AC Sizing - Actual vs. Rated

[wizzim]

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?

 

[KiwiMace]

Most HVAC Engineers will take the cooling load, desired space temperature/RH, desired efficiency and ambient outdoor design conditions, and shop these numbers out to a few vendors for selections.

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.

 

[wizzim]

Is it common to shop out the performance for residential? It seems like there should be a straightforward way to calculate that derating. I will look into the ASHRAE section though.

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.

 

[IRstuff]

Sounds like you're possibly overthinking the problem. For most single-family residential work, the rule of thumb is 1-ton AC for every 500 to 700 sq. ft., depending on how nice a guy the AC salesman is. I bought my home AC based on the sole fact that one of the salesmen made a case for why 1-ont AC per 500 sq.ft. was gross overkill, and he turned out to be correct in my case.

TTFN

FAQ731-376

 

[wizzim]

I agree that rules of thumb will get the job done but they can't guarantee energy efficiency and comfort the way a properly sized unit can. Not to mention increased footprint and the added cost from larger equipment (furnace, coil, and condenser) passed on to the home owner that isn't necessary. For how much energy buildings use in the USA (almost 40% of all energy) its amazing that we don't design them to the standards that consumers demand from almost every other industry. If you went shopping for a car and the salesman told you a Hummer would provide all the leg room and power you needed that doesn't make it the best solution. Of course this is all my own opinion but wanted to provide some background. I'm seeking high performance. Once the equation is nailed down its easy to repeat on every other job. Think hard once and breath easy forever after.

 

[IRstuff]

OK, so pick 1 ton per 700 sq.ft AC with a SEER of 20 and be done with it.

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

FAQ731-376

 

[KiwiMace]

I still think you should go to the manufacturers on this. Selecting specific coils is an option available only in larger commercial units.

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.

 

[wizzim]

@Kiwi, the manufacturer (Trane at least) provides options for many many single family residential sized C/E combinations all AHRI rated. You can pick out the coil and condenser independently. I want to make sure I have enough sensible capacity like you said - concurrent loads - so I'm trying to properly size instead of just assume 2-3 tons over capacity. I think between manufacturer and ASHRAE I will have to seek out my answer. You're right about bigger as a function of efficiency. Mostly I was talking about larger capacity = higher cost. Only sometimes does larger capacity = larger footprint.

@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.

 

[IRstuff]

You're operating under the assumption that you can calculate everything, and that's just not a realistic assumption. Case in point, it hit 113°F in Los Angeles yesterday, almost part of the normal September hot spell. Do you design for that, or not? If you do, your system will be over-specified for the remainder of the year. If you don't you'll have miserable month or two months.

On the other hand, they're selling SEER 20 systems, and ten years ago, the best you could get was about SEER 14.

TTFN

FAQ731-376

 

[EnergyProfessional]

you also should get 2-stage unit. Has better part load efficiency and better dehumidificaion in part load and less cycling (=wear)

 

[wizzim]

Discussing the finer points and best practices of energy modeling is best kept for a different thread. I'm curious to hear though what these mystery loads might be? But in short, yes you do plan accordingly for high temperatures based on TMY 30 year historical data (design to mean extreme, or 99.6%, or 99.2%, etc). Additionally its very easy to model room occupancy, appliances, lighting, windows, etc. As HerrKaLeun put it you install a multi-stage piece of equipment for non-peak conditions. Then, if you just size it accordingly the client will never be miserable and the equipment will cost less, operate better, last longer, and perform more efficiently.

 

[imok2]

Wizzim
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

 

[IRstuff]

While a detailed analysis is certainly the theoretically correct approach, the reality is that at the end of the design, you'll have design point with some range that is optimal, and your customer will conspire to avoid that design point at nearly all times.

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

FAQ731-376

 

[wizzim]

so that 1 ton per 700sf rule you recommend is ALWAYS right even with all those changing loads? seems like it should grow a little every year when new computers get introduced? Look up the heat output of a computer monitor, if that puts me over on cooling load then I've got bigger problems. i'll take my chances with modeling. again...a lot of the "unpredictable" variables you mention are in fact very predictable (its easy to find out how much insulation someone has) and balancing should be a best practice for any contractor who doesn't want a call back. i'm dealing with single family homes, maybe the set point issue is more prevalent in the buildings you work in. thank you for the input though. engineering versus rules of thumb...agree to disagree.

 

[IRstuff]

Don't forget that I gave a range of 500 to 700. I think that the desired granularity is pretty much at that level. And while any individual monitor won't break the bank, it's the total sum that starts looking pretty ugly. I call that the "Costco Effect" where you go into Costco and buy a bunch of items ranging from $5 to $10, and the final bill comes out $200, and the wife insists there's an error. But there isn't, because there's about 30 items in basket; 30 * ~$7 = $210.

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

FAQ731-376

 

[cry22]

I am all with Irstuff
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.