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Community and Q&A

Using one minisplit head (wall unit) for two small rooms

haiku_r | Posted in GBA Pro Help on

Currently I have a ducted HVAC in my urban rowhouse condo (495 sq. ft total, living room is about 220 and bedroom 180), I have long wanted to install ductless A/C & heat to remove the large duct hanging from my living room and kitchen ceiling and give me an extra closet (worth about $100-$150 month to me). That old system finally died.

Installing ductless in the bedroom is not as easy as I thought – there is limited space above the ceiling &it’s a historic facade; condensate will have to go through the entire wall of my apartment to the back.

I have been given the following solutions by three contractors:
A) 12,000 BTU outdoor unit + one head in the living room [$6250]. The one wall unit would be placed on the back wall of the living room, pointing straight towards the door of the bedroom 12 feet away
B) 24,000 BTU outdoor + one unit in the living room, if it’s uncomfortable I then have the option to add a bedroom unit later without buying a new outdoor unit [$7,500 now + $2,500 or so- later]
C) 24,000 BTU outdoor & one head in each room, run pipes from the bedroom unit through holes drilled in the studs of the wall to the back deck [$9,500], insulate the pump with foam so the noise doesn’t bother me.

I am concerned about the pump being noisy & also having pipes embedded in the wall in case of issues/leaks so I really want Option A to work (if it doesn’t I may just drop the idea of ductless). Is it crazy to think one wall unit would cool both rooms?

I don’t care much about temperature differences – usually only one room is in use at a time, & the door open. The current ducted system has always had a big difference between rooms, the air basically passes 12 feet through the hot attic crawl space before reaching the room.

Would Option B work? Or would a 24000 BTU be so oversized for this space that it would short cycle and damage the system? It would be nice to have the option if I need it later/sell. If it is technically feasible I would go with this.

Thanks so much for your suggestions. It’s difficult to decide with HVAC guys telling me different things.

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  1. ohioandy | | #1

    Haiku, you've not given your location and climate information. Would also be helpful to know what type and age of structure you're dealing with. Any proper HVAC installation should include a heating/cooling load study to determine the proper size, but that's why you solicited three bids from professionals! Did any of them perform this study?

    I guarantee you, though, that all your options are oversized. 500 square feet in a rowhouse will have far lower requirements than your bids are providing, so any of the suggested configurations will function suboptimally at best, and be uncomfortable at worst.

    I know it's superhard to find a creative contractor, but your best option might be a mini-ducted unit mounted in between the two rooms, or up high where your present duct is. HVAC contractors don't like them because their installation involves some carpentry and maybe some trimwork. It can be mounted near the floor, and the condensate line can go straight up to the ceiling, and then back.

    You'll never hear the outside unit. The inside one, if not ramping up and down repetitively (i.e. if properly sized) will hum along very quietly.

  2. haiku_r | | #2

    Thanks for your response!

    None did a formal "study," they just eyeballed the space and gave me quotes (not a lot of detail either). These guys have *plenty* of work (esp. now), this is a small project for them.

    Not sure if miniduct will work - due to large beam the current ducts pancake to about 3 inches b/w living room & kitchen - I'll check the specifications.

    Not worried about OUTSIDE unit noise but the condensate pump which would be installed in the crawlspace. As it is my neighbor's outdoor HVAC unit (compressor? ) (on my roof) & pipes drive me NUTS...also their TV gets into the wood beams and creates a resonance.

    BTW this is a 1905 rowhouse in Washington, DC converted in 1988 to 4 flats, HVAC & ductwork dates to 1988. I have top floor. It is configured like a T; living room is the lower part and the bedroom the crossbar. Bedroom is street side, has southern exposure & three windows so sunny & warmer; living room abets back patio & is cooler. Side wall 1 abuts building wall & stairwell - I assume there are studs but in some places drywall was placed directly over brick & plaster. Side wall 2 abuts the building wall then small galley kitchen with about 4 feet exposed brick, the rest of the wall is studs & drywall over brick. Ceiling-roof crawlspace slopes from about 2.3-3 ft on the street side (bedroom) to 6 inches or less on the deck side (living room).

    The load is probably on low side due to shared walls... with no AC/heat temps vary from 55-99 but it does not take a lot to cool or heat the place. Hot humid summers, moderate winters.

    With these constraints we're going for functional, not optimal. 5-10 degrees difference between living room and bedroom would be fine. Key is not difference b/t rooms, but if i want the bedroom 75 degrees, can I get it there (same with living room - if I have people over don't care how hot the bedroom is, as long as living room is comfortable). Basically you will have at most 1 person or a couple staying in the same bedroom using this space. .

