Results 1 to 13 of 13
  1. #1
    Join Date
    Aug 2006
    Location
    Windsor area Ontario Canada
    Posts
    285

    Confused External static pressure

    when measuring external static pressure with a magnehelic gauge, and you change your ESP to a CFM reading using a chart, what is the recommended CFM you should get or does it vary? im not really familiar with this? i was told your ideal ESP should be around .5 '' WC is this correct? sorry if this is a dumb question but my company does not even have one of these gauges and im pretty sure i need to get one!!!!!! any help would be great thanks!!!!

  2. #2
    Join Date
    Aug 2006
    Location
    Windsor area Ontario Canada
    Posts
    285

    ESP contd

    and also at would point would the ESP be low enough to cause freezing ?

  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    The "recommended ESP" is whatever the manufacturer's blower curve states it should be for the amount of airflow needed for a given application or system design.

    In other words, say you have an air handler with blower curve data, and you see that at .5 ESP it will move 1400 cfm, and at .2 ESP it moves 1600 cfm (both examples blower set on high speed). Let's now say it's a four ton unit and you like to set your systems to run around 400 cfm per ton. At .5 ESP you won't see 400 cfm/ton, more like 350 cfm/ton. Now, 350 cfm/ton might not be a problem, particularly in more humid regions of the country, where lower airflow produces a colder evap coil and thereby pulls more moisture out of the air. But it can be a problem when the sensible load on the evaporator coil increases, such as on a hot day with lower outdoor humidity. The unit may come up a little short on sensible capacity, and the interior space temperature begins to creep up.

    The key here is following the manufacturer's data, if it is available, for determining if the system you're working on is within an acceptable range for ESP.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Join Date
    Aug 2006
    Location
    Windsor area Ontario Canada
    Posts
    285

    Esp

    thanks for your help, so the manufacturers data will say how much CFM per ton is needed? and then you just follow the chart, pretty simple!

    If there is no manufactures data (cant read it) then is there a rule of thum i should follow or does the data need to be obtained!?

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    No hard and fast rule of thumb, but a general consensus for residential installations is to shoot for ESP staying at or below .5

    Tonnage varies with load. If a system is designed and installed perfectly and ARI rating test conditions are imposed on the system, it will run at or near nominal capacity. Real world sees a pretty wide variance in capacity of a running system, due to many factors. The 400 cfm/ton rule is nominal, but actual operating conditions will vary on either side of it, often with no detriment to system performance or longevity. However, if I did a capacity check on a system and it showed something like 200 cfm/ton I wouldn't have to think long and hard that the system might be starved for air. Other parameters you measure would also confirm this, such as your superheat and subcooling readings.

    Generally, the higher the cfm number on the cfm per ton side of the ratio, the more sensible cooling the system will do, and it will not do as well at dehumidifying. Newer systems can intentionally slow blower speed for the purpose of pulling more moisture out of the air, and can also kick into higher gear should more sensible cooling be needed.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
    Join Date
    Aug 2006
    Location
    Windsor area Ontario Canada
    Posts
    285

    Wink Great explanation and 1 quick question

    thank you so much that cleared up alot for me, im not longer in a gray area with this subject!

    One more Quick question can i use my digital manometer to take ESP readings instead of the magneuhelic gauge!?

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    Quote Originally Posted by markmast View Post
    thank you so much that cleared up alot for me, im not longer in a gray area with this subject!

    One more Quick question can i use my digital manometer to take ESP readings instead of the magneuhelic gauge!?
    Yes, definitely re: digital manometer. I'm looking to pick up the little one made by Testo...easy to carry around and it has a wide range, and doesn't need to be held level like a Mag, AFAIK.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    To get the correct cfms,which come from a Manual J load calc.,you'll need the fan chart to make the correct speed choice,with PSC motor.Otherwise could be too high or too low ,effecting the sensible and latent capacity of the equipment.

  9. #9
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by shophound View Post
    No hard and fast rule of thumb, but a general consensus for residential installations is to shoot for ESP staying at or below .5

    Disregarding the effect of a wet coil (minimal, I assume), should SP be the same for heating as well as cooling?

    AM

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by ampulman View Post
    Disregarding the effect of a wet coil (minimal, I assume), should SP be the same for heating as well as cooling?

    AM
    Only IF the cfms required are the same for heating and cooling.Disregarding the wet coil as you stated,however the wet coil in cooling increases the resistance,so air flow in the ducts will be reduced slightly ,which will reduce the static in the ducts.

    Above is for PSC motors,not variable speed.

  11. #11
    Join Date
    Nov 2005
    Posts
    367

    variable speed

    so what happens with variable speed?

  12. #12
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by dash View Post
    Only IF the cfms required are the same for heating and cooling.Disregarding the wet coil as you stated,however the wet coil in cooling increases the resistance,so air flow in the ducts will be reduced slightly ,which will reduce the static in the ducts.

    Above is for PSC motors,not variable speed.
    Dash: I'm a little confused on one point. Doesn't increased resistance across the (wet) coil contribute to increased SP? Can you have increasing SP measured at the coil and decreased SP downstream at the same time?

    AM

  13. #13
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    The term is ESP(external static pressure),so external to the air handler.The wet coil is internal to an air handler,but the wet coil still effects the air flow.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event