Thread: A/C Capicity - Austin, Texas

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500sf to 700sf per ton of cooling is a good ballpark figure. You have so many variables that can impact the cooling capacity of a system.

I would say that for a 3500sf home 6 tons of cooling would work fine. If the home was tight and well insulated you could even reduce that figure.

Originally Posted by Scott Patterson

I would say that for a 3500sf home 6 tons
of cooling would work fine. If the home was tight and well insulated you could even reduce that figure.

In many areas, or with many windows, or ... (all depends - what height ceilings, all kinds of things) you might even need 7 tons for that 3,500 sf house, maybe even 8 tons.

I don't speculate on heating or cooling capacity. That is way beyond the SOP and could get you into trouble fast. To know for sure, you need to run a load calculation. SF is only one component of a dozen or so factors needed to be accurate.

That's what I've always read. A manual J calculation must be done to properly size AC and heating systems. Too many variables for square footage to be more than an approximate guess.
So what does the J stand for? Anyone know?

IMO, we do need a range to guide us. If I find a 1500sf home with a 5 ton unit, I think that I could safely say that the system is too big for the home.
The 500sf to 700sf per ton is just a guide, but I would also say that it fits about 75% or more of the home we see.

As a general rule (and that is all) of thumb, I use 550 sq ft/ ton.

Again. That is only a quick thought while checking the unit... nothing else.

Rich

I don't even try to do the math. I would hate to tell a client that it appears that the unit is of sufficient sizing only to have them later discover that it is not and call me on it.

I will comment on a unit that appears to be grossly undersized or oversized and request that they have the sizing of the unit compared to the variables of the home evaluated to determine if the sizing is approproiate. I also explain to them that an undersized unit will not cool the home correctly and /or that an oversized unit is actually less efficient due to cyclling and will have a decreased life expectancy.

The obvious is always stated, like where the house starts out at 900 sq ft and 1.5 tons. Then they doubled the size of the house and just ran more ducts. Some of them have so much wrong that being under or over sized is the least of their problems.

But for the usual house, I take Jim L's approach. Every report has the following, whether I find no issues or a couple of dozen.

IMPORTANT NOTICE ABOUT HVAC INSPECTIONS:
You must be aware that all facets of this inspection are based upon visual observations. There are a number of important aspects of HVAC inspections that this inspection will not reveal. It requires a licensed HVAC specialist to determine weather the unit is correctly sized for the house. Correct sizing is a calculated value, not a subjective opinion. Further, routing of ducts and correct sizing of return air and distribution air are calculated items. Inspections of sealed heat exchangers, internal wiring, and A/C evaporator coils remain the responsibility of licensed HVAC professionals.
You will receive as much information as is possible from me today. However, for a complete evaluation of the system and an opinion concerning continued performance, the opinions of a licensed HVAC specialist concerning costs and additional needed repairs is always recommended. This opinion should be based upon his/her actual inspection and service, not from an estimate given concerning specific deficiencies this report may reveal.

Originally Posted by Jon Randolph

I would hate to tell a client that it appears that the unit is of sufficient sizing only to have them later discover that it is not and call me on it.

Jon,

It's not that we tell them it *is* sufficiently sized, it's being able to recognize *when it is not* sized properly (which could also mean too large).

I've always used 500-600 sf per ton, which, when you come down to it, equates to the 550 sf per ton Rich stated (except that 500-600 sf per ton gives a range, and the size should be within that range, or close to it, 550 sf per ton give a specific number, about which you create your own range).

550 sf per ton / 500-600 sf per ton = same thing.

Originally Posted by Jerry Peck

Jon,

It's not that we tell them it *is* sufficiently sized, it's being able to recognize *when it is not* sized properly (which could also mean too large). I've always used 500-600 sf per ton, which, when you come down to it, equates to the 550 sf per ton Rich stated (except that 500-600 sf per ton gives a range, and the size should be within that range, or close to it, 550 sf per ton give a specific number, about which you create your own range). 550 sf per ton / 500-600 sf per ton = same thing.

When most ceilings were 8 feet and most houses were boxes, I felt a lot more comfortable using the 500-600 rough guess. Once builder's started figuring out that they could make a house look larger just by raising ceilings, I got a little less comfortable. Then along came "bringing the outdoors in" by increasing window size and adding as much glass as possible, but the A/C's were still based on sq ft. Then we went to everybody needs a personal bath, complete with fart fan and let's put a TV in every room and refrigerators larger than my first apartment. But we still sized the same. Then we picked up more sq ft by putting the unit in and ducts in the attic. Anyway, I still start with the same rule of thumb, but I'm not too sure I should. I find myself paying much more attention to what I see, what I hear, and how i feel when I'm in the house. Damn you Dr. Joe! I was much smarter before I thought about stuff.

[QUOTE= when you come down to it, equates to the 550 sf per ton Rich stated (except that 500-600 sf per ton gives a range, and the size should be within that range, or close to it, 550 sf per ton give a specific number, about which you create your own range).


Do you try to take geographical location, window sizing, insulation levels, leakage, etc. into account when using that range? I have been told that to size a unit correctly, the HVAC guys need to at least see the home to perform a quick energy audit.

Hey John,

I stated above that I use 550/sq ft per ton. HOWEVER, that is MY information when considereing the system. Not information I share with the client to tell them if the size is appropriate or not.

There are so many factors involved to determine the proper sizing, it'll make your head hurt.

Rich

Unless you are trained to do a manual J to size the job, I would not go by any square foot idea. The way everyone is telling you to use XXXsq ft per ton may not be close for your area. Please post your area on you info as then we might be better to help. I have been trained in manual J and D and have sized a 1500-1600 sq foot house having need a 5 ton system as the house faces east/west and has a large amount of windows and doors and very little insulation. This being said the best way a HI can test the a/c to make sure it is proper is to run for a long period of time and check to make sure the coil does not ice up and there is a temp. difference on both sides of the air handler.