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HVAC system recommendation

isasych | Posted in Mechanicals on

Hello everyone,

My husband and I are in the process of building a semi-custom home in Seattle/Pacific Northwest (WA) and have no clue as to which HVAC system to go with.

The house is about 4,900 sq ft excluding the garage area. We have an accessory dwelling unit on top the garage which square footage is a little over 1,000 included in the house sq ft. We will use that space as a rental and live in the remaining 4,000. Our living room is a double story. The walls on the first floor are 9′ and 8′ on the second. We will have a gas fireplace in the living room and have triple pane windows installed.

Here are the energy guidelines we have on our plans which I understand are just minimum requirements.

GLAZING, VERTICAL (U-VALUE) 0.22 MIN.
DOOR, ENTRANCE (U-VALUE) 0.22 MIN.
CEILING INSULATION (ATTIC) R-49 MIN.
CEILING INSULATION (VAULTED) R-38 MIN.
INSULATED WALLS R-21 MIN.
SLAB EDGE R-10
FLOOR R-38 MIN.

Our architect also included a 3.5 WA energy credit notes on the plan to add more efficiency to the house. We are looking for:

1. a very efficient system
2. natural gas is an option as well as electricity
3. will ductless heat pump be an ideal solution for a house that size?
4. a combo heating/AC

Could you please give us some input as to which HVAC system can potentially work for us?

Thank you all so much!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Without the actual heating and cooling load numbers there's no way to even take a stab at it.

    Given your location and your city/state carbon emissions goals heat pump solutions are probably better in new construction than gas burners that may not be used for a normal lifecycle. The PNW electric grid is pretty low carb now, and is now required be become zero-carbon over time.

    Ductless solutions aren't usually the go-to solution for a code min or slightly better than code house, but sometimes can be if it's WAY better than code (think "Passive House") , to where not all rooms need localized heat/coolth distribution. Ductless applied to only moderately better than code houses usually end up with grotesque oversizing factors, so oversized that they can't operate efficiently. Ducted modulating heat pumps RIGHT SIZED for their loads CAN be a reasonable solution, but not the only solution, and not necessarily the ideal or best solution.

    This size of the house hardly matters (a cozy weekend getaway place, is it? :-) ) , the load numbers are everything. For a 4900' house heated & cooled with heat pumps it's common to use 2-4 separate ducted heat pumps, rather than one monster sized central heat pump distributing the output to the whole space through a medusa of ducts looking like a ball of mating snakes, zoning with duct dampers.

    Is the architect qualified to run heating & cooling load calculations? Some are, some aren't. Some who THINK they are sometimes aren't really, so don't hesitate to pay for a second opinion from a qualified professional engineer. As a general rule load calculations from HVAC contractors are crap, with only a few exceptions to prove the rule. This is even more then case when going better than code min.

    BTW: Double low-E double panes can hit or sometimes duck under your U0.22 line in the sand at a lower cost than triple panes. Multiple glass vendors make double panes with a low-E coating on surface #2, which is the sealed glass side of the exterior pane), and a hard coat low-E on surface #4, the side that is in contact with the indoor air. With argon fill the glass units themselves run about U0.20 at center glass. With only 2x6/R21 type walls (U0.058-U0.060 ) the upcharge for going triple pane would be better spent improving wall performance. The down side to the double low-E double panes vs triple panes is excessive window condensation when it hits -20F outside, a condition not seen in King County since the last ice age.

    Pella's AdvancedComfort Low-E glass is one example, Andersen's HeatLock and Low-E4 glazings are too. Many other vendors use Cardinal's LoE-xxx + i89 double low-E glass. The high solar gain of their LoE-180 + i89 glass can work quite well in the PNW, but probably a suboptimallyo high gain for large unshaded west facing windows.

    Any chance you can be talked out of the gas fireplace in lieu of a (right sized) wood stove or wood burning insert? Local air pollution issues and burning restrictions can be pretty severe in King County, but as a "Hail Mary" when the gas grid goes down (or gets ripped-up/shut-down to meet future zero-carbon-emissions goals) it's a nice backup.

    1. Yupster | | #2

      "a medusa of ducts looking like a ball of mating snakes"
      An apt description if ever there was one. :D

      1. Expert Member

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