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Best Approach to Cooling Second-Floor Bedrooms

chief083 | Posted in General Questions on

My upstairs can get 10 degrees hotter than my downstairs in a 4000 sq ft house built 1978. The air flow to the 2nd floor is not very good. I am leaning towards getting a mini split for each bedroom upstairs so at least 4. Before I pull the trigger I wanted to know if adding a 2nd hvac unit to increase air flow to the existing vents for the 2nd floor would be better? I have read about adding insulation but regardless I believe I will want to add a system to increase comfort upstairs. Any recommendation on mini split brands? I have heard Mitsubishi is the best.

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Replies

  1. DCContrarian | | #1

    What you're asking is a complicated question. The way a true pro would handle it would be to do what's called a Manual J. You go through every room in the house and measure the walls, windows and doors, look at the construction and insulation. Then you take local weather statistics and estimate the heating and cooling load. You get a load for the whole house by adding up the rooms. You size the equipment for that load. Then you do what's called a Manual D, where you calculate what size duct each room needs to get enough air to meet its load. Once the ducts are installed you measure the airflow, and then adjust it to get the flow you calculated.

    If your house was built in 1978 that work probably wasn't done. The reason your upstairs is hotter is it's not getting enough air. If the equipment you have is adequately sized for the house you may be able to adjust the ductwork to get more air going upstairs. It could be as simple as closing off some dampers. Alternately, adding insulation upstairs will reduce the cooling load and you might be comfortable with the existing airflow.

    The thing to know about this is there are no rules of thumb, no back-of-the-envelope calculations. The fact that your house is 4100 square feet tells us nothing about how much heating and cooling it might need. A very efficient house might need one tenth of the cooling of a very inefficient one. If anyone wants to install equipment without running a Manual J, run away from them. It's like a surgeon offering to operate on you without knowing what's wrong.

    There's no magic to minisplits. You have to go through the same sizing process. An improperly sized minisplit will not provide comfort. What's not intuitive is that one that is too large is more of a problem than one that's too small. One that's too large won't be able to hold temperature, it will be hot and cold, and it won't dehumidify properly.

    Generally, minisplits usually provide too much cooling for comfort in a bedroom. I know I said there are no rules of thumb but it's the exception that proves the rule.

    1. chief083 | | #7

      Thank you DC. I'm actually in Maryland under 30 minutes from DC. Do you have any local companies/contractors you would recommend? Its hard to find someone who is knowledgeable. I agree with your assessment we are not getting enough air upstairs. I know some air pressure is definitely being wasted in the basement and the house was renovated and our last contractor didn't properly seal off some of the old vents.

      1. DCContrarian | | #8

        So another quick thing to check is to make sure your ducts aren't leaking somewhere, you could be losing a lot of air that way. It's something you can fix yourself with -- you guessed it -- duct tape.

        1. chief083 | | #12

          Thanks I will check that out. I attached a page from the energy audit I had done a year back.

  2. Hammer 🔨 | | #2

    If you are looking for something quick, NY has been in a heat wave the past few days, you can open the dampers wider on second floor and make the opening lower on the first but not completely closed as why DC was suggesting. You can do opposite in winter. Get same window acs for those bedrooms or at least some fans. Ceiling fans help a lot. If you still lack comfort get a hvac pro but at least in the meantime you aren’t sweating

    1. chief083 | | #13

      Thank you! I do currently use a fan upstairs to help with cooling/air circulation.

  3. GBA Editor
    Kiley Jacques | | #3

    You should read this article: Getting the Right Minisplit. “Installing a separate minisplit head in each bedroom is a mistake,” says Martin Holladay . . . “The typical bedroom has a design heating load of 2,500 BTU/h or less — far less than the rating on a typical ductless minisplit head, which usually ranges from 9,000 to 12,000 BTU/h. If your HVAC contractor is advising you to install a separate minisplit head in each bedroom, it’s probably time to choose a different contractor.” Instead, he recommends installing a minisplit head in a nearby common room — perhaps the living room or hallway — or to select a ducted unit instead of a ductless unit.

    1. chief083 | | #14

      Thank you the article looks informative. I just found out about this site yesterday and am leaning towards getting a membership to read premium articles. The site seems to have lots of good info and active members.

      1. GBA Editor
        Kiley Jacques | | #22

        Welcome to GBA, you’ll love it here.

  4. Kyle R | | #4

    I would suggest getting an energy audit done with a blower door test. Based on the age of your house there could be considerable air leakage. Address this first, it’s possible that this addressing air leakage alone may fix your comfort problem.

    1. chief083 | | #15

      Thank you. I actually did have one done a bit over a year ago but had a lot going on at the time and was not able to follow up on the improvements. Insulating the attic better was on of the recommendations. I attached my report which shows my air leakage. It doesnt show it would improve my duct leakage although i'm almost positive my duct leakage could be improved.

      1. DCContrarian | | #18

        "The reconfiguration of the duct work performed as part of recent remodeling activities appears flawed." I'd find out what that means.

