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Thread: Blower speed

  1. #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"?

  2. #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.

  3. #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.


  4. #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"

  5. #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.

  6. #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.

  7. #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.

  8. #21
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    Conclusion. When this thread began the question in my mind was what kind of flow did Filtrete pleated filters have. The 1900 had the best flow for a Filtrete pleated filter but it was too high for the furnace so fiberglass was chosen. A Dwyer manometer was added to be able to see when the filter was dirty. Using that manometer measurements were taken that showed a need to tune the system. The Delta t was low, being about 13.5 +/- inlet temp to outlet temp. Changing motor speed made little change on Delta t. The evap coil was cleaned with a foaming cleaner purchased from Johnstone supply, rinsed with clean water using a pump up sprayer and then blown out using a long wand made from 1/4" copper tubing flattened on the end to make a fan of the compressor air. Small dents in the cooling fins were straightened using a fin comb of the correct number of teeth per inch. At high speed the Delta T rose from the 13.5 F to 15.5 F. Blower speed and CFM was calculated using ESP, and the fan speed dropped 1 speed from High to Medium-High. With this drop the Delta T rose to 20 F and the ESP was closer to where it should be max. I might note that before the coil cleaning speed changes had almost no effect on Delta T. Before and after drawings are shown below. I also learned a lot from the folks here and elsewhere about this process. For that I thank all who helped me thru this. FWIW I really didn't want to get this involved and tried to hire someone to do this for me but everyone was too busy right now due to summer increases in clients wanting services, or more interested in selling a new unit. In the end this system is working well. Humidity in the house is falling too more than before. We also find ourselves comfortable in the mid-70's instead of the low 70's. (Both of us have CPAP machines that put about a quart of water into the air each night, as well as showers and cooking adding water.)

    BEFORE .73" ESP, 13.5 F Delta T



    AFTER .57" ESP, 20 F Delta T



    CFM Chart for this model


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