PTAC and ductless air conditioning?

Discussion in 'HVAC Heating & Cooling' started by mar3232, Feb 25, 2012.

  1. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    I have a pretty small ranch house, really just 2 "major" rooms that need cooling and I'm faced with either buying a new outdoor heatpump condensing unit (trane) OR something that may be a "smarter" ? choice.

    I'm single, so really only need to cool the parts of the house I'm in at the time -- tv room etc.

    Can someone explain the PTAC and ductless air conditioning systems? I just ran across these on the home depot site (Ramsond) and know NOTHING about them.

    I read about a simple hole through an exterior wall? high efficiency?

    I know most of guys are in the business of installing whole house units so you may knock these but please be objective.

    2000 to repair my heat pump -- but I want to make sure of other options.


    Also -- another question -- I mounted a window AC unit high on a wall (cutout the wall) in my bedroom for occasional use. Of course, I'll drain it correctly and insulate it well.

    Is there any reason why I can't vent the hot air from it into my attic? (There will be a roof vent nearby). thanks.
  2. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    in other words the window unit is NOT going through an exterior wall -- it's an interior wall that I'll build a "chamber" around and hopefully vent into the attic. (which is a non functional attic)
  3. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    looking into it further, certainly don't want PTAC but interested in the ductless mini split.

    good thing is, if I got 2 air handlers, I could turn off the one I really don't need but I wonder also about overall effieciency (electrical)? what do you guys think about these systems?

    would I go ahead and seal up my ducts if I were using one? or put the ductwork in a cycling fan only mode maybe?

    I'd get it for cooling, not heating.
  4. jimbo

    jimbo Plumber

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    Location:
    San Diego
    Your window A/C unit with a "chamber" around it, will not likely work. It most likely will not get the air circulation needed and will be ineffective and suffer early failure.

    PTAC is a fancy word for a thru-the-wall air conditioner unit. Most are sold with heat, but can be purchased as just cooling units. The heat feature can be electric heat or a heat pump with back up electric. They will mostly require a 220 volt circuit. Some units with small btu and very small or no heat can be had on 120 volts. They are easy to install ( no ducts, no tubing) but are not as efficient as a central hvac system.

    The ductless mini spit systems have a more or less conventional condensing unit or heat pump unit outside, and the indoor unit is a wall mounted ( NO DUCT WORK) air handler ( blower unit). You can get an outdoor unit which is multi-zone, meaning it will feed two ( or 3 or 4) separate indoor air handlers.

    The only thing which has to pass thru the wall is the set of copper supply and return refrigerant lines, and a small electric wire.

    Most PTAC brands sell accessory duct kits so the cool/heat can be shared into an adjacent room.

    A single PTAC will be in the $700 to $1000 range. A two-zone ductless, $1500 to $2000 plus install ( requires an HVAC tech with vacuum pump, gauges, etc)
  5. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    I'm not interested in the PTAC.

    At least I could save cooling costs by shutting off my living room air handler when I'm in the TV room at night. Only 2 major rooms in this house, living room with kitchen and a "TV room".

    What have you heard about efficiency vs a wholes house (at least there isn't all that ductwork in the way (I could seal the ducts?)

    Reliability of these systems?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,009
    Location:
    New England
    The mini-splits have been around for a long time, and they just keep getting better (not that they were ever bad). The noisy part is outside, and you have some freedom on where you want the units on the wall. They work just fine when you only have one or more working, and are quite efficient. The key, like with anything HVAC releated, is to get one the proper size. Especially with a/c, you don't want one too big. They work best when running constantly on that very hot day. On/off cycles usually mean it can't dehumidify well, and comfort levels drop. Plus, those extra on/off cycles create more stress and wear on the system.
  7. Dana

    Dana In the trades

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    01609
    "The noisy part" is now whisper quiet with scroll compressors and inverter-drive/continuously variable speed units, which is quite a contrast to reciprocating 1 & 2 speed compressors. At low to mid-speed you can be standing right next to it and barely be aware that it's running unless you happen to be in the air flow or are looking at the fan. It's often quieter than your refrigerator by quite a bit. Listen to this 2-ton LG at low-speed- the 60hz hum is barely above ambient wind noise on the microphone.

