Static pressure - dash

[jacob perkins]

negative numbers, dash.

-.25 is smaller than -.05

stickaround,though im going to give him somemore of advice and need to hear your critique...

[dash]

Originally posted by davidr
In a gas furnace application:

After the filter & before the evaporator = Total external static pressure,this is the number you use to determine fan airflow from the fan performance data for that piece of equipment. These two numbers are added together disregarding the positive & negative signs.

Before the filter & after the filter = the pressure drop of the filter.These two numbers are subtracted from each other.Ideally should not exceed 20% of the fans rated capacity normally .10" W.C.

Before the evaporator & after the evaporator = the pressure drop of the evaporator.Again these numbers are also subtracted from each other.Ideally should not exceed 40% of the fans rated capacity normally .20" W.C.

Be sure to take your readings with a wet coil as this will have an impact on your readings.The pressure drop difference between a wet coil & dry coil can vary as much as 40%.

Check out http://www.nationalcomfortinstitute.com I believe they offer free downloads on how to measure static pressure.

Hope this helps.

Nice job,just like to add that if you have the mfrs.,fan data or curve,read the footnotes ,does the data ,include or exculde the filter.

Also the wet VS dry coil,and deducts from heaters in the air handler.

[davidr]

Originally posted by jacob perkins
negative numbers, dash.

-.25 is smaller than -.05

Even though it has a negative symbol in front of the number this is still an absolute pressure the fan is being exposed to,so .25 would be more pressure.

[jacob perkins]

Originally posted by bananaboy
Correct, these are the readings I took.

(0.25") without the dirty filter
0.05" when the dirty filter is "in"



Higher readnings (like 0.25" in this case) means more air is passing through my sensor, does it not ?

Yes there is more air passing,but being in the return it is measured and given a negative value to be compared or added to the positive value measued at the supply.

[dash]

+.25,+.20,+.15,+.10,+05.,0,-.05-.10,-.15,-.20,-.25



-.05 ,plus -.05 ,plus -.05, plus -.05,plus-.05,equals -.25


Just part of why we all get confused at times.

[Edited by dash on 07-09-2005 at 03:04 PM]

[MadeinUSA]

Originally posted by jacob perkins
Okay banananaboy,lets start here:
you are asking is
-.25 more than -.05?
the answer is no.
-.25 is smaller than -.05


With a negative number,the farther you get from zero on a number line the less it is

number line:
9876543210-1-2-3-4-5-6-7-8-9

With a positive number,the farther you get from zero the more it is.

Jacob:
As far as “whole” numbers are concerned you are correct. When we take readings of ESP though, yes there are positive and negative numbers, but we essentially ignore the positive and negative signs and are just looking for the difference in the numbers to gain our ESP readings.

[jacob perkins]

I was only trying to see where bannanaboy was confused.
That was the only thing I could think of
where'd he go?

[Edited by jacob perkins on 07-09-2005 at 03:00 PM]

[bananaboy]

Last question:

If I only stick my electronic manometer at the inlet of the furnace (before the filter)- and I get (-0.10"w.c.) reading, is that a usefull info ? Does it tell me anything at all ?

[Jultzya]

It tells you that the filter/return duct system is in good shape.

[dash]

It tells you that the ESP of the return system excluding the filter is .1,which if "all else is well",say it's a new system on the correct fan speed, the return is fine.


The filter,coil,etc.,as mentioned before could be clogged,which reduces cfms,which lowers duct system static.