I stumbled today upon the following scenario.
Went to a house for a regular maintenance on A/C. Turned the unit "ON" and took Temp. drop accross Evap. coil, 68F to 55F- only 12F.
Then decided to measure the ESP. Drilled two holes, just below and just above Evap. Coil. The readings were: 1. before the evap. coil = 0.92"w.c. just above the coil= 0.12"w.c.
On the other side - the Return duct showed (-0.12"w.c.)
I could not make sense of it. Is the coil plugged ?
Why below 0.92"w.c. and above 0.12"w.c. ?
Filter was O.K.
Actually, when I pulled the filter - it read 1.10"w.c. just below the Evap. coil.
From your reading ,I'd assume it's a furnace with the coil on top.
The drop across the coil is .8" of wc,,that indicates it's plugged.
Is it an "A" coil,if so the location for test points isn't at the top of the "A".
it is Clare 36,000
it is "A" Coil on top of the furnace (furnace is Carrier Weathermaker - High efficiency, did not take the Model# ==> my bad)
Now, what puzzles me is why on the Return I have 0.12"w.c. and AFTER the "A" Coil (right above it) I have 0.10"w.c., but right under the "A" Coil - it is 0.90"w.c.
I will try to describe how the "A" had been installed. Imagine the upflow furnace. The furnace opening is 17 3/4" Wide and 18 1/2" Deep. About 4" above the furnace the Plenum changes to 17 3/4" Width (notice it is the same) and 22 1/2" Depth (increased by 4" ==> 2" on each side). So the Plenum changes (increases) on Depth. Those dimensions are not 100% accurate - but prety close.
Now, I drilled a hole and inserted the Mangehelic probe right where the duct is slanted from the furnace opening to the duct transition. Yes, it is on slight angle. Is it possible that the probe is not chatching the airflow, thus giving me a "high" reading ?
One more silly question: 0.90"w.c. indicates resisitance, right ?
0.10"w.c. indicates very low resistance/obstraction, right ? or may be I am wrong.
Since, there is no other place to stick the vacuum probe - can I use the Return duct to make my conclusion ?
appreciate your help
Above is trying to idicate the points to test the coil ,on the left side .Doesn't need to be left could be right side.The test points should be near the center of ,and perpendicualr to one side or the other of the coil.
-.12 is the return ,plus + .90 is the supply,ignore the +&-,for a total of 1.02,way too high.
Think of static two ways ,first it's reading the resistance of the duct system,A coil, etc..
Second way is,your blower is capable of delivering "X" amount of cfms at "X" amount of static.This is why it's .90 before the coil and .1 after,.8 static(Pressure Drop) thru the coil was used up,leaving only .1 on the other side of the coil.
The coil when clean is likely around .35 ,in a "wet " condition,certainly no more then .45.So it's dirty and currently "using up" too much of the static the blower can produce.
At the current static the blower can't deliver the required air flow .When you clean or replace the coil,expect the readings to change a lot.If it's a good duct system it may be something like this:.20 return,.50 before the coil,and .15 after the coil,Just a guess,but you need the total of ,before the coil,and the return added together to be below,.70 or maybe even less(see manufacturers fan data chart).
Your next question might be ,how does the after coil and return static increase after cleaning the coil?It's because you now have lower overall static(.70) and ,more air (thank the lord sez the furnace,and compressor),is moving thru those ducts than before.Just as the static increased when you pulled the filter out.
[Edited by dash on 05-20-2005 at 05:04 PM]
you said a good system would have a total static of .7 in
I thought the max pressure was supposed to be .5 in. Is this not correct. In another post it was said that most houses have high static. Do you agree and what static would you tolerate before recommending a change. Does your range of tolerable static differ for a PSC or a VS blower?
We use the mfr. fan data when ever possible.Though the name plate may indicate .5 ESP on the equipment,the fan data may show that it can be higher,and still deliver the required air flow.
Lower is always better,but cost to get it lower may not be worth it.
PSC see mfrs chart,maybe as low as .35 or as high as .7.
Variable speed,see the chart,but .7 or .8 is often possible.