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Community and Q&A

HVAC zoning

FrankFulton | Posted in General Questions on

We are seeking to switch from an inefficient oil-fired furnace in our long, cape style home in CZ4 (looks more like a ranch: 2700ft first floor, 900ft 2nd floor). Room-by-room load calculations suggest total heat loss (including basement) of just over 70,000 btu/hr. (R4 stone walls cannot easily be insulated – we have made significant envelop upgrades elsewhere.) The load calcs were performed by a RESNET rater and seem reasonable – do not account for sun orientation but based on volume, glazing, leakage, and insulation. In one area of the home we have an electric baseboard heater and also a propane fireplace.

Our thought is that a single ASHP unit will cost about 10k installed, and we can use another 5k to finish our envelope upgrades. We’d prefer multiple units or minisplits, of course, but the up-front cost would double. We live in an area where nat gas is not available, but many folks have “geothermal” GSHPs.

Thus, our tentative plan is to heat the home (using the existing 1955 high-quality galvanized ductwork) with one 5-ton heat pump (i.e., perhaps Carrier 18vs or Bosch – cooling is less of a concern, because there is an old AC unit in the attic). These ASHPs can generate heat of about 60k of BTU/hr down to 17 degrees, which is our 99% design temp per EnergyStar. Then, after installing the ASHP, if airflow or heat is insufficient, we could add more ducts, mechanical zoning, or a separate zone via a ducted minisplit. We’ve just begun to seek bids.

Does this plan seem reasonable? Is there a rule of thumb for payback on zoning (how much would zones reduce our monthly costs)?


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  1. Jon R | | #1

    18VS> "When the outdoor temperature is below 10_F(- -12.2_C) the unit control may automatically shut the heat pump off..."

    So be sure you have backup sufficient to handle the entire load. Normally up to 140% of design load, requiring > 122 amps, but stone walls help. Hopefully not many neighbors do the same.

    Consider keeping the oil heat for backup.

    1. Expert Member
      Dana Dorsett | | #2

      Even at 0F the heat pump will still be putting out the lion's share of the heat- you don't need anywhere near 122A/240VAC of heat strip, it just needs to cover the shortfall. (Does that series really shut down at temps as high as +10F??? If yes, it's a terrible choice for that location.)

      Even at -5F a typical variable speed Carrier heat pump is delivering about 2/3 as much heat as it delivers at +17F, at a COP of about 1.5 or better. The 5 ton 18VS looks like it's capacity drop is faster than typical for the smaller ones, going from about 50 KBTU/hr @ +17F to about 30 KBTU/hr @ +5F. See page 11:

      There are no rules of thumb about payback (if any) from zoning. If done poorly the zoning can even reduce system efficiency to the point of negative returns. But zoning by floor is often necessary to keep both floors comfortable year-round.

      It may make more sense to heat the 900' second floor with a mini-split and only use the big heat pump for the first floor. A Fujitsu 9RLS3 or Mitsubishi FH12NA could probably manage that for much less than the cost of a second big-duct heat pump. What is the 99% outside design temperature, and what is the calculated heat load of the second floor?

  2. Jon R | | #3

    > Does that series really shut down at temps as high as +10F???

    I copied that text from below:

    Their chart shows down to +5F, but even if reliably true, this isn't sufficient to avoid shutdowns.

    In general, be sure to review heat pump system performance at records lows. This is a case to be conservative (as in if they don't say it's OK, don't do it), not aggressive (counting on operating outside specs).

    1. Expert Member
      Dana Dorsett | | #4

      That would indeed make it a lousy choice for locations and heat load of 70K! It's not as if +10F is rare in zone 4A, or even 4C.

      It may be unrealistic to think the installed price of a 5-6 ton variable speed heat pump would come in at $10K. Every Carrier Infinity proposal I've seen were at least twice that much, even for 3-4 ton units, but those proposals also included all new duct work, so it's not exactly apples-to-apples. Mini-split & multi-split solutions have always come in cheaper, but there's always a first time for everything.

      1. Irene3 | | #14

        My sister and her husband recently got a 4-ton Carrier Infinity as a replacement for an existing heat pump, so I don't think they had any duct work redone. They had several bids of which the lowest was $15K and the highest above $20K. They did get a couple of rebates which brought it down a bit from there.

  3. FrankFulton | | #5

    Dana, Thank you.

    The peak heating load for the second floor is 9500 btu/hr with a 99% temperature of 17*.

    The second floor would be ideal for ducted minisplit, but the biggest issue is the up front costs. And we’d need new ducts - we could install these in a conditioned kneewall space (but the runs might be too long, and register placements in the bedrooms would be very very far from ideal - interior walls only). Another option would be to create a ducted mini split zone at one end of the house where the bedrooms are (upstairs + downstairs), but the run length and register placement would remain a challenge.

    Wed much prefer not to spend 20k, which is why we were thinking one 5-ton unit.

