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Community and Q&A

Best HVAC options for large house in zone 5a.

Nethaniel Ealy | Posted in GBA Pro Help on

I have a custom house with 3930 sq/ft of conditioned space in a development in northern Idaho, (Zone 5a). The basement is planned to be Logix Platinum ICFs with R-28, the main and upper floors planned as 2×6 advanced framing walls with cellulose, 7/16″ OSB, Tamlyn Drainable Housewrap, 1.5″ of Terrafoam EPS, and fiber cement siding.
I’m hoping to get the owner to agree to spray foam the attic sheeting to condition it (R-38 min.)
The basement is 1830 sq/ft including a conditioned space under a large garage. The upper two floors of living space are 1050 sq/ft each.
Natural gas will be piped to the house.

What is the most efficient and cost-effective way to heat and cool this home?
What fresh-air ventilation strategy would be efficient and cost-effective?

This is my first custom home and first post on GBA.
Open to all wisdom.

Nethaniel in Idaho

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  1. Richard McGrath | | #1

    Much more information needed . Heat load , ACH , window type , size ? Does homeowner want hot air , hydronic , hybrid ?

  2. Nethaniel Ealy | | #2

    Heat load now attached.
    ACH unknown. Best we can achieve with the assembly listed.
    Triple-pane vinyl - Elevations now attached
    Hot air or hybrid - Owner originally thought hydronic, but claims too expensive and certain breakdown within 20 yr. span.

    Local Gas rates:
    Local Electric rates:

  3. GBA Editor
    Martin Holladay | | #3

    To me, this is a no-brainer. Your customer doesn't want a hydronic system. You have natural gas. It's a big house. That means you need a conventional natural-gas fired forced-air system, probably with central air conditioning.

    For anyone building a house that wants to be energy-efficient or green, here's my advice: the choice of heating equipment or fuel is much less important than the envelope details. The idea is to build an excellent envelope, so that you don't need much heat -- then it doesn't matter what equipment you use to heat the building. Try to exceed code-minimum R-values, pay strict attention to air leakage, use a blower door, and buy high-performance windows.

    For more information on forced-air systems, see All About Furnaces and Duct Systems.

  4. Richard McGrath | | #4

    No hydronic means no hydronic . Have a look at the link I am providing . Every house has a water heater and some are better than others , especially if it was made to serve 2 functions in the design . This heater has been used by others GBA readers and the reviews are good as well as what my customers are saying .
    Your customer may like the idea of saving the added gas piping running throughout his home to furnaces and opt for a unit like this to make his hot water and heat without alot of extras .

    The next link is to the original if hot water demands may be more than the LD can accomodate .

    Just a couple ideas of quality stuff to do the hybrid method .

  5. Expert Member
    Dana Dorsett | | #5

    A 46,167 BTU/hr load @ 0F for a 4124 square foot house is 11.2 BTU/hr-ft^2, which would be about right for a code-min house that size & shape, with that amount of glazed area.

    But with the triple-panes (way better than code-min) and an R20 + 6 wall (U-factor of about 0.05), a fairly simple shape (for less thermal bridging and low exterior surface area to floor area ratio) and a fairly modest glazing fraction I would think it would come in about 38-40K @ 0F. I'm curious what's driving the load numbers(?). Who did the heat load calculations? Any idea what the 10,570 BTU/hr of "other equip loads" are that boosts the heat load up to ~57K?

    BTW: The R6-R6.4 foam sheathing is a bit on the skimpy side for dew point control in a Zone 5B climate (IRC 2012 demands R7.5 minimum: ) But with cellulose cavity fill and sufficient ventilation rates to keep the interior air 35% or less in winter it should be fine. I hope it doesn't have a poly vapor retarder on the interior (?), since the exterior foam is impeding drying to the exterior a bit.

    If they're going fully ducted for the heating, at Idaho's 10 cent electricity and buck-a-therm gas it may as well be a modulating air source heat pump, or multiple mini-ducted mini-split type heat pumps, say one per floor, if it fits the 0F capacity. A Fujitsu 18RLFCD puts out about 20KBTU/hr, and has an HSPF of 11.3 (COP=3.3) which it would likely come close to hitting in N.ID, but even if it makes a COP average of only 3, with 10 cent electricity that's $9.77/MMBTU.

    Buck-a-therm gas at 95% thermal efficiency (and ignoring the electrical power used by the gas-burner) comes to $10.53/MMBTU. At 90 cents/therm all-in delivered cost it's still $9.47/MMBTU, and the air handler isn't exactly a tiny electrical load.

    Electricity prices are far more stable than gas prices too. About 1/4 of the natural gas in the US has been a by-product of fracked wells with liquids (light-tight oil, propane/butane,etc.). Now that the price of crude has crashed drilling rates have fallen off a cliff, and the depeletion rate of a shale well is steep, with 95% of the hydrocarbon output that it'll ever deliver being out by the third year of production. Fracking shale for dry gas requires a wholesale price many times the recent years' rates. Bottom line, natural gas prices have nowhere to go but up, at least until oil prices rise sufficiently to cover the cost of new drilling high rates. Cheap gas won't last more than another three years. But electricity in Idaho is primarily from legacy hydro, with 78% of the total mix being from renewables (as of 2013, according to the EIA data sets.) Having zero fuel cost for nearly 80% of the power sources makes for VERY stable electricity prices.

    The -18RLFCD slim-duct mini-split has a rated operating temp down to -5F, and will still be putting out quite a bit heat even at -20F, (but they won't specify the output at temps below -5F.) It's likely that a pair of those, one each for the two fully above grade levels and a smaller unit for the walk-out basement would have it covered, especially since the true whole house heat load isn't likely to be that high. The room-by-room load numbers make it a lot easier to spec, and are really essential once you dive into the real design.

    The ducted mini-split x 3 solution is more expensive than a simple single 1 or 2-stage gas furnace with an 3-4 ton AC coil in the air handler and a code-min compressor. But the modulating mini-splits are going to be higher comfort, with a lower operating cost., and it is naturally zoned by floor, each with it's own compressor. The cost is probably comparable to an HTP water heater based hydro-air handler + 3-4 ton coil and code-min efficiency AC solution. Key to making any of them work comfortably & efficiently is finding competent contractors to fine-tune the real heat load numbers, and who understand the products they're working with. There may be plenty of folks who can deal with the gas-furnace w/coil solution, but that's probably not the optimal way to go for max comfort.

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