air conditioning question

Eric_10

Can anyone tell the way for testing static pressure on fha systems. I know you check return then supply inches of h2o
and add them together. Im not sure what numbers to look for or how to compare the readings for trouble shooting?
Thanks for any help!!!!!!!!!!!!!!

Ken D.

Static pressure

Use an inclined manometer or a Magnehelic. Put low pressure tube in the return before the filter and the high press. tube in the supply after the AC coil but before any take offs. You will be able to read direct. The higher the SP reading the more restriction. Use manufacturers literature to determine what is normal for a particular system. Hope this helps.

Matt Undy

tubing

Couldn't you also use a length of small diamater clear palstic tubing witha bit of water in it, put one end in the supply, one end in the return, make a loop in it to form a manometer and measure the difference in the height of the water?

Matt

Fred Harwood

reading tubes

Yes, plastic tubes work also. However, an inclined tube is more sensitive, and ready-made manometers are inexpensive.

Matt Undy

I never thought about how inclinign it would make it easier to read, i just filled it with mineral spirt to increase sensitivity. If it remember its specific gravity was about .7 or .8.

Matt

Eric_10

static

I do appreciate some of this feedback but Im looking to know what readings to see to establish proper airflow.
As an example I was woking on a system today with a high crosscoil and wanted to establish proper cfm when I checked return I got -30 and on suppley I got +05 do I add these together to read 35 or 25? and how to match these numbers to where it needs to be? Thanks.

Frank_4

ESP

Take a good reeding at this.

http://www.contractingbusiness.com/editorial/serviceclinic/static.cfm

Very good info. All you need to know is there.

Hope this help.

Frank.

Paul B_3

you've got the right idea

You add them together, so you would have .35" wc. which you would then have to plug into the manufactures fan curve to determine airflow. Most residential furnaces are good for between .25 and.50" wc. external static pressure. On a guess I would say your return is a little small, it's out of proportion with the supply, assuming you weren't sensing turbulance at the fan outlet. You have to be careful not to take readings too close to the fan discharge or any duct fittings. Try cracking open the blower compartment door. If the return is restricted you should see the return static go down slightly,and the supply static should increase.

Paul

Ken D.

Static

Dwyer makes an inexpensive U tube manometer that can also be inclined. You can read direct which is a plus for someone not used to reading them. You can take gas pressure readings as well as static pressures. You can use it to check presure switches and other components. A very handy and necessary tool.

Ken D.

Static

Dwyer makes an inexpensive U tube manometer that can also be inclined. You can read direct which is a plus for someone not used to reading them. You can take gas pressure readings as well as static pressures. You can use it to check presure switches and other components. A very handy and necessary tool.

Jim Davis

Static Pressure

Your pressure on the supply side sounds low. It should be taken between the cooling coil and the furnace. Most systems are rated for .5"W.C. total static for proper air flow. Having a blower on too slow a speed can give a false low static readings, so knowing temperature rise or drop needs to be added to the equation for verification.

Al D'Ambola

Static pressure

on the supply side if this is an upflow unit should be taken as Jim Davis said between the fan and the coil. If you measure after the coil you missed the pressure drop of the coil(should be wet if possible). On the return side between the fan and the filter. Once you add these to together plot these readings on the fan curve this will tell you how many cfm this fan can move at that satic pressure.

The pressure drop across the coil(wet) shoulf be 40% of the total rated static and 20% for a loaded filter. Leaving about .2"wc for your complete duct system.

Eric_10

static

Thanks for all the feedback and Frank the website was very helpfull thanks...........

Jack_23

Static

If you chech the manuf rating plate it should give you cfm ratings at various static pressures. If you are unable to locate it then give the manuf a call.

I have a Dwyer 100-5 incline manometer which I have used for years for this app and it is just great. Steady as a rock and very good for oil draft readings (as opposed to the old Bacharach draft gauge which was all over the place and required frequent and costly calibration).

Put your sensing tube in perpendicular to the air flow and as you say, add them together. This is a nice test because if you end up with to high a static you can go out from the appliance taking readings at each fitting and frequently find the duct system problem (usually undersized returns, in my experience)

don_36

Wow!

All great replys.I also like to say boy how the wall has change for the better.

I remember not so long ago,one would leave with his head in
his hand if they brought up forceair here at the wall.

Personally,I for one have allways been fascinated by all
system that can move btus.
Thanks Mr Holohan for letting other see that there are those even know its forceair..do it right.

And thanks to all you Wetheads too.

Eric_10

static

Another option I was looking in to was the bacharach florite 800. It is supposed to calculate velocity to cfm and then you would know if you have the right amount for the tonage you are using. Has anyone used this tool? Good or bad?????

Don_39

Now

you're getting into flow rate and velocity..Personally never seen the tool,but cant hurt when you're trouble shooting airflow.

May I suggest a flow hood for balancing your system.

Now I'm not sure of your experience..but if i may offer a
few books that will help you to understand blowers and air-
side devices.

Manual D residential duct system.Understanding the friction
chart,both book offer by ACCA.

Suggestion,next time you pulled a Airhandler that still in
pretty good shape.
Hook it up on a base and pleum.Tap in some big collars with
dampers.Then run a wire from the motor to the fanrelay outside of the cabinet,for amps draw reading.

I have this setup at the shop,the crew and I get together
and play what ifs on the slow days.
just remember everything is limited and its design,because
of noise.

Just let me say Jroy,or what ever your real name,It good
to see you are trying to do it right.Good Job!