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  1. #14
    Join Date
    Jun 2012
    Posts
    14
    general rule...17 btu/sq ft.
    when sizing a heat pump option if the structure has above average heat loss or is not utilizing ac...up sizing up to 25% on the first calculation can be done.
    eg. 2000sqft x 17+3400 btu (3 ton) x 1.25 = 42500 (3,.5 ton)

    If you go with a two stage unit, ac problems from over sizing wont come into play as much,you can give the home owner a larger unit more closely matched to their heat loss and the system wont utilize the back up heat as much if at all.
    Usually a retro fit from another heat source will have questionable duct sizing which will give a noisy/higher velocity air flow from the needs of the 400 cfm/ton for the heat pump, when a two stage is used it will run on first stage most times at a reduced fan speed giving a quieter air distribution system with out costly sheet metal alterations.

  2. #15
    Join Date
    Feb 2004
    Location
    Ontario
    Posts
    700
    Quote Originally Posted by jetstream View Post
    I'm in a 2400 sq foot split level with a 35 yr old 8 SEER unit and I'd say it's sized just right.

    By your R of T I'd have a 5 T unit in here which would blow my ductwork to bits.
    I notice my post doesn't make sense without the important part which is that it's a 2 Ton system.
    Question authority!

  3. #16
    Many homeowners are starting to become aware energy costs in addition to the installation cost. There is no way that an oversized system can meet the advertised SEER. A rule of thumb that is proven for a specific area may be ok, but you must remember that design temperature and humidity levels can change drasticaly from one location in a state to another location. A ton per sq foot rule fails to consider the design temps, the building features such as insulation and windows. For quote purposes, you don't have to do a room by room, but you run the risk of finding surpises in the distribution system. The single most important factor in the installation of a HVAC system is the quality of the installer. That quality starts with the understanding of what is needed to properly heat and cool a residence.

  4. #17
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,750
    Quote Originally Posted by jmo90280 View Post
    I know theres alot to consider (light,windows, ect) but in general what would u use?

    Around here, you could have anywhere from 600 sq ft per ton to a little over 900 sq ft per ton. Varies with house type, townhouse(inside or outside), rancher, 2 story, etc. Then of course windows, and insulation along with infiltration.

    Need to learn the construction practice for your area, for the different years. Then you should after sometime get a good idea what a house might need when you drive up and look around a little.

    Doesn't take long to do a block load.

    Keep in mind that a 2000 sq ft house with a 4 ton unit moving 1400CFM with a 24 degree coil delta is only doing around 32,000BTUs sensible. Meaning a lot of its sensible and latent capacity is just being wasted keeping the compressor cool, instead of the house.
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  5. #18
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    925
    Quote Originally Posted by beenthere View Post
    Around here, you could have anywhere from 600 sq ft per ton to a little over 900 sq ft per ton. Varies with house type, townhouse(inside or outside), rancher, 2 story, etc. Then of course windows, and insulation along with infiltration.

    Need to learn the construction practice for your area, for the different years. Then you should after sometime get a good idea what a house might need when you drive up and look around a little.

    Doesn't take long to do a block load.

    Keep in mind that a 2000 sq ft house with a 4 ton unit moving 1400CFM with a 24 degree coil delta is only doing around 32,000BTUs sensible. Meaning a lot of its sensible and latent capacity is just being wasted keeping the compressor cool, instead of the house.
    I find it funny everyone points out that i said to figure a 4 ton for a 2000 sq house. I could of said 2 ton and got the same response. When we are just talking in round about figures.


    SO my main point is still valid. if its not 500 sqft. then use 600 or 900, if that is what your used to seeing in your area. That's what we are talking about, when your familiar with the area you work in giving a Rule of thumb is not out of line for a fast budget #. How many times do i get ask how much to install a complete system. Drive out there, do my calculation, spend time figureing out duct work layout. Maybe call a guy to run the electrical, ect ect... to find out from the customer ...Oh i cant pay that..


    But than sometimes they tell you their budget..But you still have to go through the same process because you need to cover yourself. If you don't they will say but YOU said bla bla bla...

