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  1. #1

    Need some new construction advice..

    I am working on the plans for a house I plan to build. It will be my retirement home and I plan to pay a little extra and build it very energy efficient, foam block basement, sprayed closed cell insulated walls, high efficiency windows, etc. I am looking at energy efficient HVAC systems and have run into some confusion. The house will be one story, 1700 sqft., and I also plan to heat and use the basement. I have calculated the heat loss and with the values for the high R values in the walls and ceiling I come up with less than 50K BTUs heat loss. The problem is, the CFM of the high efficiency furnaces I have looked at is much lower than the CFM needed for 3200 sqft of conditioned space.

    When the time comes I will get a HVAC professional to help plan and install the system but I, at least, would like to know what I need and make sure the person I hire knows what they are talking about. Do I reduce duct size to match the CFM of the furnace, need more than one furnace, increase BTUs to get enough CFM?

    I will also have an energy recover ventilator removing air from bathroom and kitchen areas and bringing fresh air to the return duct.

  2. #2
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,618
    If it was me, I'd zone this system. That way you'll have the air flow you need for each level and the temperature control you'll want for each level. Either that or two seperate systems, and that I'd not recommend. But some contractors like two systems instead of one zoned system. As to duct sizing, the experience contractor will be able to do this.

  3. #3
    I planned to zone the first floor and Basement. My confusion is with CFM. A high efficiency furnace that will provide the correct, or close, BTUs has a maximum CFM of about 950. The conditioned space will be 3200 sqft. I am questioning if 950CFM will be enough air flow for the space. I am hoping it is and hope someone who knows more than me can help.

  4. #4
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,618
    That is a really small blower you've got in that system. I'd plan on a system that will supply 1200 cfm and you should be fine. I'm assuming you're talking about 1600 sq/ft on each floor?

  5. #5
    Quote Originally Posted by wahoo View Post
    That is a really small blower you've got in that system. I'd plan on a system that will supply 1200 cfm and you should be fine. I'm assuming you're talking about 1600 sq/ft on each floor?
    Yes about 1600 sqft per floor. A bit more on the first and less in the basement because I don't plan on conditioning the whole basement. All the 60-70K BTU high efficiency furnaces I have looked at have a max CFM of around 900-1000 I need about 50K BTU output based on the heat loss calculations. To get 1200 CFM I would have to go with a bigger furnace around 90K BTUs almost double my requirement.

  6. #6
    Join Date
    Jun 2006
    Location
    Richmond, VA
    Posts
    2,992
    What's your cooling load?

  7. #7
    Quote Originally Posted by beshvac View Post
    What's your cooling load?
    I don't know that. The total heat loss for both levels at 72 design temp is 42,000BTUs. The entire conditioned space is 31,000cuft. I think that's about 1.5 Ton?

  8. #8
    Join Date
    Jun 2006
    Location
    Richmond, VA
    Posts
    2,992
    Quote Originally Posted by RFP52 View Post
    I don't know that. The total heat loss for both levels at 72 design temp is 42,000BTUs. The entire conditioned space is 31,000cuft. I think that's about 1.5 Ton?
    Above you said your heat loss is about 50k. What is the heat gain in the summer?

  9. #9
    Quote Originally Posted by beshvac View Post
    Above you said your heat loss is about 50k. What is the heat gain in the summer?
    I don't know if I figured this right but..35,000BTU?

  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    www.energywisestructures.com

    They know how to do a heat gain/loss on a tight home. Don't let a dealer unfamiliar with foam insuation size it, it will be way too big. Got a buddy who foams and customers who don't listen to him have run into issues, particularly humidity problems in the summer with too much cooling.

    You can get 45K 95% furnaces with a 3 ton blower. But you need to set cooling CFM for the size of the A/C. If you move more than 400 CFM/ton you don't dehumidify well.

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,432
    Location ? Electric rate? $ _._ _ / kw

    Natural Gas rate? $ _. _ _ / therm

    You may wish to review whole house ventilating, dehumidifier to
    address off peak cooling periods and IAQ.

    http://ultra-aire.com/index.php?opti...d=15&Itemid=44

    Looks like you're developing a Great System through early planning.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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