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Thread: Enough Flow ?

  1. #1
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    Enough Flow ?

    Have a single 2.5 ton a/h v/s and a 2 ton compressor.

    The fan speed is set at 350 cfm per ton or 700 cfm total.

    This is a 1 1/2 storey house w/ 1,000 s/f down & 600 s/f up stairs.(Spray Foam Insulation)

    To cool the upstairs, I have to all but close every supply duct damper that feeds the 1st. floor.---> 10 ea. 6" rigid pipe....( as a side note there are 3 ea. 8" return ducts on the 1st floor.)

    Supply ducts feeding the upstairs (dampers wide open) are as follows--->2ea. 8" rigid pipe, approx. 40' in length...and approx. 5 ea. 90 degree fittings on each duct.......also 1ea. 6" rigid pipe going upstairs.

    Upstairs there is 1ea. 10" return main trunk w/ 2ea. 8" pipe splitting off the main pulling air from the 2 bedrooms.

    With dampers positioned as they are, I get a 3 degree differential between up & downstairs



    My question is....With a educated guess (without doing a s/p test)with the 1st floor duct dampers being closed approx. 85 to 90% will this harm the system?

    thanks for any imformative response...

  2. #2
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    Best guess is yes,it will harm it over time.

    Have it tested and see about some duct remediation ,for the second floor.
    Last edited by dash; 06-20-2008 at 05:58 PM.

  3. #3
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    I'd guess yes also.

    Neither the compressor or the VS motor are going to last long like that.
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  4. #4
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    Thanks for the replys !

    Do you think if I opened all 10 of the 1st floor 6" supply dampers to approx 1/4 to 1/3 from fully closed position,

    that I would increase the airflow enough that I would be safe as far as static pressure or any thing else goes

  5. #5
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    Guessing at that could cost you a compressor.

    Have the system checked and the ductwork corrected as needed. Compressor change outs aren't cheap.
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  6. #6
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    You can rig it any way you want but the end result will be the same. An expensive motor module repair and(or) more expensive compressor. When all else fails do it right.

  7. #7
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    It's a variable speed motor people. As long as the motor is not huffing from too much static resistance, there really is no damage being done. The variable speed motor is going to attempt to achieve the set cfm rating no matter what. Only if the motor is dropping down and then ramping up continuously, it is doing it's job properly under the adverse conditions.

    With a variable speed motor providing constant airflow, just what could be damaging the compressor? If the coil is not freezing up, there is enough air, so just what adverse affect is being created for the compressor?

    Is this the best situation? No. Is it damaging the system? Doesn't sound like it.
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  8. #8
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    My question is....With a educated guess (without doing a s/p test)with the 1st floor duct dampers being closed approx. 85 to 90% will this harm the system?
    IMO, that is a dangerous situation to be playing around with, it could be costly! That is a high risk gamble I would NOT take, even for a NY minute.

    First,let me be clear, you will need a competent HVAC company to do all that ductwork design layout & sizing according to Manual D.

    It always helps if the evaporator coil has a thermostatic Expansion Valve (TXV) metering device. TXVs help control flood-back but lose superheat control when airflow & load drops too low.

    Also for a 2.5-ton condenser, 350-cfm per/ton of cooling is 875-cfm, not 700-cfm. A 700-cfm total would be only 280-cfm per/ton.

    You would have to make a lot of ductwork sizing changes to damper off most of downstairs SA & Block all Return Air downstairs.

    From upstairs, I would want a 16" main Return Air duct
    I didn't figure what it would need on he Supply Air side upstairs, because you need a competent HVAC company to do the figuring for you.

    They have to know what your blower will deliver at say, 0.5" of External Static Pressure (ESP).

    Edit: Robo may be correct, it may not damage the compressor under normal load conditions, but when the filter(s) get loaded & there is a light heatload, bingo, freeze-up.

    Using 875-cfm through the 10" main return my software gets 1605-fpm RA air velocity & 0.48" Friction Loss per/100' on the RA side.
    When the SA problems are added to the RA problems, I would not want a variable speed blower to be ramping up to the ESP overload.
    What is the top end on that v/s blower?


    Using manual, they can draw up the requisite sizing layout before doing any work & put a normal load on the v/s a/h.
    Last edited by udarrell; 06-22-2008 at 01:06 PM. Reason: I did not address the V/S A/H

  9. #9
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    my furnace has vs motor, chart say sp 0-.8 will give me 1330 cfm max so what sp should i use to set up duct work

  10. #10
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    Quote Originally Posted by iowa tech View Post
    my furnace has vs motor, chart say sp 0-.8 will give me 1330 cfm max so what sp should i use to set up duct work
    Check the difference in motor wattage at .3 and .8 static,and you'll want .3 .

  11. #11
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    Damage or not??

    We don't know what the static is ,and it's an air handler that could be huffig away in the attic.Would the OP know what that sounds like anyway??


    High static,no huffing ,and water could be flying off the coil into the motor/module.

    I'd want it checked,plus lower static lowers operating cost.

  12. #12
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    Quote Originally Posted by RoBoTeq View Post
    It's a variable speed motor people. As long as the motor is not huffing from too much static resistance, there really is no damage being done. The variable speed motor is going to attempt to achieve the set cfm rating no matter what. Only if the motor is dropping down and then ramping up continuously, it is doing it's job properly under the adverse conditions.

    With a variable speed motor providing constant airflow, just what could be damaging the compressor? If the coil is not freezing up, there is enough air, so just what adverse affect is being created for the compressor?

    Is this the best situation? No. Is it damaging the system? Doesn't sound like it.
    Check your fan data for a VS blower at 1.2" and 1.4" static. The air flow is much lower. And at lower ambients, you can slug liquid back to the compressor.
    And the VS won't be huffing.
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  13. #13
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    Quote Originally Posted by iowa tech View Post
    my furnace has vs motor, chart say static pressure @ 0.8" will give me 1330 cfm max so what SP should i use to set up duct work
    The new External Static Pressure (ESP) is what has to be found in order to know what CFM that air handler will deliver with both the existing & the new renovated ductwork.

    There should be a chart or graph that will tell you what that air handler will deliver at various ESPs.

    That is a job for a competent licensed & insured HVAC contractor, NOT a Retired Contractor.

    We are NOT supposed to do all the work for DIY'ers for free, while contractors' lose the business they deserve to have.

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