    With inverter drive mini-splits in fact you DO want to oversize it (by ~50%, maybe slightly more) if highest efficiency is the goal, since the part-load efficiency can be 2x as efficient as when it's running flat-out. The turn-down ratios of lowest:highest speed are on the order of 1:5 to 1:3, so the duty cycles are long and nearly continuous at moderate loads, even if oversized (within reason.) This is very different from what makes sense for sizing 1-2 speed AC systems. At 50% oversizing a minisplit will spend the vast majority of it's time running at half-speed or less, where it's most efficient. The key to getting the most out of it is to not crank the setpoints up & down often or turn it off, since the lower efficiency of running at max speed on the recovery ramps will usually end up using more power than letting it run with truly phenomenal efficiency at low speed just maintaining temp. See figure 14 in this document. The COP at 87F outdoor temps at min-compressor and min to mid fan speeds blows away the (still quite decent) efficiency at max speed, and the mid-speed COP @ 95F outdoor temps is still more than 50% better than at max-speed. At ~50% oversizing and a "set & forget" control strategy they should outperform their (already outstanding) SEER numbers.

    At more than 50% oversizing the air volume requirements on the interior blower can become an issue, and at 2x oversizing it can be a bit too breezy for comfort at mid-load or higher, even if it's slightly more efficient.
  8. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Wow -- that's quiet.

    So, if it were you and you had a 1500 sq ft ranch house, single story, brick -- well insulated -- what would you do?

    Replace the heat pump outdoor unit (installed about 7 years ago) or put in a ductless unit (which I could do myself).

    I'm single, so like I say, I'm usually just in one section of the house at a time (tv at night etc) and the house really has just 2 major sections with an open doorway between them.

    So, 2 wall units should handle it.

    Would there also be any benefit to put a thermostat in that simply recirculates the air, say 2 or 3 times an hour? Or completely seal the ducts?

    As I said, for winter (what winter? -- none this year) I just setup a new wood stove and I'm surrounded by woodlands. (Bad for summer humidity).

    I just now measured and my 2 main living areas are about 650 sq ft each.
  9. Dana

    Dana In the trades

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    If it's even-money or even 30%-50% more I'd go with the mini-split. The comfort/efficiency/noise factors are all much improved over most ducted systems. Unless the outdoor unit used R410A (aka Puron) you may not be able to use the same refrigerant lines & coil in the air handler with an R410A compressor and likely stuck with an obsolete & lower efficiency solution. If you end up replacing the outdoor unit, independently of refrigerant type, a unit with an inverter-drive variable speed scroll compressor will be much quieter and more efficient (and usually more reliable) than a 1 or 2 speed reciprocating compressor.

    In a 1500' ranch with a relatively open floor plan, if you can leave the doors open you can probably do it all with a single head mini-split in the area with the biggest heating/cooling load (often the south or west side of the house), or at most a 2-head unit. If the open doorway has much cross sectional area to it and you put the head in the higher-load room the temperature differences between those sections won't be very big, and you can always drop a box fan on the floor to speed up the convection between them if desired, getting a bit of the wind-chill benefit at the same time.

    While it's possible to do a DIY mini-split install, getting the highest efficiency out of it requires the right tools & training to fully test & commission the thing. Yes, they often come with the refrigerant pre-charged in the lines, but it isn't always dead-right even out of the factory, and can leak during storage/shipment/installation. They can "work-mostly" at relatively poor efficiency if not set up correctly. With an experienced tech and a bit of DIY pre-work setting up the power, etc, they can be installed & tested in well under 4 hours, sometimes under 2.

    The benefit of using the existing air handler & ducts to recirculate the air between zones isn't all that great. In a ducted AC or heating system the air entering the ducts is well below (for cooling) or above (heating) the setpoint temperature of the room, so it takes a lot less volume of air movement to meet the load. If you're moving merely-comfortable air from the better controlled zone to a different zone it will be many times the volume and many times the air-handler power use to cool or heat the remote zone. A box fan in an open hallway can move more air for less power, but may not be necessary except during the hottest of days. The mini-split will still dry the air in the total volume of the house if you keep the doors open to the common areas, which is half the comfort aspect of air conditioning in an Indiana summer. At 40% RH a temp of even 82F isn't too tough to take, which could be about where you'd be at the other end of the house if the room with the mini-split head is kept at 75F/50% RH. If you take the RH down 10% lower, even 85F starts to feel pretty reasonable.

