# HVAC design flaw?



## rblaine (Aug 30, 2010)

Hello,

New to this forum. I hope someone out there might have some advice. My home was built in 1999, so it's not that old. 2 story 2800 sq ft. Has always had issues with the temperature being 10-12 degrees different between up and down. There is only one thermostat upstairs. I just avoided being downstairs during the winter. My wife and I married in 2004 and she complained about the difference and now we have some money to get it fixed. After having two contractors take a look at what I have, they both tell me the ducting is designed wrong. There are only two returns, 20x25 and 18x18, which they told me will allow only a little more than 1200 CFM and my 5 ton system needs 2200 CFM. So i need to install a new return and then install a zone system with a thermostat downstairs.  The whole thing will cost $10K. My question is shouldn't the builder be responsible for this since it's appearently a design flaw? Any advice would be greatly appreciated.


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## kok328 (Aug 30, 2010)

I don't know if the builder would be liable or not but, chances are pretty good that the builder is long gone.


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## oldognewtrick (Aug 30, 2010)

Typically a builder will warrant a structure for a period of 1 year, unless you can prove willful intent to defraud, you are probably out of luck. A lot of contractors rely on their sub-contractors to provide and install systems. A lot of them don't fully understand the dynamics of house construction. You will spend a lot more than 10K trying to prove and recover the cost of fixing the original system.


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## rblaine (Aug 30, 2010)

Thanks for the replies. I was thinking that since car issues that are a design problem are repaired by the maufacturer (think Toyota) even decades after the car was built, the same might apply to an improperly built/designed HVAC. In my view this is something I would not have detected within the one year warranty. The builder is still around, so I'm thinking I might just ask him about it and maybe he can at least help me out with lower cost.


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## kok328 (Aug 30, 2010)

So would it be the builder (general contractor), the subcontractor or the local code inspector who signed off on the build?
Unfortunately, comparing automobiles and homes are like comparing apples and oranges when it comes to defects. Not to mention that the 1yr home warranty applies only to the original owner.


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## om55 (Oct 5, 2010)

your probably  not going to get anything from them and what you need is to find an affordable answer to your problem. have you asked your heating contractor about zone dampers with different controls  that would shut off air to one section and let it continue in the other. I would think that an honest contractor could do that for less than ten grand. granted the duct work may still be not the way it should be but make it work now and rework the duct when it comes time to replace the whole unit.


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## classic (Oct 20, 2010)

An excellent rule of thumb for return sizing is filter length x filter width x 2 = capacity, so 20 x 25 x 2 = 1000 cfm and 18 x 18 x 2 = 648. You have a nominal capacity of 1648 cfm and a 5 ton unit needs 2000, so you are 20% undersized.

Your real problem is heat trandfer 101 - hot are rises, cold air falls.

Lets pretend that your 2800 sq ft house is the same up and down.
Each floor is 1400 sq ft, so you need the same ammount of air for each floor,
right?

WRONG!!!!!
Hot air rises, so in the winter you need about 70% of the heat downstairs because the downstairs heat will take care of the upstairs.

Summer is just the opposite. The cold air downstairs pushes the heat upstairs, and the attic is hot as hell, so you need 65% of the cold air upstairs

you need some sort of damper system to make the switch between summer and winter needs.


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