Sizing AC System for 1500 SF Interior Office Space

Tom_99

I need a new air conditioning system for 1500 square feet of office space in a small garage building in New York City. But I am having trouble getting a calculation of the cooling load from contractors that have given me proposals.

Before I say “whatever” and take the 5 ton system that three contractors have recommended, can someone refer a contractor or engineer who would be willing to give me a serious load calculation, notwithstanding the relatively small size of the job?

Short of that, how could I make a good guess of whether the 5 ton estimate is accurate rather than, say, 4 or 3.5 tons? Also, could I hedge by getting a two-stage AC unit, like the American Standard Allegiance model and/or a variable speed air handler to compensate if 5 tons is, in fact, oversized?

The building is two floors, 7,000 SF each floor, in the middle of a city block with similar sized buildings on both sides. The first floor of the building is an auto repair shop; 5,500 SF is the repair area; 1500 SF is for the shop’s offices that I need to condition. The office area has no exterior windows. The building is a steel frame with masonry walls built in about 1930.

Two people work in the office, with just one PC computer. A big part of the office is dedicated to an inventory of car parts. Occasionally, customers from the auto repair shop are in the office for short intervals, dropping off or picking up their cars.

35 years ago, a 7.5 ton compressor, air handler, and ductwork were installed. Looking at the architectural drawings for that system, it appears that the system was designed to be several times over capacity. (The drawings reflect required supply CFMs of 494 and actual supply CFMs of 3,600).

The AC system has died and needs to be replaced. I have read about the problem of having an oversized AC system: the system cycles frequently cycles on and off and it does a poor job of removing the humidity.

One contractor recommended replacing the system with a 7.5 ton system because the existing duct work would be too big for a system that is any smaller. Alternatively, he recommended a 5 ton system with a smaller duct system. Two other contractors have also recommended a 5 ton system.

The 5 ton estimate may, in fact, be accurate. But in each of the three proposals, the estimate appears to be the product of the contractor’s professional judgment, not a detailed load calculation. At Alpinehomeair.com, a website that Google found for me, I found an interactive thing that takes the square footage and zip code and gives an estimate of the size system that may be needed. The site appears to intend for such an estimate to be compared with a more detailed Manual J calculation, not as a substitute. The interactive site spit out an estimate of 2.5 tons.

My problem is that summer is fast approaching in NYC and the repair shop owner wants to be cool now, not after I find someone who can perform a more detailed Manual J calculation.

Any suggestions about a contractor or engineer who will do a load calculation?

Or how to guesstimate whether to go with 5 tons? Does the 5 ton sound like a good estimate?

Or could I hedge by getting a two-stage AC unit, like the American Standard Allegiance model and/or a variable speed air handler to compensate if 5 tons is, in fact, oversized?

Thanks.
END OF THREAD

Brad White_173

While a Gut Check

says that the 5 tons should be fine (and how 7.5 tons is too much for consistent dehumidifying operation), hiring an engineer for a couple of hours would be well-spent.

As a ballpark check figure (danger Will Robinson!), a preliminary estimate at 300-350 SF per ton for an open office plan is about right. As you noted, it may still be a tad over-sized, especially on milder days. Still, due diligence says you have to have defensible paper to back it up. You are really limited by the commercially available increments, so it gets down to the best match between your evaporator and condensing unit. Your presumably 5-ton unit may wind up being de-rated for certain installation conditions -the penalty helps sometimes, I hate to say it.

You may also consider some of the newer 2-stage condensers in R410A. They are available in split systems; not sure about packaged DX systems. But what a good way to keep the dehumidification going rather than cycling off on low-load.

Regardless, one or two stage, use a TXV system for best control.

John Mills_5

Wow

5 tons for 1500 sq ft internal office with just a couple of people seems B I G. Our office is 2000 sq ft, lots of glass, 7 people, computers, copiers, and 3.5 ton does it though struggles a bit.

I would sure want to see an accurate load calc done. If the existing ducting is too big, should be changed.

schiller

TRY THIS, PICK THE CONTRACTOR YOU LIKED BEST AND OFFER THEM $250. FOR THE LOAD CALC #s. ASK THEM IF THEY GET THE CONTRACT WILL THEY DEDUCT THE $250 FROM THE TOTAL.
IT'S A WIN WIN IF THEY DID THE LOAD CALC TO BEGIN W/
TROY