Static Pressure.

[GarnetJax]

One of the measurements I make during Heat inspections is that of static pressure. The ideal static measure is: l 0.5l inches of water. So in a balanced return vs. supply it should be: l-0.25l + l+0.25l=l0.5l or simply 0.5 in wg. But this is rare if ever. Most of the time the return will be significantly higher than the supply. (-0.4/+0.1). Or since that heat is run at a lower speed the sum is significantly less than 0.5 in wg (-0.2/+0.07). Then on rare occasions I will get readings in the opposite extremes (-0.15/+0.4).
I would appreciate feed back for the following:
1.What diagnostic can be made from these example readings.
2. What potential equipment damage can result if left unaltered.
3. How would the supply and return be altered to change these readings to meet the ideal ranges.

[acwizard]

I am not sure where you get that the ideal pressure is 0.5. All fans have a fan curve that compares static, cfm, and hp. The ideal static would be when the fan is operating at its most efficient point delivering the required airflow and not exceeding the rated hp.Furnaces should be checked for heat rise, blower amperes against nameplate ratings. If the recorded results are within specifications then the actual static will be inside the rating of the blower.Excessive static is usually caused by filter and evaporator coil maintenance or design problems associated with poor duct design.

[beenthere]

As the wizard said, .5" is not the ideal. Some air handlers are rated at .3"wg.

Fitting loss readings are sometimes required to make a decision on which alterations will make the most improvement in air flow.

[cuchulain]

The unit itself usually has a static pressure number on it and as wizard and beenthere have said it's not always .5. Also in the units manuals and such there is an airflow data chart which shows you based on the fan being one speed and the ESP being x then your getting x amount of cfm. If that number ends up being less then what the unit is rated for then you'll need to make adjustments accordingly.

[GarnetJax]

0.5 in wc is from an inspection template given to me by my company. The comparison the temp and IFM amp to the name plate is something I also do.

So the blower wheel static rating is posted on the blower itself?
Thanks for the feedback.

[54regcab]

Are you taking the supply reading before or after the A/C coil?

[second opinion]

0.5 in wc is from an inspection template given to me by my company. The comparison the temp and IFM amp to the name plate is something I also do.

So the blower wheel static rating is posted on the blower itself?
Thanks for the feedback.

the 0.5 on the furnace name plate is the design static pressure, the 0.3 on the air handler includes the evaporator coil. Most coils have around 0.2 to 0.3 that needs to be subtracted from the furnace 0.5

Not all gas furnaces will have a coil added to the system.

A fan curve chart is supplied showing the selected fan motor, fan cfm, and at what pressure drop, or static pressure.

[Redwood650]

Static pressure is something my trade school didn't go over as much I thought they would have. I have similar issues with SP as the OP. Anyone have any good info on SP that i can read up on? My work requires I check it on all calls, and I would like to know alot more about static pressure then I actually do...

[second opinion]

Static pressure is something my trade school didn't go over as much I thought they would have. I have similar issues with SP as the OP. Anyone have any good info on SP that i can read up on? My work requires I check it on all calls, and I would like to know alot more about static pressure then I actually do...

for some help. Unfortunately most trade schools do not teach what students need to know.

[Desuperheater]

I tried to answer this but timed out due to there being so darned many things to say about ESP, TSP and design.
.5 ESP is a middle of the road number. A decent duct design and a fairly well matched ahu/furnace running adequate CFM will typically fall into this area.
Less ESP at equal CFM means the blower is working less, might last longer and will use less energy. Saving money on duct design and construction will always spend it on electricity and fuel. Always.
Too much return pressure can be from dirty filters or filters of higher than normal efficiency. Restricted return registers or dirt and corrosion in the ductwork.
Too much supply pressure is usually a design problem. Too much flex duct. Too long a run. Too many turns.

