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  1. #1
    Join Date
    May 2005
    Posts
    71
    I wish to update my 37 year old HVAC system in my house and am finding that it is not a simple venture. The problem is that the system (all original) is a commercial Trane Climate Changer air handler with 6 zones and air flow up to 7,500 cfm. Heat source is a boiler (300,000 btu input) and the motor for the air handler runs at 1,200 rpm during heating and 1,800 rpm during cooling. The boiler is oversized because it also supplies another Trane air handler (Torrivent) for a 1,000 sq. ft. solarium. Cooling is from two 5 ton Trane air conditioning units but the second is never called on. The system is switched to either heat or cool at one time. Air flow is the same in each zone when heating is called for or not and when cooling is called for or not. The house is ~3,500 sq. ft plus ~1,200 sq. ft. lower level which we use often (rec. room) for a total of 4,700 sq. ft. (not including the solarium). We are located in North Central US and have large overhangs with no direct sun inside (except the solarium).

    The basic problem is that I have read that ducts can be too small and also too large. If we convert to residential unit(s) we won't have the airflow that the ducts and vents were designed for. I asked for 3 bids to start and they ranged from 1 unit to 3 units including zoning. I am confused at this point and am unable to proceed. I realize that this is not an easy question but what do we have to consider when updating our system especially in regard to the duct/register size?

  2. #2
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,510
    the way i see it is
    decide what it is you want. one zone or sevaral zones.
    chance are the duct is to big to use over again.
    start at the begining and get a manual-J done on your house to see just how many tons of cooling you need and how many btu of heating you need.now you will have a place to start. devide up your systems to your load. one zone,two zones,or threezones. if you want at this time you can go one system and zone it with ducts.
    after you find out the size and decide how you want to split it up have a manual-Ddone so you will now know what size ducts you will need and the rest is how it is installed. find a contractor that is willing to work with you on this and listen to all your conserns and willing to advise you with all the aspects of the job is the guy to use. this is going to cost you and price is only a small concern the contractor is the biggest.
    so start at the begining as if the system wasnt there
    and look for you contractor first

  3. #3
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    Duct size is determined by the required airflow, which is determined by equipment size, which is determined by the heat load. There's the rub. What's the heat load? It's obviously not ten tons because you say that you don't use the second system. So you need a Manual J (heat load calculation) performed by someone who understands Manual J (as opposed to a fearful nincompoop who adds in fudge factors and would come up with ten tons again). Once the Manual J is done then a Manual D can be done (duct size calculation). From there you can figure out which of your existing ducts can be used and which can't.

    In my opinion the problem lies in the "free estimate". How much investigation, engineering and design work can be done for free? A typical sales call might last a couple hours if you're lucky. I could easily spend an entire day mapping out your existing system, performing the calcs and designing the new system. Even some of the ones that do know how to do all of that won't offer because they know the negative response they'll get. EVERYONE does free estimates. Surely the contractor that wants money for one must be a crook! Man... lol.

    Anyway, on rare occasion I'll make a deal with a customer to perform a real survey and [/i]real[/i] calcs for a certain amount as a separate transaction from the installation. If you can find a knowledgeable contractor who really understands design then perhaps you could do that. It's a work in progress, but some of the things you need to know are here. Obviously the dry climate recommendations don't apply if your area is humid.

  4. #4
    Join Date
    May 2005
    Posts
    71

    Another question and info.

    Should I find a mechanical engineer to do the calculations or an HVAC contractor? Mechanical engineers likely won't be as familiar with modern residential equipment with 2 stage heating and cooling, variable speed fans and modulating dampers but it might be tough to find a contractor able to do the calculations correctly. What do you think?

    Regarding changing ducts and vents: this is difficult because access is difficult or impossible (under concrete).

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,997

    Question RE-Zoning

    Originally posted by rumn8r
    the system (all original) is a commercial Trane Climate Changer air handler with 6 zones and air flow up to 7,500 cfm. Cooling is from two 5 ton Trane air conditioning units but the second is never called on.

    Air flow is the same in each zone
    4,700 sq. ft. ... located in North Central US and have large overhangs with no direct sun inside

    If we convert to residential unit(s) we won't have the airflow that the ducts and vents were designed for.
    Six equal flow Zones with 7,500 CFM total ...
    so each Zone is currently sized for 1,250 CFM or about 4 Tons. However, you only need total of <= 10 Tons as determined by experience.
    ...

    Is model TRANE model MCC-08 with 4,700 CFM on 4 zones and two 5-ton condensers feasible?

    http://www.trane.com/download/equipm...lb006en_r1.pdf
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    A good (compared to most of the hacks, he could be called elite) contractor is your best choice to design the system. He'll be able to adapt to the ducts that aren't accessible.

  7. #7
    Join Date
    May 2005
    Posts
    71

    Clarification

    Sorry, It was confusing the way I wrote the following:

    "Air flow is the same in each zone when heating is called for or not and when cooling is called for or not. "

    Air flow ranges from 380 to 2500 across the zones based on the capacity of the registers on the blueprints. I wanted to convey that the airflow within a zone isn't changed when the themostat call for air (or heat) or not. Airflow within the air handler either flows through the colling coils or the heating coils. Because only heat or air is on at one time (by manual switch), air flow is not changed when the thermostat calls for a change. Dampers within the air handler are actuated by Honeywell Modutrols.