    On mini-splits it's worth the extra money to get a cooling + heating version, even if you're on the gas-grid and heating costs are low. During the shoulder seasons when the heating loads are low most can deliver average COPs of 4+, and at low blower speeds delivers better comfort than 1-speed ducted hot air furnaces. Most of them can still put out quite a bit of heat even at 0F outdoor temps or a bit below too, (albeit at a COP <2.)
  10. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Thanks very much Dana. I should probably close the ducts then? Fact is it wouldn't be much work to stick some fiberglass in them every Spring (and maybe all year as I just installed the wood stove).

    Maybe I can do "most" of the mechanical work like the holes through the wall, mount the brackets, electrical etc and have a guy just come set it up for me.

    You "must" install the units high on a wall? (That's the only thing that bothers me -- from an eyesore standpoint).

    I may just try one unit at first and create some in wall circulation into the other room -- you like LG? any others?
  11. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Also Dana, if you don't mind me picking your brain again -- what do think (overall) of portable units? I suppose it's possible I may be able to supplement a bit with one to ease the load on the mini split.

    I would imagine they must be more efficient than the old school window units? (and certainly a more pleasing appearance) -- they had some running at Fry electronics last summer and I couldn't believe the cool air those things were throwing out.
  12. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Portable units work fine. You do realize you have a 5" diameter flex hose which must go into a window or through the wall???
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,009
    Location:
    New England
    The reason(s) you want the system high on the wall is for better coverage and temperature gradient control. The cool air is denser, so it falls. You can push it up, but it's better to mix it up with the hot air that naturally buoys up. Nothing worse than a hot heat and cold feet!. By having it high, it's also pulling the hottest air in to cool it so you don't find you stand up and all of a sudden you find yourself hot. Not sure if anyone makes a head that can be ceiling mounted, but that may be an option. If you're going to use it for heating as well, ideally you'd reverse it, have the hot air come in from below. Some of this can be alleviated with the fan, but in the wintertime, moving air makes you feel cold.
  14. Dana

    Dana In the trades

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    Fiberglass doesn't seal, that's why it's used in filters. If you want an insulating plug to stuff in there as an air seal, use rigid foam, and make it a tight fit. If you use the mini-split for both heating & cooling, what would the purpose of the ducts be? Heating with mini-splits is comparable in cost to heating with natural gas in some areas- it depends on your electric & gas rates, and climate, of course. In a truly tight well-insulated house in IN it may be possible to handle 90% of the load or more with a 2- 2.5 ton mini-split. (An easier call in southern IN than up on the MI border.)) Depends on your outside design temp, mostly. What's your ZIP?

    I have no direct experience with LG, but read online of at least one instance of a mis-install where the pre-charge refrigerant an an LG was either less than spec or it leaked during installation, and the installer (a pro, not a DIYer) was not competent enough to realize it at the time of installation OR diagnose later why it's performance was sub-par (later rectified by a better pro.) Fujitsu's compressors and some other fundamental components are used in other vendor's products and are quite reliable. Mitsubishi & Daikin have models with specified heating output at -20C (-4F), or even -25C (-13F) for the Mitsubish H2i series. Those would the main three vendors to look at- the number of local installers with factory certification and proximity to the distributor would be factors to look at beyond the SEER & HSPF specs.v Sanyo has a couple of models with very respectable specs too, but I know less about them.

    The 1-ton Fujitsu 12RLS has been third-party tested with VERY good efficiency results in both heating & cooling modes- better than SEER and HSPF spec comparisons might have implied. If you're looking to just handle one mid to large sized zone that might lead the list, but it's probably not enough to heat & cool your whole house, even if it works for this guy in a super-insulated house in MA.

