AC sizing


Many local HVAC companies just guess the size of the AC unit and therefore I decided to run my new house data through a Manual J compliant software. Depending on design summer temperature I obtain the following AC size (in tons) recommendations
Summer Temp Size 1 Size 2 90F 4.76 6.02 95F 5.04 6.46 100F 5.32 6.89 105F 5.61 7.33
The house is being build near Seattle where the max summer temp rarely exceeds 105F but gets to 100F for a week or so during summer. 90F can stay with us for few weeks per summer.
I am a bit confused about two sizing recommendations:
Size 1 is based on sensible and latent heat gain Size 2 is based on 75% Sensible Capacity
I do not want to oversize my A/C unit but I do not want to significantly undersize it either. What do you think? Thanks.
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Martino wrote:

First off, you have to use the summer design conditions for your area! Washington, Seattle-Tacoma airport, Official listed Summer Design is 80 dry bulb & 64 wet bulb, or around a low 41% relative humidity.
Therefore, you are using Summer Design temperatures that are way too high! Run the above 80-F dry bulb & 64-F wet bulb temps through your manual J software, then tell us what you get. How many square footage is your home? Did you figure the heat-gain correctly? The ductwork & airflow is critical to efficient operation! http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
- udarrell
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This is where my knowledge was (way) off, I guess. The software's Seattle default summer dry and web b. values were 82 and 66 respectively and I overwrote the higher one. With 80 and 64 (per your comments) the output is as follows:
4.14 tons (based on latent + sensible) 5.15 tons (based on 75% sensible capacity)
My house is fairly large (5,700 sq. ft) and has large window areas and a number of cathedral ceilings. Knowing that many units are 5 tons max (as one of you also mentioned) I wonder if a 5 tone unit will be adequate.
Thanks for pointing the error

http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
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I have never lived in a house with an AC that was "too big" but have been in many with an AC that was too small. Some of them have to run nearly continuously to struggle to maintain a decent temp. Whatever a sensible design program comes up with I would round up another half ton unless I was really really sure it's answer was good enough.
On Thu, 8 Feb 2007 19:21:30 -0800, "Martino"

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I thought conventional wisdom was that, if it's running "nearly continuously", and not "continuously", it's big enough. The only time an AC unit or heater is too small is if it's running constantly, and STILL can't keep up, or if you're after a faster recovery time from setbacks.
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On Thu, 8 Feb 2007 07:38:47 -0800, "Martino"

If it's 105 degrees outside, do you really need it to be 72 inside, or will 82 be good enough?
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Good point but if it is possible to get down to a more comfortable 75F (which I used for the design param) then why not. Looks that with the adjusted dry/wet bulb values I will be able to achieve it with a 5 tons unit. Thx
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The why not is comfort. To get that much cooling for a week a year, you may be oversized the rest of the year. Not to mention that operating costs go up rapidly the greater the differential you are trying to achieve. If you are in and out frequently, you may also find the differential to be too much of a change adding to overall discomfort.
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Seems like a 2-stage unit would take of this situation. Operating at a low-speed during lower temp periods and kicking into high speed when temp approaches the max design one.
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On Thu, 8 Feb 2007 19:24:50 -0800, "Martino"

Let me rephrase: How much are you willing to pay for the difference between 72 degrees and 82 degrees, on the three to six 105 degree days you might encounter per year?
Is that more or less than the price difference for the next smaller AC unit?
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If your neighbors have similiar design size homes why not ask ?? Make friends and learn from their mistakes:)
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The reason I run the analysis is to avoid such a comparison. Different room arrangements, different insulation, different windows, different equipment at home, different number of people inside, different ceilings arrangements, ..... -> different AC need

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Martino wrote:

I cool over 900 sq ft in an older home down to 76-F, 50% Relative Humidity or less, with a Half-Ton or 6,000-BTUH window unit, even at 104-F heat index! Read how I do it: http://www.udarrell.com/airconditioner_current_temperature_btuh_charting.html
Do NOT oversize the system! - udarrell
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OK. I get the point it's certainly worth considering. Hard to assign the $ value to the comfort but easier to assess if extra $xxx is worth being comfortable during these few days. In reality over a longer period of time (even only 10 years) the per very hot day cost might be not that high and therefore the return on investment might be there. I need to see what the price diff is between the units and associated costs. Thx
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Martino wrote:

It might even make sense to factor in global warming. Extreme temperatures will be more common as the planet suffers the impact of all those SUVs and the destruction of the rain forest.

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On Thu, 8 Feb 2007 07:38:47 -0800, "Martino"

Since the standard size residential unit has a max of 5 tons, your solution is simple. You need two units, a 5 ton and and 2 1/2 ton unit.
You will find it most economical if you run the highest SEER unit 95 percent of the time and only kick in the other unit when it is extremely hot and the single unit can't keep up. And you do that by setting your thermostats appropriately.
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Martino writes:

It's like politics and elections. The issues are complicated, but all that complexity reduces to 2 or 3 choices. (Although unlike politicians, one is usually happy with a choice of air conditioning system.) If your hunch between one or the other choice is usually right, then it may not be worth performing a costly analysis. And as your calculations show, the results depend on some noisy assumptions, so your results are no less noisy than that, and your hunch may be just as well grounded, especially if you do a lot of installations, and extra especially if you're replacing an existing system of known capacity and knew how that performed.
While the Manual J process is sound in physical principles, it also serves a marketing role. A clever salesman can manipulate assumptions and other inputs to yield the result he prefers.
Another marketing role was that only "experts" could do it, and any other competitor was therefore a "hack", but that is becoming less of a working distinction as the software and publications become available freely on the Internet, as they should be.
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