General AC sizing question

[John D in CT]

Hi all, first post.

No, I'm not going to ask you to size a system for me. I just have a general question about what I think might be a slightly "gray" area when it comes to sizing.

If you did a load calc and it was exactly midway between two sizes, would you go up, or down, and why?

If you went down, would the system then be unable to achieve the "design" indoor temperature on the hottest day(s) of the year? If it couldn't (and I don't see how it could), would this seem like a worthwhile compromise to you in order to maintain system efficiency for the majority of the cooling season?

If you went up, would you be sacrificing some energy efficiency and dehumidifying capacity (due to increased cycling) in order to achieve sufficient cooling on the hottest day(s) of the year?

It seems to me that the "perfect" size is achieved when the system can just exactly keep up with the load on the hottest day(s) of the year, or maybe just barely exceed cooling demand, with very little cycling.

It also seems to me that it's very difficult to hit this exact spot, especially if a properly-performed load calculation lands right between two sizes; say, you come up with 33,000 BTU's, right between 2.5 and 3 tons.

So, which way do you go - up, or down?

I'm thinking that down might be the way to go, and maybe just be slightly uncomfortable for a few days a year, while maximizing system efficiency for most of the cooling season.

Any thoughts you could share? Thanks in advance.

[acwizard]

This is a very common problem. Other factors need to be considered , like occupancy load or any other high latent loads.Minimum ventilation rates and air changes per hour. I would always in a commercial application lean towards the larger unit. As a contractor it is very difficult to please that irate customer when they call and say it is too hot. Another point is that the load calc was accurate with the info given, but the customer has now added more lighting, refrigerators etc.

[john dalton]

Dear John D in CT,

In a perfect world you would have a valid point, but of course we do not live, or work, in a perfect world. Performing either an ACCA Manual J or N heat load is still, after all based on a perfect structure, which of course doesn't exist either.

Insulation (thermal) construction practices (appropriate word practice) leaves a lot to be desired based on almost all post completion thermal scans of the structure.

As acwizard posted.......size up my friend.

Respectfully Submitted,
John J. Dalton CM

[dandyme]

you would chose condenser & id coil based on load, you'll get really close


just because the mfg rates their unit at 24k, 30k, etc., you had better look at the real capacity of "unit combination" or you could shoot yourself in the arse