Need help with ducts or whatever

Problems related to residential installations.

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- Need help with ducts or whatever

Post by pughimag » Fri Feb 06, 2015 12:59 pm

I have a similar concern. We are building our retirement home in zone 3 at the coast in North Carolina. I've come to know more about some of the design considerations for HVAC thanks to this site and I'm looking for some reassurance given that all our savings are going into this project. The house is 2,453 sq ft with a 433 sq ft bonus room above the garage. We have 9ft ceilings and 2x6 walls and the cavities have been well sealed. We have low E windows and doors and we're planning dense-pack cellulose in the walls and R38 in the ceiling. The roof is vented. Top and bottom plates have been sealed. We have a tight envelope with OSB and house wrap on the outside. We're using LP TechShield on the roof deck.

The Lennox HVAC system that has not yet been installed delivers 0.11 air exchanges and an average of 800 CFM (May peak higher). It is a 3-ton system with three zones and has an MERV16 filter, UV light and a catalyst (Lennox Pure Air-PC014-23). We're also using an ERV that delivers 200 CFM and is hooked into the master bathroom and outside air.

I am concerned about the low number of air exchanges and total sustained CFMs. With windows that an be opened but will likely stay shut like our current house I would like to have 4 air exchanges an hour and more like 1800 CFMs? What do the experts think? Should I be concerned about humidity control and stratification at low air exchange rates with this system? Will Lennox guarantee no mold issues and relative humidity of 45 - 55% 90% of the time for a tight house.

The heat load has been calculated at 29,389 for the ground floor and 5,747 for the bonus room. The corresponding cooling loads are 26,752 and 4,818. The HVAC engineer calculates heat loads of 33,400 and sensible cooling at 24,640 for the whole house.His pipes come out of the A/H at 19" to 14" to 10" to 4"/7"/8" pipes feeding the rooms. It seems like a waterfall is moving into a trickle but I'm no expert.There are three returns sized at 16"(693 CFM) for the great room, 12" for bedroom zone (bath laundry, closets, toilet) and 12" (329 CFM) for the bonus room.

Why does the code call for 4.0 AEH in commercial buildings with sealed windows but not in tight houses where windows are not opened and the air is controlled?

Any thoughts would be appreciated.

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- Need help with ducts or whatever

Post by Freon » Fri Feb 06, 2015 7:27 pm

I can't begin to comment on your design. It's way to complicated and theoretical. My one suggestion is get a very well written agreement with your installer that he/she will make the design parameters a reality. If not, know what remedies you can look for.

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- Need help with ducts or whatever

Post by LennoxHVAC » Mon Feb 09, 2015 11:12 am

It is a very nice part of our country for retirement life.

I concur with Freon.

Assuming the garage is temp controlled too, but not a brick house.
The code requires modern HVAC units to pull-in and circulate certain amount of fresh-air during each cooling or heating cycle. Learn about Green Building.

Make sure installation is easy accessible, and beyond what code says.
Tonnage is adequate.

The secret of comfort is in air-mixture, and re-circulation.
We installed VELUX Skylight in 2001... love its many benefits.

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