Mini split sizing

Discussion in 'HVAC Heating & Cooling' started by dorlando, Jul 15, 2020.

  1. Dana

    Dana In the trades

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    BTW: I ran a quick LoadCalc on the 250' room in Orlando assuming

    -- R13 in the walls, under the floor, & above the ceiling with 10' ceilings

    -- 140 square feet of framed R13 north wall, 140 square feet of south wall, 125 square feet of west wall (which has all the window area subtracted out)

    -- 55 square feet of west facing single-pane with no blinds, no exterior shade/overhang

    -- 10 cfm of ventilation/air leakage plus two people and it still came in at 8125 BTU/hr. With zero people it's 7275 BTU/hr

    You're totally safe with a 9K head there.

    If that room is leakier than that or has lower R-values than indicated, from a comfort point of view it's worth fixing those deficiencies rather than just throwing a 12K head at it. An R13 ceiling under a flat roof radiates a lot of heat at the occupants even if the air temp is reasonable.

    How deep are the rafters in the flat roof?
     
  2. dorlando

    dorlando Member

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    I believe they are 2x8’s. The room does have two five foot wide openings into the home. I would like to make sure that room air blends in well with the house air. In other words I don’t want the house unit to have to help cool that room because that will make the house unit run more
     
  3. wwhitney

    wwhitney Well-Known Member

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    Assuming that your main house A/C has enough capacity, the way to solve that problem will be some combination of: (a) better routing of the duct to avoid kinks (b) switching to rigid duct (c) larger diameter duct (d) more insulation on the duct (e) better air sealing of the duct or (f) a booster fan for the long supply run.
    What's wrong with that? If your house unit is fairly efficient, running it more is the least capital intensive and most efficient solution, until you get to the point that the house unit can no longer keep up with the load. But if your house unit is never running 100% of the time, you're not there yet.

    Cheers, Wayne
     
  4. Dana

    Dana In the trades

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    A 2x8 is nominally 7.25" deep, which would yield about R27 if filled with cellulose, about R30 if dense packed (1.8lbs per cubic foot minmum) with fiberglass. If what's in there are some tired R11s or R13s it's definitely "worth it", to do an overblow with cellulose, completely filling the cavity. That will cut the peak cooling load to under 7K using the conservative LoadCalc tool, but more importantly, it'll cut the peak ceiling temp.

    Got a ZIP code? (For more accurate weather data, design temps, etc.)

    The amount of "extra" load on the central air conditioner represented by that room even if it only had a half-ton head is really "in the noise". The extra load would be effectively zero most of the time even if the head were undersized- the 1% design load is only exceeded 1% of the time, and the average cooling load even on hot days would be within the capacity of a half-ton head.

    Does the central air keep up with the cooling load, or is it marginal? Are any of the ducts or air handler in an attic, above the insulation layer?
     
  5. dorlando

    dorlando Member

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    If we leave the doors Open to the bonus room on a really hot day the central unit runs a lot more and yet doesn’t effectively cool that bonus room. That room gets very hot during the day and so some air from the house is going into the room but since no duct is out there it can’t effectively cool it. I don’t think an option is to let the central unit call the room. That room really needs a mini split that would work in tandem with the central unit.

    I’ve spent a lot of time trying to figure out that long flex run. I’ve added additional insulation around it. It could be that there is a restriction at the end of the run with the size of the vent. The airflow is very strong so I’m not sure a booster fan would work but I have thought about it. The air is getting through but it’s just picking up a lot of heat
     
  6. Dana

    Dana In the trades

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    If the flex duct is installed fully stretched out tight it's impact on flow is pretty low. The restriction at the end will give a nice high velocity for good mixing, if tapered reducers are used prior to the register boot.
     
  7. wwhitney

    wwhitney Well-Known Member

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    "A lot more" doesn't sound like non-stop. And if you're not directing cool air in there, it makes sense it doesn't get cooled sufficiently. As a test, you could try putting a box fan in the doorway to the bonus room and see if the extra air mixing is enough to get the bonus room cooler and run the main house unit more. If that works, is there any way to get a duct to the bonus room?

    Cheers, Wayne
     
  8. dorlando

    dorlando Member

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    The flex has 25 feet of 8” and then reduced to 7” for another 22 feet.

