Results 1 to 13 of 13
  1. #1
    Join Date
    Jul 2009
    Posts
    4

    Opinions on a new install

    I live in Southern California and I'm about to replace the existing HVAC in my home. The home is 15 years old and still has the original system installed by the builder. A 3.5 ton Condenser (Carrier off-brand) and a 4 ton Carrier Weathermaster Furnace. We've been in the house for 3 1/2 years now and the HVAC has given us problems the last 2 years ... everyone that has serviced the unit has said that it is undersized. The house has a very open floorplan with the main living room being open to the second floor with lots of windows. It is 2000 square feet with around 750-800 of it on the second floor.

    Now, I've been lurking on this forum for about 3 months now as I've been researching systems and what to look for in installers. The first thing I've read time and time again is that all good installers do a heat load calc to size the system .. I've had 9 estimates ... only 1 guy did the calculation (Sears) everyone else has ballparked it ... of course while letting me know how long they've been doing this .... Ironically half have suggested a 4 ton and the other half a 5 ton.

    I've also had conflicting opinions regarding zoning the system .. a couple of guys swear by it and others say that the floorplan of the house will negate any benefit. Some have blamed poor ductwork while others have no problem with it ... to make a long story short ... I'm pretty confused.

    Well, the last guy to give me a bid finally convinced me .. even though he did not do a heat load calc he seemed to be on the ball with regards to everything else. I won't go into prices here as I understand it is not allowed but his quote was somewhere in the middle ... His company is the Home Depot installer of choice in this area and that's where I found him.

    His quote seems to cover all of the bases. Upgrading electrical panel, Installing dampers in the ductwork, Hanging the furnace to code using metal rods and a metal base, new pad for condenser, installing a new copper line to handle the bigger condenser, enlarging the return and even changing the registers (the builder used stamped everywhere).

    His quote is for Lennox equipment ... a XC14 and a G61V furnace .. the upgraded furnace is there so that I can qualify for the $1500 Federal tax credit ..

    We are planning on staying in the house for a good long time .. I'd even like to add a room in the future ... 250 - 300 square feet max.

    I've really tried to do as much research as I can but as you probably know a little knowledge is a dangerous thing .... I'd love to hear any feedback with regards to my situation .. my biggest question is the actual size of the system .. his explanation was that the open floor plan, while not actual square footage, still requires cooling for the air .... makes sense to me ... as opposed to the zoning idea which left me scratching my head .. how do you control the upstairs when it is so open.

    Sorry for the long post and thanks to everyone on this forum ... I really learned more than I'd ever wanted to regarding HVAC systems.

    All the best,

    Gtrplyr1

  2. #2
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    My house is 50 years old, 2000 sq. ft. single story with 1 pane aluminum frame windows, R11 walls and R26 attic. The ductwork is in a soffit not in the attic. I have never had an air conditioner larger than 3 ton in the 25 years that I have lived in this home. The local design temperature is 109 degrees Fahrenheit and my wife keeps the temperature at 78*F. See http://www.rssweather.com/climate/Arizona/Yuma/ I suspect that your home is better built than mine and your climate is milder.

    So why would you need anything larger than 3 ton? The answer is WASTE. Ductwork in the attic wastes a lot of cooling. LEAKY ductwork in the attic wastes even more. The DOE says 25 to 40% of the heating and cooling energy is wasted by air leakage...on the average, and MORE if the ductwork is located in the attic. (Mine is not.)

    We also keep our system clean and well maintained. Dirt accumulation and incorrect refrigerant charge also reduce effieciency and capacity. Oversizing the system adds waste due to short run cycles and it covers up other waste. But it makes life richer, safer and better for the sellers. Duh!

    Check out "Better Duct Systems for Home Heating and Cooling"
    http://www.nrel.gov/docs/fy05osti/30506.pdf

    Then find a contractor who already understands this. If they don't, they are not professionals and they don't have YOUR best interests at heart.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  3. #3
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    I decided to add to my first post to address some other issues beyond sizing.

    Quote Originally Posted by gtrplyr1 View Post
    The first thing I've read time and time again is that all good installers do a heat load calc to size the system .. I've had 9 estimates ... only 1 guy did the calculation (Sears) everyone else has ballparked it ... of course while letting me know how long they've been doing this .... Ironically half have suggested a 4 ton and the other half a 5 ton.
    NINE estimates! I can see why they don't waste their time with heat calculations. What size did the Sears Guy come up with? How about the Lennox Guy with the "bigger condenser"? How big?

