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Are duct losses really losses if all ducts are in conditioned space?

rocket190 | Posted in Mechanicals on

Hi All,  

I’m in the preliminary planning stage of a new house build in Climate Zone 6A.  Shooting for PGH goals and/or NZ which an ACH at less than 1.0

Preliminary house design will be a one-story ranch with a walkout basement.  The basement will have two habited bedrooms, thus it will be imported to have this area fully conditioned except for a small storage/mechanical area.  My area doesn’t have natural gas, so combustion appliances would run on LP gas.  My electric costs are somewhat high, averaging around $.15/kw counting all fees.  LP gas costs has been very low the last few years….Less than $.80/gallon.

I’m leaning toward a conventional hvac system with a 98%+ efficient mod-con forced air furnace setup.  With the house design as such, all ductwork (with exception to HRV exhaust venting) can be installed in the floor joists or interior partition walls.  Besides friction loss from the blower motor  (electric losses), what are the heating losses through the ducts?  Anything that leaks from the ducts would be leaked into conditioned space.  What am I missing that would make this setup less efficient than using minisplits.

On a side note, are heat pump hot water heaters still more efficient than a mod-con unit like a Polaris?

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  1. Expert Member
    Dana Dorsett | | #1

    Are the duct losses truly losses?

    From a total energy point of view, no- the duct losses accrue to offsetting the whole house heat load, and are not truly lost.

    From the point of view of delivering that heat to the intended room or zone, yes.

    >"What am I missing that would make this setup less efficient than using minisplits."

    Ducted systems necessarily induce room to room pressure differences, and some of the return path will inevitably be via "The Great Outdoors". The tighter the house and the better balanced the duct system is, the less that really matters. So if you're hit your <1ACH/50 goal and the ducts are balanced to where even at max blower speed the room to room pressure differences are always <3 pascals whether the room doors are open or closed (= Energy Star standard for ducted HVAC) the impact of a ducted vs. ductless system is really "in the statistical noise" .

    The efficiency and comfort impacts of suboptimally oversized ductless heads is far greater than those of reasonably sized ducted system with a modulating compressor & blower.

    But sizing really matters. Don't assume than an LP furnace with a 2:1 (or smaller) turn down really makes sense here- it probably doesn't. There is no substitute for running the load numbers before specifying the equipment.

    The heating & cooling loads of a PGH are quite low compared to traditional construction- even the loads of tight code-min construction make most LP furnaces too big for a ~1500' ranch over a 1500' walk out basement. If the basement doesn't have a lot glass doors & windows there are no LP furnaces (except maybe the smallest Dettson) that would make sense for heating the basement of a PGH house. A dedicated cold climate 3/4 ton ducted mini-split would probably be a better bet for the basement, and depending on where the load numbers fall a separate ducted mini-split would work for the upstairs as well. The Carrier/Bryant/Midea 3/4 tonners are fairly inexpensive, and are good for over 11,000 BTU/hr @ +5F, and have extended temperature capacity tables down to -30C/-22F:!/product/26241

    The 38MAQB09R--3 compressor unit also has a pan heater & controls for managing defrost water re-freezing issues unlike the comparable Fujitsu units. Like the Fujitsu ducted units the 40MBDQ09---3 ducted cassette can be mounted in an upflow orientation unlike the comparable Fujitsu units. They don't handle as much static pressure as the Fujitsu's, but that's a duct design detail. With the low loads of a PGH the house doesn't need the high cfm of small LP furnace, and the ducts don't need to be super oversized to get there within the static pressure limits. Just be sure to use large oversized pleated filters to keep the static pressures low (even if going with the default LP solution.)

    With upflow oriented ducted minisplits the "mechanical room" is really a mechanical closet, and not even a full-depth closet at that. A 3/4 tonner won't eat up even 10 square feet of floor area.

    LP won't stay that cheap forever- it's a byproduct of the oil & gas biz (there's a glut of LP from fracking in some areas) , which is likely to slip into structural decline (like coal is right now) before 2030. Also getting combustion out of the house completely solves a lot of indoor air issues, a real concern in a house that tests <1 ACH/50. Even at 3ACH/50 active ventilation would be a necessity, not a luxury, but at <1 ACH/50 balanced ventilation (like an HRV) becomes the only safe option, and more urgent when there are combustion appliances in the house. A heat pump water heater would help manage dehumidification (necessary in an occupied tight house) and eliminate the holes in the building envelope otherwise needed for a condensing tankless LP water heater. (Atmospheric drafted LP water heaters simply won't work.)

    If you're still dedicated to the notion of an LP solution, a condensing gas TANK water heater (or a tiny boiler + buffer tank) and hydro-air coils (ducted or ductless) for the different zones has a better shot at right sizing the heating system than a standard LP system. With the buffering mass of the tank it can even be micro-zoned room by room. See the illustrations on p.3:

  2. rocket190 | | #2

    Thanks so much for the detailed response. Nothing is set at stone at this point, so I will keep investigating other options. Regardless of what direction I go initially, I plan to install wiring and linesets for future minisplit installation.

    You are correct about the furnaces. Most of the high end 98% efficient units are 60k input, with the lowest turn down of 24k. I haven't started a diligent search, but someone must make a 40k input unit? I was hoping to avoid the the complications of infloor heat, but those air coils look interesting. Any idea how these price out?

    1. kyle_r | | #3

      Goodman makes a 96% 30k btu/hr 2 speed and 96% 40k btu/hr variable speed furnace. Other than that look at Dettson.

      1. rocket190 | | #6

        Thanks. I have that Goodman 40K unit in my current house. It's been phenomenal for me so far. No expenses other than an annual checkup for over 10 years. Knock on wood.

