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Community and Q&A

Zone 1 home addition — Dual-zone HVAC strategy?

Mark McFarlane | Posted in Mechanicals on

I’m building a 1,700 sq. foot single story addition to my home in Houston, TX. 1,000 sq foot are a woodworking shop, 700 sq feet are living space. Living space is multi-room, think small apartment (kitchen/den, bedroom, bathroom, closet). Shop is one big room with a separate mechanical room. My current HVAC system won’t handle the 700 sq feet of living space, which is also on the opposite side of the shop (maybe 100′ from the current hot-attic system).

Current house is traditional stick frame, brick veneer with batt insulation and blown-in in attic. Gas available for heat.

New addition will be open cell SPF in 2*4 walls and rafters, brick veneer. Primary load is cooling but some heat will be needed for month or so in winter (at least it is needed in my current uninsulated garage).

I’d prefer to not share air circulation between the new shop and the new living space, but spending for two separate central HVAC, ERV, and dehumidification systems for this small space seems cost prohibitive and I don’t know if I could even find small enough compressor to operate efficiently for 700 sq feet.

Mini-splits aren’t very common in Houston yet, particularly multi-zone. My GC has no experience with them, and the reviews I’ve read from end users are not very promising (could be install issues).

Anyone have a creative idea for efficiently heating, cooling, and dehumidifying these 2 spaces? My gut feeing right now is just to share air between the 2 spaces. I can control when I do ‘noxious’ work in the shop, like spraying,… It’s a hobby shop, not a production shop.

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  1. Expert Member
    Dana Dorsett | | #1

    Getting to the "right" solution starts with the room by room load calculations. Without knowing the load most HVAC installers end up swatting flies with sledgehammers, grossly oversizing systems to guarantee that they'll work, but often at the expense of efficiency and comfort.

    Slab on grade, or ...???

    Is there much west facing window area? (or can it be minimized?)

    Does the current system use ducts that are outside the pressure & insulation boundary of the house? If yes, it it possible to re-commission it in a fashion that is fully inside of conditioned space?

    What means " system..." in the 'merican vernacular?

  2. GBA Editor
    Martin Holladay | | #2

    First of all, Houston, Texas is in Climate Zone 2, not Zone 1.

    Dana is right that the first step is to perform a Manual J load calculation. You didn't tell us what type of attic insulation or ceiling insulation that you plan to install. Nor did you tell us much about your windows.

    The better your envelope, the lower your air conditioning bills will be. Invest in airtightness measures (confirmed with a blower door test), low-SHGC windows, and thick ceiling insulation. Minimize windows that face west, or shade the windows with a wide porch.

    I don't know why you wrote that "the reviews I've read from end users [of ductless minisplits] are not very promising." On the contrary: the vast majority of minisplit users who post opinions on GBA are extremely satisfied with the units.

    To provide cooling for your 700-square-foot apartment, my guess is that you'll want a single ductless minisplit unit (or perhaps a window-mounted air conditioner if you want to save money).

    It will be hard to beat a ductless minisplit (or two?) for your shop as well.

  3. Mark McFarlane | | #3

    Thanks Martin and Dana for taking the time to respond, I really appreciate it.

    It is slab on grade construction.

    There are 3 walls to the addition, 4th wall is existing home's garage. One new wall is NW facing with 40 sq feet of windows and a single car insulated garage door in the shop area. NE facing wall has no windows, SW facing wall is ~130 sq foot of windows.

    The current HVAC system in the existing home is in unconditioned attic space (That's what I meant by 'hot attic'). Insulation in existing home is batts in the walls, blown in cellulose in the ceiling joists.

    The new addition's current plan is for open cell foam in the walls and roof rafters, i.e. the new attic will be conditioned space. The existing house is unconditioned in the attic. There will be an insulated wall (barrier) between the two attic spaces.