  3. GBA Editor
    Martin Holladay | | #3

    The usual advice applies: The first step to designing a heating and cooling system is to perform a load calculation. More information can be found in the following articles:

    "Saving Energy With Manual J and Manual D"

    "Who Can Perform My Load Calculations?"

    If you end up with one indoor head in your living room, you may or may not be comfortable in your bedroom. Factors that improve comfort include the following: a low rate of air leakage to the outdoors (indicating good air sealing work), high-performance windows, and high-R-value walls and ceiling.

    Factors that reduce comfort include the following: Large unshaded east-facing or west-facing windows (since this type of window increases solar heat gain during the summer), a high rate of air leakage to the outdoors, and low R-values.

  4. Expert Member
    Dana Dorsett | | #4

    It's highly unlikely that unless you have no insulation and a full glass walls on the exteriror that the total heating or cooling load of a 500 square foot row house would come anywhere near 24,000 BTU/hr. Even the 12,000 BTU/hr unit is probably overkill.

    With as little as R19 in the attic and double pane windows (or single pane + storms) and a "normal"15% window/floor ratio most 500 homes would have a heating load of 12,000 BTU/hr or less @ 0F outdoors even as a freestanding house. A row house would be lower, usually under 9000 BTU/hr, and the cooling load would probably be under 9000 BTU/hr too, even if your 99% outside design temp is 0F.

    You're probably looking at a 9000 BTU/hr unit. (Cold-climate 9000 BTU mini-splits put out more than that at 0F outdoors.)

    Worst casing it assuming a 25' x 20' row house with the 20' being exterior walls, and all R2 walls & windows (U0.5) and 80 feet of exterior wall 10' high you have 800 square feet of exterior wall. At a temperature difference of 70F (0F outdoors, 70F indoors) the heat loss from the exterior walls would be:

    400' x U0.5 x 70F= 14,000 BTU/hr, less interior heat sources plus whatever it going out the insulated ceiling & floor.

    If your outside design temp is 20F (the 99% outside design temp for Washington National Airport) , that would be more like:

    400' x U0.5 x 50F= 10,000 BTU/hr

    But that's basically a single-wythe brick wall with no plaster & lath, no insulation. The wall is more likely to be at least R4 (U0.25), even if uninsulated.cutting the wall losses by half (but not the window losses.) A Fujitsu -9RLS3 or Mitsubishi -FH09NA both have at least 12,000 BTU/hr of capacity at +17F outdoors, +70F indoors, and 9000 BTU/hr of cooling.

    BTW: The $6250 quote for the 1-ton feels excessive. In competitive bidding a state of the art brand name cold climate 1-ton will come in under USD$5K (sometimes under $4K) in my neighborhood, a 3/4 tonner would usually be under USD$4K.

  5. davorradman | | #5

    WTH is with the mini-split prices in USA?
    How come everything else is so much cheaper than in Europe, but mini-splits are so atrociously more expensive?

    Who takes all of the money?

  6. GBA Editor
    Martin Holladay | | #6

    Calm down. It's not a conspiracy.

    Labor rates in the U.S. are higher than in Serbia, Croatia, or Portugal -- but probably not higher than Germany.

    Parts costs depend on volume, and ductless minisplits are more common in Europe than in the U.S. (and are therefore more competitively priced in Europe).

  7. haiku_r | | #7

    Thank you all for the helpful information. Dana, you are right, this place is *snug*. Barely need heat in winter, for A/C I often turn it on 10 minutes, then turn it off & it stays cool for an hour.

    Problem is there's a wall b/t living room & bedroom so one head in living room may or may not be comfortable in bedroom. Tech A says definitely if I place as he wants; others say maybe or definitely not.

    Tech B recommended I overinstall now as insurance in case in case I wanted the second head later (the higher capacity now would be just a few hundred dollars more, but installing a second outdoor unit install later would be super expensive, requires crane, permitting, etc). They don't make less than 18,000-20,000 BTU outdoor units that will support 2 heads so I would be way over what I typically use.

    Question: will having system run so far below capacity (20,000 - 24,000 BTU when 9000 would work) cause less comfort now & damage to system? The article Martin posted suggested it had *less* negative impact on efficiency and comfort than people thought.

