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newbie
06-17-2009, 06:34 PM
Hi, I have a question about static pressure. From what i have thought about it , I think static pressure is the pressure placed on the fan and the duct by air that is not moving. It is measured through water column and water column tells you the difference in pressures between the return and supply so that you can tell how much air isnt moving between the two and basically getting clogged in the system making the blower work harder etc. Lets say you filled a one liter bottle up with water and then drilled two holes either side of the bottom allowing the water to pour out. Static pressure would be the water at the top of the bottle pressing on the water at the bottom of the bottle trying to relieve pressure from gravity? And the more you fill the bottle up the faster and farther the water will squirt out of the holes and the emptier the bottle the slower and less far from the bottle the water would come out?


thanks for your help , on my own trying to figure this stuff out.

newbie
06-17-2009, 10:11 PM
is this just crazy talk? or am i on the right track?

device
06-18-2009, 12:03 AM
is this just crazy talk? or am i on the right track?
i could be off..
but is static pressure the resistance the surface of the duct has on the air moving through it..

Basher
06-18-2009, 02:19 PM
i could be off..
but is static pressure the resistance the surface of the duct has on the air moving through it..

Bang on my friend.

Basher
06-18-2009, 02:20 PM
Water column is just a way to measure pressure. IE. One PSI is equal to 27.68 Water Column.

danski0224
09-26-2009, 06:11 AM
Hi, I have a question about static pressure. From what i have thought about it , I think static pressure is the pressure placed on the fan and the duct by air that is not moving. It is measured through water column and water column tells you the difference in pressures between the return and supply so that you can tell how much air isnt moving between the two and basically getting clogged in the system making the blower work harder etc. Lets say you filled a one liter bottle up with water and then drilled two holes either side of the bottom allowing the water to pour out. Static pressure would be the water at the top of the bottle pressing on the water at the bottom of the bottle trying to relieve pressure from gravity? And the more you fill the bottle up the faster and farther the water will squirt out of the holes and the emptier the bottle the slower and less far from the bottle the water would come out?


thanks for your help , on my own trying to figure this stuff out.

I think the airflow part of your analogy is sound... not so sure about the water part.

By definition, static pressure is the pressure exerted in all directions. Think of an inflated balloon with the opening closed. The air within is exerting static pressure on the skin of the balloon. Open the end of the balloon, and the result is velocity pressure.

Inches of water column is just a way to measure the pressure. Measuring in PSI would be just as useless as measuring in inches of mercury if you want to measure static pressure in a typical residential/light commercial environment.

If things like ductwork and filters are too restrictive, then your static pressure goes up. Restriction comes from poor fitting design, long runs of flex, duct that is too small, those "high efficiency" 1" pleated residential furnace filters... etc.

Fans are rated to move a given quantity of air at a certain static pressure.

If a furnace fan is rated to deliver 1600 CFM at .5" of water column, and you measure .8" WC of total external static pressure, that fan may be moving only 1200 CFM of air... yet you are paying to move 1600 CFM. Makes a big difference in equipment efficiency- and that is before figuring duct losses (air/insulation leaks) or duct gains (ducts in an attic poorly insulated/return ducting not sealed).

Here is an example- that 4 ton air conditioner needs roughly 1600 CFM of airflow to work properly. If the ductwork is only moving 1200 CFM of airflow then you are only getting 3 tons of cooling out of a 4 ton system at the equipment, before measuring system losses. Putting a bigger AC unit in to "solve" a poor cooling complaint without addressing the ductwork will make the problem worse- not better.

Those "high efficiency" 1" pleated filters are not a good thing for the typical residential installation unless they have been accounted for in the duct design. Very easy for the filter to be responsible for a significant pressure drop and resulting reduction in airflow... which leads to poor cooling performance and heat exchanger problems due to low airflow... which gets worse as the filter loads up.