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  1. #14
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    5,658
    You're talking about putting in 5.5 tons for 1100sqft! That's waaaaay overkill. Have you priced this out yet? IF you can find a contractor that would install that then its going to be astronomical in $$$. If its oversized that much inverter units are the only option or they will never run long enough for it to drop rh and be comfortable. With 5.5 tons of inverter capacity they would all run at the lowest speed all the time and still short cycle. 18k for the big room and a small ducted minisplit (12k) for the bedrooms/baths should be more than enough for 110* outdoor temp.

  2. #15
    Regarding redundancy... I just like that. Some of the overkill has to do with the 3/4 ton units in the 900 ft3 rooms and the other is due to redundancies.

    Yes, I've priced the systems. Since I'm going with individual ductless systems per room the price is "fairly" consistent regardless of system size... 12K BTU units don't cost much more than 9K BTU units and 2x 12K BTU units don't cost that much more than a single 24K BTU unit... and I like the redundancy... if one breaks then I have the other... plus, if only one unit needs to run then only one will power on. This deals with effiiency and humidity issues as well as one 24K variable speed unit. The cost differentials are far greater between SEER ratings. In fact, I believe I can buy two 12K standard" units for the same or less cost than one 24K variable-speed unit.

    I do realize I'm spending more than I must... but please keep in mind I have health issues requiring a very cool environment so I can function at least partially. Also, I'm installing a full PV system so, other than PV system size (initial cost), power usage is of secondary concern. I just need to keep functioning the best I can for as long as I can.

    BTW, I'm having a second "dumb" power meter installed BEFORE "Smart Meters" are mandated in this area. I'm also installing a manual switch to cut me off of the grid completely if I so choose. I'll be darned if I allow anyone to mandate how much of "my own" power I can use. I'll run off of propane generator in the evenings if I must.

  3. #16
    Regarding BTU size for this 1100 ft2 building...

    If the large room downstairs needs "at least" 1.5 ton (for normal cycling at 105 degrees with 40 percent humidity) then the two smaller downstairs rooms (combined) require another 1/2 ton. That's 2 tons downstairs. Is it silly to assume I'll need a total of 3 tons upstairs given the design of this structure? That's a total of 5 tons.

    I really don't know... I'm guessing. But this is direct observation too. Maybe once the upstairs is cooled I'll only need 1 ton downstairs... I don't know. Even so that's still a 4 ton requirement for this structure. And, if I decide to not cool the upstairs because I can't get up there, I'll still need the larger units downstairs.

    Please keep in mind also that I can leave doors open between rooms and only utilize systems as I need them.

    I've done some analysis regarding system cost and have found individual one-zone systems to be as cost-effective as multi-zone systems and the latter doesn't have the redundancies I prefer plus overall system tonnage is reduced... may be a good thing if I goof elsewhere and in efficient usage, e.g. leaving doors open between rooms to share unit usage between rooms (leaving one system off).

  4. #17
    Join Date
    Jun 2013
    Location
    orange county, ny
    Posts
    135
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 07-13-2013 at 06:57 PM. Reason: non AOP Pro Member

  5. #18
    Okay, I'm convinced to have a heat load calculation done. Or maybe I'll just buy a giant deep-freeze and move into that.

  6. #19
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    24,940
    jp125

    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.

  7. #20
    One thing I failed to mention is the windows. Most are small double-pane and fairly well insulated. All are completely shaded. However, there are two very large north-facing panoramic windows, one on each floor, that are approximately 3x10 feet in size. These are thick tinted glass but single pane.

    I found on-line calculators and it appears that 24K BTU is probably about right for the large first floor room if I have three visitors... occupancy of four is the maximum and is not common but does happen.

    The calculator recommended 20K BTU and this didn't include any cooking nor lighting.

  8. #21
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by dunno_HVAC View Post
    I'm intentionally buying over-sized systems because I expect them to lose efficiency and cooling ability over the next ten years... and I need it very cool so I can function well enough. I might have a heat load calculation done but, frankly, I don't trust many technicians (of any trade) in this area. Finding a knowledgeable/honest one (of any trade) can be a crap-shoot.
    I strongly urge you to reassess your assumptions regarding oversizing. You may end up doing a worse job than the technicians you don't trust.

    With mini-split systems there isn't a lot of choice for the two smallest rooms except either a complete system for each or add a vent between the two and use an exhaust fan in the bathroom to pull air in from the adjacent room. A 9K BTU system is plenty for that but a separate system (might??) better mitigate moisture issues without worrying about a balancing act between the two rooms. Perhaps the best solution is to vent adjacent room A/C into the full bathroom pulling in cool air with a high-volume exhaust fan then add a small dehumidifier?? Maybe separate systems AND a dehumidifier?? Maybe I'm making this too complicated??
    If you had a larger house I'd suggest multi-split VRF might suit you better. But at 1,100 square feet, with what I hope is a reflective metal roof, you would be making a grave error oversizing anything. If you like humidity control, don't oversize. I don't care if every system in your house has inverter driven compressors and infinitely variable capacity. DO NOT OVERSIZE.

