RULE OF THUMB FOR A/C

[CESSNA1]

HELLO ALL:

IS THERE A RULE OF THUMB FOR AIR CONDITIONING SIZE IN THE NORTHERN SAN DIEGO AREA OF CALIFORNIA. I HAVE COME ACROSS 500 SQ FT PER TON. WHICH MEANS THAT AN 1800 SQ FT HOUSE WOULD NEED ABOUT 3.6 TONS OF A/C. I BELIEVE THAT IS TOO MUCH. THE BUILDER SIZED THE PIPING FOR 3 TONS. I THINK THAT ALSO IS TOO MUCH. I BELEIVE IT SHOULD BE BETWEEN 2 AND 2.5 TONS FOR 1800 SQUARE FEET. ANY OTHER OPINIONS?

THANK YOU

 

[CRG]

From "HVAC Equations, Data, and Rules of Thumb," residential heat load: 500 to 700 Sq.Ft./Ton with a SHR of 0.80 to 0.95.

Without knowing all of the details (# of windows, their orientation, shading, building design and orientation, etc.). I would say 500 Sq.Ft./Ton is a reasonable figure. Rules of Thumbs should not be used to size A/C equipment; however, they are useful for a sanity check of calculations.

CESSNA1, why do you believe that the builder is incorrect in sizing equipment?
Are there low internal loads compared to an average house?
Are there fewer windows than a typical house in the area?
Are most of the windows shaded?

 

[CESSNA1]

DEAR CRG: THANK YOU FOR YOUR RESPONSE. THE REASON I ASKED IS THAT OUR LAST HOUSE, THAT WAS 3 MILES FROM THE PACIFIC OCEAN WAS 2000 SQ FT AND HAD A 2 TON A/C THAT DID A FINE JOB. THIS HOUSE IS 11 MILES FROM THE OCEAN AND IS 1800 SQ FT. I JUST THOUGHT THAT A 3 TON UNIT WAS ALITTLE TOO HIGH AND I WAS LOOKING FOR OTHER INPUTS.

THANKS AGAIN
DAVE

 

[jsummerfield]

My 2000ish square foot in Houston had three-ton; now has four-ton. I wonder, are there different requirements, based upon outside air temperature and humidity?

John

 

[CESSNA1]

DEAR JSUMMERFIELD:

CRG IS CORRECT, THERE ARE MANY FACTORS INVOLVING THE SIZING OF HVAC SYSTEMS. HUMIDIDITY IS A BIG FACTOR, IT IS CALLED LATENT HEAT. RESIDENTIAL AND COMMERCIAL A/C UNITS ARE LIMITED BY WHAT IS CALLED THE SENSIBLE HEAT RATIO (SHR). IT BASICALLY IS THE ABILITY OF THE UNIT TO PROVIDE A COMFORTABLE ENVIRONMENT. HIGH LATENT HEATS(HUMIDITY) AFFECT THE ABILITY OF THE UNITS TO MAKE THE OCCUPANTS FEEL COMFORTABLE. HIGH SHR'S ARE USUALLY PRESENT IN RESTAURANTS/MOVIES/CONVENTION CENTERS/AND LARGE HOME GROUPS. IN GENERAL IT IS BEST TO SELECT A UNIT THAT IS EXACTLY OR SLIGHTLY SMALLER THAN WHAT THE CALCULATIONS SHOW. GOING TOO HIGH CAN INHIBIT THE ABILITY OF THE A/C UNIT TO DEHUMIDIFY. THIS IS BECAUSE THE LARGE A/C COOLS THE AIR BEFORE IT CAN BE DEHUMIDIFED AND THEN SHUTS OFF.

THANKS FOR INPUT
REGARDS
DAVE

 

[imok2]

cessna1 Try this:

http://www.heatload.com/unico/special.html

 

[quark]

Here is a link for capacity calculator

http://www.warmair.net/html/window_air_cond.htm

...and check the tonnage calculator at

http://www.voltasac.com/split_ac_home.htm

 

[ClydeMule]

CESSNA,

That's the fun of living in North County, just drive from SE Escondido to San Marcos and you might drop 10 deg. F

To really size the HVAC properly you have to do a heat load calc. Using xyx ton/sq ft would only get you into the realm of what size you have, like 1 ton vs. 10 ton. You can't distinguish between 2 and 3 tons with a rule of thumb.