    The bad reviews on mini-splits weren't on GBA, they were on other consumer oriented sites like Amazon, which is why I mentioned some of the issues may be due to installation issues. FWIW, I have had mini-splits in my last 2 homes outside the USA and they have worked well for cooling.

    The 700 sq feet of living space is being designed as an apartment for resale purposes, but its use for the next 20 years (Assuming I live that long) will be as a recording studio. This my my retirement home.

    One concern I have about a single zone mini split in the living space is its ability to cool the bedroom (recording control room) when the door is closed, which it will need to be during those hot summer afternoon recording sessions.

    My second concern about mini-splits is the heating cost, which I suspect will be a lot higher than a gas heater, but considering the location and the plan for a tight house, maybe the heating cost won't be that bad.

    I'll try to find a place to post a floor plan, that may be helpful

  4. Reid Baldwin | | #5

    A shared ERV will not cause air mixing between the spaces when it is running. The ductwork for it would create an air pathway between the spaces that could allow for some air exchange when the ERV is not running if there is a pressure difference between the spaces. You might be able to eliminate that possibility with systematic placement of passive backflow dampers.

    Do you have an appetite for using this opportunity to improve upon the H and AC aspects in the existing structure?

  5. Mark McFarlane | | #6

    The addition is already running considerably over my initial estimate, so I don't have much excitement about changing the existing house. It's only 8 years old and actually my summer AC bills are fairly low considering the size of the house. We live in essentially a forest so we get a lot of shade from 100' pine trees.

    On the flip side, if I could avoid the cost of a second new AC for the 700 sq foot living space by instead spraying open cell foam in the rafters and eves of the current attic and extending the current AC to cover the addition,... (assuming a reduced demand in the old house after spraying SPF). That could be interesting,.. It would be about a 100' run to the far edge of the new addition from the current attic HVAC system, and I'd need to duct both sends and returns.

  6. Expert Member
    Dana Dorsett | | #7

    You would almost certainly be able to cool the exiting 700' house with the existing system if you air sealed and insulated the roof deck with even 6" of open cell foam. Having uninsulated &/or leaky ducts above the insulation adds about a full ton to the load. Whether it would be enough to recommission it with new and extended ducts to cover the new 1000' addition as well as the original 700' kinda depends.

    If you made it all code-min the typical cooling load would run about 1 ton per 1000' of space, so if your existing AC is 2 tons or bigger, there's a real shot. OTOH, a single 1.5 ton mini-ducted type Fujitsu or Daikin might work for the whole shebang too (or a 2- ton 2 or 3 zone multi-split)- it really depends on where the actual load calculations come in. Eliminating or minimizing west facing windows might be the make or break factor. Roof overhangs or awning shading south facing windows make a difference too.

    East facing windows are a cooling load, but the don't usually factor into the peak load since the solar gains occur during a cooler part of the day, and before the roof has heated up in the mid-day sun.

    Heating with mini-splits is only slightly more expensive than heating with gas in my high-gas price high electricity price location. What are your gas & electric rates?

    What is your current split AC capacity / model / age?

  7. Robert Hronek | | #8

    A 100' run is a long way. Would the existing blower even work also I think you would have problems with comfort in the addition. How about a ducted mini split with 2 heads.

  8. Mark McFarlane | | #9

    Dana, The current ducts are all insulated flex duct, outer wrap is foil, and they are only 8 years old. From a spot visual inspection it appears they were well installed.

    I only have a few small windows facing north west, the window descriptions are in the pictures above. The house is rotated 45 degrees from North, so the front of the house is actually 45 degrees NW, The SW side is connected to the main house so there are no windows there, just an unconditioned garage that gets hotter than outside, so it will bridge heat into the workshop.

    The existing house is about 2800 sq feet and HVAC system is 4 ton, 96,000 BTU. condenser: Nordyne FS5BD-048KA / furnace: Nordyne FG6RA096C

    NE Windows are plentiful, but also protected by tress from direct sun. Those windows may never get direct sun, but the roof will mid day and afternoon.

