Looking to get central air installed- questions for experts

[BigJon3475]

Originally Posted by K'ledgeBldr

Which you would be able to size with a Manual D calc sheet.
However, when the ductwork is already existing you have to tweek the Manual J calc. Manual D is hardly ever used; maybe on a custom home to build, but kindda worthless on an existing house.

Consequences of not making a Detailed and Accurate Load Estimate

Errors (accidental or an effort to manipulate output) in the load estimate filter through the entire design and cause the instillation to miss the mark.

Manual J Dont's

* Do not add a "safety factor" to the Table 1A design conditions.

Manual J 8th Edition(Hank Rutkowski, P.E.)While it may be true that Manual D is "hardly ever used" the same could be said for Manual J several years ago. Now, if you're going to be guaranteeing your work you have very few other choices but to do it the right way the first time otherwise you'll be opening yourself up to lawsuits and the destruction of your reputation.

If you're going to be "tweaking" Manual J or any of its sister publication you mise well go back to using X number of square feet for X number of tons to design systems.

Why are duct design calculations important?
Achieving occupant satisfaction is the principal goal of
any HVAC design. For residential air duct designs
ACCA’s Manual D is the procedure recognized by the
American National Standards Institute (ANSI) and spe-
cifically required by residential building codes. Air is
the first word in air conditioning. If the network of
ducts carrying the air is not properly designed then the
health and safety of the occupant are at risk, the equip-
ment could fail more quickly, the energy costs could
rise, and occupant comfort might be sacrificed.

What problems come from wrong sized ducts?
In order for home owners to be comfortable a duct sys-
tem must be designed to carry the right amount of air,
at the right speed, into the right room.

If the ducts are
the wrong size then the wrong amount of air will enter
the room and may cause:
•
The room to be too warm or too cool
•
The air to be too drafty and disturb people while
they sleep, eat, read, etc...
•
The air to be too noisy and drown out conversations,
TV or radio programs, etc...
•
The air to be too slow – the conditioned air will not
circulate or mix well in the room.
•
The fan to work harder,
possibly fail sooner, and
use more energy to move air
•
The furnace or air conditioner safety devices to stop
equipment operation
•
Pressure differentials that may increase energy costs
by pushing out conditioned air or drawing in un-
wanted air

http://media.iccsafe.org/geo/docs/AC...rification.pdf

[MrWillys]

I don't want to be involved in a construction pissing contest, but will add this. No mention has been made of the age of the existing FAU (forced air unit)? More than 20 years old it should seriously be considered for replacement. Also, register outlets that are low in cold country is acceptable. Mine are in the floor, with the return in ceiling, but we only used ac 6 days last year.

[wit-nit]

We had an old system and wanted to upgrade it for better efficiency and had a Trane dealer come look it over. They were great in checking everything out. They did some minor duct work and installed a new 19 seer unit and man did that make a huge difference. Cut our bill by about 50%.

[miscus555]

Thanks for the responses
I'm currently getting a couple consultations
Ill see what they have to say about the use of my old forced air ducts