        I find the heat/comfort provided by the unit to be impressive, which is why I am still thinking of going with a conventional setup. My annual heating expenses have been around $500-600 per year, in a locale where we need heat from Late September to mid May.

  3. Expert Member
    AKOS TOTH | | #4

    Duct losses inside conditioned space only become a problem if the air doesn't make it to where it needs to go. Common enough to see ducts through conditioned crawlspace that are so leaky that the rooms one floor above don't get enough heat while the crawlspace stays nice and toasty. With decent install, this should not really be an issue though.

    If going on LP route, Dana's suggestion of running off your water heater is spot on. These are local to me, but there are other similar units out there:

    Going by your current price, LP is cheaper but not all that much. If your electricity goes down to $0.10 it will be even, so even adding in a small PV array could get you to the point that LP is not worth it. The cost of a small array will be comparable to the LP tank install, so it might be a better low carbon option down the road.

    Even with an LP furnace, you still need to install an AC system, the cost of that plus the labor to install is comparable to a heat pump, so not much is saved by not going the heat pump route in the first place.

    1. rocket190 | | #5

      I would like to add a moderate size solar array in the future. Something along 12-15 kv. I have plenty of roof and yard space to make it happen. Unfortunately, my local power company has a terrible grid share agreement. They don't net meter, and they only buy-back excess generation at around $.04/KW. Was thinking it would make sense to prep for an in-home battery and hope that costs continue to fall for them.

      Do you think that geothermal would still make any sense? I have an existing 4 acre pond that is 20+ feet deep in areas, and I also own/operate an excavation business, so any earthwork expenses are minimal to me.

      1. Expert Member
        AKOS TOTH | | #7

        Even without a decent net metering, most likely your electric heat will be very low cost to you with an array that big. With $0.04/kW, your shifted PV power comes back to you $0.11 which is pretty close to break even for LP heat, during the day with some sun, you heat/cooling will be pretty much free.

        I don't know enough about geo to give you an exact answer. With the cost of a decent 2 ton cold climate ducted mini split being around $2k, I can't see geo being cost effective even with the earth work for free. Every time I say hydronic, just pumps/fittings/valves costs more than $2k. Well set up might be more efficient, I doubt there is any ROI on the extra part cost.

      2. Expert Member
        Dana Dorsett | | #8

        What Akos said- particularly regarding geothermal.

        I don't personally have a lot of useful data points, but the most recent project I had any hand in with geo was a 4 ton WaterFurnace Series 7 (fully modulating) retrofitted onto an existing duct system (with almost no modifications) that came in at an upfront cost well north of $50,000 (reduced by US Federal tax credits & local incentives to <$30K.)

        That price point included desuperheater & potable buffer tank for supplementing hot water, but there is a legacy oil fired water heater doing the heavy lifting. The plan is to replace the water heater with a heat pump water heater once the oil tank is empty (which should be any day now.)

        I had recommended either 2.5 or 3 ton modulating air source heat pump, such as these:\M_SUBMITTAL_PVA-A30AA7_PUZ-HA30NKA_en.pdf\M_SUBMITTAL_PVA-A36AA7_PUZ-HA36NKA_en.pdf

        With enough heat strip to cover the shortfall during Polar Vortex disturbance cold snaps, which would have come in at about half the after-incentives out of pocket cost of the geo. Based on this past winter's performance of the Water Furnace 7 a 3 ton cold climate heat pump definitely have worked even without heat strip (though a 2.5 tonner might have been more efficient.) Even at temps more than 5F cooler than the 99% outside design temp the WaterFurnace never hit anywhere near it's max output, or even it's nominal output. The only time it ever ran full blast was during the commissioning tests, and a few times in early days when the homeowners were experimenting with deep overnight setbacks. With a "set & forget" approach it would.

        In new construction spending the difference in cost between a couple tons of geo (probably >$30K unless heavily subsidized) and 1.5-2 tons of ducted cold climate mini-splits (~$15K in my neighborhood) is better spent on rooftop PV (& battery where net metered only at wholesale bulk electricity pricing.) Even with a DIY pond loop it's going to be pretty expensive, and comes with a lot of system design & implementation risk that potentially reduces seasonal efficiency & capacity something much less than advertised, whereas air is air and it's not rocket science to design reasonable duct systems. It may be possible to pay a competent geo designer to spec every pipe pump & valve in the system if you REALLY want to do it DIY for about the same cost as a ducted mini-split solution. It's not going to have significant "pay back", but there is some entertainment & education value to doing geothermal systems as a hobby, which is worth something.

        If your all-in fully delivered electricity price it under 15 cents it may be worth putting off the PV + battery decision. Both PV and batteries are still on a steep learning curve- in 10 years they will be a lot cheaper.

        For dedicated hobbyists who know what they're doing that have plenty of room to spare for solar arrays there are now USED PV panels available for < $0.25/watt still operating at 80% of rated power or better, and salvage yard battery packs from wrecked EVs (or retired EV battery packs) at a fraction of the cost of brand new lithium ion batteries. Battery packs that can no longer take the Model S from 0-60 in 3 seconds (a MASSIVE peak power output) can still run most houses just fine. Per kwh of output the older ~12-15% (initial) efficiency panels running at 80% of rated power take up about twice the area of better class new ~20% efficiency PV going onto suburban rooftops, but where space isn't a problem it's a dirt cheap way to go. Getting permitting in place for connecting a DIY-hacked system to the grid might be the biggest hurdle, but even with a brand shiny new grid tied string inverter a DIY 16-20kw array with 10 kwh of DIY used battery on a backup inverter & transfer switch with a can still come in around buck a watt or less.

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