    There are always tradeoffs. Having ability to keep bedroom comfortable at night with the door closed is worth paying more a bigger outdoor unit, but not if it causes the whole system not to function. Efficiency is not a big issue here, issues are size of the unit (ducted system uses A LOT of space), avoiding breakdown/leak nightmares, maintaining value of property, comfort.

    Thanks so much for all the information! I did talk to 4 techs but it's hard to get detailed answers & unclear how objective the info is (vs. making the sale or selling their product). It's like the wild west.

  8. haiku_r | | #8

    RE the prices - I was surprised too: $6,250 was the LOWEST bid (an freelance tech gave me a ballpark estimate of around $5,800 but never submitted an estimate) which is the easiest setup (mounting on an exterior- facing wall about 10 ft from the outdoor unit). That includes pulling down the duct in my living room & hauling away the old system.

    From Yelp reviews of these companies they mark up parts/equipment about 60% or so, labor is probably billed at $180-200/hr.

    One said a crane/permits cost $600 but then it was in the budget at $1000. Another didn't mention a crane at all.

    One was billing for an electrician to rewire; another said he wouldn't need an electrician because there was already connection to my roof.

    The budgets they've given me are not itemized and barely describe what they plan to do.

    The more established companies have almost as many 1 star as 5 star reviews which I found very surprising. How do they stay in business? Those with great reviews either are too busy, don't service my neighborhood, or don't do ductless. It is discouraging.

  9. Expert Member
    Dana Dorsett | | #9

    Oversizing "just in case" is ALWAYS a mistake!

    The ongoing cost of oversizing is in both lower comfort and lower efficiency, as well as the higher up-front cost. Short cycling the thing silly doesn't really damage the thing- the only thing destroyed is the efficiency. You'll do better on efficiency with a small window-shaker or a 2-hose portable heat pump AC unit than a ludicrously oversized ductless head.

    The cost of going with a dual-head compressor is also in both comfort and efficiency, since the heads don't modulate with load when hooked up to a multi-split, they only cycle on/off. The loads of the doored off rooms are TINY compared to the output of the smallest heads, and just the minimum refrigerant flows through the head when the head's blower is off can overheat/overcool the space. With a single head fully modulating system the blower runs nearly continuously at minimum speed most of the time, and ramps up when the load is higher, delivery ultra-quiet operation and very stable room temperatures.

    A 2x4 partition wall runs about R4 (U0.25). If the doors to the other space are open there will be convective heat transfer at a very reasonable rate, and the room temperature differences won't usually exceed 5F. Cranking up the setpoint temp up (or down) by a few degrees can usually make the remote rooms comfortable when the outdoor temps are low (or high). For the heating season comfort it's cheaper and more efficient to install right-sized resistance heaters (baseboards, panel radiators or cove heaters) that are only used during the temperature extremes than to install a 5x oversized ductless head that cycles itself into abyssmal efficiency.

    In your location oversizing also results in poorer latent load (humidity) handling. Even if the main room is set to 75F and the other rooms hit 80F, a temperature of 80F and DRY is still more comfortable than 75F and sticky/muggy which is where you would end up due to the low duty cycle. It's fine to set it to 70F if that's what it takes to keep the remote rooms sufficiently cool. A modulating system will run almost continuously even under light loads. The Mitsubishi -FH09NA is particularly good at this since it can throttle back to ~1600BTU/hr (when on it's own dedicated modulating compressor, but NOT on a multi-split.)

    Bid / pricing issues:

    It doesn't take a crane to install a 1-ton mini-split compressor/condenser on the roof (seriously?). They only run about 100 lbs- that's including the weight of the shipping box/crate. (Without the packaging they're only about 85lbs.) In 99 cases out of 100 this would be a completely manually hoist-able operation, or even a ladder operation with a beefy worker on the roof belaying it while another pushes it up the ladder. (Don't sic OSHA on me for this, but I've seen crazier ways to get 100lbs of equipment up on a roof :-) ).

    The installed costs in my area includes electricians running the wires, the condenser pad (or wall mount bracket), line sets & covers etc- it's the "all in" pricing. A $5800 price for a 1-ton might come about with a particularly awkward installation. I've seen quotes that high (and higher), but when same-for-same equipment is put out for bid to multiple contractors it has consistently been cheaper. It's also been the case that the lower bids were from seemingly more competent contractors too (probably not an accident), from both larger contractors and 3-5 person outfits.

    On a project I was involved with a handful of years ago three individual 1.5 tonners wall-mounted on brackets 4' above grade had multiple bids in the $13,000 range (for all three). That's less than $3000/ton, less than $4,500 per mini-split.

