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  1. #14
    Join Date
    Mar 2006
    "By the way, from what I'm aware, "Bonus Room" is a realtor-coined term used to describe a room in a house that can't really be considered a bedroom (maybe floor joists are spread too far apart for a conventional load, or the ceiling is angled, etc.), and that previously couldn't legally be considered as a bedroom of the finished home. It may or may not be considered part of the finished square footage, but a realtor certainly wants to point out that somone *might* be able to use the room. It may have been previously known as "storage" or "additional room" or even "useable heated attic space". Maybe it's an Ohio thing."

    Where I'm building it's related to a septic field issue. You technically can have multiple bathrooms in a home, but the number of bedrooms is dictated by the septic field perc test. As long as you don't have a closet in a room it's not defined as a "bedroom" in NC. Also, a true bedroom has to have an egress window.

  2. #15
    Looks like "bonus room" has many definitions, but the general idea is that it's a room, but not a room that fits a given definition. The bedroom closet/egress thing is right-on for the common definition here, too. Typically, around here, a bonus room is a room over a garage that has angled ceilings, and probably no closet.

  3. #16
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
    Being a high eff. air tight home in a green grass climate, suggest a whole house ventilating dehumidifier to provide filtered, fresh air ventilation. This is important for purging indoor polutants, replenishing oxygen, and providing make-up air for exhaust appliances. The supplemental dehumidification will take care of excess moisture during low/no cooling load conditions. We do many with reports of excellent results. The dehu TB

  4. #17
    Join Date
    May 2000
    Indianapolis, IN, USA

    Check out these folks. They know how to do a load on a foamed building.

  5. #18
    Join Date
    Dec 2005
    Originally posted by ohioac123

    For spray foam insulation:
    Sensible Gain: 33,308 BTUH
    Latent Gain: 4,899 BTUH
    Total Gain: 38,207 BTUH (3 tons)

    For standard "average" insulation/infiltration:
    Sensible Gain: 36,111 BTUH
    Latent Gain: 7,498 BTUH
    Total Gain: 43,609 BTUH (3.5 tons)

    As Steve already said, you calculated what the LOAD on your house was. This does not mean that is the size of the EQUIPMENT you need. The equipment is rated at 95* outside, and 80* inside. If you plan on a temperature lower than that, the equipment will need to be larger. Also, you need to determine exactly what the selected equipment will deliver.

    Here are the specs for a Rheem RARL system. This is for a nominal 4 ton:
    RCHL-48 / Sensible 33,600 / Latent 10,900 / Total ARI Capacity 44,500

    As you can see, even though it is a 4 ton system, it delivers less than 48,000 BTU. This 4 ton system matchs your load calcs pretty closely - if you keep it at 80* inside. If you want it cooler, you'll need larger equipment. Also, I'm not sure if the program you use takes account of other heat producing items in your house such as lights, TVs, computers, refrigerators, etc. All of these items are creating heat that needs to be removed by the AC system. So once you factor in equipment derating and other variables, you may end up needing close to nominal 5 ton equipment.


  6. #19
    Join Date
    Apr 2002
    NW Indiana
    Originally posted by mrbillpro
    Just call me a dumb Texan but what is a bonus room?
    My friend here in indiana that is a room originally designed for storage in an attic above a 20 degree unheated garage in the winter time now converted to additional living area where as you now field the complaints "the floor feels cold on my bare feet."
    "The Bitterness of poor quality remains long after the sweetness of low price is forgotten"

  7. #20
    The HVAC-Calc program does allow you to add miscellaneous btu to rooms, such as kitchens, etc. I added 1200 BTUH to the kitchen for this reason, and it's included in the above specs. I also added 5 people to the family room and 5 people to the kitchen, also included above. I didn't include people in the other rooms, since during most occasions, there will be no more than 5 people in the house, and the 5 people from the family room will migrate to bedrooms, etc.

    Thanks for the info regarding the 4-ton unit.

    How can I get the specs are for a Heil 13-SEER 4 or 5-ton Quietcomfort, just for grins? I think it's a DX1300, but I can't seem to find any specs online. I think this is where my confusion comes in -- I can only do calcs based on the info I can find. I believe I have the all the correct room size info for HVAC-Calc, but I don't have ANY info on the actual equipment capacities and CFM's etc. to even compare.

    Thanks again!

  8. #21
    Join Date
    Jan 2004
    Ok, just for kicks I checked and, a York CZB048 13 seer with standard A/H with psc blower will meet your sensible load at 75 ID, and 95 OD temp, for tight construction, but not for average.

    Thats why everyone is telling you 3 tons won't work.

    The lower you keep the indoor temp under 80, the less heat the A/C can remove.

    Contractor locator map


    How many times must one fix something before it is fixed?

  9. #22
    Join Date
    Feb 2006
    ARI (Air conditioning and refrigeration institute) has an online database allowing lookups of ARI-rated equipment.

