Here is what I think I want: to retire one old 3.5T Intertherm A/C outdoor unit and relocate a 3.5T Trane XL-12 to replace it from the other half of the house. The Trane would be replaced with something a bit smaller than 3.5T. Is it terribly abnormal to ask to swap an A/C rather than just plug in a new one?

Here is why I would want to:
1) The Intertherm is so old we cannot ID its actual model but based on motor size I am told it would correspond to a "light" 3.5T capacity. Any replacement would be more efficient.
2) The Intertherm is sized correctly, and the Trane is oversized in its present location.
3) The Trane would be replaced with something modern, lower capacity, and better suited to the ductwork which exists. That would solve a problem with duct velocity and excessive noise.

While I have not done Manual D calculations, I have added up the sum of the cross section of the ducts leaving the plenum. It is around 400 SqIn for the Intertherm half of the house, and around 300 SqIn for the Trane half. The number of supply outlets is about proportional. When we installed an A-S (Am.Std.) VS air handler, it originally was set for 1400 cfm on both sides, on the smaller ductwork it made too much noise (some ducts beyond 600 fpm, and it is flex) and I reset to 350 cfm/ton or about 1200 cfm. Getting down to 1050 cfm would be more comfortable still, from a mechanical and noise point of view.

I know that Manual J is the classic Grade-A method of determining sizing needs, and I would plan to have a room-by-room Manual J done before taking action, to verify that downsizing is proper. At first I only did whole house Manual J load using HVAC-CALC, and it tells me 4.5-5.0T is sufficient (vs. 7.0 presently). But I am pretty sure what the room-by-room Manual J will say, as logging data on that AC tells me it runs no more than 73% of the peak hour on genuine hot summer days. I am expecting Manual J to tell me the design load is either 2.5 or 3.0 tons capacity for the current Trane half of the house.

This is near Houston where hot summer days are pretty much all alike. I worked for the utility looking at load patterns and have learned that once you get to the days of 75F low and 95F high, that is the full summer load. May 21 was officially 96F, to give one pre-Memorial-Day example. As I remember HVAC-CALC recommends 94F design temperature. That's where we are right this week.

Tentatively this project is planned for around the end of summer. If finances dictate, maybe postponed from that so long as the Intertherm keeps working.

So... does this sound reasonable to a professional? Any other steps I should take before pulling the trigger?

Thanks for all the things you professionals teach us homeowners!
-- P.Student

[Edited by perpetual_student on 06-16-2005 at 07:38 AM]