A good, reputable HVAC company should be able to determine how much airflow (CFM) is needed in each room. The designer can then determine friction rate and size the ductwork according to the friction rate, required CFM and length. The friction rate is determined for the longest run and a properly sized system will have dampers installed to properly balance the airflow to rooms that are closer to the air handler. Being in Texas myself, be very careful if the contractor designs your system with a .1 friction rate, especially if you are using flex duct which is common here. Most installations I see with the flex duct have a friction rate from .05 to .07 and airflow suffers. Additionally the contractor should have an idea for the maximum External Static Pressure (ESP). Too much static pressure and the unit will never operate within design parameters. All this said, it is unlikely any contractor will put this much work into a quote and give it to the homeowner so that a lowballer can bid against him. You may be able to negotiate all this work and receive all the supporting paperwork once the bid is accepted and a deposit paid.

Once again, thanks for all the input.

The more companies that come out, the more it sounds like my duct work is undersized even for decreased heating and cooling load after the window and door replacement. This is an existing house with ducts in difficult to reach areas (lots of drywall) it will be expensive to fully retrofit" but there are still some significant improvements that can be made.

Given the comment that poor ductwork can have a significant impact on system performance and looking at performance comparisons for air source vs geo for my climate... It sounds like I would get better return on my $$ with air source and investing some of the up front 'savings' in duct rework. There is no way I can afford geo and new ducts. Am I headed in the right direction on my thought process?

So far none of the folks have done any significant calcs on the ducting, but measurements of all the pipes coming into and out of the unit have several of the potential installers concerned.

In a refit, does one calculate the load, install the equipment, and then balance (and fix) the ductwork?

Still having trouble finding company i am fully happy with. So far the one company that has spent the most time discussing options, getting list of trouble and potential trouble areas, crawling through the attic, tracing ducts, etc is one that doesn't do manual j. This is someone that only does air source.

Load calcs to determine each room's air flow requirements (heating and cooling) are not rocket science. I do them weekly.

With design numbers in hand I use a balometer (flow hood) to measure actual airflow. That specifically identifies starved and overblown rooms. Sometimes minor mods to existing ductwork are all that is needed, particularly if steps have been taken to lower house load.

It's easy stuff, but hard to get someone to do it.

Posted By aus on 04 Feb 2011 10:28 PM
Once again, thanks for all the input.

Still having trouble finding company i am fully happy with. So far the one company that has spent the most time discussing options, getting list of trouble and potential trouble areas, crawling through the attic, tracing ducts, etc is one that doesn't do manual j. This is someone that only does air source.

No Manual J to me is a red flag.  As an HVAC contractor heavily involved in the replacement market, at least 50% of the systems I replace are oversized and underducted.  I can often replace a 5 ton with a 3.5 ton and upgrade the ductwork for a price very close to a straight 5 ton replacement and no duct repair.  I have had customers literally call me after a change-out and let me know that for the first time in 5 - 10 - 15 years that they are now truly comfortable.  Often times with these customers I will also get a call after they receive their power bill exclaiming that even though they have lowered their temperature (during summer) that their electric bill is lower during periods of similar weather. 

There are products available on the internet that grant a temporary license to figure out your own manual J loads.  All you need is a tape measure and have the ability to measure your insulation depths.  The programs will walk you thru all the calcs and give you a room by room analysis.  The cost is nominal ~ I think about $50.

The TX HVAC contractor license requires the applicant to have at least a basic understanding of Manual J.

Engineer's tag line says it all...Absent data you only have an opinion. 

I'm looking at a 1999 house in Jax - 2800 SF with a 5 ton ASHP. All the return air comes through about a 16" flex crushed in half or more by addition of some plywood in the attic for storage. I don't know why the unit isn't tripping out on overload in heating mode.

There's the usual raft of high bills and uncomfortable rooms.