    Getting ducts in bonus room is probably not possible due to it having a flat roof, accessibility and other factors
     
  9. dorlando

    dorlando Member

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    One more wrench. Let’s assume I only get a mini split for the bigger room and go a different direction in bedroom. My utility company only gives rebates starting at 1 ton units. Since a pioneer 1 ton ramps down to 4k btu and at peak times I will probably need lets say 8k, maybe a little more if we have people over, would it hurt to put a 12k in there? That way I can capitalize on the rebate.
     
  10. Dana

    Dana In the trades

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    If the Pioneer 1-ton multi-split compatible head can modulate lower than the full min-outout 4K it's probably fine to put 1-ton heads in each zone. That may require a 2-ton compressor. If not, it's probably not great for the 150' bedroom zone, but fine for the 250' zone.

    It's often easier and cheaper to go with two single zone minisplits, which usually simplifies refrigerant line routing when heads aren't in adajacent rooms.

    The single zone 1-ton Pioneer WYS012GMFI19RL ramps to 3700 BTU/hr, which is lower than their 3/4 tonner in that series(!).

    The Gree GWH12YD-D3DNA1A/O + GWH12QD-D3DN***/ (which I believe is also a re-labeled Midea- could be rong, offen am ) ramps down to 2900 BTU/hr/.

    The Senville SENA/12HF/OZ + SENA/12HF/IZ ( also made by Midea IIRC) drops back to a bit under 2800 BTU/hr.

    That's still a lot more than the ~1 KBTU/hr minimum output most LG models seem to manage, but not too bad.

    BTW: If the single-pane windows are still in pretty good shape, adding low-E storm windows will cut the solar heat gain coefficient (SHGC) by a quarter or so at a fraction of the cost of a code-min replacement window, and would out perform an IRC 2018 code-min U0.40 window with very little drop in visible light transmission (much less light loss than window film solutions.) See Tables 1 & 2. Tight storm windows also reduce the noise transmission, and protect the original sashes & sills from the elements, and eases the cascade of cold air draft coming down the window on those somewhat rare sub 40F days in FL. The Larsen low-E storms distributed through box store chains aren't bad, but there are others. (I'm not sure if box stores in FL would carry them as a standard product but they can probably be ordered through the box stores.) Without the low-E glazing option the solar gain is only cut 10-15%, but with low-E glass it's usually on the order of 25%, sometimes more. That is not as dramatic as the highest heat blocking window films, but definitely a measurable performance upgrade with very little loss of natural light.
     
  11. dorlando

    dorlando Member

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    So I’ve been thinking about this and wonder if I need to go a different direction. Since that room has a vent the room only needs a boost of cold air for a few hours a day. I think a mini split would be overkill and I’m not too thrilled about a window unit.

    how about a portable AC unit? I have heard mixed things about these. Is there one that someone would recommend?
     
  12. WorthFlorida

    WorthFlorida The wife is still training me.

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    It sounds that your home was built in the 1970 or 80' well before miniumum SEER ratings were mandatory. Several things you are fighting.

    1) With nearly 46 feet of 8" and 7" flex duct, for todays higher SEER units (14-16) a new install they would be at least 10" flex ducts. Today new units use more volume of air rather than velocity to get the efficiency out of it. For those who suggest ridge duct, they virility do not exist in Florida homes. You might find ridge ducts in homes built in the 1950 or 60's. For Florida flex ducts work good if installed correctly. Very large homes will usually have two AC units and that in itself keeps the length of duct lengths reasonable.

    2) You did not mention if this bonus room is first or second floor and what is the wall construction. If its cement block & stucco, at best you may have 1.5" of insulation in the walls, however, most drywall is hung with 1x2 furring stripes at 3/4" thickness for insulation. CBS walls are also is a huge heat sink. They soak up a lot of heat especially if the home is a dark color exterior paint, and after the sun goes down if the walls are warmer than the ambient air, the heat will dissipate both outward and inward. Load evaluations are good but the right type of wall needs to be factored.

    If it's a second floor, most likely it is wood frame with 2x4's. As DANA suggest, what color is the flat roof? If it is dark, it soaks up a lot of heat and radiates it downward, a light gray color would be less.

    Right now my son's house second floor here in Orlando, the HVAC guys are increasing the size of the duct from the air handler to the warmest and furthest bedroom (attic access). The AC tonnage is OK, just short of air volume. For his home office on the first floor, same issue, furtherest from the air handler and it's under this warm bedroom, the HAVC company is adding a mini split unit since it would mean ripping out the ceilings for larger ducts and the floor joist would limit the size. FYI, both rooms get a lot of heat from the sun, the walls are east and south. CBS first floor and wood frame second floor. This a home built in 2006.

