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Community and Q&A

3rd Party HVAC design

dirkgently | Posted in Mechanicals on

So am a carpenter who is trying (and failing) to design my own hvac system. I am totally stumped on the Manual D calc portion of the project and wanted to find a third party to design the system. Third party design is referenced a lot in other discussions on this site but I have had no luck finding some one to do small residential. Can someone point me in the right direction on this?
should I be looking for an engineer?
I am trying to install my own duct work to save money. I have access to a metal shop. Punching holes in ceilings to run ducts is no big deal (dumb work) I need help with the smart work (design)
My plan is to keep the main trunk as short as possible so perhaps things like friction loss may not even come into play if I use mostly metal work.
I can provide any details on home needed. I really would like a standard split system (not a mini split) so I can get make up air into the fairly tight home.
Would blower door test results be used to help in calculations? The Manual J software I used did not have a setting for “fairly tight house” only tight or leaky.
I am also considering a High velocity system because they do offer some design support.

Tom Fraser

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  1. CramerSilkworth | | #1

    Hi Tom,

    Well first and foremost where are you / the project located? How big is the house, and what's the overall plan for the enclosure (R-values)?

    And I'm not sure how a "standard system" and makeup air are related - can you clarify?

    A blower door test would certainly be helpful, if you can do it at the right time - while the air barrier details are still accessible. Then you have the ability to make it tighter, otherwise if you wait til the end the number's only good for posterity.

  2. dirkgently | | #2

    Thanks so much for the interest. The home is in MA. It is a 1,700 sq ft. cape (1956) which has had extensive air sealing and insulation upgrades. Blower door test was in at 1542 @ 50cfm before my last round of spray foam insulation which turned all cape dormer areas into conditioned space. Walls are a combo of Fiberglass/dense pack cellulose and rigid foam.
    If I turn on my kitchen exhaust I have negative pressure in the home and my boiler back drafts down the chimney.
    My manual J calcs came back with total heat gain of 15,692 BTUH
    I was honest when entering the data and went for higher heat gain if anything.
    I would like to locate my blower unit in the now conditioned triangle shaped space of 2nd floor dormer attic knee wall areas. It would be near the middle of the house and keep trunk line I am not sure if friction losses come into play here. I also would like to use a minimum of flex duct (and use metal elbows if I do use flex for 90 degree bends).
    Any help appreciated. I really think there is a market out there for Manual D consulting. Most of the HVAC guys I know just guess.

  3. davidmeiland | | #3

    There must be residential HVAC installers there locally who could design the ductwork for you. There are people you could hire remotely, but you need a complete, detailed, accurate set of plans to send them.

  4. dirkgently | | #4

    I know a few HVAC people. But I get blank stares when the term "manual D" comes up.
    Being in the trades I would not expect any of the bigger shops to do "just a design" I would think they want it all or nothing.
    I got a quote thru the Electric company's energy saving program from a large shop and it was for 20k. and I thought the system was way over sized with 2 mini splits for 2nd floor and HV system for first floor. According to my Calcs I should be able to do the whole house with 1.5 Tons. But I would feel better rounding to 2 Tons.
    I did a submittable form for quotes on another green web site and was hoping that someone who responds for the RFP will be willing to do design only. Wait and see.

  5. watercop | | #5

    I use Elitesoft RHVAC for Man J, and if I enter the load data room-by-room, it'll return register, run-out, zone, and main trunk sizes. I can specify materials, roughness factors, min and max velocities, friction rate,heating and cooling delta-Ts, max CFM per register and it'll adjust sizing.

    Sounds like a two stage two ton two zone system would more than suffice. Carrier's Infinity zoning would handle intelligently splitting the air flow, but is a bit pricey. A simpler solution would be a basic Honeywell zone board with commodity motor dampers.