[knott]

All equipment has it's own tesp....don't just go by what your company says go by the rating on the furnace.... And you should take it before the coil on a furnace

[hvacrmedic]

I tried to answer this but timed out due to there being so darned many things to say about ESP, TSP and design.
.5 ESP is a middle of the road number. A decent duct design and a fairly well matched ahu/furnace running adequate CFM will typically fall into this area.
Less ESP at equal CFM means the blower is working less, might last longer and will use less energy. Saving money on duct design and construction will always spend it on electricity and fuel. Always.
Too much return pressure can be from dirty filters or filters of higher than normal efficiency. Restricted return registers or dirt and corrosion in the ductwork.
Too much supply pressure is usually a design problem. Too much flex duct. Too long a run. Too many turns.

Ducts can be too large. Consider heat gain in a nonconditioned space. Heat gain increases with duct surface area and with slower moving air. You could easily lose more energy than what you save by oversizing ducts.

[Redwood650]

I have also been told to check SP without involving the evap coil. Meaning for example, an upflow furnace with the blower on the bottom and the coil above, you would check SP at the bottom, and just before the evap. Is that true? Every tech I have rode with has check after the evap coil and they claim that is how you can also tell of the coil is dirty...

[energy star]

little bit of pressure.

[cuchulain]

gas heater check just prior to blower and prior to the coil. Reason for that is a heater is designed to push x amount of air. The designers don't know what coil your putting on it so they do it for the heater itself. for an airhandler (coil and blower in same cabinet. ) check incoming and outgoing. Reason for this, is that the designers know what coil is in the cabinet and designed appropriately.

[Gib's Son]

Wow.....allot of questions being tossed out here.

To the OP, if your return sp is greater than supply the return is undersized or you have dirty filter. Systems that I do I always have the return sp is less. No rule of thumb, but I want to keep return sp low because it's easier to push the air than draw the air.

There is talk of pd across the coil, bear in mind the coil will have a pd for wet and dry conditions....always design for wet conditions and a dirty filter. The only real way to know if the coil is dirty is take a reading across the coil, I.E., before and after. Hopefully you will have mfr literature to tell you what the pd should be at the cfm your system is providing, and coil status (wet or dry)

The question was asked about literature to read...I will think about that and see if I an find something, but SMACNA manuals or ASHRAE fundamentals comes to mind. If you have a specific question, post and I will try and answer.

I disagree that dirty filters etc. result in higher operating costs the system is working more. It is actually working less. Take a blower and run it wild with no restrictions....amps go up. Stick board in front of the outlet and restrict flow and amps go down...it's moving less air and doing less work. Your losing efficiency, which wastes energy and is a loss of efficiency causing the system to run longer.

What is important is tesp. If the coil is cut in, add that to tesp when looking at blower curves. If the coil is integral to the air handler, disregard unless your trying to find out if its dirty.

[JBM1000]

Attachment 216462Before the coil period, and as close to a good center of R/Air at heater cabinet with filters installed. Your after the fan wheel input and output with a ****Wet Evap Coil****. This is full load evaluation at the home installed instead of just a factory chart. Remember Factory charts are bench test best guides to help the tech, it's up to you to make a real on site decision of many factors like supply vents closed and furniture to low to allow stratification of airflow, returnable airflow to the return grills and many many more more only real world experience techs finally learn home to home new or used!

[Redwood650]

Hm, sounds like there are alot of factors for SP and many routes to take...

Maybe just a general checklist of things to check for Low readings, and high readings.

Low SP causes & High SP causes.

I do understand now the purpose of checking SP without the coil in the way because the Manufacture designed the Furnace W/O the coil in place. What do people usually do in uplflow/downflow/horizontal applications for checking the supply side? Remove the high temp limit or drill another hole?

[davidr]

There is a member here by the name of rundawg who made a static pressure manual and posted it here some time ago.

It was a great piece on the topic.

Do a search on his name and I'm sure you'll find it.

If anyone is interested send me an e-mail or pm me and I'll send you some of the static pressure training material we use in our classes too.

[beenthere]

"hvac r us 2" made a video that will help many.