    Don't waster your money with portable units. You need to have a window open and framed in with the unit exhaust duct. It dumps the heat outside, similar to a window unit. Do go with a split unit. If you main unit ever ever quits on you one day, you'll have a room to escape from the heat. Like right now it's been brutal in Central Florida and the HVAC guys are busy. A mini split will also allow the bonus room to be used with the doors closed, such as for guest staying over, etc.
     
    Last edited: Aug 3, 2020
  13. Dana

    Dana In the trades

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    What WorthFlorida said regarding portable AC units. Double or triple that so for the single-pipe portable systems. With a single hose portable AC 100% of the condenser coil's air flow is drawn from indoors, which increases the LOAD on the building by driving a massive infiltration load whenever it's running. For that reason the parasitic infiltration load increases with outdoor temperature and humidity, making it's rated capacity something an absolute lie- it's adding load to the rest of the house while cooling the room where it's located.

    A half ton hotel-style PTAC or PTHP (heat pump version) would cover your intermittent load. Many are available with wired or wireless wall-remotes that behave like thermostats. In a simple 2x4 framed wall the installed cost is comparable to a low-end mini-split, but involves cutting and framing in a hole for the mounting sleeve. An inexpensive pre-charged DIY type mini-split can sometimes be easier/cheaper, especially if with masonry or stucco siding, since it only takes a 3-4" diameter hole, with no framing or siding trim details to attend to.
     
  14. dorlando

    dorlando Member

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    Thanks for the input. I will stay away from portable units. For my 250 sq ft bonus room it is solid block under the windows and they face the pool so a hotel style unit wont work

    I will go with a mini split. Just have to decide on 9k or 12k btu. Most bids have been for 12k. I don’t think they are jacking up quote as it is only $100 more than 9k. I want to be sure i Dont oversize. The 12k modulates down to 4k.
     
  15. Dana

    Dana In the trades

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    Whether the capacity is rated at 9K or 12K, the one with the lowest minimum modulation is the better choice. A 4K minimum isn't terrible for a space that size.
     
  16. dorlando

    dorlando Member

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    When an inverter unit starts it ramps up to near Max capacity and then it lowers itself until it reaches the setpoint and then shuts off? That’s how it was explained to be by the manufacturer is that correct?

    so if I got the 12,000 it would ramp up to maybe 11,000 until it is close to the temperature set and then lower itself
     
    Last edited: Aug 5, 2020
  17. WorthFlorida

    WorthFlorida The wife is still training me.

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    dorlando? I'm assuming you are in an Orlando zip code such as I am. OUC has a rebate for window film. I had mine done by a contractor five years ago but you can still get a rebate for DIY job. DUke Energy doesn't seem to have one for window film.

    Another small improvement you can do is the window screens. I was at HD looking to redo a window screen when I saw a solar screen. It is a very dense screen that blocks more light from passing through. From the outside it is darker than a standard screen but from the inside you barely can notice it. After I replaced the four screens on the second floor bedrooms, all facing south. I took the inside glass temperatures. The top half with window film and no screens compared to the lower half with the solar screen and window film was a 5 degree difference.
    https://www.homedepot.com/p/Saint-G...coal-Solar-Window-Screen-FCS10391-M/203063628

    https://www.ouc.com/residential/save-energy-water-money/residential-rebates-information
    Window Film or Solar Screen ($.55/sq. ft.)
    Installing solar window film on pre-existing homes can help reflect the heat during hot summer days. Your cooling unit will become more efficient. Incentive Requirements: 1. Only east-, west- and south-facing windows qualify. 2. Shading Coefficient (SC) of film or screen must be 0.5 or less and noted on the invoice. 3. ENERGY STAR qualified double pane windows do not qualify for this rebate. For work performed by the customer: Provide a copy of the manufacturer’s specification to prove 0.5 SC or less and provide the dimensions of each window where the film was installed for square footage calculations.
     
  18. dorlando

    dorlando Member

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    Yes. Orlando. I have put up some solar shades on those windows and they make a big difference. That is part of the reason why am wondering if I need to put a 12,000 be to you in there or 9k
     
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