    Here is the cumulative CFM of the Titus registers in each zone:

    Living room: 2500 cfm
    Entryway and study: 510 cfm
    Kitchen: 700 cfm
    Diningroom: 380 cfm
    Rec. room (lower level): 605 cfm
    The above zones share one common return

    Bedroom wing: 1110 cfm
    Another common return for the three bedrooms

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,997

    Lightbulb Re: Clarification

    Originally posted by rumn8r
    Here is the cumulative CFM of the Titus registers in each zone:

    Living room: 2500 cfm
    Entryway and study: 510 cfm
    Kitchen: 700 cfm
    Diningroom: 380 cfm
    Rec. room (lower level): 605 cfm
    The above zones share one common return

    Bedroom wing: 1110 cfm
    Another common return for the three bedrooms
    2,500 ......380
    .. 510 ......605
    .. 700 ....1,110
    3,710.....2,095 ...
    5,805

    The 5,805 CFM Total for 6 zones noted above
    is a L O N G Way from "... upto 7,500 CFM"

    Why 2,500 CFM for the Living Room zone?
    ________ Sq. Feet

    ...3,500
    .+ 1,200 Rec Room
    .= 4,700 Sq. Feet Residence

    5,200 CFM for 3,500 Square Feet

    I wonder What Air Flow the existing Climate Changer can actually deliver?
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #9
    Join Date
    May 2005
    Posts
    71
    The 7,500 cfm for the Trane Climate Changer is from the mechanical blueprints. I think the 2,500 cfm for the living room is because it is a large room (~1,200 sq. ft.) with cathedral ceiling and one long (>40') wall that is all windows (floor-to-ceiling). There are 5 windows that are 8'x8' with one 7.5' long register in the floor under each. However, several years after the house was built a solarium was added along that wall so that the wall of windows is no longer an outside wall. In the winter, that zone doesn't seem functional because I have the thermostat set at 70 F and it seems that it is always ~72 F or so. I haven't noticed in the summer. I should mention that the return air plenum for all but the bedroom wing runs the length of the LR and all rooms but the bedroom wing are open to LR. I don't have a clue what the actual airflow is. I guess that could be measured?

    [Edited by rumn8r on 06-01-2005 at 07:24 AM]

  10. #10
    Join Date
    Apr 2004
    Location
    Lubbock, Texas
    Posts
    267

    Re: Another question and info.

    Originally posted by rumn8r
    Should I find a mechanical engineer to do the calculations or an HVAC contractor? Mechanical engineers likely won't be as familiar with modern residential equipment with 2 stage heating and cooling, variable speed fans and modulating dampers but it might be tough to find a contractor able to do the calculations correctly. What do you think?

    Regarding changing ducts and vents: this is difficult because access is difficult or impossible (under concrete).
    If a contractor can not do these calculations he has no businness being a contractor. Do your homework there is mostlikely some true proffessionals in your area
    Learning never ends and everyone has something to teach. Some people teach me what to be like others teach me what not to be like!

  11. #11
    Join Date
    May 2005
    Posts
    71
    So what can I expect if the equipment is sized correctly with Manual J but the ducts and registers are left as they are (too big).

  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,997

    Question Sizing and Recommendations

    Essentially 320 sq. feet of glass (8' x 8' * 5) was taken out of the direct radiation path. This load is about 15,000 BTUh or > 1.5 Tons. In other words, one past mod may have had a ~15% impact on cooling load.

    Therefore, with this rather unconventional replacement/ upgrade that a contractor does NOT deal with everyday, one should hire ($1K- $2K) a Mechanical Engineer to develop a FULL set of Equipment Plans, Specs, and Duct Layout Drawings.

    _ _ _ _ _ _ _ Worth it in the Long run to have a Wholelistic Approach for the Equipment Sizing / Performance.

    Testing responsibility may also be assigned to an independent air balancing firm as recommended my mechanical engineer.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  13. #13
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    Originally posted by rumn8r
    So what can I expect if the equipment is sized correctly with Manual J but the ducts and registers are left as they are (too big).
    It's not likely that much will happen because of oversized ducts. The velocity through the ducts will be slower than if they weren't oversized. But the only potential problem I'm aware of is increased heat loss or heat gain. If the air is lingering longer in the duct before it finally gets expelled then obviously there's more time for heat to escape to or infiltrate from the concrete or whatever it is that surrounds the duct. But it won't be a big deal unless you have incredibly poor insulation on the ducts.

    I would encourage valid criticism from my peers on that point because I haven't thought much about the drawbacks of oversized ducts. Oversized ducts are almost as rare as dodo birds.

    The big deal is the velocity at which the air leaves the diffuser. You need good velocity out of your diffusers to ensure proper mixing. If your current diffusers are too big I'm sure they could be altered. You obviously don't have to dig the concrete up just to change a diffuser. If the size difference is big enough that turbulence is an issue then I'm sure someone could custom fab some inserts to make a smooth transition from the larger duct to the smaller diffuser.

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