PTAC v. Through the Wall AC + Radiator?

Discussion in 'HVAC Heating & Cooling' started by suziqed, Jun 4, 2013.

  1. suziqed

    suziqed New Member

    Messages:
    2
    Location:
    New York, NY
    Hello,

    I will be buying my first apartment in NYC this summer and want to reconsider the HVAC. Currently there are through the wall AC units and radiators in the living room and bedroom Bedroom.jpeg . They are enclosed in a box that vents into the room -- heat on top and AC through the side -- which seem terribly inefficient for the AC (it must be creating a very cold space inside the box, but little cooled air makes it into the room). Here's a pic of another unit in the building without the box Unenclosed HVAC.jpg . The AC units are very old and I assume inefficient. I was at the new place last week and while I was there a fuse blew because both AC units were on, which isn't great, but there is little I can do at this point other than install something more efficient.

    I am wondering about whether to consider installing a PTAC unit to replace either the AC and/or the AC and radiators? I know they are inferior to mini-splits, but having the half unit outside is impossible in the building (completely not allowed). I have seen Amana mini-PTACs advertised that heat and cool, but I'm not sure that the heat function is necessary because the radiators are essentially fine (and much, much cheaper while operating). Still, I'm not sure that the simple through the wall AC will be as powerful as a PTAC. The room sizes are about 250 sq. feet (bedroom) and 350 sq. feet (living + kitchen).

    I would love to install a full-sized PTAC but don't have the proper sized hole in the wall. I am concerned about enlarging the existing hole because a) I'm probably not allowed and b) the apartment is on a high floor and we won't be able to access the wall from the outside.

    I would really appreciate any advice or feedback. Should we just replace the through the wall AC units with new ones, with mini-PTACs and rip out the radiators too, and/or plan to remove the covered boxes around the HVAC?


    Thanks!
  2. Dana

    Dana In the trades

    Messages:
    2,804
    Location:
    01609
    The upcharge for a PTHP (Heat Pump) from an PTAC (Air Conditioning only) isn't huge money, and well worth it. Look for units that have variable speed scroll compressors & blowers and they will hit mini-split type comfort and quiet sound levels, if not quite identical in efficiency.

    Whether the radiators are actually cheaper to operate than a PTHP in heating mode depends on the fuel, boiler efficiency, and distribution losses. In many places (even higher-priced electricity markets) a mini-split is cheaper to heat with than with an oversized 80% AFUE boiler on a system with significant distribution losses (say, the boiler is in the basement, serving a second floor apartment, with uninsulated or barely insulated heating pipes.) During the shoulder seasons when the temps are in the mid-40s or higher the efficiency of variable speed heat pumps SOAR, and will almost always be the cheaper to run than covering those loads with a fossil-burner.

    All good heating system designs start with a careful heat load calculation (ACCA Manual-J method, or similar). Apartments typically have very small heat loads compared to the output of ANY steam or pumped hot water boiler, and it just might be cheaper to heat with PTHPs, but we really can't say until the calculations are run. The heat loads of an apartments other than the top & bottom floors are a function of the exterior wall construction and window types, and the square footage of both the wall & windows, not at all about the size of the apartment, only the exterior surfaces, their U-factors, and the anticipated temperature difference. NYC's 99% outside design temp is +15F, so if you assumed a 70F interior temp that's a 55F temperature difference. You could design for a colder temp/bigger difference, but that won't make you any more comfortable, and leads to lower efficiency if oversized by more than ~1.3-1.5 or so. (Most cast iron hydronic boilers would be 5x oversized for a typicial apartment's heat load, and running well below the nameplate AFUE efficiency.)

    Cooling loads are a bit more complicated, with solar gains as well as parasitic gains from internal sources. If it's a top floor apartment the amount of ceiling area and the insulation in the attic space or roof matter too.

    Measure it up describe the wall & window construction a bit, and we can probably ballpark it pretty reasonably. I'm guessing that any PTHP with sufficient cooling capacity will be oversized for the heat load, which would make a modulating version even more desirable.
  3. suziqed

    suziqed New Member

    Messages:
    2
    Location:
    New York, NY
    This is really helpful, thanks Dana. It sounds like a PTAC with a heat pump is the way to go. Will it be more efficient with the PTAC over the window AC? I hope the fuses will be less likely to blow.

    Here is a floorplan that shows where the windows are located. The apartment faces north. Ceilings aren't very high, I think probably about 9 feet. The apartment is on the 10th floor in a 14-story building. I'm not too concerned about the heat -- every apartment I've ever lived in in NYC has been too hot in the winter with radiator heat and we end up leaving the windows cracked.

    Let me know if you have any more questions. Thank you!

    Floorplan.png
  4. Dana

    Dana In the trades

    Messages:
    2,804
    Location:
    01609
    How is the radiator heat supplied & paid for? Is it built into the co-op/condo fees and split pro-rata on apartment size or something?

    Assuming that's a window in the bath area, it looks like you ave about 50' of exterior wall x 9' of height for about 450' of gross exposed area, about a third of which is window. Are the windows all double-pane glass? Is this a 1' thick brick wall, or something else?

    If you get a PTAC or PTHP with a modulating variable speed scroll compressor and variable speed blower the start up surges are a fraction of that of a rotary/reciprocating compressor, and you'll be less likely to blow fuses, even if the full-rated amperage is the same.

    North facing glass has much lower solar gain than east or west (or south) facing glass, so your cooling loads are probably going to be less than half of what it would be if the apartment faced west without taller buildings to shade the windows. You probably don't need more than ~6000 BTU/hr of rated cooling out of either one of those PTACs to handle the peak cooling load, but if you let the place heat up all day then crank it on when you come home at 6pm to an 85-90F apartment the catch-up time could be pretty long with units that small. The efficiency & comfort joys of a continuously modulating version is that they can be left on all day, bumping the setpoint up maybe 3-4F when you're gone and they'll sip power quite efficiently while you're gone, and even if you come home to an 80-82F apartment the air will be DRY 40-50% relative humidity, (which is pretty comfortable compared to 85F & 60%RH +) and won't any more power than a 2-3x oversized unit that would be necessary for cool-down times if you were turning it off all day to save power.

    If you can find the ratings plates on the existing PTACs, what are their BTU and current (Amperage often abbreviated "A") ratings?

    There are probably dozens of places to get them in NYC, but I have no direct experience with modulating PTAC/PTHP units, and can't really recommend one over another, and it seems to be hard to find the more detailed specs for them online.
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