        1. chief083 | | #19

          He mentioned he wasn't an hvac specialist but we discussed it in person. Basically we knocked down a wall to open up an area and it contained ducting that went to the 2nd floor. They rerouted this ducting with smaller duct work than the original and it doesn't look very professional. Also it seems like some of the duct work may have not been properly sealed. I knew I already needed to but am planning on following your recommendation of checking all the ducts to seal with duct tape. I definitely suspect some air is leaking being wasted in the basement. For example there was a vent on the ground 1st floor that was removed but I can still feel cold air on the wood floor when I stand there so basically they just put floor over it without sealing it and air is being dumped there. Thats just one example...I can likely address some of this myself but was hoping to find someone local with knowledge to really get everything fixed up for me. Maybe just going in the basement and tracking the places the air is leaking and not sealed and then adding insulation to the attic could do the trick. I know basements are always coldest but my basement definitely feels very cold in comparison to the rest of the house.

          1. DCContrarian | | #20

            Definitely fix the leaks,, but smaller duct work could be most or all of your problems. Air flow is very sensitive to the size of the duct, they sized them that way originally for a reason, you can't just go in and stick in whatever fits. Or you get what you're getting.

            Putting in bigger ducts isn't necessarily a big deal. An HVAC pro might look at it and say it's an hour's work. How big were the original ducts and how big are they now?

        2. chief083 | | #23

          I am attaching pictures of my original ducting and the modified ducting. You can see the difference in size. I took these pictures today...you can see in one up close pic near the blower that it is sweating heavily and my house was at 71 even though thermostat was set at 65. It was 90 here yesterday but still...The non duct picture is where the wall used to be where the vents went vertically through and it was re-routed to that column on the left in smaller ducts that then I think go at a right angle and reconnect to the vents in that overhang you see where the wall was. I am not very confident in the ducting which is why I was considering mini splits for upstairs.

          1. DCContrarian | | #24

            I can't really make out the pictures, but it looks like they took a 6" round duct and dropped it down to a 4" duct so it could run inside of a 2x4 wall. That 4" duct is going to have about half the capacity of a 6" so yeah, there goes half your capacity.

            A 3-1/2" x10" rectangular duct has the same capacity as a 6" round and would fit inside that wall. That would give you your capacity back.

        3. chief083 | | #25

          Ya I guess they went with those because of the tight area they worked it in. I definitely think HVAC wasn't their expertise. So you think getting that ducting redone would be better than getting mini splits? I am worried about paying a good amount to fix it and the AC to not be much better.

          1. DCContrarian | | #26

            Whenever you get anything fixed, whether it's a car or an appliance or whatever, there's always the risk that the repair will cost more than what the item is worth. I can't tell from here what the repair would cost or what your current system is worth. There's also the risk that I've mis-diagnosed the problem and the repair woudn't fix it.

            Let me ask you this: before the ducts were moved, did the system do a good job of keeping the second floor comfortable? Because the most you can hope for is getting it back to where it was.

            There's also a problem that the housing market is going crazy and construction is bonkers and it's hard to get people to do anything.

        4. chief083 | | #27

          Yes thats the issue we had the wall removed before we moved in the house so I am not sure how well it heated or cooled before we made the modifications. I know even in my last house which was built 1995 the 2nd floor could get 5 degrees hotter than the 1st floor. So I am expecting even the ducting was fixed the 2nd floor would still be less comfortable than the 1st. I am sure for efficiency it would be good to fix eventually but for long term improved comfort I am thinking I need something extra for the 2nd floor whether its a unit that provides more pressure to the 2nd floor or a mini split system. I have heard mini splits are very cost efficient and since they don't require ducting I felt that they could best provide what I am looking for. I think based off feedback here and my energy audit that adding insulation and sealing the recessed lights etc in the attic combined with some mini splits might be the right combination for me. The information about some splits providing too much cooling to be used in a bed room was something I hadn't considered so that was useful information too. I have a contractor who is about to do work on my house starting this week for other improvements so I could probably get them to do the attic insulation and I could go over my options for the mini split install.

  5. Expert Member
    Dana Dorsett | | #5

    What they said- run load calculations, and don't oversize by more than 50% (if you can), even with modulating equipment like mini-splits.

    A fairly handy tool that was explicitly created for implementing ductless (and ducted) heat pump equipment is the freebie developed by the Better Built NW folks:

    http://hvac.betterbuiltnw.com/

    It requires signing up for a (free) account, but it's a very straightforward and simple tool to use, and delivers reasonably accurate numbers. (There is also a simplified duct design part of the tool that's best left to the HVAC pros unless you're willing to learn enough about duct design to play "junior HVAC engineer".)

    Unlike other free (or professional) Manual-J-ish tools it's default U-factors are appropriately aggressive, and require the user to manually override them, making it a bit more idiot-proof. Other free online tools such as LoadCalc (dot net) and CoolCalc (dot com) can be pretty good in the hands of engineers or people with more experience, as long as the user is appropriately aggressive on the input data, and the oversize factors are kept to a minimum (or even slightly undersized relative to the tool's load numbers.)

