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Community and Q&A

Heating a Large Home with Minisplits

tylerrencher | Posted in Mechanicals on

I’m designing a pretty good house in central Utah for my family. It’s about 4500 sqft, so definitely large for this crowd, but not crazy. Main level plus full finished basement on a 60×40 footprint. We are fairly set on the layout to capture the views and support our lifestyle, but are still open to changes. I’m looking closer at HVAC now. We are building in zone 5B.

I designed with provisions for a traditional HVAC system with ducts in the conditioned space. Then I modeled the house in BEopt and have found that the main level gets up to temperature in the winter quickly when the sun comes out, then the heat doesn’t run for many hours. That’s a pretty normal winter day here. The basement stays uncomfortably cool when modeled with a single zone traditional heating system. About 62-64 degrees for 3 months.

Now I’m looking at zoning the levels separately which has naturally lead me to mini splits. I’ve run heat loss calcs on the house with 3 different tools and it consistently comes out to a heating load around 25000 BTU/h including the ventilation system losses. I’ve done a room by room estimate which I’ve labeled in the floor plan pictures. I’ve also labeled some potential head locations. I think it might be possible to use 2 of the Fujitsu ducted medium static pressure units for the whole house, but given the size of the house, and my limited understanding of duct design, that may not be feasible.

We tend to leave doors open, so air will have opportunity to mix naturally. I’m not opposed to electric spot heaters. The nursery off the master and the library in particular seem like good candidates. We are ok with some room to room temperature variation.

My thoughts on this are to have a head on the main level in the dining room over the office doors or under the dining room windows. Then in the basement family room and another head in the hall among the bedrooms. 3 heads would be a pretty straight forward install with no ducting. We plan to heat to 68F, but if we heated to 70 or 71 upstairs that would help with convective flow to the master suite and library.

I’m still searching for local professional help, but this kind of knowledge is a bit sparse in rural Utah. One guy wanted to install a 6 ton propane furnace… My biggest concern is that I need to keep this build moving and I’ll end up with a central system designed and end up with a low duct somewhere I don’t want it. Professional full design work has a delivery date about 12 weeks out currently, so I’m in a bit of a pickle here. I may end up biting the bullet and purchasing a license to Wrightsoft or Elite, but I’m not sure how well they handle mini split designs.

Thanks in advance for your thoughts.

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  1. nickdefabrizio | | #1

    I know folks out there who built houses with many bedrooms because they occasionally have visitors, not because the bedrooms are always occupied. They use electric baseboard or convection heaters to bring the rooms up to 68-70 Fonly when in use, otherwise the main HVAC units cover the space but the bedrooms are slightly lower temps.

    1. tylerrencher | | #6

      If this was our situation I'd definitely do the same. We have a few children with more coming and the bedrooms will eventually all be occupied and doubled up. It's a blessing and a curse haha.

  2. kyle_r | | #2

    From a load perspective one ducted mini split per floor would work well. Whether you can fit the ductwork without bulkheads will depend on what your floor system is.

    If you can’t find a qualified Fujitsu installer in your area you might have better luck with Mitsubishi or Carrier.

    1. tylerrencher | | #7

      Thanks a great idea to look for other reps. I looked at the unit specs for those 2 brands and it looks like they have caught up quite a bit to Fujitsu.

      1. kyle_r | | #10

        In Zone 5B you may want a pan heater. Mitsubishi and Carrier offer that on 1:1 ducted units, but Fujitsu does not.

        1. tylerrencher | | #13

          I knew they didn't have one, but our climate is so dry that I was doubting that it would do anything. I'll have to look into it. Wouldn't want to find out later that I need it.

  3. Expert Member
    Akos | | #3

    Having a basement simplifies your life quite a bit as you can install the equipment down there and run the ducting through the floor. It still requires good design to integrate it into the house but it does make it much easier.

    Large floor plan and longer ducts simply means slightly larger ducting. Most ducted units around 30BTU/CFM, so even your largest room only needs 90CFM. Generally if you want to keep loses low, you want to aim for around 500FPM duct velocity, that means the 90CFM can be supplied by a 6" duct. Add in a bunch of fittings and turns and you are still well within the 0.6" WG of the PAED head.

    Provided you keep the ducting simple, you can do most of the design in a spreadsheet. The simplest way is to home run each duct to a central plenum right on the air handler. This does mean more duct runs but a simpler design.

    Use a simple short straight run to a single central return in the house with an oversized filter. You can do jumper ducts or door undercuts for the returns on each room.

    You can fit both air handlers in the basement room where you show the ducted unit. You can vertical mount the basement unit with a low return right through the closet wall and mount the main floor unit to the ceiling. You might have to give up some of the bedroom 1 closet space for ducting.