    I have no experience with the roll-around portable AC units. Seems uglier than a mini-split wall-wort though, and takes up more floor area than a window-shaker unit. ;-)

    They make flush-mount ceiling cassettes and suspended ceiling variants of the interior unit, if you can't find a suitable place on a wall where it doesn't destroy the aethetics. They're more commonly used in commercial applications though, and at the low-output end you might not be able to use them with the smallest of mini-splits:

    [​IMG] [​IMG]
  15. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Now I'm going to sound like a real idiot but I didn't realize they heated as well -- but now you mention it, it is like a heat pump system in theory?

    Anyway -- thought they were just for AC -- so definitely have to get a system setup now. Those ceiling units are great! Good thing also is if I use (2) room units I can put the outside unit within 5 or 6 feet from both.

    I'm going to seal the ducts and go this route -- seems to be for smaller ranch homes like mine, it's the way to go. Thanks for your help.
  16. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Ok, now I see that some have heating mode and others don't although it really doesn't show much difference in price when I compare. However, I'm really more interested in the best AC for the money so should I go with just the AC only units? Supplemental heat would be a real plus but AC efficiency and RELIABILITY are what I'm after.

    Also, maybe (2) single zone units rather than a dual zone? If one goes down, I still have AC and the pricing seems to actually favor (2) single zone units.
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,009
    Location:
    New England
    They're not 'like a heat pump' they are! Likely have backup electric resistance heating, but in many climates, you may not need them. You're more likely to need the resistance (heating strip) backup if you setback...you may find it works better with a small or no setback unless you're boing to be away for a long time.
  18. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    Incredible SEER numbers on that Fujitsu 12RLS -- I'm starting to think of one unit located close to the doorway of my 2 main rooms and do some through wall venting with a fan that cycles on/off.
  19. Dana

    Dana In the trades

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    Location:
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    Mini-split heat ARE very much heat pumps, but they're ductless, heating/cooling the space with the interior head directly. Inverter drive mini-splits (like those in that third-party test document, and most of the high-SEER units out there) have continuously-variable compressor speeds and controls to adjust refrigerant volumes/flows to precisely match the heat flow through the system to the compressor speed, optimizing efficiency. Some have continuously-variable blowers on the interior heads too, others are 2-speed.

    Most mini-splits do not come with resistance heating of any kind- they handle the defrost cycles by reversing the loop. Since most high-HSPF units have a COP of 2.5 or better at +15F, sizing it for the whole house heat load is often possible size them at 100% or even 125% of the 99th percential design heat load for small to mid-sized homes of reasonable tightness & efficiency in locations with design temps over +10F.

    A 1-ton mini-split is going to be over $3.5K, installed, (my mother was quoted over $4K for a Fujitsu 12RLS, went with a 1.5 ton Mitsubishi instead for similar money). A 2-ton 2-3-head multi- (eg: Mitsubishi MXZ3B24NA, or Daikin 3MXS24JVJU, both with SEER>16, HSPF>9) is likely be cheaper that a pair of pretty-good 1-ton units.

    I wouldn't worry about the reliability aspects- the number of moving parts is lower than old-school heat pumps and the reliablity record of Fujitsu/Daikin/Mitsubishi are all pretty good. Installation by a competently trained techs with experience and backup from local distributors is key. HVAC outfits that only install mini-splits as the 5th line on their their line-card at fewer than 10 units a year aren't in the same class as those who install 25-50/month with an installed base of hundreds still under warranty. It's worth figuring out who is installing lots of them- calling the local distributor is and asking them to recommend a contractor isn't a bad approach- they know more than anyone who is taking them by the dozen, and who keeps calling the tech line with stupid-attack type issues.
  20. JB40

    JB40 New Member

    Messages:
    2
    Location:
    Albany
    You shouldn't count out the PTAC unit just yet. There's a reason these are found in most hotels across the country - simplicity. No special refrigeration equipment which means no $125 per hour service calls. Hotel staff doesn't need to rely on 24 hour maintenance contracts, they simply keep 5 on hand at all times. The trades required to install a PTAC unit are a carpenter and electrician. You can expect to get 10-15 years out of them, and when they die you simply buy a new chassis and slide it into the existing wall sleeve. Sound levels contiune to decrease and efficiencies continue to increase. http://www.h-mac.com/brands/cooling-brands/amana.html
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