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Reducing Static Pressure in HVAC System

noew | Posted in Mechanicals on

We are replacing our gas furnace in the basement with a centrally ducted heat pump (the air handler will be the Mitsubishi PVA-A24AA7). My contractor is proposing some modifications to the 1st floor ducting, whereas the 2nd floor (supply only) will remain as is (total cost for ductwork $6,500).

Based on reading articles on this site and others, I have specified that the return “drop” should be replaced with a larger size to reduce the static pressure in the system. I wasn’t quite sure about the size that I should request, so I just specified that it should be able to hold a filter box for a large surface MERV 13 filter in horizontal installation. Using Bailes’ rule-of-thumb, I requested a filter with a 4 sqft surface area.

My contractor questioned this idea. He says, “The standard external pressure dropped at any air handler or furnace blower motor is .5”.  If we put in the system, as the duct system sits right now, it would be well beyond .5 inches. That’s why I’m curious, it seems like a little excessive to have such a giant return drop, when the rest of the Duct system is standard/traditional.  Having a rather large return drop is great, but if the rest of the system is standard, we’ve gained little. [….] what are you looking for, total external static pressure? It will likely require rebuilding the entire duct system.”

Does it make sense to insist on a bigger return drop, despite the “standard” duct system? How should I answer his question about the expected total external static pressure? He says that a 16″x25″x5″ filter is “total cake for a small 2 ton system”.

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Replies

  1. kyle_r | | #1

    For low static pressure slim duct air handlers you have to watch your static pressure very carefully. The PVA air handlers are standard static pressure air handlers.

    Is your old air handler still installed? Have the contractor measure the static pressure of that unit and look up how many CFM that unit was rated for. Compare that to the unit you are going to install. I would guess that the new unit has a lower rated air flow. As long as the static pressure is lower than what the PVA can handle (0.8 in I think, but check) at an equal or higher flow rate than the PVA’s max flow you are good. You still need to account for the new filter, but your contractor should be able to account for this.

  2. Expert Member
    AKOS TOTH | | #2

    Like Kyle said, check first. In most cases the existing duct is fine for a high static air handler.

    The big filter is needed for a low static air handler, the 16x25x5 should be fine for the 2 ton PVA. The only time I would go for larger is if the static pressure of the ducting is borderline, which case a larger filter will buy you some headroom. If the static pressure is still too high, you can put a grill on the return in the basement as well to help.

    Either way, $6500 is beyond silly for a run from the basement to the main floor. I would get the installer to put in the heat pump as is than have a GC come and price out running a return to the main floor and up to near the ceiling of the 2nd floor. Without a largish high return on the 2nd floor, cooling tends to suffer. There is not much point in upgrading just the main floor. Lot of times you can run the duct in a hallway corner or the back of closets for a simple and clean install.

    The PVA is a commercial unit. I would go for the residential version the SUZ-KA24NAHZ + SVZ-KP24NA. Performance is close enough to not matter and a couple thousand cheaper.

    1. noew | | #4

      Thanks for these reassurances and suggestions Akos, particularly the idea of having a GC do the ductwork.

      The problem with the SUZ-KA24NAHZ + SVZ-KP24NA combo is that it is not on the MassSave approved list, so I wouldn't be able to get the (recently much expanded) rebates. Do you know of any other comparatively sized system with hyper-heat capability, other than the P-series? We have a heating load of approx. 27k at a design temp of 0F.

      1. Expert Member
        AKOS TOTH | | #6

        You can look at the half size up the SUZ-KA30NAHZ + SVZ-KP30NA.

        Fujitsu, LG, Daikin and Midea all make one. This is a good resource for searching, select "Single Zone Centrally Ducted":

        https://ashp.neep.org/#!/product_list/

        The is a Carrier branded Midea unit that has great performance but would be a bit undersized. It would work quite well with a small strip heater added.

        https://ashp.neep.org/#!/product/33276

  3. walta100 | | #3

    The first question in my mind is do you need MERV 13 filtration?

    Sure it would be nice but want vs need. For a few people with medical issues, it would be a need.

    I tend to leave my doors open for me returns are overrated. In fact, after a year of living in my home I discovered the contractor had forgotten to cap off my return ducts in my basement so all the return air was coming from the basement and not the grills upstairs. After the ducts were fixed any differences remain imperceivable to me.

    The way I see it bigger duct are always better than smaller one. The air will move at a lower speed and will make less noise all good things the problem is big ducts take up space that may be in short supply and cost more money to buy.

    To my ear $6500 sounds like a I don’t want to do this job price tag.

    Walta

    1. PBP1 | | #9

      Agree with Walta, why MERV 13?

      Mitsubishi supplies the SEZ ducted with a thin woven mesh filter. Though, as with many, I had filter boxes installed teamed with low pressure drop washable filters. For the low static SEZ, size of filter and type of filter makes a significant (huge) difference on airflow (I have anemometer measurements that prove that point).

      Here's a good article: http://www.homeenergy.org/show/article/nav/issues/page/4/id/667
      "Filters were originally conceived to protect heating and cooling equipment—for example, to prevent large particles from clogging the air passages of coils. "

      See the plot for filter pressure drop at 492 FPM, the Web Lifetime MERV 8 is below 0.30, which is great for low static systems and its a "lifetime" filter. It's a step up from the Mitsubishi OEM filter.

      For IAQ, portable air cleaners (PACs) are great, when needed. Here's an excellent study on IAQ, including thermal stratification during heating such that IAQ in the people occupied space/level became an issue: https://pubmed.ncbi.nlm.nih.gov/34477251/

      By using high MERV / high pressure drop filters, one risks lower FPM/CFM and increased thermal stratification, which operates much the same as an inversion over a valley where air quality becomes quite poor at the valley floor. In the study, PACs were used to both improve IAQ and breakup thermal stratification.

      If IAQ is an issue, may be worth considering a PAC rather than burdening the air handler.

      I see 16″x25″x5″ filters are about $20 a piece or more. Beware that zigzag is not a good substitute for cross-sectional area. There are very detailed studies on how pleated filters actually work, along with pros/cons.

      From Dr. Bailes: https://www.energyvanguard.com/blog/do-high-merv-filters-always-reduce-air-flow
      The final result I’ll mention here is that they didn’t see as much difference as they expected for filters of different depths. For example, “the 4-inch Filtrete 1550 (MERV 12) was only marginally better than the 1-inch Filtrete 1700 (also MERV 12) and the two other [1-inch] MERV 11 filters of the same brand (1000 and 1085).”

  4. kyle_r | | #5
    1. noew | | #7

      Thanks for this, Kyle. I couldn't get real information from the official Carrier website when I looked. Thanks also for your instructions about static pressure measurement above.

    2. noew | | #8

      Thanks for this suggestion Kyle.

      I am very interested in the 38MARBQ24AA3 + 40MBAAQ24XA3 combo, but the performance data here looks quite odd: https://ashp.neep.org/#!/product/33600/7/25000///0

      Do you know whether that strange dip at 17F is some kind of mistake? Because the stated performance from their own materials doesn't show it: https://ca-manufacturer-documents.s3.amazonaws.com/CarrierDocs/38MARB-03PD.pdf

      1. kyle_r | | #10

        I have found many errors on the Neep site, I would trust the manufacturer’s literature as they are the ones that report it to the Neep.

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