    Last, should we should stay away entirely from the 18vs?

  4. Expert Member
    Dana Dorsett | | #6

    With an outside design temp of +17F the number of times and the number of hours it drops below +10F is pretty small, but it would be up to you how to deal with the down-side risk of the 18VS during those events. With a heat load as high as yours there aren't a gazillion options, but it might only shut itself down for a few hours per decade. With enough heat strip to keep it from freezing indoors during those events you can probably make it work.

    The Trane XV18 series is similar, but has a rated capacity down to -10F, and might be a better choice. I know of one XV18 installation near me at a location an outside design temp of +2F, but I haven't pondered the technical manuals. (You might want to.)

    Depending on the air handler option the 5 ton versions in those series should be good for about 50KBTU/hr @ +17F or a bit more. I'm not sure what the differences are between the 4TTV8060A vs. 4TWV8060A really are.

    The notion that the registers of a ducted minisplit have to be close to the exterior walls isn't well founded. With a proper design the diffusers/registers will be directed for maximum throw & mixing even at the lowest speed with sufficient throw at high speed to break up convection drafts at the cold exerior wall. With Fujitsu's ducted mini-splits long straight runs of duct aren't an issue, as long as care is taken to use long-radius ells at the turns to limit turbulence, keeping the velocity high. (N0 sharp throated ells or tees, with turning vanes if the radius has to be small.) The 3/4 ton 9RLFCD has plenty of capacity at +17F, and is rated for 14,000 BTU/hr @ -5F, which pretty much covers your load at that temp too. In my area that would cost something like $5-6K, all-in, depending on the complexity of the duct design and how difficult it is to implement in cramped kneewall spaces, and how hungry the contractors are. YMMV, but even in a gold-plated high cost neighborhood it probably wouldn't hit $10K in competitive bidding.

    1. Mark Walker | | #18

      Dana, Could you post a website that sells the types of ducts that you recommend for ducted minis?
      Where do you go for the adaptor to run four ducts off a compact (slim) duct?

      1. Expert Member
        Dana Dorsett | | #19

        Ducts are ducts. People who do the duct math carefully enough can even use flex. The "slim" part is the really the cassette (air handler +coil) which is half or less the thickness & length of bigger-deal split systems.

        The typical way of managing 4 ducts off a ducted mini-split is a short fat plenum on the cassette with 4 separate duct runs, sometimes with balancing vanes to adjust flow. Here are some pics of a 1.5 ton Fujitsu heating & cooling a whole house in California on 4 duct runs using flex:

        But trunk & branch can work too. At the limits you may need to swap out sharp throated ells & tees to get the necessary flow, but a competent duct designer would be able to spot any deficiencies needing remediation.

        John Semmelhack (who posts here frequently) uses these regularly in his new home designs, and can tell you much more about them than I can. The first picture on this blog piece was one of his:

        Other comments from John appear here:

  5. FrankFulton | | #7

    Dana you are a font of knowledge! To be clear, we'd love to do multiple ducted mini-splits (probably better than GSHP), but given the length of our house, the cost would be prohibitive for the 4-5 ducted units we would need (including the large basement).

    Please clarify: is comfort the only reason for the extra zone (ducted mini-split upstairs)? My wife likes the bedroom warm, so it might be worth it to create a two-story zone at the end of the house with our bedrooms on 1st and 2nd floors. We could use a ducted minisplit which would address the cold weather performance and also might let us get away w a 4ton central unit. Thanks again.

  6. Expert Member
    Dana Dorsett | | #8

    Zoning with a second heat pump is both for comfort and efficiency. The heat loss and solar gain characteristics of upper floors are very different from other above grade floors, and operated as one zone you end up overheating/over cooling parts of the house to be comfortable in others. The heat loss characteristics of basements are also very different from above grade floors. (If your basement isn't already finished, insulating the basement walls from the interior to current code min can probably take more than 10K off your heat load.)

    Using a ducted Fujitsu for the second floor is good for both efficiency and comfort due to the very wide modulation range- a about a 6:1 turn down ratio at +47F, so it will run nearly continuously at part load for very high efficiency during the heating season. That is compared to only a 2.5:1 ratio for the Carrier (which will not be able to modulate down to the second floor load's actual design load when the first floor isn't calling for heat.)

    It might also be reasonable to install a 4 ton, (up to 8 zone) Mitsubishi cold climate multi-split here. The zone heads won't modulate, but if sized correctly for the zone loads it will still be pretty efficient and comfortable here. An MVZ series air handler can probably handle most of the main zone. The MXZ-8CNAHZ is good for 54,000 BTU/hr @ +5F in any number of ductless head or air handler configurations:

    The 1-ton MVZ-A12AA7 full-size air handler can handle your upstairs zone even without heat strips, and while oversized not ridiculously oversized:

    It can be mounted horizontally in a kneewall attic space if need be, and with it's bigger air handler power than mini-duct cassettes can use standard ducting & register designs.

