Why DIY is necessary. - Page 5
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  1. #53
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    I designed and installed the duct work before I had a copy of HVAC calc. I expected an energy efficient design, but not that low. But, I did install dampers in the duct work so that I have manual zoning control and can increase the throw at some registers by shutting others off.

    Calculating actual numbers for the total static pressure seemed like a real waste of time. On the supply side, they had to use enough flex duct to connect up to all of my hard metal duct, so I know there is no problem there. The biggest restriction in the system is the 14" flex duct for the return air.

    The variable speed blower means that a very low static pressure will just lower the energy requred by the blower.

    I did include the fresh air intake on my HVAC calculations.

    Besides the 2 ton central system heat pump, I also have a Fujitsu mini-split inverter driven system with variable capacity between 5,500 BTUH and 19,000 BTUH.

    So, the total rated cooling capacity for the house is variable between 5,500 BTUH and 43,000 BTUH for a house that HVAC calc says needs 16,137 BTUH of cooling. I did not see any point in looking at derating factors.

    I do have radiant barrier decking on my house. I VERY seriously looked at going with a hot roof and it was part of the plans for many months. I really wanted it to work for several reasons, but it just did not work for our design.

    My wife and I spent a lot of time looking at new houses while designing ours. We wanted to make sure that we spent our dollars in ways that added real value to our investment. The ONLY place we ever saw sheet metal duct work was in modular housing where the duct work was under the floor and sheet metal was required by code.

    Houses that cost more than 10 times the amount we are spending on ours still had all flex duct. The only way that we could fit the sheet metal duct into our budget was for me to DIY.

  2. #54
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,273
    Out of curiosity, what was your reasoning behind using a basement for a house in north central Texas?

    And by "hot roof"...do you mean tar and gravel?

    Did you frame this house yourself or did you sub that out?

    The blue strip seen in the photos winding around the top plate...is that to check infiltration or something?

  3. #55
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    >>Did you account for the additional sensible and latent load a 6" fresh air intake will impose on the cooling coil when you did your equipment size selection?

    Paul42 has done a most impressive amount of design before building, but I would like to ask how he determined this airflow. I always thought it was highly dependent on return vacuum. A damper to adjust for whatever conditions? It is fascinating to read the back and forth discussion of design issues here.

    >>If you sized each supply outlet using a friction rate for 1,800 cfm vs. 800, you're likely to have insufficient throw and way too much drop at each supply register... will dribble cold air out into the room and generate cold draft complaints for occupants near the outlet. Other areas of the room may become too warm and stuffy.

    Shophound, why is this not a severe problem when certain 2-stage ACs are installed? Or is it, and we just seldom hear about it? If one planned to use a ceiling fan or other fan in the room, would that make the problem disappear, in your judgement?

    >>While 99% of local residential installations use attic mounted ductwork with ceiling supply registers, almost none use round ceiling diffusers, which is almost ideal for a cooling climate.

    Shophound, I would be very pleased if you would try to explain why. Is this something I will find if I finally buy the Manual T book?


    Thanks to all who have contributed to this thread! -- Pstu

  4. #56
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    Quote Originally Posted by shophound View Post
    Out of curiosity, what was your reasoning behind using a basement for a house in north central Texas?

    And by "hot roof"...do you mean tar and gravel?

    Did you frame this house yourself or did you sub that out?

    The blue strip seen in the photos winding around the top plate...is that to check infiltration or something?
    A "cold roof" is a conventional ventilated attic with insulation on top the ceiling. A "hot roof" is where the insulation is either part of or just below the roof decking leading to a conditioned attic.

    It is not a true basement, all of it is actually above grade. The tax appraisal district calls it an unfinished basement. It is actually a very tall conditioned crawl space with a concrete floor on 80%. The crawl space is 6 to 10 feet tall. The rest is a very steep slope of fill dirt under the garage / breezeway that is covered with a vapor barrier - which was a real pain to put in.

    The house was built on a slope and to fit the house between the huge live oak trees, it was necessary to go with a pier and beam foundation. Fill dirt for a slab foundation would have covered the roots of the trees and killed them.

    I found a very good framing contractor for my house - he was the most expensive bid, but my wife and I both believe that we saved money in the long run by going with him.

    The blue strip around the lower top plate is a gasket to seal between the top plate and the drywall. It was another DIY project. The method was copied from a builder back east that does energy efficient houses - and does blower door tests to check their work. The gasket is made from 6" wide sill gasket. A simple jig made from a 2x6 and some utility knife blades was used to cut it into 4 strips the right width for this task.

    I am also using duct sealer (for sealing electrical boxes) to seal the back of all of the electrical boxes. After the drywall is installed, I will use caulk to seal the edges of the electrical boxes and the gap between the drywall and the floor.

    I will do some initial infiltration tests with my sawdust vacuum connected to one of the exhaust vents on the house. After I have found and fixed all the leaks I can find, I will pay for a blower door test - I've already found a contractor for that.

  5. #57
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,273
    Quote Originally Posted by pstu View Post
    >>Did you account for the additional sensible and latent load a 6" fresh air intake will impose on the cooling coil when you did your equipment size selection?

