Static pressure - dash

[bananaboy]

I do not understand the concept of measuring the Static pressure.

Today, I went to an A/C call, found a totally plugged filter and decided to do a little experiment. To be specific, I decided to take Static pressure reading with and without the "plugged up" filter.

So, I drilled a hole above Evap. coil and one on the Return duct very close to the filter (2-3").


Here are the readings I took:

- above Evap. coil ==> 0.09"w.c.

- just before the filter ===> (-0.25"w.c.) that is without the filter.

- when put the "plugged up" filter in ==> (-0.05"w.c.)


Seems that when the "plugged up" filter is IN, the readings are less. And I was expecting to see higher readings.

That tells me that when the air flow is slowed, the readings are less (smaller).
But when the filter is OUT (of washed) there is more airflow passing through the my "hooks-sensors", thus the higher readings.

I am totally confused.


If I block all my Return openings in the entire house - what kind of readings should I expect before the filter ??? Higher or smaller .... readings ,,,,, ???

[Jultzya]

So, let me see if I understand your info...

No filter, return = .25wc

Dirty filter installed, return, on inlet side of filter = .05wc

Is this correct?

If so, take a measurement between the filter and the furnace return inlet.

Or a pressure drop across the filter.

[bananaboy]

Correct, these are the readings I took.

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

Sorry, can not measure the drop before and after the filter since I left the customer's house.

I am trying to understand the concept of measuring the Static pressure.
If there is a strong flow - will my "hook sensors" show higher readings ? Sounds logical to me. But that is contrary to what I've read here and in some literature.

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

[Jultzya]

Correct, but if you are measuring the filter inlet and not the furnace inlet, you will have a lower reading with a plugged filter.

But if you measure the furnace inlet, the reading will be higher.

[Mr Bill]

Originally posted by bananaboy
decided to do a little experiment.

I use to have the time for experiments many,many moons ago, tried walking on the sheetrock once instead of the ceiling joists, needless to say it failed had to much static weight. :D

[dash]

With the dirty filter installed,checking the "before" the filter,it will be lower because you are moving less cfms.If you had checked it "after" the filter it would be higher with the dirty filter.

You would see the same result if you lowered the fan speed,ESP would be less,because you're moving less air,raise the speed and ESP will increase ,due to increased cfms.

On an older system:

1.If the blower is dirty,ESP will be lower ,due to the blower moving less air.

2.With an air handler,if the coil is dirty,ESP will be lower,due to the dirty coil PD,causing less air flow.


It's always best to check the static ,before and after, the coil and filter.Be sure the blower is clean.

[davidr]

One situation that can give you static readings that are out of line is on a 90% furnace.
If the recouperative coil is plugged up it will give you lower than normal static pressure readings.
This gives the misconception the fan is moving more air than it really is.

I am not aware of pressure drop info for these since they are included in the static rating for the furnace.
The access to obtain this reading would be difficult to say the least.
This is a good reason to measure static pressure on start up & on every call to establish some type of trend data over a period of time.

[bananaboy]

O.K. so my Total ESP would be between *** before the Filter to After the Evap. coil *** which in this case is the sum of 0.09 and (-0.25") disregarding the + and (-) signs, which is 0.34" it is within the 0.5" specified by the manifac.

So, can I assume the Evap. coil is clean ?

I gather that the ESP is to be measured across Evap. coil, or accross Filter, or accross Furnace itself. In esence, it shows the resistance (if any) that the air flow is experiencing due to some obstractions.

[dash]

Total ESP should be measure at the inlet and outlet of the indoor unit.


Air handler,inlet and outlet.


Furnace ,inlet and oulet(between furnace and coil).


It's best to take the inlet ,just after the air goes thrru the filter,so the return ESP includes the PD of the "clean" filter.

The before and after the coil is to get and idea,if the coil is dirty ,impacted ,etc.Really helpful if you have the factory static/PD of the coil.

[jacob perkins]

Originally posted by bananaboy


Here are the readings I took:

- above Evap. coil ==> 0.09"w.c.

- just before the filter ===> (-0.25"w.c.) that is without the filter.

- when put the "plugged up" filter in ==> (-0.05"w.c.)


Seems that when the "plugged up" filter is IN, the readings are less. And I was expecting to see higher readings.

That tells me that when the air flow is slowed, the readings are less (smaller).
But when the filter is OUT (of washed) there is more airflow passing through the my "hooks-sensors", thus the higher readings.

I am totally confused.


If I block all my Return openings in the entire house - what kind of readings should I expect before the filter ??? Higher or smaller .... readings ,,,,, ???


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.

that's the way they taught us in prison,unless you are are on death row thats the way we count...

[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.

[Jultzya]

First, we need to know what kind of equipment you are working on... as there are different test points for different equipment.

Second, no you can't assume the evap is clean, unless you take a pressure drop accross the coil and know what the drop should be for a given coil (wet or dry).

[dash]

Originally posted by jacob perkins

Originally posted by bananaboy


Here are the readings I took:

- above Evap. coil ==> 0.09"w.c.

- just before the filter ===> (-0.25"w.c.) that is without the filter.

- when put the "plugged up" filter in ==> (-0.05"w.c.)


Seems that when the "plugged up" filter is IN, the readings are less. And I was expecting to see higher readings.

That tells me that when the air flow is slowed, the readings are less (smaller).
But when the filter is OUT (of washed) there is more airflow passing through the my "hooks-sensors", thus the higher readings.

I am totally confused.


If I block all my Return openings in the entire house - what kind of readings should I expect before the filter ??? Higher or smaller .... readings ,,,,, ???


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.

that's the way they taught us in prison,unless you are are on death row thats the way we count...






.25 is smaller then .50 ,not .05

[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.