    The 3-ton MVZ-A37AA7 air handler could handle the load of the first floor, but may need some amount of heat strip option to be fully covered:

    The MXZ series are fully characterized down to -13F, and won't turn off until it's colder than -18F.

    The MXZ-8C48NAHZ may be cheaper to install than a modulating Carrier or Trane + Fujitsu (or even just the Carrier or Trane.)

  7. Steve Grinwis | | #9

    This is anecdotal, but I wish I had of gone with the Trane XV20i instead of the Mitsubishi split system we ended up with.

    It was significantly cheaper, and it's not cold enough enough of the time to make the Mits system pay off. The HSPF is the same, and the turndown ratio is much better.

    If the Mits dies an early death, I'll definitely be replacing it with a Trane.

    1. Expert Member
      Dana Dorsett | | #10

      Is your system broken up into zones?

      1. Steve Grinwis | | #11

        Yes. We have a triple zone split / ducted system.

  8. FrankFulton | | #12

    Question, as we await quotes:

    What are pros/cons of keeping oil (vs heat strips) as a backup to an 18VS or Trane mentioned above, or Bosch inverter unit (which has condenser made by mitsubishi)?

    1. Expert Member
      Dana Dorsett | | #13

      If the heat pump is still running and the heat strips are covering the shortfall the bulk of the heat will still be leveraged by the heat pump. So-called "dual fuel" systems can only flip from one source to the other and cannot operate both at the same time. It's almost never lower cost to run a dual-fuel system unless electricity is higher than the national average, and/or the heat pump is undersized for the load.

      Multi-zone mini-splits don't modulate the heads/cassettes with load, which is probably the root of Steve's dissatisfaction. This is especially problematic when the heads/cassettes aren't reasonably matched to the zone loads. The compressors step up/down based on which heads are currently active, but doesn't ramp the heads, which have a fixed nominal output. With single zone mini-splits (ducted or ductless) the compressor and heads are constantly adjusting to match the load, and are extremely efficient at part-load.

      FWIW: A 1.5 ton Fujitsu -18RLFCD mini-duct mini-split puts out more heat at +5F than the 2 ton Bosch does at +17F and almost as much as the 3-tonner, with a much bigger modulation range. A pair of them puts out more heat than the 5 ton Bosch. If breaking it up into two zones, with good condition hard-piped ducts may be cheaper & better to go with a pair of Fujitsus than a 5 ton damper-zoned Bosch with limited modulation range. They don't have heat strip options (the way bigger air handers do), but they do have an control output for activating auxiliary heat when they are running full-out and not able to maintain the setpoint.

      1. Jon R | | #16

        > If the heat pump is still running...

        This is a key qualifier that also applies to dual fuel - the HP might not be running when it gets too cold.

  9. FrankFulton | | #15

    Dana, Thank you.

    Please confirm: you are suggesting that we create two smaller zones using our 1950 ducts, which are in reasonably good shape, and then use the Fujitsu as central heating/cooling, correct? Depending on up front price, nice, we will explore this option.

    FWIW, the first bid came in for a 4-ton Carrier 18vs, installed, at $10,250 after a factory rebate. This included removing oil furnace (but not tank) and heat strip backup - we need all of 5 tons for heating. This installer didn't measure the ducts well enough, imo, but the ball is rolling. The other two installers measured the ducts nearest the furnace and reported we can move 2100 cfm.

    Last, we've more or less decided that if we need supplemental heating and cooling on the second half-floor (Cape layout), we will use a ducted minisplit to cover 2 of the bedrooms, and then a single head to cover the 3rd bedroom. The layout prevents connecting all the rooms on ducts, unless we stay in the unconditioned attic. That leaves the bathroom unheated, but perhaps we could leave that on the downstairs zone.

    1. Expert Member
      Dana Dorsett | | #20

      Yes, I'm suggesting that you at least explore the possibility of breaking the ducting (or new ducting) to create two independent zones, each with their own Fujitsu. It's not a slam-dunk obvious solution- the room by room load numbers and duct design needs to be verified, but it can be pretty comfortable and efficient, no zone dampers to mess with.

      The individual zone load of a single bedroom are usually well under the max output of a single head ductless. If it's impossible to run all three bedrooms off one ducted mini split the choices for the single bedroom zone are limited. The half-ton Mitsubishi FH06NA modulates down to 1600 BTU/hr @ +47F, but that's probably well above the load in that room at +47F. The 3/4 ton LG LSU/LAN090HSV5 can modulate down to ~1KBTU/hr, but may or may not have good distributor/installer support in your area.

      The nominal 2100 cfm the installers talked about was probably specified at typical 0.5-1" water column pressures bigger air handlers are designed for. The smaller Fujitsu xxRLFCD cassettes can be set up for between 0-0.36" water column, so the cfm numbers delievered won't be quite that high (but they don't necessarily need to be.)

  10. FrankFulton | | #17

    Dana? Thanks.

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