How Do You Know When A Central Area Unit Is Sized Right?

SuperiorBuilt

I was always curious on how do you know when a central air unit is sized correctly for the space to be cooled?
Thanks

Diesel65

The correct way is with Manual J calculations for residential or Manual N for commercial. I do not care who tells you "X" ton's for "Y" square foot, it's not right you need the correct load calculations done.

bjg5240

What can be done to correct a cooling problem on the 2nd floor of a 20yr old Colonial home? Short of closing some of the supply lines on the 1st floor to boost pressure, Can a larger blower be installed to give that extra boost need? Or do you normally have to go the more expensive route by installing a second unit in the attic. Sorry if I hijacked the tread

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Diesel65

Was the unit installed to provide a/c / heat to both floors originally ?

mdshunk

Short of doing the calculations, if the system is installed already, you can observe its operation. A properly sized system will run flat out constantly on the hottest design days of the year, and just barely keep the temp at the thermostat setting. If there's a dozen or so days a year where the system runs constantly, the system is properly sized. Keep in mind, however, that there are only certain sized "steps" between sized of ac equipment. You can't get a 3-1/4 ton unit, for instance, if you calc calls for that. The contractor might go the next size up to avoid a call-back on the very hottest few days of the summer, or he might drop back a size if economy is the main issue.

Most of the new air handlers have ECM motors in them now, so the "computer" will automatically put the airflow within spec no matter how poor your design is. Well, within reason, that is.

Second Look

I believe oversizing is bad because you get cooling without enough moisture removal so the air feels clammy.

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mdshunk

You got that right. I was in a double-wide home the other day that was plenty cool enough inside, but felt like a damp basement. It was very clammy. A very odd feeling. I took a visual of the outdoor unit, and it looked huge to me. I'd be willing to be that it was a couple of tons oversized. The cooling setpoint gets satisfied way before the unit runs long enough to wring enough moisture out of the air.

SuperiorBuilt

Thanks for the info. You always have the answer

troubleseeker

Technically, only when a "J chart" has been calculated, taking into account all of the factors affecting the efficiency of the unit, such as window size and exposure, insulation values,etc. , but I believe the great majority of units are sized from one of the general "rules of thumb" that most ac guys are familiar with relating square footage to btu output. If you are referring to an existing set up, and the house is cooloing without feeling "clammy" to the skin, your unit is evidently within an accaptable range. If the air inside feels damp, it is a sign that the unit is oversized and cooling the air to the thermostat setting before it has run long enough to remove sufficient humidity from the air.

#CARRIERMAN

Hi SuperiorBuilt

I read some of the responses to your question. Some of them are very good. But the ACCA manual J has more to do with the heating side of the field. It does have guidelines for A/C and for the area its used in. You have now stepped into the side of the field that I was a master at. Find someone who can do enthalpy calculations accurately. This will give you the maximum BTU value needed. The next thing that needs to be done is the charge properly set to get the maximum BTU output of the condensing unit. The enthalpy calculations will also include the blower speed with the LBS of air calculation. Trust me, this has to be done by someone who knows their stuff. You cannot guess at the charge by putting a set of gauges on it, no one in the field has this ability period. To be truly accurate you have to have the temperature readings. Hope this helps answer your question.

Good Luck
Rusty

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Dustball

I've always wondered this...

If I knew that hot weather was coming the next day but it was still fairly cool at the moment, which is more efficient? Turn on the A/C the night before and have it maintain the current temp or to turn it on when it's already warming up outside and have the A/C cool the house down?

troubleseeker

I've nerver heard of this before, but I am not an ac man.It sounds technical enough to make me believe you, but I envision nothing but blank stares from any of the typical residential ac installers I have known.

hf05

rusty!!! i would say charging by weight if correctly figured out is more accurate than temp. and those charts,ya they are good to understand but very rarely come into play in resi a/c service

MechanicalDVR

Hell from what I see of "residential a/c service" in my area, most of those attic butchers don't even use a vacuum pump and you think they would know what a psychometric chart is all about, I'm surprised Carrierman. And as far as charging by exact weight, by the time you measured the line set and and all, it's easier to take temps and go by superheat or sub cooling.

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hf05

Ya your probably right ,you don't really need a "load" with cooling!You just set up all your work when its 90 deg. out.I don't think its a big secret that weight charge is more accurate.Just residential remember!!!!!

bjg5240

Came original with the house. Also, is there any truth that keeping the fan "on" instead of auto is better for air quality purposes?

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Diesel65

A couple more question's

Did it ever work to your satisfaction ?

Have you kept up on the maintenance, check up's, filters etc ?

Have you made any alterations to the system ?

It does nothing to leave the fan running unless you are running an air cleaner to filter for allergies and such (many manufacturers have their own design for these), a standard filter will not do the job.

It does sound to me that it may be time for a new unit, 20 years is a long time, have you had a pro out to check things over?

kevin Bessette

Install Attic fan

aldematt57

It's important to note that a manual J heat loss/gain calculation is only a part of the sizing process.
Manual D duct sizing is also of paramount importance to satisfying load considerations for each and every room in resi apps.( man Q in commercial apps)
I also noticed that one post suggested that the majority of indoor blower motors are ECM (variable speed) types.
Unfortunately, this is not the case. The prevalent type of motor still used in our industry is the PSC (permanent split capacitor) motor.
Although variable speed is a better uption, it is more costly.
It provides better performance, more options and better dehumidification.
Variable speed does provide better airflow when existing ductwork is incorrectly sized, but it cannot compensate for grossly undersized ductwork.
Bottom line...there is no rule of thumb when sizing equipment and ductwork.
It must be done according to the guidelines provided in man's J & D.
It is what separates the pro's from the pretenders.

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bjg5240

There is an attic fan installed. As far as the servicing of the unit, I did have a local hvac company come out & clean & check everything when we first purchased the home in May 2007. Other than the electronic air filter was a filthy discrace, he didnt mention anything about it being on its last let. I removed the electronic air filter & replaced it with a paper filter. I just don't think there is enough air pressure coming out of the upper vents. I tried closing off some of the lower supply's, but I havent seen much of a difference. Is there a way to install a larger blower motor without replacing the whole unit?

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