    Our May electric bill was $140 for 1320 KW, so thats $0.106 per KWatt. I don't have access to my gas bill, but based on some government stats for Texas, the rate seems to fluctuate between $8-$22 per thousand cu feet on a seasonal basis.

    Robert, Here is the location of the existing HVAC system showing the 100' run. .

    I hadn't seen ducted mini splits before, but did some quick research and a 2 head system with the one in the living space being ducted sounds pretty ideal. Do you have a particular manufacturer/model that you have had a good experience with? The ductless system is also aesthetically much more pleasing.

    Another thought, if the multi-headed systems are similarly price to 2 separate systems, maybe 2 systems make better sense from a redundancy perspective.

    Thanks again everyone for the help.


  9. Expert Member
    Dana Dorsett | | #10

    I guess I was confused by your first posting, which I took to mean a 1000' addition onto a 700' house, not a 1000' addition, 700' of which is living space, with a 300' shop. (I'm still working on parsing 'merican, I guess. :-) )

    With 4 tons of compressor it should be able to keep up with the cooling load of a 2800' house without too much trouble if it were all inside the thermal & pressure envelope of the house. It looks like you have more SE facing windows than SW facing, but the gain from the SW windows drives the peak load. At a ton per 1000' with a 1-ton penalty for the ducts and air handler being in the attic that comes in at 3.8 tons, but if you insulated at the roof deck and air sealed the attic it would be more like 2.8 tons of load assuming it's all working correctly. It's probably going to be a stretch for covering the load of the addition though. It might make it, but it it would likely be marginal at best.

    But a 1.5 ton mini-ducted Fujitsu could easily handle the load of the addition, and would be more appropriate than a 2-head multi-split. The -18RLFD will modulate down to about 3100 BTU/hr out in both heating and cooling modes, so even if it's max cooling output is oversized for your peak loads, it will still modulate most of the time at high efficiency rather than cycling on/off, which takes a toll on efficiency.

    But the 1.5 ton AOU18RLXFZ compressor (the smallest 2-zone) has a minimum modulated output of 6100 BTU/hr in cooling mode, which is probably more than half the combined cooling load of the shop + living space, and probably close to the peak cooling load of the 700' of living space, which means it will cycle rather than modulate, which is both less efficient and less comfortable.

    This is unfortunately typical of multi-splits- with a half ton minimum. Ideally the modulation range would be down to something like 1/4 of the peak or lower, which you do get with Fujitsu's mini-duct cassette. IIRC Daikin's 1.5 tonner is comparable for modulation range, but with a less power full air handler. With mini-duct systems it's always better to hard-pipe it rather than flex duct, since these tiny air handlers have limited power. Fujitsu's mini-duct cassettes are better than most in that regard.

    If going with two separate systems, either Mitsubishi FH09 or FH06 would handle the shop, and modulates down to 1600 BTU/hr out in cooling mode, which isn't too bad. There are some pretty efficient and attractively inexpensive 3/4 ton LG heat pumps that modulate down to about 1000 BTU/hr, but industry scuttlebutt & web complainers indicate they may not have fully solved their QC issues yet (ask the local installers how good the local support is.)

    Gas at 1000 cubic ft has about 1,020,000 BTU of source fuel, and burned in a 98% efficient furnace delivers about 1,000,000 BTU (1 MMBTU) of heat to the house (with additonal air handler power cost). So at $8 that would be $8/MMBTU.

    A mini-split with an HSPF of 12 or 13 delivers about 12000-13000 BTU per kwh in a zone 4 location, but if sized so that it still modulates well with the load when it's in the 40s outside will deliver more like 16000 BTU/kwh in a zone 2 climate in heating mode. So per MMBTU that would take 1,000,000/16,000 = 62.5 kwh, which at 10.5 cents/ kwh would cost $6.56/MMBTU, slightly cheaper than $8 gas.