There is a lot more to it than the square footage; insulation, windows, orientation of the house, etc.

When you say piped, is there just a lineset, or an indoor coil as well? If there is a lineset only could be the builder was covering his behind and putting bigger piping in, but there is not much difference in the lineset sizing from 2 to 3 tons.

As a courtesy, you should turn off you caps lock. It makes it seem as if you are shouting.

Clyde

 

[CESSNA1]

Thank you everyone:

I sincerely appreciate your input and information. Clydeset I usually type in all caps because it is easier for me.

Regards
Dave Hall

 

[CESSNA1]

Clydemule: I erred in your name above, sorry. The linset piping is sized for 3 tons. Right jnow there is no coil in the air handler. I am contemplating getting an A/C, but I want to make sure it is properly sized. I agree that builders sometimes lean to the high side for sizing piping. What concerns me is the the A&E that did the design apparantly did not size it properly.

Thnaks again
Dave Hall

 

[ClydeMule]

If it is a cookie cutter house, than they usually just pic an average design and apply it to all houses.

My old house in Corona (KB Home) was 2300 sf, with the back of the house, two stories, about 8 windows facing west. The upstairs rooms were always blazing even with the AC full tilt. Neighbor across the sreet, no problem. House was always cooler. When the next pahse was built (mine was first phase) 5 feet away, it blocked the sun a bit, and the view, and it seemed to help. But I can't complain, if I wanted a house with a high end A/C system, I wouldn't have been able to afford it.

If it easier to type in all caps go for it!

Since you obviously live in North County, what is your opinion on humidity control concerns here? The statement about oversized A/C not dehumidifying well is of course true, but I wonder if there is a point where the location's average RH is not high enough for slightly oversized A/C to be a problem

Whaddya think?

Clyde

 

[CESSNA1]

CLYDEMULE: I live in North County, where are you? It seems that the hummidity is getting a little worse each year. Probably due to spa's/pool's/etc. Yesterday RH was about 84% temp was about 90 deg F. Usually only a few weeks out of the year are really bad temperaure wise. This is about the time. When we first moved here in 1987, everybody said " you don't need A/C here." Not totally true, especially with my east coast mentality, where everything is air conditioned. I do not think a slightly oversized A/C would be a problem, but like everything else caution and judgement are advised. In our area we only "need" A/C about 1 -2 months per year, and even then not everyday.

Regards
Dave

 

[IRstuff]

Generally, most A/C suppliers will offer a slight larger unit to start with, but we decided to get one that was 20% smaller than recommended 4 ton vs. 5 ton for 2200 sq ft house in north Orange county. So, I think that there's usually enough design margin to downsize the A/C at least 20% without too much hassle.

The big issue with the A/C isn't really the tonnage per se, but the sizing of the ducting. Even with the smaller unit, the audible noise when the A/C is on is quite loud.

Our house has pretty poor insulation overally, so you might also consider upgrading the insulation, installying attic fans, etc., since they may actually have positive payback.

TTFN

 

[ClydeMule]

Dave,

I live in Escondido, off of San Pasqual Valley Road. About 10 min from the Wild Animal Park, which is commonly the hottest place in the county during the summer. My shop is by the Escondido Auto Park.

Humidity yesterday was lousy. When you see those giant thunderheads out west you think it is going to rain cats and dogs.

Clyde

 

[ClydeMule]

Uh, I mean East.

 

[DaveGriesel]

I live in Tucson with a 2400 square foot house and had a
company install an air conditioner. We ended up with a top
of line 2 speed 5 ton unit. After 2 years of paying high
bills I disconnected the 2nd stage such that it only ran
at 2.5 tons. This saved about 15% on the air conditioning
cost. The reason is the A coil temperature is higher there
fore condensing less water at the lower power setting.
A trade off of somewhat higher humidity at a lower cost.
I sized our 1300 ft sq. townhouse in TierraSanta for a
1 ton / 2 ton 2-speed unit with the idea of using it mostly
as a 1 ton. This turned out to be not very cost effective
as the price difference of an expensive unit versus a very
cheap unit was more than could be saved in a low utilization
situation in a cooler climate.