    With a low load and a lot of rooms, you don't wall mounts. The only time I would look at it is if one of the rooms has very different thermal profile from the rest of the house. Say a room with a lot of west facing windows that you want to keep cool without chilling the whole house.

    Once you have the design, I would quote the ducting and HVAC install separately. This avoid the magical pricing you get as soon as you mention minisplits. If you are talking about two zones with a total of 2 tons, you are only looking at around $3k to $5K in heat pump equipment.

    P.S. Unless it is something like a raised bungalow, your basement loads seem high.

    1. tylerrencher | | #8

      This is great and very similar to what I was thinking would be a possibility. I haven't figured out how to do the return for the upstairs unit that could be mounted on the ceiling. I had planned on sending ducting through the closet of bedroom 1. Will probably lose a foot or so of headroom in there. But in a 9' tall closet, that's just unusable space anyway.

      Great suggestion on keeping the ducting and equipment purchasing separate. I didn't realize that pricing issue was still a problem.

      I included the rim joist heat loss in the basement numbers since that heat would most likely be replaced by the downstairs unit. There will only be 1' of basement wall exposed above ground. Years (like this year) when we have lots of snow on the ground, the ground stays warm and doesn't even freeze. Other years we don't get much snow and the ground freezes a bit of the way down. I tried to take that into account. Either way, a unit with specs like the Fujitsu ARU12RGLX paired with the AOU12RLFC outdoor unit have plenty of higher end heating power at my temperatures, plus excellent modulation, and good COP.

      1. Expert Member
        Akos | | #12

        For the main floor return, I would make the wall behind the fridge thicker and run a 8x24 duct up to the ceiling with a return register above the fridge.

        Looking more at your layout, I think a better spot for the basement unit is the closet of the bathroom closes to mechanical room. From there you have an easy run for a low return to the family room and simpler runs to all the rest of the rooms. This would put the bedrooms at a pretty far run, so look at running an oversized trunk across the house just inside the basement load bearing wall with simple branches to feed the rooms along the way.

        Most Fujitsu units can do much more than their rated output, the AOU12RLFC is capable of almost 18K at 5F, a fair bit oversized in this case:!/product/25349

        Having seen the ice pile that can build up under the outdoor unit during the winter, I would not install something without a base pan heater in cold climate. I really don't know why the Fujitsu units don't come with it, strangely enough the control board even has an output for it.

        I would look at Mitsubishi 9k PEAD (good for 12k at 5F):\M_SUBMITTAL_PEAD-A09AA7_SUZ-KA09NAHZ_en.pdf

        or Carrier/Midea 12k unit:!/product/26455

        I believe the 12k Carrier unit is low static though, so you have to take a bit more care with ducting.

        1. tylerrencher | | #14

          That's an interesting idea for ducting behind the fridge. Your suggestions gave me some other ideas for mounting them vertically in under utilized spaces. Looks like those Mitsubishi units can't be mounted vertically. They have to be horizontal. I modified a closet on the main level to house the air handler vertically. Return on the bottom, supply on the top. I'll have an access door for it concealed behind a picture. The red line is the duct path. I dropped the ceiling in my model to make the run to the library and I thought I would hate it, but I think it looks ok. The run seems ok, but I'll have to learn how to size duct and velocities and all that and find out.

          Edit: I just found the design and installation manual and I'm putting the upstairs one in way to little space.

          As far as pan heaters, how long does it take for ice to build up in the bottom of the unit? We sometimes have a week where we don't see much sun, but typically we're getting some decent sun every day. Enough to heat up metal even on really cold days. My outdoor units will be on the south side of my house so they will see any sun that's out and will warm up because of it. Whether it's enough to do any kind of melting, well, who knows. I noticed on some of the Fujitsu models they are starting to advertise pan heaters, so perhaps they are figuring that out now.

  4. Jon_Lawrence | | #4

    I have a similar set up to yours with an open kitchen/dining area/family room on the 1st floor and another family room and bedrooms on the 2nd (similar to your basement). I placed a slim duct Fujitsu unit on the first floor we you have yours. That does a good job of conditioning that space and a decent job conditioning the rest of the first floor which includes a formal dining room, bedroom and a den. I do have a ductless unit in the den which will condition the dining room and bedroom when the doors are open, but for the most part I just rely on the slim duct. The den unit is the only multi-zone I have with the other head in the basement. The basement gets cold in the winter if that unit is off because I have a HPWH down there. I find the first floor is much more comfortable if I keep the basement unit on from December to March. On the 2nd floor, I have a ductless unit high on the wall in the family room which has a cathedral ceiling and the bedrooms are served by a medium static ducted unit. The ductless unit is rarely used, except when really cold or really hot, maybe 6 weeks/year. The ducted units are great and given the static pressure allotment, they can handle long runs. If I had it to do all over, I would go all ducted. They are extremely quiet and they keep the temps to within 1 degree of the set point.