    I haven't used HVAC-Calc yet, and an earlier poster suggested considering internal building heat loads. We have approximately 8-10 kwh daily consumption that puts heat into the house that AC has to remove during the cooling season. That's comprised of refrigeration, lighting, computer and media equipment. Most families are probably higher than us since we have only one TV, have laptop instead of desktop computers and CFL lighting throughout. A more typical house might gain 2,000 Btuh from these loads. As we all get more gee-whiz gadgets, these numbers will rise.

    There are internal latent loads from people, plants, showers, cooking, etc, as well.

    Pay attention to earlier posts about equipment ratings that assume an indoor design temp of 80. Most folks want it cooler, and bringing temps down just a few degrees to 77 or 75 will require substantially more nominal tonnage. The governing calculation is Manual S (equipment Selection)

    Consider oversizing the ductwork to allow for a 5 ton unit but go with a 2 stage condenser and a variable speed evaporator blower. A 2 stage 5 ton system would run at around 3 tons most of the time, kicking into high gear only when needed.

    Teddy Bear is trying to softly sell you a Thermastor whole house dehumidifier, and that, budget allowing and properly installed, will give the best total comfort by providing dehumidification during times of low or no cooling load. I know I want one when we replace our present house.

  10. #23
    Join Date
    May 2002
    Be aware that HVAC-Calc doesn't account for shading of windows by eaves or other overhangs. The reduction in load can be considerable.

    HVAC-Calc is based on Manual J, 7th Edition, and does not handle the newer low SHGC windows that are becoming standard these days due to the IECC. Just specifying low-e will overstate AC load from windows if you have these newer ones.

    I suspect that a PROPERLY DESIGNED, INSTALLED AND CHARGED 3-ton system would keep you comfortable more of the time than will a 5-ton system. And it will save you money up front and over the years.

  11. #24
    Join Date
    Mar 2004
    Yep, equipment capacity is rated at 95 degrees outside and 80 inside. You're close enough to the 95 outdoor design to ignore that, but the inside temperature makes a big difference.

    The equipment capacity has to be derated by the following formula:

    (80-indoor design temp) * nominal system CFM/1000 * 835

    So a four ton system (1600 CFM) with a 75 degree indoor design temperature has to be derated by 5 * 1.6 * 835 = 6680 BTU. When you consider that in addition to the fact that many systems fall well short of the capacity you'd expect from their rated tonnage (four ton systems with 44,000 BTU capacity, etc.) your equipment selection will not and should not literally match the output from HVAC-Calc. That number is just an input into another calculation.

    To cover a 38,300 BTU load (the foamed case) with indoor temp at 75, though, you will need a nominal four ton system with a rated capacity of about 38,300 + 6680 = 44980 BTU. Most any four ton will suit that. But your average infiltration load at 43,600 BTU- once you adjust for the equipment derating- no four ton system will be able to do that, not at 75 degrees. I see no reason that a foamed house would have "average" infiltration, though, so if you do foam it, four tons should be right. Most three ton systems would not even hold 80 degrees against that load. (There is not supposed to be a "safety factor" for cooling, incidentally, at least if you get your numbers right.)

    So to really do equipment selection right, you have to correct equipment capacity to suit your indoor and outdoor design conditions, and you can't look strictly at the nominal tonnage. That will get you close, but final selection needs to be based the actual rated BTU capacity of the exact condensing unit and coils you select. They can vary quite a bit from the number of tons * 12,000 rule.

    Do also check on your window shading and window treatment selections. It may be that you are overestimating your heat gain there, as mentioned above, and possibly to the extent that you could end up with a proper equipment selection in the 3.5 ton range. Square footage per ton in ultra-efficient new construction SHOULD be off the charts based on the old-school rules of thumb. If you talk to a contractor who can't catch up with modern tech, just keep shopping.

    [Edited by wyounger on 08-11-2006 at 02:01 PM]

  12. #25
    Join Date
    May 2002
    Here is an example of the engineering data that most manufactures make available to their chosen few. The Expanded Ratings Tables give you the capacities at common indoor conditions, outdoor conditions and air flow rates.

  13. #26
    Thanks for the info... It's starting to become clearer. But, I'm still a little confused as to where your CFM number (1600) used in your calcs comes from. It seems like it's pulled from somewhere, but I don't know where.

    If I decide to go with the foam insulation, I may also have to have fresh air makeup, which will also effect the numbers. Either way, what I'm wondering is if the final numbers come out to right around 4 tons, would going with a 5-ton unit be a big mistake? I've also seen the note above about a 2-speed condenser unit (which I believe the Heil DX1300 is). Would it be accurate to say that a 5-ton 2-speed conderser fitted with a 5-ton air handler will do the job, as opposed to a straight 4-ton unit, if the 2-speed 5-ton unit can cycle down to a lower speed and provide lower tonnage such as 3-tons, or is there some other problem with that reasoning?

    So much to think about.

    Also, thanks for the links for Lennox numbers. I can't seem to find anything for Heil, Whirlpool, etc., though, at least in the 13 seer range. Any help?

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