    Paul42 has done a most impressive amount of design before building, but I would like to ask how he determined this airflow. I always thought it was highly dependent on return vacuum. A damper to adjust for whatever conditions? It is fascinating to read the back and forth discussion of design issues here.
    6" fresh air return with a damper is a common way to install such a system. The damper allows a bit of fine tuning for the proper amount of make-up air needed. Make-up air is sized for recommended air exchange rates...if the house can't make up enough fresh air naturally it will need mechanical help...that's where a fresh air intake comes in.

    A fresh air intake must also have provision for exhaust or the house will not see sufficient air exchange rates...and will become pressurized to boot. An ERV could be used to accomplish this need, but so could a whole house dehumidifier that dries the incoming fresh air before it is injected into the house environment. A low cost method for exhaust with a system using fresh air intake only is to tie a kitchen or bath exhaust fan to energize on a call for heating or cooling...as long as the equipment runs there will be an exchange of air.

    >>If you sized each supply outlet using a friction rate for 1,800 cfm vs. 800, you're likely to have insufficient throw and way too much drop at each supply register... will dribble cold air out into the room and generate cold draft complaints for occupants near the outlet. Other areas of the room may become too warm and stuffy.

    Shophound, why is this not a severe problem when certain 2-stage ACs are installed? Or is it, and we just seldom hear about it? If one planned to use a ceiling fan or other fan in the room, would that make the problem disappear, in your judgement?
    With properly installed two stage systems, we're talking about ductwork sized for the high stage...and there is not a significant CFM change between stages, typically. That is, provided the system in question varies blower speed with staging the refrigerating capacity (compressor). If Paul designed and built a duct system capable of flowing 1,800 CFM of air, and will only be flowing approx. 800 CFM, that's a significant difference in dynamics being imposed on the duct system and air distribution outlets.

    The only use I have for a ceiling fan is when I'm in the room with it. When I leave, it gets shut off. It is also handy should I walk into the room after either being outside on a hot day, or being active indoors and wanting more air movement to feel cooler.

    With a properly designed air distribution system, the room is enjoying the benefit of evenly distributed cooling even when I'm not in the room, so when I walk in, if I haven't been active to where I feel hot, I may not need the ceiling fan at all. The air and the objects/surfaces in the room are sufficiently cool to give me a feeling of comfort.

    >>While 99% of local residential installations use attic mounted ductwork with ceiling supply registers, almost none use round ceiling diffusers, which is almost ideal for a cooling climate.

    Shophound, I would be very pleased if you would try to explain why. Is this something I will find if I finally buy the Manual T book?
    Manual T states the benefits and drawbacks of each air distribution method clearly. It includes drawings for each type that show in basic form how each supply air introducton method affects air movement throughout the room. When I first read this info, it was a big eye opener for me. In the house where I was living prior to purchasing my own (my in-law's place) I used the info gained in Manual T to modify the supply registers throughout the house from the stamped steel cheapo crap to curved blade registers, aiming the airflow to flow along the ceiling and wash the exterior walls.

    This is accomplishing to a degree what a round ceiling diffuser does...an attempt to mitigate the natural upward convection that occurs at windows, being window surfaces are in summer warmer than the room air and will set up a "stack effect" of upwardly rising air in front of each window. Left unchecked, this air will accumulate at ceiling level, combined with the ceiling material being warmed from above due to an overheated attic. The ceiling diffusers...or a high sidewall register aimed between 5 to 15 degrees toward the ceiling, will move this air and cause it to mix with the rest of the room air before the return outlet removes it from the room to be conditioned.

    In my new house, the round ceiling diffusers are not smack dab in the middle of the room. They are consistently set a bit off-center, toward the windows, where the discharge toward that side mitigates upward convection from the windows. The rest of the diffuser distributes air throughout the entire room, washing every wall. With the location favoring the windows, the exterior walls tend to get more "washing", which they should.

    When we get settled I will possibly add transfer ducts to assist in return air volume, provided the door undercuts are not sufficient. Since we removed all of the carpet throughout the entire house and are installing laminate bamboo, the door undercuts may be sufficient in this case. Only time and testing will tell.

    I would suggest to you, since your curiosity about HVAC system design remains perenially high, that you visit the ACCA web site bookstore and purchase Manual T for your own perusal and enjoyment. Read it through thoroughly, even for systems not like your own. If I ran a contracting shop I'd have every technician and installer in my employ read that book, and pass a test on it. I wouldn't jump their case if they didn't get it right away...for many it takes awhile for the concepts presented in air distribution literature to sink in. It's not emphasized nearly enough in our trade...but needs to be. Paul would not be tossing around the DIY coin if he had found someone who "got it" regarding air distribution design and installation.

  6. #58
    Join Date
    Oct 2007
    Location
    Milwaukee,WI
    Posts
    1,068
    Quote Originally Posted by paul42 View Post

    I am also using duct sealer (for sealing electrical boxes) to seal the back of all of the electrical boxes. After the drywall is installed, I will use caulk to seal the edges of the electrical boxes
    ___________________________

    Chicago is an indian word for stinky!!!!!!
    -supertek65

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