Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

duct problem

Mike Member Posts: 94
Not sure what you mean by 18" hieght of duct? But, I would look for open branch ducts or missing end caps. Pressure is way low. Check blow and filter, make sure they're clean. Amp blower motor, see if you can rev it up.


  • zeke
    zeke Member Posts: 223
    duct problem

    Have a system with suction pressure at intake plenum .15" and pressure after heat exchanger .1". Understandibly no or little air out of second floor registers. I get a calculated pressure drop of .2" due to the height of the air 18". How can I fix it? Thank you for your input.
  • JimGPE_25
    JimGPE_25 Member Posts: 7

    I believe you are not getting many responses because your question is not very clear.

    First, there is no pressure drop associated with a column of air like there is in an open hydronic system.

    Second, what kind of system is this? A residential furnace? A commercial air handler?

    Third, I assume it is an existing system or else you would not be measuring air pressures, correct?

    The straight forward answer is if you are not getting the correct airflow, either change the duct system (eliminate airflow resistance) or spool the fan up and generate more static pressure. The problem with spooling up the fan is that you run out of horsepower very quickly. HP required goes up with the cube of the fan RPM - add 10% to the RPM/CFM the horsepower goes up 33% - and normally there is not that much extra HP in a fan motor.

    Care to add some more info?
  • zeke
    zeke Member Posts: 223
    correction 18feet, not 18 inches

    Typo error,
    Thanks for,the replies.
    I meant that the supply registers on the second floor are 18 feet above the output static I measured of 0.1" and according to my calculations you lose at least 0.2" due to the height of the air column, so you should have zero air on the second floor. Please correct me if I am in error.
  • Brad White_122
    Brad White_122 Member Posts: 9
    Not applicable really

    Truth be told you will have a slight chimney effect such that you will draw air by buoyancy and actually help airflow. (If your logic were followed, you would be having backwards airflow.)

    The static pressure rate (decay rate) is often expressed in "inches WG per 100 feet of duct". What is your best guess at CFM? What is the duct size? I can take a stab at it, but whatever it is, it would be 20% or so of the rate per 100 feet. In other words, if you are running a rate of 0.10" per 100 feet, the register run would lose 0.018". That pressure loss is proximate to one MSF- Metric Squirrel Fart.

    Now, from what you mentioned in your first post, it seems that the pressure drop of the heat exchanger is 0.05". Tell me, Zeke, were the pressures negative or positive? If you could draw a mental profile of the system for me, starting with the return plenum, then filter, then blower (or is it draw-through?), Heat Exchanger, discharge. And what is the total pressure measured across the fan?

    Help me help you.

  • zeke
    zeke Member Posts: 223

    Thanks for responding. The sp readings were taken at the intake plenum just before the filter and in the main trunk just after the airhandler and were negative 0.15" and 0.1" respectively. As of yet I have not taken the pressure rise across the blower, nor the motor current. The blower is positioned just after the filter.
  • zeke
    zeke Member Posts: 223

    Jim, Brad and others responders,
    Jim, on thinking this over you are absolutely correct. There is no change in the flow due to a height differential, since the higher floors discharge into correspondingly lower ambient pressure.
    However,I still have the low flow problem and with .1" static at the takeoff; one would think that should have been enough to get the proper flow. I have no access to the duct and can only surmise an obstruction or necking down in the run. Do you think an inline addon blower would be effective to correct this?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Going Negative

    If as you say, Zeke, your pressure after the air handler is also negative, I have to ask if you checked your blower rotation? If the polarity is off it may run backwards which can produce the lackluster pressures you see.

    Can you see the fan as it is running? Start there and then we can hunt down duct leaks, unicorns and other invisible beasts.

  • Empire_2
    Empire_2 Member Posts: 2,343
    HI Zeke:

    Just a thought,...Did you happen to pull the blower housing? There is a baffle plate that protrudes into the discharge of the squirrel cage. With out that you will tend to free wheel loosing your delivery static that the blower is supposed to produce.

    Mike T.
  • zeke
    zeke Member Posts: 223

    Thanks for the reply. I did not pull the blower housing but I might now do so at your suggestion.
This discussion has been closed.