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Thread: Blower speed

  1. #1
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    Blower speed

    High end Carrier R22 residential split system built 2004. Question on blower speed.

    Dealer set it up so the AC is 2 1/2 ton into 3 ton air handler with variable speed motor. Low and high heat calculated to CFM properly with Carrier white papers I have read online. But all the white papers and folks on forums suggest the handler should be set to produce about 400 CFM per ton of AC, so for 2 1/2 tons of AC the furnace motor would be set to about 1000 CFM. The dealer set it up to run full speed at 1440 CFM when it was installed in 2004. I asked the dealer today if I should set it to about 400 CFM instead, and the response was short, "best to leave it on high speed." That correlates to about 580 CFM per ton. Anyone care to comment what the logic might be to set it so high? My dealer doesn't like to explain their logic so asking them won't get any further answers.

  2. #2
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    If you don't live in a real humid area, you could probably leave it as is. If you need more humidity control, say your humidity level is above 50-55%, lowering the speed would probably help.
    __________________________________________________ _______________________
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  3. #3
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    humidity inside stays right at 50%. Temperature split inlet to outlet (R6 flexible ducts new in 2004 with new insulated sheet metal manifold) was 13 F - 14 F at 1440 CFM (576 per ton).At 406 CFM split is now 16 F, and humidity is slowly falling from a high of 55% today in the house to 46%. I don't know if that information helps.

    Sacramento area gets all its rain from November to May. June thru October are dry with some humidity when fronts pass thru or tropical storms monsoon north but we do not get any rain from those systems. Outside humidity is 28% right now, and 93F, 5pm. Rather cool summer day for here. 100F to 105F is typical this time of year. Taking a shower, cooking etc., raise inside humidity.

    I have 3 speed choices, 576 CFM per ton, 472 CFM per ton, or 406 CFM per ton. That assumes low restriction filters. The other two speeds are slower and are dedicated to the gas furnace portion. Heat uses low (43,000 BTU) and medium-low (66,000 BTU) speeds.

  4. #4
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    Quote Originally Posted by Sgkent View Post
    humidity stays right at 50%. Temperature split inlet to outlet (R6 flexible ducts new in 2004 with new insulated sheet metal manifold) was 13 F - 14 F at 1440 CFM (576 per ton).At 406 CFM split is 16 F and humidity is slowly falling from a high of 55% today in the house to 46%. I don't know if that information helps.
    You could tinker with the airflow, but if your humidity is @55-46% if it were my home, I would leave it alone.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law

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  5. #5
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    You must be at 11,444 foot elevation.
    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
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    Quote Originally Posted by Sgkent View Post
    High end Carrier R22 residential split system built 2004. Question on blower speed.

    Dealer set it up so the AC is 2 1/2 ton into 3 ton air handler with variable speed motor. Low and high heat calculated to CFM properly with Carrier white papers I have read online. But all the white papers and folks on forums suggest the handler should be set to produce about 400 CFM per ton of AC, so for 2 1/2 tons of AC the furnace motor would be set to about 1000 CFM. The dealer set it up to run full speed at 1440 CFM when it was installed in 2004. I asked the dealer today if I should set it to about 400 CFM instead, and the response was short, "best to leave it on high speed." That correlates to about 580 CFM per ton. Anyone care to comment what the logic might be to set it so high? My dealer doesn't like to explain their logic so asking them won't get any further answers.
    are these actual #'s of guesstimates

  7. #7
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    Thanks Bill. It is 50% - 55% is with the speed set to 576 CFM per ton. It has fallen from 55% to 46% using the 406 CFM speed. Everyone tells me that 13.5 F temp split between the inlet grille and closest outlet to the air handler is too low. That temp split is using the 576 CFM per ton speed. The differential increased to 16 F at 406 CFM. Would you put it back to the 576 CFM setting and run with the 13.5 F temp split or slow the fan down and try for a better temperature split?

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    are these actual #'s of guesstimates
    They are from the Carrier manual for that specific system. The charts give the CFM per fan speed and in. wc. The temperature measurements are with a HVAC probe thermometer left in the air flow at the inlet grill and the closest outlet for a few minutes to stabilize. The humidity comes from a 5% electronic gauge I keep in the house. I don't have a sling psychrometer.

  9. #9
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    Quote Originally Posted by Sgkent View Post
    The charts give the CFM per fan speed and in. wc.
    Have you actually measured your inches of WC?

    Do you have the full model number of this unit?
    Instead of learning the tricks of the trade, learn the trade.

  10. #10
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    Quote Originally Posted by Sgkent View Post
    I don't have a sling psychrometer.
    Well hopefully all of us would understand that. Here's the $64,000 question, how does it feel in there for you? If you feel comfortable, I would just leave it alone, and not worry about all this science.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law

    "When the teachers become unteachable we're all in trouble"

    "It's what you learn after you know it all that counts." ~ John Wooden



  11. #11
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    Wish I could make more sense but I will post the results.

    right now I am running on the medium speed. It was originally set to the high speed. At the speed these readings were taken the CFM should be about 405 CFM per ton. It was set up by the dealer at high speed which is probably about 576 CFM depending on in wc at this speed.

