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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. Originally Posted by Sgkent
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

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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. 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

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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.

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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.

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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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BEFORE .73" ESP, 13.5 F Delta T

AFTER .57" ESP, 20 F Delta T

CFM Chart for this model

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