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
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.
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
The ductwork & airflow is critical to efficient operation!
WISDOM PRINCIPLED EMPOWERMENT COMMUNICATIONS -
THE REAL POLITICAL ISSUES and PEOPLE EMPOWERMENT
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
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
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"
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.
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
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.
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?
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
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.
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
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.
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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