    Worst casing it, even if it were "only" getting the HSPF 11 efficiency in heating mode (a mini-ducted Fujitsu in climate zone 4 delivers and HSPF of 11.3 ), that's 1,000,000/11,000= 91kwh, which at 10.5 cents costs $9.56, slightly more than $8 gas. If you were using an 82% efficiency gas-burner instead of a state of the art 98% AFUE furnace, it would be almost exactly the cost of the mini-ducted Fujitsu, until you add in the air handler power use of the gas burner, tipping it sligthtly in favor of the mini-split.

    So, as in many local markets, in your area heating with mini-splits really is comparable in marginal cost to heating with gas- it all depends on the as-used as-sized AFUE or HSPF.

  10. Mark McFarlane | | #11

    Thanks Dana,

    regarding: 'I guess I was confused by your first posting, which I took to mean a 1000' addition onto a 700' house, not a 1000' addition, 700' of which is living space, with a 300' shop.'

    Let me try again. Current house is 2800 sq feet living space with a 500 sq foot garage. New addition is 1700 sq feet, of which 1000 sq feet is a workshop and 700 sq feet is living space. So in the end there will be 3500 sq feet of living space, 1000 feet of shop and 500 feet of garage, or 5,000 sq feet total under the roof.

    regarding: '(I'm still working on parsing 'merican, I guess. :-) )'
    I keep hearing George Bush Junior in my head... :)

    regarding: 'But a 1.5 ton mini-ducted Fujitsu could easily handle the load of the addition, and would be more appropriate than a 2-head multi-split. "

    I'd prefer to not share air circulation between the workshop and living space in the new building due to use of acetone, mineral spirits, lacquer, spray paint,... in the new workshop. I think the single head mini-ducted systems all share the return, correct?

    Thanks for the gas-electric comparison. Not what I had expected at all.

    I'll research the units you mentioned. The multi-headed Mitsubishi VRFZ systems seem to be able to operate at 10%, but that may be an average across all units, not a particular model, and they may not have a 1.5 ton VRFZ.

    Sorry, if I am getting some of this wrong, I'm just a homeowner, not a pro.

  11. Mark McFarlane | | #12

    One additional point. Considering the living space will actually be used as a recording studio, quiet indoors operation is important.

    The Fujitsu AOU18RLXFZ or AOU18RLXFZH looks great, other than the 6100 Btu lower power mode. Good dehumidification is also a goal, Houston is miserably humid. It looks like the AOU18RLFZ can be used with two mini-duct cassettes, but I guess that does not solve the 'low modulation' problem...

  12. D Dorsett | | #13

    If you're going to do it with two heads, separate half ton or 3/4 ton Mitsubishi FH-series units are better than a dual head multi, since they modulate down to 1600 BTU/hr each. 3200 BTU/hr total, or roughly half that of most 2-head multi-splits. That way the both can still run nearly continuously even at low sensible loads, which is the best way to keep the humidity down.

    For more money the Daikin Quaternity series mini-splits are designed to dehumidify to independent humidity and temperature setpoints, and can even dehumdify when the sensible load is zero. The min-modulated output of those is 5300 BTU/hr (EACH, so 10,600 BTU/hr minimum for a pair of them) so they'll be cycling a lot and thus won't hit their efficiency numbers, but they will definitely nail the humidity to wherever you set them.

    (The halt ton Mitsubishi FH06 for the smaller space, and an FH09 for the bigger, or just a pair of FH09s would be a better solution.)

    The compressors of Mitsubishi multi-zone systems won't modulate below 6000 BTU/hour in cooling mode, even though individual heads can, so they are not much better of than the Fujitsus when your peak load is about 12,000 BTU/hr.

    A Mitsubishi single zone SEZ/SUZ KA09 mini-duct only modulates down to 4800 BTU/hr, which is higher than the Fujitsu 9RLF's 31000 BTU/hr.

