Sizing of A/C 1

[macmet]

Just a question that is more out of curiousity. When sizing air conditioners for houses how do you determine the heat(cooling) load? Is it simply xxx btu per sq ft?

Can anyone point me to any online guidance for this sort of thing?

 

[DonPhillips]

The building code requires the following:

M1401.3 Sizing. Heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.


Don Phillips

http://worthingtonengineering.com

 

[willard3]

Heat gain/loss is a great deal more complicated than X btu/square foot.

Read an ASHRAE fundamentals volume.

The trace program occupies 3.2 gig on my hard drive....this is not a simple problem.

 

[IRstuff]

Hmmm, your typical AC saleman will tell you 1 ton/500 sq ft, although, at least in So Cal, you can get away with 1 ton/700 sq ft.

TTFN

FAQ731-376

 

[macmet]

Thanks IRstuff that's more or less what I was looking for.

Cheers.

 

[willard3]

Square feet/ton is the sure way to a lawsuit. It is a number simply for checking other's work and is usually off by 20-30%.

 

[IRstuff]

Not necessarily. The commercial A/C ROT is almost invariably oversized. You might sue if you get too hot, but if your electricity bill is too high, you simply scale back.

One would expect that unless the customer reveals an abnormal occupancy condition, like a server farm, that there's be plenty of capacity for most situations. Therefore, they probably have a few tiers of capacity densities for 95% of the applications, and will only crank numbers if the application is grossly out of bed.

TTFN

FAQ731-376

 

[imok2]

With the amount of data required to do an accurate load calculation, the possibility of errors is increased. Even the computerized methods of load calculation are lacking in error checking procedures that could catch operator errors. For example, window areas can exceed wall areas, or wall areas facing north can be one square foot with a south wall of 300 square feet and east and west walls of 200 square feet. Many of the methods also oversimplified the process and gave insufficient options for climate, building assemblies (windows, doors, walls, etc.), and shading. Then there’s the effect of duct leakage that has only recently been investigated to any significant extent. As a result, cooling loads due to duct leakage are not included in any of the methods. Duct leakage has three effects on design cooling load. First, a supply leak is a direct loss in capacity. Second, a return leak will often bring in superheated attic air. Third, the difference between supply leakage and return leakage will cause increased infiltration. While it is tempting to treat duct leakage as additional infiltration, the effect is actually more complex and the answer is to always check out the duct work and repair the deficiencies. But attempting to get the proper size of air conditioning by the sq.ft. method is like throwing darts at the bulls eye by a novice player.

 

[IRstuff]

But, that's the whole point. The standard ROT is gross overkill, specifically to ensure that load errors are negligible. It's not any different than always asking for a 2.5-ton truck for carrying groceries; even in the most prodigious day at Costco, you'll still have spare capacity.

You can do the detailed calculations and come up with the exact load. Then, you'll apply a design safety factor to ensure that your client won't sue you, and, guess what? You'll be a hair's breadth from the standard ROT.

TTFN

FAQ731-376

 

[willard3]

You would be surprised how many times I have the client's employees tell me what the cooling load is by rule of thumb. They get very prickly when I don't take their advice and calculate the load and find the ROT 30-40% off from what the cognicenti have told me.

If all you need is rule of thumb, you don't need an Engineer.

 

[ChrisConley]

What I like about rule of thumbs is they let you know if your software is incorrect (more likely the person using the software). Without knowing approximately what the answer should be, how can you verify that your carefully calculated number is right? GIGO (Garbage in Garbage Out) means that as a design professional you have to understand your outputs.

 

[IRstuff]

I have no issue with doing calculations; far from it, since I crank calculations on a regular basis. My own A/C experience is solely from the perspective of a homeowner dealing with to a bunch of sales guys.

Obviously, most of us who are mere mortals tend to need the calculations to get the "correct" answer. There are a few souls who can do the decrements and increments in their heads in a blink of an eye to tell you a non-standard value that works.

I'd would certainly agree with Chris that having an ROT to gauge your calculations makes lots of sense. Every good engineer needs to be able to check his results independently.

TTFN

FAQ731-376