    If you are burning propane or oil for heat, a heat pump rather than straight cool is probably a better deal down to 30*F +/-, depending on fuel unit costs. If nat gas is heating fuel, straight cool AC might be the better choice - system is simpler, no reversing valve, cheaper (~$400 less) and more efficient in cooling mode (presence of reversing valve reduces efficiency)

  6. dirkgently | | #6

    Thanks Curt,
    I will check out the Elitesoft. If it does all that it will likely be worth the price.
    I used HVAC Calc and found it very easy but, not sure I trust the duct sizing and has no data on length of duct run, or # of turns etc...Perhaps if the duct runs are short that will not matter?
    Most of the friction loss data I see is based on 100 ft. in the charts. That is longer than I need.
    A friend just purchased software for his ipad made by Carmel but, not sure if it does ductwork.
    My HVAC friend also is familiar with the Honeywell zone but I will check the Carrier out as well.
    I am heating with Nat Gas hot water baseboard and see no reason to convert it. spend less than $900 yearly for heat and hot water.

  7. claumergiddens76 | | #7

    Hi, Tom.

    To get the complete story, it takes a "conversation" about the building envelope, your living habits, your energy use expectations, orientation, surroundings, equipment preferences, how many people, pets and plants, yada, yada. All of this has an effect on the load and the zone diversity in your home. Rotating the home 15 degrees can make the ducts in one room bigger or smaller because the load in just that room can vary quite a bit as the heat gains and losses change.

    Knowing static pressures, acceptable friction rates, etc., are also big factors in designing a duct system. The equipment can be adjusted to accommodate longer duct runs, when a short run isn't practical, even when that's desirable. Shorter duct runs do perform better, as long as they're laid out properly.

    There's no simple answer, and no matter what software is used, all of these factors and more have to be evaluated to make sure the program is following the Manual D protocol and/or best practices.

    The advantage of 3rd party, as you have undoubtedly read on this site, is that the designer is unbiased about the cost of the equipment and it's installation. They should know the approximate costs of things, and factor that in to the design process. It's impossible for designers to know what a given contractor will charge, but we have a pretty good idea what they shouldn't charge.

    By the way, several mini-split manufacturers have started offering ducted systems. Mitsubishi has been doing so since 2007. I have specified many over the past couple years that, when installed correctly (like any system), are performing very well. Homeowners are comfortable and pleasantly surprised at how little energy is required to be that comfortable. The technology (inverter compressor, VRF) are unmatched in terms of efficiency, and having several small air handlers, as opposed to one, especially in a high performance home, performs much better.

  8. Expert Member
    Dana Dorsett | | #8

    In a 1700' cape you can probably do pretty well with mini-ducted cassettes to distribute to the smaller/doored-off spaces on a 1.5-20-ton multi-split rather than a fully centralized cooling duct system. And it WOULD be worth spending the extra $500 or so to go with a heating & cooling version.

    At current MA electricty & gas pricing it's cheaper to heat with ductless than with natural gas if you're on one of the lower-cost municipal utilities, but even at NStar and National Grid pricing it's pretty much dead-even on operating cost with 78-83% efficiency gas boilers, assuming the gas boiler isn't more than 1.7x oversized for the load. At 3-4x oversizing on the boiler a right-sized ductless would still be cheaper.

    The fact that you're backdrafting on the boiler flue is an issue to be resolved, but the solution isn't building in make-up air with a standard split system, the better solution is dedicated mechanical ventilation (either HRV or ERV) which is always a good idea under any circumstances, but essential if you're house was testing at ~1500cfm/50 BEFORE the last round of foam went in.

    Many mini-splits have rated outputs a -4F these days, which is below the Manual-J 99% outside design temp for all but mountaintop locations anywhere in MA, and at your level of tightness you're probably well within the range of a 2-ton ductless. There are some pretty-good 2 & 3-head Mitsubishis & Fujitsus out there that can take a variety of interior heads. It'll be more expensive than a bargain basement 2-3 ton 2-stage AC, but it'll be worth it on overall comfort, and in the shoulder seasons would be cheaper to operate than even condensing boilers (which you clearly don't have), with much stabler room temps.

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