    I've also had conflicting opinions regarding zoning the system .. a couple of guys swear by it and others say that the floorplan of the house will negate any benefit. Some have blamed poor ductwork while others have no problem with it ... to make a long story short ... I'm pretty confused.
    Zoning is great if the contractor understands duct design. Most don't. Most contractors prefer adding additional systems to achieve zoning. Simpler and more money for them. Not zoning a 2 story house one way or another is a mistake if you care about comfort.

    Well, the last guy to give me a bid finally convinced me .. even though he did not do a heat load calc he seemed to be on the ball with regards to everything else. I won't go into prices here as I understand it is not allowed but his quote was somewhere in the middle ... His company is the Home Depot installer of choice in this area and that's where I found him.

    His quote seems to cover all of the bases. Upgrading electrical panel, Installing dampers in the ductwork, Hanging the furnace to code using metal rods and a metal base, new pad for condenser, installing a new copper line to handle the bigger condenser, enlarging the return and even changing the registers (the builder used stamped everywhere).

    His quote is for Lennox equipment ... a XC14 and a G61V furnace .. the upgraded furnace is there so that I can qualify for the $1500 Federal tax credit ..
    Sounds way better than average, but he is doing it too cheap if he is in the middle on pricing.

    We are planning on staying in the house for a good long time .. I'd even like to add a room in the future ... 250 - 300 square feet max.
    Figure out NOW how you will build the addition and include it in your design and heat load.


    ..my biggest question is the actual size of the system .. his explanation was that the open floor plan, while not actual square footage, still requires cooling for the air .... makes sense to me ... as opposed to the zoning idea which left me scratching my head .. how do you control the upstairs when it is so open.
    If a contractor can't explain that to you, it is probably because he doesn't understand it himself. (It really isn't that hard. Your plumbing and electrical systems are zoned.)
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  4. #4
    Join Date
    Jul 2009
    Posts
    4
    Yes I realize nine estimates sounds like a lot. Not sure what you meant about wasting their time ... I didn't have them come at the same time and they didn't know I was getting other bids, I would hope most people at least get a few. I originally planned on having three .... all three were so wildly different it left me scratching my head. These are all reputable companies ... not people from craigslist ... believe me the last thing I wanted to do was keep calling people to come out but when you hear directly opposing views it makes you wonder ....

    My house is two stories .... and like I said very open floor plan ... if we had a second story over the living room area that is so open it would easily add 400+ square feet. We also have a lot of windows ... they are double pane but still lots of windows and Southern exposure. And yes the ductwork is in the attic ... the unit we've settled on is 5 tons ... but I wasn't pushing for that ... once again half of the contractors suggested this ... that's my biggest reservation at this point.

    Thanks for the links .. I checked out the one on ductwork .. unfortunately there is no way to do that at my house .. I don't have a crawl space (Concrete slab) and adding soffits would mean major reconstruction ... this is a typical tract house. FWIW , not one of the nine recommended anything smaller than 4 ton ... so I'm quite confident for my area and layout and ductwork we need between a 4-5 .... I wish I had the plans for adding a room but I don't ... I know where I'd like to do it and around how many square feet to add ... the original plan was change the HVAC system at the same time as the addition .. but the economy didn't want to play ball ... and summer is killing us right now with the current system ...

    I didn't mean to imply the guy we are going with is cheap ... he's not and has said as much .. but compared to the others he seems knowledgeable and fairly priced ...
    Last edited by gtrplyr1; 07-15-2009 at 07:33 AM.

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,614
    Chances are the current duct system can't handle the 3.5 tons you have. So hopefully if you are going way bigger you are adding lots more duct capacity to support it!

  6. #6
    Join Date
    Jul 2009
    Posts
    4
    Is there a rule of thumb sizing with regards to ductwork and size of HVAC ?

    Thanks for the reply .... I guess I'm not the only one who can't sleep right now ....

  7. #7
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    Chances are: the duct system is too small for the unit you have. (as Baldloonie says) Most duct systems are undersized (and leaky). Uping the size without replacing the entire duct system is not a good idea.

    Rules of thumb for sizing duct systems can be dangerous because they do not address the roughness (friction) of differing ducting materials and they do not address turbulence created by duct fittings of various types, especially when the air inside must change direction or pass over obstacles like the coil, a filter, grilles and dampers.

