AC Sizing - Actual vs. Rated
AC Sizing - Actual vs. Rated
(OP)
I am trying to confirm the exact procedure for properly sizing condenser and evaporator coil based on my Man J calculated cooling load.
Question 1: I want to verify how to derate the capacity based on actual air entering indoor cool (EATa=75F) versus test conditions (EATt=80F). I was told (-835 BTU/h)*(deltaT)*(Actual cfm)/1000 (?)
Question 2: Is it true that 50% of excess latent capacity can be added to the sensible capacity?
Things I think I already know:
a. I understand the derating based on outdoor air temperature using the manufacturer performance tables and applying basic interpolation.
b. The derating based on actual vs. tested cfm is straightforward using values given by the manufacturer.
Any guidance for questions 1 and 2?
Question 1: I want to verify how to derate the capacity based on actual air entering indoor cool (EATa=75F) versus test conditions (EATt=80F). I was told (-835 BTU/h)*(deltaT)*(Actual cfm)/1000 (?)
Question 2: Is it true that 50% of excess latent capacity can be added to the sensible capacity?
Things I think I already know:
a. I understand the derating based on outdoor air temperature using the manufacturer performance tables and applying basic interpolation.
b. The derating based on actual vs. tested cfm is straightforward using values given by the manufacturer.
Any guidance for questions 1 and 2?





RE: AC Sizing - Actual vs. Rated
Packaged and split DX equipment as used for residential and small commercial is produced in given sizes with a limited range of C/E coil pairings for given conditions. Which is to say, that a given package rooftop unit, will perform to a certain capacity at a given outdoor condition, and there is not much your calc can do to change that. Being able to accurately determine a coil size is not something that is usually cost effective to do.
Having said that, it is a good exercise and you should read the DX section in ASHRAE and walk through a calc procedure.
I don't understand what you are getting at with Q2 however. Sounds like a wrongism.
RE: AC Sizing - Actual vs. Rated
I read the 50% of excess latent capacity rule in ACCA Manual S. Background...In California where I work our average RH = 32%. Therefore our sensible loads are much higher than latent loads (sometimes 95/5 sensible/latent ratio). A typical Split System is closer to 60/40 sensible/latent ratio. If I look at sensible capacity of a split system and try to match with sensible load it often requires a full ton more capacity on the condenser just to satisfy sensible load - leaving much excess latent capacity and requiring larger blower motors, etc.
RE: AC Sizing - Actual vs. Rated
TTFN
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RE: AC Sizing - Actual vs. Rated
RE: AC Sizing - Actual vs. Rated
Picking something with the bare minimum of spare overhead is just going to make everyone at home miserable, because there will ALWAYS be unpredicted factors and forces at play that will make a mockery of your calculations, and you'll find your system underpowered for your actual conditions.
TTFN
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RE: AC Sizing - Actual vs. Rated
Remember also that in air-conditioning you deal with concurrent conditions. In CA, the days where you get the 5%RH are also screaming hot Santa Anas. Latent cooling you don't need a lot of... true, but you will sure miss that sensible capacity you skimped on.
The evap temp but also number of rows of the E coil will largely determine sensible/latent heat removal. Altering the coil rows is something the manufacturers can do to assist.
C coils get bigger when the efficiency needs biggering, not directly attributable to load.
RE: AC Sizing - Actual vs. Rated
@IR, I just don't agree with you. No engineer in his right mind would make this claim. Region-to-region climate, construction quality, insulation values, equipment performance, et al vary so much you can't believe that rules of thumb are good engineering practice. I'll leave those to the contractors who aren't guaranteeing energy savings and comfort. A good energy model and proper planning can easily account for "unpredictable" factors. These are single family homes not conference halls. You can provide adequate cooling without over sizing. There's a cost, performance, and environmental impact difference between engineering and rules of thumb.
RE: AC Sizing - Actual vs. Rated
On the other hand, they're selling SEER 20 systems, and ten years ago, the best you could get was about SEER 14.
TTFN
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RE: AC Sizing - Actual vs. Rated
RE: AC Sizing - Actual vs. Rated
RE: AC Sizing - Actual vs. Rated
aording to ACCA quality maintenance and installation you are correct in saying that doing a heat load should always be done because rules of thump are noe honest to the customer Keep up our reputation
RE: AC Sizing - Actual vs. Rated
Case in point, cars used to be designed for maximum fuel efficiency at around 55 mph, but a giant percentage of people didn't even come close to driving in that range.
Likewise, people do not regularly clean their air filters, or sweep out their ducts, or use the optimum setpoints, or continually balance their system loads, or insulate their buildings. At every building I've worked in, there is one conference room that is the "icebox" and another that is the "oven." People are constantly tweaking the setpoints because some people are uncomfortable at whatever setpoint someone else picks. It used to be that facilities guys were often keep busy 100% tweaking dampers throughout a building. Things got better with zone cooling and controls, but only barely. Invariably, the zones extend across some random min/max configuration of the building, and the end result is a setpoint that makes one group too hot, and another group too cold.
Additionally, many buildings are grossly underinsulated, and whatever efficiencies you gain in the design of the AC is completely lost in the lack of insulation.
Finally, heat loads are constantly changing over time. At one point time, every person might have had a single PC with a 14-in monitor, which presented a particular heat load. 5 yrs later, everyone has a BIGGER PC, and multiple monitors; a couple of people in my group at work are running 4 monitors on 2 to 3 computers, but a large percentage of people have at least 2 monitors, usually in the 20-24 inch class.
Whatever AC load calculations might have been done when we moved into our building, our overall heat is substantially larger now, but we didn't go and replace our AC with a resized one.
TTFN
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RE: AC Sizing - Actual vs. Rated
RE: AC Sizing - Actual vs. Rated
So, sure one monitor, no biggie, but guess what, I've got about 6 computers operating throughout the house, at least 1 large screen TV, etc., etc., etc. You can spend the calories to hunt down each piece of equipment's spec panel, go into the attic and measure the insulation, get some sort of instrument to determine whether the walls are insulated and measure the insulation efficiency of their windows, but is the end result going to be that much better, particularly after you've spent how many hours in the client's home? Is he even going to let you spend that much of his time?
At the end of the day, if you have sufficient experience, you take about 20 minutes to tour the house, guess at the insulation and windows, and give the guy a range of options, and you're out of their hair.
While detail all sounds good from our engineer's perspective, it's ANNOYING from a customer's perspective. When I was shopping for new windows, there was one sales guyg that burned about 2 hrs of my time, explaining in gross and gory detail about why his particular vinyl windows were so much better than the others. But, in the end, they weren't really that different, and ultimately, I was more turned off by the extra hour that the guy stole from me.
TTFN
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RE: AC Sizing - Actual vs. Rated
make sure you undersize residential units, in the 700 to 800 SF/ton range, especially if you have low-e glass, your unit will work like a horse and will dehumidify your space, if not, you will have that damp smell in the basement.