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Community and Q&A

Troubleshooting heat pump ducts

lmosakow | Posted in Mechanicals on

I’m trying to “debug” my recently installed slim ducted heat pump system with two trunks and several takeoffs. I did the Manual D myself.

The installer did not install it exactly to spec due to some constraints when installed in practice, and I’m now finding that certain takeoffs are not outputting the expected CFMs, which I’m measuring using an air velocity measurement device.

I have several hypotheses for why this is happening, one of which is that the longest run’s total effective length (TEL) is longer than expected.

A) Is there a way I can “measure” the effective length (EL) of different components of a specific takeoff—for example, a bend or a supply boot—so I can compare it to my Manual D assumptions and target my system adjustment where the mismatches are greatest?

In other words, assuming I’d have to use a manometer to measure static pressure differences before and after the component, is there a way to convert the static pressure difference to effective length?

B) Is it possible to put vanes or something that has a similar effect into a an existing plenum? I’m worried that my supply plenum effective length is much worse than it should have been and vanes would help reduce turbulence?

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Replies

  1. lmosakow | | #1

    Bumping this, would love the community's thoughts on this

  2. Expert Member
    Akos | | #2

    No need to figure it out. Measure the flow at each duct, if the total is what you need and the pressure is less than what the unit is rated for you are mostly there.

    From there start closing the dampers on the branches that have too much flow. Once you get each branch to the required flow, check static pressure again. If good, you are done.

    These branches are never fully balanced so you always need some adjustment.

    1. lmosakow | | #3

      Thanks! I have a central large return with a filter grille. Ie, the filter is not inside the duct downstream of the return grille. At which points in such a setup should I be taking my static pressure?
      A) outside the filter grille (not in the duct)
      B) After the return filter grille in the return duct before it
      C) in the supply plenum
      D) in a supply trunk after the plenum before the first take off?
      E) towards the end of the longest effective length takeoff before the supply register?

      Luke

    2. lmosakow | | #4

      So I did as you advised. I had the total number of CFMs I needed. I balanced the dampeners so each duct outputted what it was spec'd for. I measured the static pressure loss to be 0.25 in wc when measuring just before and after the blower. The unit is rated for 0.36 (my design anticipated a loss of 0.11, so things are more than 2x off, but still within the 0.36). Note: I don't have any grilles set up yet, so that might add a bit more loss. I'm measuring a 0.18 loss over my 12" by 24" central return filter (which will eventually be in a filter grille, now it's taped in).

      Is this within the acceptable bounds despite being way off from my intended pressure loss? Is the filter clearly the problem to focus on or not necessarily?

      1. Expert Member
        Akos | | #5

        0.25" is pretty decent, if you are getting the right flow at the register you are good.

        The filter drop seems a bit on the high side especially since it will increase as it gets loaded. I would bump that up to something like a 16x24. Otherwise I think you are ok.

        1. lmosakow | | #6

          Thank you so much! I should have mentioned that the 12 by 24" filter is a 2" thick filter whose size I determined from this article where it says you should have it's area be 2 sq ft per 400 cfm (my 12k unit is about that), which my filter is: https://www.greenbuildingadvisor.com/article/supply-air-filtration-and-airflow-pressure

          It is dirty though, so that may explain a lot of the problem.

          I'll increase the surface area anyway just in case.

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