    More recently, a co-worker of mine had three separate mini-splits, two Mitsubishi FH12s and an FH09 installed on condenser pads + stands (to keep them above the snow line) for about $12K. That's ~$4300/ton, or ~$4K per mini-split. There were a couple of bids in that range for that equipment- he went with the contractor who seemed to listen & explain things better. There were also insane proposals north of $20K with ridiculously oversized (and too much) equipment that would have left him much LESS comfortable (and poorer.) This was for a ~1700' 2x4 framed 1.5 story house with a 99% outside design temp under +10F (but over 0F- don't remember the exact number.)

    So, start by figuring out realistic load numbers- you might only need the half ton Mitsubishi FH06NA (not that it would be dramatically more efficient than the FH09NA, since their minimum modulation numbers are the same.) If you can help it, don't go with anything that has a minimum modulation @ +47F than is higher than half your design heat load at +20F (or whatever your actual local 99% outside design temperature.

  10. GBA Editor
    Martin Holladay | | #10

    I had a job interview for my first roofing job. There was just one question: "Can you carry 100 lbs. up a ladder?" I wasn't sure, but I said yes.

    It turns out, fortunately, that I could. I used to carry cardboard-wrapped 100-lb. barrels of solid asphalt on my shoulder up the ladder to a flat roof, where the kettle was set up to melt the asphalt for tar-and-gravel roof repairs.

  11. Expert Member
    Dana Dorsett | | #11

    What, you didn't rent a crane? ;-)

  12. Expert Member
    Dana Dorsett | | #12

    BTW: If you're building an extra closet at the partition wall between the 220' space an the 180' space you could take 5-7 square feet and devote it to a vertically mounted 3/4 ton (or 1-ton, if the numbers really support it) Fujitsu 9RLFCD mini-duct unit and have very short supply ducts to both spaces.

    Other vendors' mini-duct units are horizontal-mount only, but could be installed in the top of a closet, under the ceiling, but maintenance access is easier with a vertical mount. To get a visual as to what I'm talking about see this (a 1.5 tonner, which is the biggest in the series- bigger than you need) take a peek at the attached pictures. The big grille is the return plenum, the supply plenum is above the mini-duct cassette, with flex ducts going to remote locations. If yours is mounted in the back or side of a closet the duct length could be effectively zero in one direction, and roughly the depth of the closet in the other.

    This would more money than a ductless unit, but should be quite a bit less than the ridiculously oversized 2-ton units proposed, and maybe not much more than the overpriced 1-ton ductless proposal.

    The 9RLFCD (or 12RLFCD) will modulate down to 3100 BTU/hr out @ 47F which is probably still less than your actual heat load @ +47F, which means it will modulate at very high duty cycle at very low speed most of the time.

    This takes a bit more planning and coordination with the framing plan, but it's probably closer to the ideal solution, since the room to room temperatures can be balanced with a balancing vane in the section that has the stubby duct run, or two balancing vanes right where the ducts exit the plenum, before the register boots.

  13. haiku_r | | #13

    Thanks Dana, this is interesting - it looks like it takes quite a bit less space than the old unit I had so sort of has the best of both systems. The horizontal orientation at the top of the closet would probably work better (I'm reluctant to do anything which makes the space look smaller or takes away wall space). It might be the best option in terms of balancing temperature between the two rooms, the other side of the wall is kitchen cabinets, but the duct could go out a couple of extra feet and miss the cabinets (roof is sloped, higher on the front side of building where bedroom is). Anything more than a couple of feet past the wall there would be minimal space b/t ceiling & roof (photos below show bedroom closet on one side of wall, the full living room/kitchen, and the back of the living room - the proposed location is just to the right of the patio doors a little higher than where the coat hanger is. )

    I doubt this would cost less though - any kind of work just seems to be more expensive in DC, maybe even more so for HVAC because they need certification. They're doing so many complete renos (there are three row house to condo conversions on my block alone, plus someone building a new 4 story behind me) plus new construction that smaller projects aren't too attractive.

    I think I will go with the single ductless you described - it will be perfect for the living room which is the most important since I use it during the day and evening. If the bedroom is a little warmer or cooler it's not that important. I need to get it done - it will probably be over 90 in a couple of days and then will be really hard to get someone out.

    Thanks again for all the useful suggestions and comments! I feel much more confident about the decision, without your advice I probably would have gone with the bigger system.

  14. Laither | | #14

    Hi haiku_r, I have the same issue and considering a single head for two small bedrooms, can you give us an update as to how things worked out for you?

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