    He ya go you price shopper. 2000sqft house...cost XYZ...could be more, could be less. Can you handle that? just don't waste my time...

  6. #19
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,750
    I find it funny everyone points out that i said to figure a 4 ton for a 2000 sq house. I could of said 2 ton and got the same response. When we are just talking in round about figures.
    I think you missed the point. I used 2000 sq ft to show that it could be 500 to 900 for a 2000 sq ft house in my area. No one sq ft ROT covers every 2000 sq ft house in this area.
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  7. #20
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    925
    Quote Originally Posted by beenthere View Post
    I think you missed the point. I used 2000 sq ft to show that it could be 500 to 900 for a 2000 sq ft house in my area. No one sq ft ROT covers every 2000 sq ft house in this area.
    I understood what you meant and agree with what you said.

    but its still funny how many hard points of view we got when we are just talking in round about figures.

    I got a 4000 sqft house made of Styrofoam 4 feet thick and can cool it with a ice cude and a fan

  8. #21
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,750
    [QUOTE]I got a 4000 sqft house made of Styrofoam 4 feet thick and can cool it with a ice cude and a fan.QUOTE]

    LOL...
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  9. #22
    Join Date
    Oct 2004
    Posts
    202
    Here in South Florida we get people all the time kicking the tires asking for quotes. Depending on the age the house was built... if it is well insulated or not...depending if it is a rancher or a cube shape...depending on exposure...etc...etc... I usually go average 600 sq feet per ton...

    I quote a price....get a down payment and fine tune from there....otherwise....I let them keep shopping....

    My 2100 sq foot rancher built in 1977 with a South West exposure... CBS construction ...not the best insulation...some original windows with lots of sliding glass doors ....called for 4.5 ton....I installed a 4 ton...corrected the duct work which had a properly sized supply and a return grill only good enough for a 2.2 ton....

    My electric bill went down 50%.....as we upgrade the windows and insulation....I might just be a pinch oversized... with humidity control.... all should be o.k...
    It's all about heat transfer...

  10. #23
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by dlove View Post
    500 sq.ft. per ton

    2000 sqft house 4 ton unit. In most cases after you do a study you will fall somewhere more or less close to that.

    Did i just hear an ME curse?
    *curse*!!! Actually I do use rules of thumb to check my work and for bids since I don't want to do the load calcs before the project is awarded.

    This is what I got from ASHRAE Principles of HVAC 1994 (attached is a longer list of buildings).

    Houses:
    Cooling: 17 BTUH/Sq. FT
    Heating: 3.0 BTUH/Sq. FT
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  11. #24
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    925
    Quote Originally Posted by hcong View Post
    *curse*!!! Actually I do use rules of thumb to check my work and for bids since I don't want to do the load calcs before the project is awarded.

    This is what I got from ASHRAE Principles of HVAC 1994 (attached is a longer list of buildings).

    Houses:
    Cooling: 17 BTUH/Sq. FT
    Heating: 3.0 BTUH/Sq. FT

    Thats what im Talking about

    I know it don't take much to do a calcs when the job is yours. But, how many times are you going to do a calculation or give a detail estimate than have the customer turn around and handed to the next guy, before you start doing round about estimates?.. call me when your serious...

  12. #25
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    5,348
    I think ROT's are used much more often than actual loads. So called "older guys" will say they've been doing it this way for years and why should I tie up my time with a load just to lose the job to another contractor.
    I think younger contractors are much more likely to do a load because they are more familiar with computers and more comfortable with the results. Computers seem like a natural place to get information for them.
    I often see posts here looking for load software when it's just as ez with a simple paper form but they've learned to trust computers. I'm sure it's easier to get an older contractor to use the paper form than a computer. Different generations.
    I wouldn't expect many older contractors to change any time soon. Their system will probably work ok and no one's going to yank it out to show how a different system will be better as long as the house is fundamentally similar to those where ROT worked before.
    Tracers work both ways.

  13. #26
    Join Date
    Jun 2012
    Posts
    14
    What is a computer and where can I get one, i hear they are the way to go....

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