    With that warning in mind...See
    http://efficientcomfort.net/Rules_an...ct_Systems.pdf
    http://www.udarrell.com/external_sta...enough_Airflow
    http://www.udarrell.com/check_ac.html

    "Quick Check for Sizing Units to enough Airflow

    Actually, even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.

    Then take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.

    Then do a quick estimate of airflow per equipment tonnage.
    To find area of a round duct; Duct diam is 7"; 7"X7"= 49-sq.ins., X's .7854 = 38.04845-sq.ins divided/ by 144= 0.2672541-sq.ft. area X's FPM Velocity 600-FPM = 160.35246-CFM X30 = 4,810.5738 each 7" run X's 6 branch runs = 28,863-BTUH, or airflow for 2.4-ton.

    That would also be good for 2-ton; at 550-FPM velocity X's 0.2672541= 147-CFM X 30-BTUH Per-CFM = 4,410-BTUH each run X 6-runs = airflow for 26,460-BTUH. (12,000-BTUH /400-cfm per-ton = 30-BTU per cfm ratio | / 450 = 26.666-BTUH per-cfm)

    Never sell units requiring more airflow than the duct system will support! - Darrell"

    This is about as simple as it gets to do ir right. Static Pressure across the fan bearing unit should be between .25 inches water column and .5 inches w.c. When you sip water though a straw, you create a pressure difference of 3 inches to 10 inches water column. Atmosheric pressure is what pushes water up the straw into the lower pressure area created in your mouth.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  8. #8
    Join Date
    Sep 2002
    Posts
    13,552

    Rule of thumb for sizing ductwork -

    The best short cut possible is:

    Hire a guy who knows what is required and how to design a system to achieve it. That is the really best short cut.

    PHM
    --------


    Quote Originally Posted by gtrplyr1 View Post
    Is there a rule of thumb sizing with regards to ductwork and size of HVAC ?

    Thanks for the reply .... I guess I'm not the only one who can't sleep right now ....
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  9. #9
    Join Date
    Sep 2002
    Posts
    13,552

    BTW: what I would suggest is this:

    I would leave the existing A/C system as-is.
    I would do the heat load for the second floor.
    Then install a high-velocity-duct system as the second floor zone.

    Unobtrusive.
    Excellent humidity control.
    No major surgery required
    Zoning with area-use-specific control of it. Additional cooling when you want/need it.

    Let the existing system as it is and only alter it when something major goes wrong with it at some time in the future.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #10
    Join Date
    Jul 2009
    Posts
    4
    Thanks ... I will call the installer today and get more specifics regarding the ductwork.

    The existing system is not working very well ... as a layman I don't know the problem but the condenser is 15 years old .. I'm told the furnace is in good shape.

    As of now ... the system is not working .. it's basically blowing warm air . Over the last couple of years it's had a habit of tripping the 30amp breaker on hot days ... I would just reset it ... but I did call to have regular maintainance done .. coil was cleaned last August and the entire system was serviced ... I'm pretty certain there is a freon leak as the plenium (sp?) is not even cold.

    I should also state ... the room I'm planning on adding (250-300 squrare feet) will be a music studio ... lots of warm equipment so the AC needs might be more than a regular bedroom and I'm planning on spending time there during the day ....

    Thanks again to all who have replied. I really want to do this right and am at the mercy of what the Pros who have seen the house are telling me .. when there are two opposing views it's really not that easy to know who to trust .... I don't get the feeling I'm being ripped off but there are no shortage of opinions in this business ....

  11. #11
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    5 tons is WAY too large for a duct system designed for 3.5 tons. "Rules of thumb" range from 1 ton per 400 sq ft to 1 ton per 1000 sq ft (so you can see that there's more than just square footage to take into account. When an existing size unit is not cooling the house, I think many contractors will suggest an oversized condenser "just to be on the safe side". I'd be VERY surprised if the sub hired by Sears did a proper Manual J heat load calculation. Get one done - oversizing your condenser is going to cause all kinds of problems.

  12. #12
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    Trust a contractor who has satisfied, or better yet, VERY satisfied customers.
    Good Luck
    Lynn
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  13. #13
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    Quote Originally Posted by sktn77a View Post
    5 tons is WAY too large for a duct system designed for 3.5 tons. "Rules of thumb" range from 1 ton per 400 sq ft to 1 ton per 1000 sq ft (so you can see that there's more than just square footage to take into account. When an existing size unit is not cooling the house, I think many contractors will suggest an oversized condenser "just to be on the safe side".
    I agree
    Lynn
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event