Central Air Questions for Old Home

Discussion in 'HVAC Heating & Cooling' started by bjferri, Jun 25, 2009.

  1. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    I'm in the process of getting estimates but want to do a little homework first.
    Home: 3-bedroom, 2-bath 1920's colonial/craftsman style w/ cast iron baseboard heat - no duct work. First and second floor totals 1400 square feet of living area. Please tell me I can be okay with one zone efficiently because I cannot afford 2. I have full unfinished basement and attic to work with.
    Based on 1-zone, is it better to install in attic or basement?
    Which manufacturers should I be looking for?
    Which manufacturers to stay clear of?
    Any duct material / size, etc better? I don’t know the options but with an older home I’d rather have small as possible…
    A SEER recommendation? I want to go 16 or above for the tax rebate/credit.

    I don’t know what else to ask so if you can give me some insight please…

    Thanks in advance.

    Brian:confused::confused::confused:
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    Before you do anything, either you or a potential supplier must run a heat load analysis. This requires measurements of the glass area, building insulation, size, and orientation and takes into account the local weather. Just taking sq footage will not likely produce the most efficient or comfortable solution. If the calculation comes in-between standard sizes (normal), you'll usually be better off choosing the closest smaller unit. It will run longer, removing more moisture, and even if on the hottest day it runs continuously, the house will feel better because it will be drier. The worst thing to do is have one too large, since you'll be cold, clammy, and spending lots more money than needed. It's much better to be a little warmer on the hottest day of the year, and be much more comfortable the remainder. And, that may only be a degree or two. Because you'll be nice and dry, you may not even notice.

    The compressor must be outside, but your question of where the rest goes depends. If you can place ducts in the ceilings of the areas to be conditioned from the attic, then placing the air handler there may be best since the cost of ducting will be minimized.

    If the ceiling registers would be too obtrusive, you can go with a high velocity system ($$). These put what almost look like recessed light cans as outlet registers in the ceiling. It uses a diffuser and high velocity to get enough cold air into the room.

    If you want to avoid ductwork, have you looked at any of the mini-split systems? This leaves the compressor(s) outside, and some can have up to 3 outlets, each individually controlled. No ductwork to run. The unit is bolted to the wall and depending on the model, is maybe 5" thick. Many can be controlled with a remote, so you do not need to be able to access (except to periodically clean the filters). Mitsubishi makes some nice ones, and others may have some additional recommendations. this allows you to easily only cool those area(s) you want.
  3. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    Ductless mini-splits. Mitsubishi, Sanyo and I think Carrier may even have one but we typically spec the Sanyo units for commercial work. Some can even have a single duct attached so you could do two rooms.
  4. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    I don't want to go mini-splits but thank you. High velocity we're still contemplating...

    We are going 1 zone...I think 3 tons.

    Is there a big difference in saving on electricity by using 16 Seer rather than 13 or 14? I was told I'll never make the money back on using a 16 Seer.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    Depends on your electric rates...

    Do NOT just take a WAG at the size needed. Without a proper heat load analysis, you are asking for problems.
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Last edited: Jun 28, 2009
  7. T_Hartigan

    T_Hartigan New Member

    Messages:
    35
    Location:
    Ohio
    I have a 2 story ~2,000 sq ft old farm house with stand alone radiators (no duct work). I had central air installed 3 years ago. We had a Trane installed with the air handler in the attic. With the doors open to the upstairs (2 stair cases) the unit cools the downstairs pretty well too. Usually about 2-3 degrees above the upstairs. We have a small window unit downstairs to help during the really hot days. We are going to install another unit for downstairs, but the system works fairly well, so we are putting it off for awhile.

    Tim
  8. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    Knowing the cents/kwh I still don't know does a 13 or 14 Seer cost me much more to run versus a 16 Seer?

    Also, one estimate wants to install Rheem 3-ton or Trane XL15 2.5-ton (I forgot which one had the 2-stage cooling system) but both are variable speedair handler. Rheem I'll get a $1000 rebate from, Trane no rebate. Then $1500 tax credit.

    $2000 more I can go High Velocity with no rebates.

    Having to be practical, any ideas???


    Thanks.
    Last edited: Jun 29, 2009
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    "For example, consider a 5000 BTU/h air-conditioning unit, with a SEER of 10, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days).

    The annual total cooling output would be:

    5000 BTU/h * 8 h/day * 125 days = 5,000,000 BTU
    With a SEER of 10, the annual electrical energy usage would be about:

    5,000,000 BTU / 10 BTU/W·h = 500,000 W·h
    The average power usage may also be calculated more simply by:

    Average power = (BTU/h) / (SEER, BTU/W·h) = 5000 / 10 = 500 W
    If your electricity cost is 20¢/kW·h, then your operating cost is:

    0.5 kW * 20¢/kW·h = 10¢/h"



    It's 12,000 BTU/hr per ton so with the SEER and some estimating [as to hours per season] you can figure out the breakeven points for your different options.
    I'd use a spreadsheet for this.

    13 SEER at 2.5 ton = [({30,000,000/13} w-hr)/1000]($0.13) = $300 elec. cost per 1000 hr season.
    13 SEER at 3 ton = $360 " " "
    16 SEER at 2.5 ton = $244 " " "
    16 SEER at 3 ton = $293 " " "

    At best, with a $360-$244 = $126 per year difference you'd need $2000/$126 = 16 years to pay it back.
    Half the homeowners move out of their current house after 7 years.
    Last edited: Jun 29, 2009
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    There is one size that will work best for your particular house on your particular lot. Seems like you are getting multiple opinions. Have each of them show you their calculations on how they determined which was best. It's not just square feet. If you don't take into account the windows, insulation, orientation and the expected outside and desired inside temperature, you are going at this wrong and setting yourself up for a poorly operating system.

    SEER is a measure of efficiency - the higher the number the more efficient it is; sort of llike MPG for your car.
  11. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    One contractor did count the windows and asked about insulation but I'm sure they calculate other variables as well. These are very competent companies but I guess they individually have their way of doing it.

    I've been in the house 10 years and will be here longer - great house/location. We don’t want to move.

    Thanks for all this valuable information, especially the calculations. I try to do things on a practical matter especially if I can get a return on it. I don’t plan on selling the house anytime soon. I don’t mind using window units at all. 2 units, 1 upstairs and 1 downstairs cool the entire home and we can lower the temperature on the first floor when we go to bed. I’ll see what the other estimate comes in as.

    Last question. Are energy efficient window units more efficient to operate than central air? If I took a SWAG at it I’d say window units will be less expensive to run per month.
  12. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    They shouldn't be, due to Economy of Scale.

    Something else occurs to me.

    If the Yabba brand HVAC costs $10,000 and $400/month to run and
    the Dabba brand costs $12,000 and $200/mon,

    after (12,000-10,000)/(400-200) = 10 years
    you will have spent an equal amount ($10,000 + 10($400)) = ($12,000 + 10($200)) = $14,000 on both, so

    if you plan to leave after 10 years buy Dabba, else buy Yabba.
    Last edited: Jun 29, 2009
  13. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    LOL! Flintstone fan?

    Definately Dabba if I doda.

    Or is that dooda...hmmm.
    Last edited: Jun 29, 2009
  14. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
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