    If your existing system is implemented with flex duct, inspect the installation to ensure that the ducts are stretched as straight & tight as practical, with only wide turns, no kinks. If it looks like a ball of mating snakes up there the "effective length" of those ducts can easily be twice or more the actual physical length due to excessively turbulent flow. If it's a mess, re-commissioning the ducts with better installation practices can often fix room to room temperature differences.

    Whether flex or hard piped, make sure that all seams & joints are sealed with duct mastic, including the seams of any duct boots, and making sure that the duct boot is well sealed to the ceiling/wall/floor where it enters the room. Hard piped duct seams may look and feel air tight, but they aren't (even when new), and can open up quite a bit over time from the expansion & contraction of thermal cycling.

    If any bedrooms have supply ducts only (no return grilles) part of the problem might be the high impedance return paths restricting flow when the doors are closed. If there is only a common return in the hall or stairwell, adding "jump ducts" to lower the back pressure of the return path can improve flow by quite a lot. These can take many forms- a light-& sound baffled transfer grille in the transom above the door, utilizing partition wall stud bays as ducts, etc.

    1. chief083 | | #16

      Thank you for the response. I definitely think my duct work could use an overhaul. That's why I was considering the mini splits for the top floor so I would not depend on my ducting. The bottom floor doesn't get unbearably hot during peak heat and it always warm enough in the winter. I am thinking the cooling to the upstairs wasn't great to begin with and after our contractor had to re-route some ducting when we removed a wall that supplied some ducts to the 2nd floor it made the situation worse. I am guessing that it would be a lot of work to redo all the ducting though? Would it be cheaper than getting mini splits? I am going to take DC's advice and see what I can seal up with duct tape in the basement as I know some of the ducts they did not seal properly. I attached a copy of the last energy audit I had for reference too. I haven't been able to do any of the improvements yet.

  6. qofmiwok | | #6

    There are mini-splits now which modulate down to 1500, which is better than 9000 split among 3 (and in order to get one that allows 3 heads, you can't get a small enough outdoor unit to get much lower than 9000.)
    Isn't the answer a true VRF multi-split?

    1. DCContrarian | | #9

      Generally they run most efficiently and with best dehumidification at full load. You don't want to size them so that at design load they're running at full modulation, it defeats the purpose of modulation.

      1. qofmiwok | | #10

        Not clear on your answer. That are you referring to by "they" and "them"?

        1. DCContrarian | | #11

          Modulating mini-splits.

    2. chief083 | | #17

      Thank you for the response qofmiwok. I am definitely considering mini splits. At first I thought it was logical to put one in each room but from what i've gathered its not good to have one in a bedroom. I have an upstairs living room which is connected to my bedroom I could place one in and maybe put another in the upstairs hallway? I want to make sure all the rooms are adequately cool though and am wondering if we typically leave our doors shut how much cooling the other rooms would get from having a mini split in the hallway.

    3. Expert Member
      Dana Dorsett | | #21

      >"Isn't the answer a true VRF multi-split?"

      Show me the model that has sufficiently low modulation and up front cost to make that work!

      The best answer can't really be known until the loads are calculated.

      >"There are mini-splits now which modulate down to 1500, which is better than 9000 split among 3 (and in order to get one that allows 3 heads, you can't get a small enough outdoor unit to get much lower than 9000.)"

      There are many multilsplits with compressors that can drop to the 6000-7000 BTU/hr range with reasonable efficiency, but even that might not be optimal. Many single zone 9000 BTU/hr ducted mini-splits can dial back to under 3000 BTU/hr @ 82F, (albeit some with a lousy COP at minimum modulation) eg:

      https://ashp.neep.org/#!/product/36663

      https://ashp.neep.org/#!/product/47329

      ... but some with vapor reinjection compressors are still decent at that low low output:

      https://ashp.neep.org/#!/product/26454

      Some with vapor reinjection compressors can dial back to under 4000 BTU/hr @ 82F with SUPERB efficiency though, and others with still eg:

      https://ashp.neep.org/#!/product/25310

      Bottom line, multi-splits have less modulation range and are more likely to be oversized (TBD) due to the limitations of the compressors. Three separate mini-splits that modulate to under 2000 BTU/hr can suffer low efficiency at minimum modulation, others not. But a pretty-good ducted mini-split with the right compressor technology is more likely to be right-sized for the load, if going that route.

      I'm still advocating verifying /rectifying return path issues and fixing duct leakage & floppy flex as the first (and cheapest) plan of attack for fixing the problems. But whether minisplits or other, it's always good to have the load calculations in hand before making any major changes/purchases. A "ductless head per room" approach is almost always a mistake from a first cost and efficiency point of view.

      1. chief083 | | #28

        Thank you for your follow up again. Do you recommend any brand for mini splits? I was told Mitsubishi is one of the best but I see you linked some other brands like Carrier...any preference? I am going to try to check out my duct leakage tomorrow or this weekend and hope it will help but just based on past experience from second floor heat levels i'm thinking even if I improve the cooling I am likely to enjoy more comfort with something to boost 2nd floor cooling like a mini split. I understand now from the groups feedback the one split per room is a bad idea, which will be good to know for when I potentially consult with a mini split contractor for my property. Also I am going to try to get the additional insulation added to my attic.

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