    Any way you cut it, you won't have much modulation range if you make one of the zones a mini-duct cassette, but you can if you make it all one zone, or use a pair of low-modulating wall blob types such as the FH09s to make it a 2-zone, or a pair of LG LAU/LAN- 090HYV1s that modulate down to 1023 BTU/hr (2046 BTU/hr for the pair):

    Before picking the equipment, do some research to see how many installers handle that vendor in your area, where the nearest distributor & tech support is (in case there are problems). In my area Mitsubishi's resources dominate (with a regional installer training & design center 20 miles from my house) , Fujitsu still has enough resources, but the rest are a bit spotty. Daikin's sales center for mini-split equipment is in TX, but I'm not sure what level of service & tech support they have there. Most vendors have "contractor finder" pages on their websites:

    If you have a half-dozen or more installers in the Houston area it's a good sign that there is local product support for that vendor. If there is only couple, or the nearest one is in Lubbock, maybe not.

  13. Mark McFarlane | | #14

    Thanks Dana, that's all very helpful information for me to do some additional research, including local support which has been one of my concerns.

    FYI, I just read a GBA article (that you are quoted in) that suggested Mitsubishi GE09s for warm climates, and FH09s for colder climates.

    I also read on some reseller's site that the FH09s are for 300-360 sq feet rooms, but they didn't discuss the building or ambient temps at all, so I shall ignore that recommendation.

    Is there a safe/efficient way to soffit mount an FH09? I think I read somewhere that you can do this, but you should have it protrude a few inches out of the soffit if you have much heating load.

    I also have the issue in the recording studio side (700 sq feet) of the building where the 'small room' is the NW facing room with a window, and the large room only gets a little early morning sun. This is visible in the plans I attached earlier. Placement of a single FH09 in this space may be tricky, particularly if I want the small room (control room) door closed, which one does during recording sessions, so that the small control room will likely heat up a lot in the summer afternoons.

    Good air circulation is 100% at odds with air sealing a recording studio's control room (many have airlock door arrangements,...) This is a hobby studio so I'm not going that crazy, but the more acoustic isolation the better.

    There appears to be no perfect solution. Small multi headed systems don't appear to modulate as low, ducted systems aren't nearly as efficient,...

    I'm learning. I really appreciate your helpfulness on this Dana. :)

  14. Mark McFarlane | | #15

    Regarding local support (within an hours drive) near 77356

    Mitsubishi: 5, including several 'diamond contractors' which I assume have some factory training.
    Fuji: 2
    Daikin: 1 at ~1.5 hour drive
    LG: 2, the same company, different offices.

    So I'll focus on the Mitsubishi.

    On a related issue, I've read a lot of consumer complaints on the Mitsubishi control circuits getting repeatedly fried by power surges/lightning. We have flakey power in the forest I live in, and horrendous lightning storms.

  15. Expert Member
    Dana Dorsett | | #16

    I know of at least one Mitsubishi installer in the Pacific Northwest who advocates installing lighting & surge protection with every mini-split, but nobody I know in that region who used that installer has taken them up on it. (The youngest installation of which was 3 years ago, with no failures to report. Sample size = 3) .

    But gulf coast lighting storms out scale anything seen in that region. If you have the worst grid power quality in Texas and there are lightning scars on the local trees & houses it wouldn't be insane to install both power conditioning and lightning protection (for the whole house, with or without a mini-split to protect. )

    It also wouldn't hurt to check your home owners insurance policy regarding what sort of storm damage is and isn't covered. Most won't cover damage to plug-loads from low quality power or lightning damage but many would cover damage to hard wired HVAC equipment.

  16. Mark McFarlane | | #17

    Thanks again Dana for the tips. We have trees in the neighborhood loosing limbs all the time from lightning. Hurricanes and thunderstorms are part of gulf coast living. I'll have a chat with our insurance company and research whole house lightning protection.

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