I became concerned about which size is right and ended up purchasing HVAC-Calc. I feel like I carefully entered all of the data and it is calculating a 3 ton unit so now I'm totally confused. I know dealers sometimes go to a larger unit so that people don't complain of inadequate cooling. Should I do the same and be certain I get a 3.5 ton unit or should I trust HVAC-Calc and look in to a 3 ton system? HVAC-Calc says that the above report is "In accordance with ACCA Manual J." Thank you for your time.

 

[ball a/c]

I'm not familiar with HVAC-Calc. We have folks in our company using professional manuel-J calculators, sometimes come up with a 1/2 ton difference. The input has to be correct.
The other challenge is that you have to make sure the equipment meets the sensible load, not the total load only.

 

[t527ed]

we have been using HVAC-CALC for many years, when we bought the program it came on 3 1/2" floppies...........

have never had a problem sizing by the numbers it gave us. i would trust it more than some "rule of thumb" sizing.

call around for more estimates but make sure they will do a load calc before you make an appointment.

 

[jwiehagen76]

You should not try to out guess the program. As you have stated alot of contractors would rather go big. No one wants a return call. That is why a good evaluation is so important "garbage in you get garbage out!! If you feel you were pretty accurate on your evaluation then go with it.

 

[beenthere]

Some 3 ton units, won't meet your 28,895 Sensible BTU load.
So a 3.5 ton may be needed, to meet that sensible load.

Many systems are matched systems with the indoor unit being rated at a ½ ton larger capacity then the outdoor unit.

 

[cliebold]

your design conditions are listed as 75 inside with 91 outside summer design.....is that what you want the a/c to do on that 91 degree day? My design conditions in virginia beach va are 76/91....but i design for 75/95...change your design conditions and look at the results.

 

[skippedover]

The load calculation is like the foundation for a house. It's the very foundation of all the additional information needed. That is, the size of the equipment, the size of the ducts, the number and size of supply and return outlets, the selection of the equipment. All are predicated on a good, accurate load calculation. I admire you for attempting to fill in the gaps of having non-complient contractors bid your job. But I ask you, would you even entertain a general contractor who suggested you use hiim as a builder when he is not going to put a foundation under the home? Would you try and fill in that gap by putting in the foundation yourself, not even knowing if you're doing it correctly? I think you'd run away from the GC as not being a very bright GC. I'd recommend the same with the HVAC companies you've interviewed. Attached is my suggestion on how to properly select an HVAC company and save yourself a whole lot of time and aggrivation along the way.

 

[motoguy128]

Garbage in = garbage out. If the R values and other data don't reflect reality then the calculations won't be correct. This isn't rocket science. Take insulation values, design temperature differential and surface area and you can determine the rate of heat loss. The problem I have is that at some point you have to determine how "tight" the home is. When you have a older home (>20 years) this becomes a guessing game and at some point, you would need to look at the duty cycle of existing equipment to at different outside temperatures to determine real loads. I have plenty of air leaks, and the basement is partially finished and partially heated.

At some point you have to throw in some best guesses, and ultimately, units are sized in 1/2 Ton steps. A smaller home will have a larger surface area to volume ratio so sizing by square footage will not be linear.