    With low resistance fiberglass filter in:
    Across just the air handler .54 in wc
    Across just the A-coil .40 in
    Across filter .15 in

    bottom of furnace to above A-Coil .27 in



    With filter removed:

    bottom of furnace to above A-Coil .18 in
    bottom of furnace to ambient room .10 in
    Return duct to ambient room .05 in

    right now I am running on the medium speed. It was originally set to the high speed.


  12. #12
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    I put it back on high so there are no accidental freeze ups tonight. To me maybe the coil needs cleaning but these numbers may be totally normal for a 2 1/2 ton Carrier system. These number below in the graph were just taken on the high speed it was originally configured with. They aren't as accurate as a high end test instrument might be but they are close, and taken with a Dwyer Mark II. These numbers are different than the ones prior in this thread because of the speed being put back to where it was.



    [/QUOTE]

  13. #13
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    Quote Originally Posted by Sgkent View Post
    The dealer set it up to run full speed at 1440 CFM when it was installed in 2004. I asked the dealer today if I should set it to about 400 CFM instead, and the response was short, "best to leave it on high speed." That correlates to about 580 CFM per ton.
    Quote Originally Posted by Sgkent View Post
    These number below in the graph were just taken on the high speed it was originally configured with.
    Your CFM isn’t anywhere near the 1440 cfm or 580 cfm per ton you stated in the previous posts.

    From your drawing, your TESP is .73”wc (.45” + .28”wc).

    On the high speed tap you are have approx - 425 cfm per ton (2.5 ton unit).
    Instead of learning the tricks of the trade, learn the trade.

  14. #14
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    The .45" is between ambient atmospheric pressure, and the top of the furnace. The difference across the furnace is .70" . When calculating TESP is the amount .45" and -.28 = .73", or is it .70" and .28" = .98"?

  15. #15
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    Quote Originally Posted by Sgkent View Post
    The .45" is between ambient atmospheric pressure, and the top of the furnace. The difference across the furnace is .70" . When calculating TESP is the amount .45" and -.28 = .73", or is it .70" and .28" = .98"?
    You are measuring everything external to the furnace, so its +.45" on the positive side and -.28" on the negative side, which = .73"wc
    Instead of learning the tricks of the trade, learn the trade.

  16. #16
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    Thank you. Now to get my HVAC folks motivated to find where the excess pressure is. .5" is the manufacture's tag so .73" is excessive I think.


  17. #17
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    It is all about the %RH in the living space during significant sensible cooling loads and high outdoor dew points.
    You contractor saw the duct work and may have recognized the need for additional blower speed. 50%RH is comfortable for most. Higher air flow = less moisture removed.
    Of course during low sensible cooling loads, less moisture is removed. If the outdoor dew point is high and the space is occupied, expect indoor %RH to go higher. If +60%RH for extended days, suggest supplemental dehumidification to avoid mold/dust mites in damp areas.
    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  18. #18
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    Thank You Teddy Bear. When I was a child we lived in Washington DC - 1952 - 1956. During that time Smokey the Bear came to live at the Washington Zoo, where we would go sometimes to see him and picnic. Small Bears have a fond place in my heart.

    Do you have any suggestions on things that can be done to lower the static pressure? I am thinking one item would be to change the supply side ducting to lower the .28, but I am not sure about what else to ask my HVAC contractor to do. I pulled the cover off the A-coil and will probably run over to Johnstone to get some cleaner for it, my HVAC guy has not gotten back to me, I suspect he might be on vacation. The coil is free of hair and lint but there is 15 years worth of dust on it. Maybe a cleaning will help a little with the pressure and delta t.

  19. #19
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    finished cleaning the evap coil. The filter has .02" in more across it than before so my guess is that the system is passing a little more air. Hopefully the delta t will improve and the static pressure drop a little. If so maybe I can lower the speed from high to medium-high and that will help a little too. Also there were some bent fins four or five spots the size of dimes to quarters. I used a fin comb of the correct pitch to straighten those up.

  20. #20
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    So here is the end results. Using fiberglass filters is the best solution because the pleated filters drop the CFM too much. After cleaning the evap coil the delta t got better by 2 F, going from 13/14 to 15/16. The Static pressure at the furnace outlet remained .45" but the inlet fell from .28" to .27". I lowered the blower speed from high to medium-high based on suggestions here, and the Carrier AC manual for my compressor that states use 400 CFM per 12,000 btu. The actual compressor is 28,000 BTU so that is 2.33 tons aka 2 1/2 tons. 2.33 x 400 = 932 CFM. 2.50 X 400 = 1000 so somewhere between 932 and 1000 is optimal. AT the medium-high speed the furnace outlet fell to .40" and the inlet fell to .26" That is .66" external pressure. High but until the inlet ducting is changed it is what it is. At .66" the CFM from the performance tables shows 995 CFM. There is a footnote that if the internal filter is removed then subtract .1 so that makes the total .56" which is 1057 CFM. So - the CFM is set to somewhere between 995 and 1057 - which is right where it should be for 2 1/2 tons. At the new medium-high speed the delta t is 20 f. done. Someday when we have time, I'll have the sheet metal improved to lower the pressure a little and maybe use a better filter.

    Thanks to everyone who helped educate me thru this.

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