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  1. #1
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    I'm in the process of replacing my HVAC system, and would sure appreciate some guidance on a couple of issues.

    My existing system features a 100K in/92K out Heil furnace with 4 ton blower. A/C is 2.5 ton Heil 5000. Furnace is factory-wired for 1585 cfm cooling/1425 cfm heating (@0.5 ESP). Main trunk is only 18.5" x 7.5", which after reading this forum, seems too small??? Total trunk length is about 50 feet, with two reductions along the way - last 32 feet are 14" x 7". Total of 11 6" round branches. So my questions are:

    1. How mismatched is the current blower to the ductwork and A/C unit? What are consequences of mismatch?

    2. Assuming new equipment is properly matched to existing ductwork, is there any capacity left for additional branches?

  2. #2
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    I doubt you are pushing the 1585 cooling CFM, or even the 1425 heating CFM through that duct system. If the blower is actually set up for that, whatever the airflow actually is would be very loud due to its velocity.

    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Air speed in those ducts

    As I understand the classic A/C airflow is 400 cfm/ton, sometimes up or down 50 depending on design goals. You should expect about 1000 cfm for cooling. Hopefully someone who better knows heating, will offer an estimate about what is reasonable for your furnace output.

    ACCA Manual D is supposed to contain all the answers you need for duct design, but it's not an easy book to read without first learning some early stuff -- which I haven't learned yet. But one of the key points they state is that hard pipe ducts (not flex) should be designed for air speed of 900 feet/min, with a maximum limit maybe 1000 (going from memory).

    Your 18.5 by 7.5 main trunk has 139 square inches, which is 0.964 sqft in cross section. That would allow up to 864 cfm airflow and still respect that 900 feet/min recommendation. If indeed your cooling airflow is the greater one, and you use 400 cfm/ton, then you are a little over the recommendation but not extremely so.

    Your 11 6-inch round ducts total 2.16 square feet cross section, so it appears your main trunk is much more of a limiting factor than the branches. Even if they are flex duct, which is limited by Manual D to a 600 ft/min recommendation, it would be OK as far as I know.

    If indeed you were pushing 1585 cfm airflow, then I calculate you would be up to 1645 ft/min which is extremely high. At modest too-high speeds the result is excessive duct noise. I dunno what to expect at this high a speed.

    I am a homeowner and decidedly an amateur with this. Most of my learning is so that I can understand what is happening with my own system and know what needs fixing. Absolutely if some of the better experts contribute something that contradicts me, you should listen to them and discount what I say.

    Hope this helps -- P.Student

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Our "student" has summed it up very well.You really have a very undersized duct system,however with a 4 ton drive on high speed ,you may get enoygh air for cooling(1000 cfm),but the heating is likely not near enough to prevent damage.

    Test is yourself,what the temperature in and out of the furnace,after 15 minutes of run time?


    The correct temperature rise is likely on the furnace door.That's about all you should do,then call a Pro.

  5. #5
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    Thanks for the detailed inputs -

    mark beiser: The airflow rates are from furnace technical manual. Blower motor has 4 speed taps: HI, MED HI, MED LO, LO. Factory wiring is cooling to HI, heating to MED HI. This is how I found it, so factory settings were unchanged. As for noise, I don't have anything to compare it to.

    P.Student: Thanks for the analysis. This helps it make a bit more sense. Without instrumentation, can't confirm the speeds, so I suppose noise is best indicator of excess speed? Again, I don't have any point of reference...

    dash - By damage, do you mean excessive delta T damaging furnace? Furnace lists delta T 40 - 70 F. I'll take your advice and measure it. Also advice to use pro for changes. Not a DIYer (at least not for HVAC!), just a home owner trying to understand my situation to better interpret recommendations from contractors.

    I do appreciate the input...

  6. #6
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    dash - ran the temp rise test, and, as might have been expected, it measured 90 F, exceeding specification.

    So, is excess rise due to insufficient airflow across the heat exchanger, due to limited capacity of ductwork?

