Need help on which air conditioning system to install

Discussion in 'HVAC Heating & Cooling' started by Canaligator, Mar 31, 2014.

  1. Canaligator

    Canaligator New Member

    Messages:
    2
    Location:
    Ma
    I am looking to replace two 25 year old air conditioning systems. I have received two very comparable bids using different equipment. One bid is using an American Standard Model 4A7A3030G1000A 13.00 Seer 2.5 ton condenser, with an American Standard Model TAM4A0A30S21SC Air Handler, and a BAYWAAA05SC1AA Hydronic Coil. The other bid is using a Trane Model 4TTB3030G1000A XB-13 13.00 Seer 2.5 ton Condenser, with an ADP Model BCRMB6636S3N3 3 Ton 13.00 Seer TXV Nonbleed 120V3 Row 3 Speed Air Handler w/66K Coil. I also have a 3 ton unit to replace using similar equipment. Both contractors also priced upgrading to 16 Seer units. The American Standard contractor's price to upgrade to 16 Seer units was actually a little less than the rebate I could receive from the utility company, so that's a no brainer. The Trane contractor's price to upgrade to 16 Seer units was about twice as much as the rebate I would receive, so that's a tougher decision. I am leaning towards the American Standard units. I did a little research and found the Trane XB-13 was their low end unit, but still good. I am not at all familiar with the American Standard units. Another issue I have is that I need to have a 110 volt air handler, which is why the Trane contractor went with an ADP air handler. Is the American Standard Air Handler listed above compatible with 110 volts? Also, both quotes involved the installation of a media air filter. Which is better, Aprilaire, or American Standard Accuaire. Any input or help would be greatly appreciated.
    Thanks.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,343
    Location:
    New England
    Unless the house is huge, those are likely to be WAY oversized. If you're just replacing size-for-size of existing, they're almost certainly oversized. You really should get a proper heat load analysis done - you'll be more comfortable, and save money on both the equipment and the on-going energy costs. An oversized a/c unit can pull the house down fast, but once there, it won't run frequently enough to dehumidify the house....nothing worse than a cold, clammy interior. You'll generally be more comfortable even if the a/c can't keep up on the hottest day of the year because the house will be MUCH drier.
  3. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    Oversizing AC is mostly a comfort issue (noise & latent cooling/dehumidification), but any time a system is being replaced it's worth doing a more formal load calculation to get it right. Insulated houses with no/low shading factors that don't have a gazillion square feet of west facing windows typically come in at around 1-ton of cooling per 1000 feet of conditioned space but it could easily even more feet/ton, or slightly less depending on the particulars. Other things like running ducts in an unconditioned attic above the insulation layer (rare, but not unheard of in New England) can add a ton or so to the total.

    If you think you're up for it you can do pretty well using this online calculator, which is based on ACCA Manual-J methods. The 1% dry bulb design temps in MA are in the mid to high 80s except at higher elevation locations where they're in the low 80s. If in doubt, use +87F.

    Only with an accurate load number can you make reasonable equipment choices. For instance, a 3-speed is GREAT for comfort if the load at the 1% outside design temp is in the output range of the highest speed, or even on the edge of the mid & high speed. But it's kinda worthless if even at lowest speed the thing is oversized for the average (rather than the 1%) load, since a 1- speed system that is right-sized to the load will operate at about the same comfort level.

    Another thing to consider if you're currently heating with oil or propane, it may be worth the signficant upcharge for a fully variable speed heat pump that can also provide high efficiency heating at least down to +15F or so outdoor temps (eg Carrier Greenspeed or Lennox XP25 with a CBX series variable speed air handler.). The cost per delivered-BTU of a better-class variable speed heat pump is about half the cost of heating with propane or oil at the recent 5- year average prices for fuel & electricity in MA. Even if it's too small to handle the whole load at your 99% outside design temp, you can operate the pre-existing heating system as the "Hail Mary" backup for when the heat pump won't keep up. Most of the better ones will still be delivering cheaper heat at 0F (but not necessarily enough heat) than 85% efficiency propane or oil. If you're heating with natural gas the heat pump approach is more expensive except during the shoulder seasons when it's above 40F and the heat pump efficiency really soars. A 2-ton Greenspeed can deliver about 25,000BTU/hr @ +15F, which is about 80-85% of the heat load of my ~2400' circa 1923 antique at that temp. (But at +5F it's only mustering 20,000BTU/hr or so, and would need some supplemental auxilliary heat.) It would be slightly oversized for my cooling load (I get some afternoon shade), but being variable speed with a good turn down ratio would still operate in an efficient & comfortable mode.
  4. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    American Standard makes good equipment that should be trouble free for many years. 2.5 ton is probably too much but, the big question is the ductwork itself. The ductwork should be fairly closely matched to the CFM of the air handler. Too small and the coil ices up. Too big and you don't get proper air flow and delivery and the unit cycles more often that it should. Assume though that your system has been operating just fine for a few years so chances are pretty good that a 1:1 replacement will perform just fine.
  5. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    Just because an oversized piece of equipment managed to cool the place doesn't mean replacing it with a similarly oversized unit is the right thing to do. Doing it right means assessing the whole picture, starting with a load calculation.
  6. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    I don't necessarily want to put smaller equipment into much larger ductwork and I doubt he is going to want to re-do the ductwork
  7. hvacsystemtalk

    hvacsystemtalk New Member

    Messages:
    1
    Location:
    Thai
    If duct is too big. You will have trouble about flow. Good luck
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,343
    Location:
    New England
    Personally, for an air handler, I prefer a variable speed fan. The Trane unit I have has 16-speeds, and it almost never runs at top speed - it works fine at those lower speeds, and when maintaining temp on a mild day, does a vast improvement in removing moisture from the air. It's smart enough to adjust the air speed to prevent icing, too. Plus, when starting up it slowly ramps up to speed, making almost no sound. It only gets to max speed when it can't cool enough at the low speed, but it still ramps down at the end after the compressor stops, so it can extract all of the cooling you've paid for. IMHO, well worth the premium. Trane had an ad long time ago that showed two identical tract houses with the same outside unit; one with their 'normal' air handler and one with the variable speed one. Over the course of a year, the variable speed version extracted enough extra water out of the air to fill a typical backyard swimming pool...it is that effective when setup right.

    One of the biggest comfort factors in a home is the humidity level. A too-big a/c unit will not need to run as long, and that limits the amount of humidity it can extract. It's more comfortable, and the upfront costs are lower along with the energy use, if you do not have one that is too large. You can often be more comfortable at a higher room temp if the humidity levels are low. Unless you're doing strenuous exercise, you may find you're perfectly fine running a smaller one that dehumidifies better at a higher temp most of the time; lowering it if you want to get your exercise in. If you've ever been in the desert, you'll remember not being all that hot when the temperature was up (shade certainly helps there, though). Much different than being on the seashore even when the air temp may be in the low 70's.
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