    I like the idea of using two smaller units in the largest downstairs room because it's more efficient to operate only one if that's all that's needed. It also allows for a spare unit should one fail. I currently have a new 12K BTU window unit in that largest room and it's been keeping it "almost" cool enough with outside temps at 95 degrees with 90 percent humidity so I think I'll need 24K BTU at 105 degrees with 90+ percent humidity. That's 2x 12K BTU units. This is strictly "observation"... not proper testing.
    What you "like" and what may actually unfold in practice may be at loggerheads. Of course you can do what you "like", that does not mean it will be a sound design, nor guarantee you'll be comfortable.

    The two areas upstairs will require slightly larger units per ft3 because, even though it has a metal roof and is fairly well insulated, it has no attic space... it's a barn-style roof. This stated, I'll be adding a PV system that (I hope) the rafters can support which will, in effect, completely shade the entire sunny side of the roof.
    HVAC is typically not sized by cubic feet. It is sized by the expected heat gain (for cooling) to the structure on the normalized most extreme heat a region will see in a given year.

    Bottom line is, I think I'm pretty close regarding sizing... BUT I've been wrong about many things so, if I'm being cheap and stupid, please let me know. It won't take much convincing. I'm hard-headed but I do listen.
    From what I've read you're not being cheap. How amenable to reason remains to be seen....

    BTW, I use A/C even in the dead of winter here because it's common to reach 80 degrees during the day even in January so temps inside can reach 90 degrees.
    If you're seeing 90 degree temps inside your house on an eighty degree day in the dead of winter, there is something seriously wrong with your building envelope design.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #22
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by motoguy128 View Post
    I don't think it ever gets 95F and 90% humidity anywhere in the world, 105F and 90% RH defintiely doesn't exist without lots of steam leaking from an industrial process.. except inside industrial facilities... and that would be considered cool is some locations where I work. It's probably more like 50-60%RH and that's still very humid.
    95F @ 90%RH = 92F dew point. 105F @90%RH = 101F dew point. Meteorology is one of my hobby interests of study, and from what I know of it (and from HVAC psychrometrics) the warmer a parcel of air is, the more expanded its volume, and the lower its RH will be. So if you could warm the 95F @ 90% parcel of air at 92F dew point to 105F at the same dew point (meaning no change in absolute humidity content), the RH would drop to 67%.

    Another aspect on this from meteorology: the more moisture content a parcel of air has, the more difficult it is to raise its dry bulb temperature. This is due to evaporation of moisture within the parcel, which tends to cool the dry air around it. If a fixed amount of solar energy is falling on our parcel of air, only so much is available for warming the air, and the remainder goes to evaporation. All this said without going into mixing of air, buoyancy and mass of air parcels, etc. After all, this is an HVAC forum.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #23
    Regarding seeing 90 degree temps on 80 degree days... yes... something is very wrong. I'm currently in a cruddy old trailer with lousy insulation and metal siding and roof. It's in full sun as are all the windows. Of course, the new structure is far better in every respect. This could be skewing my perspective.

    I'm now convinced to seek professional advice. I'll ask a minimum of three seasoned experts who've listened to customer kudos and complaints for many years.

  11. #24
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by dunno_HVAC View Post
    Regarding seeing 90 degree temps on 80 degree days... yes... something is very wrong. I'm currently in a cruddy old trailer with lousy insulation and metal siding and roof. It's in full sun as are all the windows. Of course, the new structure is far better in every respect. This could be skewing my perspective.

    I'm now convinced to seek professional advice. I'll ask a minimum of three seasoned experts who've listened to customer kudos and complaints for many years.
    I suspected that your present domicile may be driving your design decisions for your new dwelling. Big mistake, as you have bravely admitted here. It's a hard bias to overcome, granted. Even professionals who are used to working with typical conventional home construction get thrown for a loop when they encounter a house design where thermal envelope performance is highly emphasized.

    The basis for sound design is driving some design parameter stakes into the ground. Pick a target indoor temperature and humidity level you want in the house. From there you analyze the building envelope, and how you expect it to perform in extreme weather. Between that and the normal expected type and level of activity inside the building, that informs the equipment sizing and design.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #25
    Goofy old south Texas... I called Sears to ask for an evaluation and they don't even offer mini-split systems for my area so no evaluation from them. Also, I've been searching on-line for calculators and have only found a handful of "somewhat" useful tools that aren't "completely confusing" for a stupid novice like me. So, other than seeking advice from smaller companies, I'm again on my own.

    The above stated: The calculators I've found usually suggest a minimum 18K BTU or maximum 24K BTU unit for the one largest downstairs room which is 15x24x9. This is inline with my "guesswork".

    The more I research this the more I become frustrated. I'm thinking my (guesses from direct real-world observation) are about as close as I'm going to get.

  13. #26
    BTW, my old trailer is running on one 25K BTU unit right now at 11:00 AM but it'll require both 25K BTU units by 2:00 PM and this is with me sitting in my underwear while doing nothing physical. I'm also doing fairly well medically at the moment... and the small office area I'm in will require an additional 4K BTU so that's 4.5 ton total and it's not very humid and not at peak summer temp outside. So... this old trailer will require a minimum of 6K BTU in peak summer heat so I can function "reasonably" well.

    I'm moving out of the trailer and into the new cabin though. And this is where I need the calculations...

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