    After reading this forum, it seems the required airflow for nominal temp rise (55 F) is calculated as: 93K btuh/1.1/55 = 1537 cfm. The MED HI blower setting for heat is 1425 cfm. The HI setting is 1585, so I'll try this and see if temp rise is in spec. Of course, we've already determined that capacity of trunk is well below this (assuming nominal velocity of 900 fpm).

    1st question is: If the main trunk is not sized to accept output of blower, is the velocity in the duct simply increased, or is there also some secondary effect that impacts temp rise?

    2nd question is: What is the impact of excess temp rise on unit?

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by muswell_hillbilly
    dash - ran the temp rise test, and, as might have been expected, it measured 90 F, exceeding specification.

    So, is excess rise due to insufficient airflow across the heat exchanger, due to limited capacity of ductwork?

    After reading this forum, it seems the required airflow for nominal temp rise (55 F) is calculated as: 93K btuh/1.1/55 = 1537 cfm. The MED HI blower setting for heat is 1425 cfm. The HI setting is 1585, so I'll try this and see if temp rise is in spec. Of course, we've already determined that capacity of trunk is well below this (assuming nominal velocity of 900 fpm).

    1st question is: If the main trunk is not sized to accept output of blower, is the velocity in the duct simply increased, or is there also some secondary effect that impacts temp rise?

    2nd question is: What is the impact of excess temp rise on unit?

    1. Velocity is increased,but when the fan reachs it's Static limit ,the cfm's are decreased,and the temp rise ,is above the spec.,as yours is.

    2.Cracked heat exchanger,which is very dangerous,in fact I have it tested ASAP.

    The motor spec,ie speed=cfms,is based on a certain Static Pressure,your static is above that Static Pressure.

  8. #8
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    Thanks for the info, dash. I'll definitely have it checked out, just to be safe.

    We're replacing the furnace anyway, so I guess it doesn't really matter, but I'm curious how this mismatch could be resolved it were weren't replacing it?

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If you put in the same size furnace ,it will or is very likely to have the same problem.

    The mismatch is,an undersized duct system,for the size of your furnace.

    It probably can deliver the cfms needed for your 2.5 ton A/C.

  10. #10
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    Just wondering what you are replacing your old furnace with? According to your specs for the Heil furnace that is a 92% efficient furnace. What is your reason for changing the furnace? How old is the unit? If it is operating well and in good shape I don't think I would replace it.

  11. #11
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    The "plan" is to replace with another high efficiency unit, either XR90 or XV90, 80K.

    We've gone back & forth about whether to replace it now or later. We need additional duct capacity to service new living space, and are either going to add a secondary trunk, or move the furnace to the center of the existing trunk and split it into two. Given the current layout, it's simpler to just move it, since the routing for another trunk would be complicated.

    The Heil unit is from 1986. We're haven't had, and are not having any particular problems with it, other than now realizing it's not sized properly. Since we're going to be tearing into the system anyway, we thought it might be the appropriate time to just replace it. Haven't decided for sure, but we're leaning that way.

  12. #12
    A blocked return or too small of a return would increase the temp. rise. I assume you checked for a plugged filter.
    possible just overfired because of gas pressure. Not everyone does a good job of setting up furnaces on start-up. Not even in 1986. What is your altitude? Also check blower fins for build-up. If you have had poor filtration over the years these could be plugged and would limit blower output. Are all eleven registers open?

    Need to know actual system S.P. before you can determine anything about ductwork.

    [Edited by hvacservice on 04-03-2005 at 07:24 PM]

  13. #13
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    Can confirm filter is O.K. - Space Gard 2200, just changed filter.

    Raw capacity of return is >= supply, but access to it may be a problem? Large "common" return services 3 bedrooms + bath (i.e. no returns in these rooms, common return in hallway). 2 other returns service balance of load - 1 in kitchen, 1 in living room. I noticed a large (15" x 7") return register cut into side of return trunk right at furnace, at entry point to filter. Is this common?

    Milwaukee is at about 650 ft above sea level. Regarding blower condition, it appears very clean, perhaps due to Space Gard filter? All 11 branches have in-line dampers, all wide open, although furniture positioning may be restricting free airflow is some.

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