    Solar heat gains can be a controversial topic around here. My house faces SE and I have a lot of glazing on that side. I wanted lots of warm, bright sunshine coming through in the wintertime and I wanted to be able looks outside and not feel cold. I wanted high VT and high SHGC. I am very happy with the results. On a sunny winter day, the heat pumps turn off around 9am and the solar gains will keep the house warm until about 5pm. At that point I start a fire in my EPA certified fireplace which will heat the open area to 74 and the other parts of the first floor to 68. The fireplace has a blower and I set the slim duct to fan mode which does a good job of moving the air around. The Zehnder ERV helps mix the air too. I set the slim duct back to heat mode when I go to bed and it will kick in around 3am. The only caveat with harvesting solar gains is that shades are a must for the shoulder seasons, otherwise you will overheat. I needed shades for privacy anyways, so it was not an issue for me.

    1. tylerrencher | | #9

      It's great to hear about your experience. I'm happy to hear that the ducted unit is as quiet as the spec sheet says it should be. I saw a video recently of one operating and I kept waiting for it to cycle on, but it was running the whole time.

      My house will face slightly West of South. I've got medium to high SHGC glass spec'd on the South wall. and a good bit of it, but not crazy amounts. I'm really looking forward to the sunny winter days warming up the house for me. My wife like to feel the house temperature vary in the winter and I'm hoping this will give her that experience and not freeze me!

      I'm hoping to not need shades. We'll be living on private acreage, but we might have glare issues or unwanted heating. Frankly though, I'm fine with running the AC in the shoulder seasons if needed. We will see.

  5. Jon_Lawrence | | #5

    Solar heat gains in action. Luckily no fading on my floor after 2.5 years.

  6. remarkablehomes | | #11

    I have installed these before with no issues and low energy bills. The noise may be due to a short line set. The instructions call for a minimum of 9’. Noise and performance issues will result if you are below 9’.

  7. user-6623302 | | #15

    Have you considered high validity air. I have a similar heat load and use a UNICO air handler and mini ducts. The heat source is my DHW tank. You have lots of small room which would lend it self to this system. Maybe two air handlers, one each zone, 50 gal BOCH gas/oil water heater. DHW and heat in one.

    1. tylerrencher | | #17

      I haven't, but I just looked at them a bit. Their website seems high on hype and low on information. Do you have any source of good information about it?

  8. user-6623302 | | #16

    Just remembered, UNICO provides free design services. Looks like they include a room by room heat loss/gain.

  9. user-6623302 | | #18

    I think you have to just dig into the manuals under Resources on the web site. I am happy with my install which I did my self and I am just a DIY guy. The layout is probably the most important and they will do that for you but I did my own using the design manuals. I would at least ask for a design. Use them to check you heat loss calculation. I see these units used all the time on This Old House. I do not know if that is good or bad but I thought it looked good so I gave it a try. Twenty years now, it just works.

    1. Expert Member
      Akos | | #19

      High velocity units are great for retrofit but are not worth the extra cost for a new build. The air handlers are spendy and the ducting and fittings can add up very quickly.

      Since you are moving air at higher pressure, the system uses a much larger blower which also means higher electricity use.

      Unless you are very tight on space, I would stick to standard ducting. There are better places to spend an extra $7k to 10k.

      Keep in mind the only small ducting are the local branches. You still need a main supply trunk that is on the order of 8" or 10".

  10. tylerrencher | | #20

    For anyone reading this, I bought Wrightsoft and did a full design model. That was absolutely worth the money and time. I built the house using an 18k and 24k Fujitsu ducted minisplit, one for the upstairs and one for the basement with all ducting in conditioned space. They were supposed to be sized at 12k and 18k but I couldn't find anyone willing to install that small of units. The basement would have been fine at 12k. The 18k would have been ok for the main level, but the model and now real world experience confirms that there would have been very little capacity remaining. It would have worked, but I appreciate the peace of mind knowing that it doesn't really matter how hot or cold it gets, we will be warm. The only thing is that it cycles too much in the shoulder seasons so during those times I manage it by hand if it is annoying me.

    The house tested right where it should have for air leakage and the power usage data from the HVAC units indicates that that whole insulation and air sealing strategy is performing as expected. Do the math and trust it, watch the details, and enjoy your pretty good house!

    One additional note, I put a solid fuel stove in the basement as a backup heat source hoping it would be able to heat the whole house through convection. For my particular home design it does not work well. The basement can be kept warm and comfortable, but the upstairs doesn't get enough heat to be comfortable. It will keep everything up there from freezing, but the diurnal swings are way too strong where we are to not have a second backup stove on the main level. The winter sun in the windows keep the heat from coming on down to about 20F daytime temperature, but on stormy or cloudy stretches the basement stove can't keep up alone.

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