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Manual D on existing house if all ducts have dampers?

aunsafe2015 | Posted in Mechanicals on

I’m having some home performance diagnostics (blower door, duct leakage, etc.) completed on my house (that was built in 2010), and the contractor has suggested that he perform some HVAC design tests including manual J, manual S, and manual D.

My question concerns manual D.  All of the ducts are the kinds that have the dampers on them where the duct attaches to the plenum, so the amount of airflow to the duct at the plenum can be controlled.  Duct leakage was within acceptable ranges as shown by the duct blaster test, and with my existing HVAC system (which I am thinking about replacing), air flow and distribution seems fine, all rooms are generally within a degree or two of all other rooms, etc.

Given that I don’t anticipate significant duct work, and all ducts have dampers for some amount of additional control, is it worth it to pay the extra money for a manual D?  Or could I skip that?


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  1. Expert Member
    Dana Dorsett | | #1

    Are they dampers, or balancing vanes?

    Even with balancing vanes on the supply ducting a Manual-D is still in order. Manual-D is as much about balancing supply with returns as it is about getting the room-to-room supply correct. Deficiencies in return paths can often be corrected without breaking into the duct system or adding new ducts, but if you don't know where they are or how bad they are you're probably not going to think about it (but you'll be paying for it in higher energy cost & or lower comfort.)

  2. Jon_R | | #2

    > some home performance diagnostics

    Make sure they include checking room-to-room pressure differentials.

    1. Expert Member
      Dana Dorsett | | #3

      That's a good idea, but may require other equipment. To meet Energy Star duct system would be balanced to less than 3 pascals (0.012" water column) of room to room pressure difference under all air handler speeds, room doors open/closed. That isn't likely to happen if it doesn't meet Manual-D (but it's also not a guarantee that it does meet Manual-D.) But manometers normally used for duct diagnostics don't have resolution quite that low.

  3. aunsafe2015 | | #4

    Thanks for the responses. Any thoughts on ballpark reasonable pricing? For manual j, s, and d, I'm looking at $1300-ish for a 2-story, ~3,000 sq ft house with 2 HVAC systems. The contractor seems knowledgeable and is the same guy who would actually be installing and commissioning the new HVAC.

  4. GBA Editor
    Martin Holladay | | #5

    Aun Safe,
    For information on the cost of performing a Manual J, S, and D calculations, see this article: "Who Can Perform My Load Calculations?

  5. NormanWB | | #6

    Is he going to rebate the cost of the analysis when you have the system installed? This is a relatively common practice in my area.

  6. aunsafe2015 | | #7

    He didn't mention any rebates. I guess I'll get at least one more estimate and compare.

    I do know that most contractors in my area, even the ones with good reputations, do not do these sorts of design tests. In fact, one guy proposed replacing my 2.5 ton unit with a 3 ton unit even after I told him the 2.5 ton unit never runs more than 5 minutes at a time, and never more than 30 minutes in an hour, even on 96 degree days! (I live in Central NC, so design temps are closer to 90 around here)

    1. Jon_R | | #8

      A Manual S will show you that sizing isn't a simple as you imply.

      1. aunsafe2015 | | #9

        So you are saying a properly functioning unit that never runs more than 30 minutes in an hour even on greater than 99% design days might not be oversized? Please explain.

        In any event, the guy that proposed replacing it with a larger unit did not do any design tests. He said because the new unit would be variable speed and the old one is single speed, it would be fine. Obviously not how a system should be sized...

        1. Jon_R | | #10

          Fan flow rates, latent vs sensible mix and register throw are some reasons that you can't just use current max duty cycle to select a properly sized new unit (which could mean any mix of compressor/evaporator/fan/ducting.)

          1. aunsafe2015 | | #11

            Fair enough. I was not trying to imply that the process is simple. I was just saying that when a unit is operating for only 30 minutes out of 60, at the peak heat of a day on a day that exceeds the 99% design temperature, it's highly likely that the unit is oversized, and to propose replacing it with a larger unit w/o even doing manual j, s, d is probably absurd.

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