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Help Estimating BTU needs for attic bedrooms

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Hello all,

I am preparing to install a Mini-Split system in a 2.5 story house. The upper floor has three small bedrooms, about 140 to 180 sq ft. each. that are integrated into the attic level with dormers.

I plan to use a Mitsubishi Heat-Pump system for the whole house and I am trying to size units for the attic levels. I am leaning toward the 6k BTU Units because of their footprint, and the size of the rooms. I would think this is adequate, and I want to avoid oversizing the units. I did have 6000k window units which were able to cool the space in the summer but they stayed on pretty consistently.

My understanding is that the cooling capacity of the 6k units is rated for 250 sq ft. which is definitely more than what I need, but that's under normal conditions without the additional heat gain of adjacent attic space etc.

The building is located in the the Bronx, NY where summers can be hot and fairly humid. The third-floor ceilings and walls are insulated with R-20, dense-packed cellulose but the attic rafters just above are uninsulated and these attic spaces become very hot under black asphalt shingles. The building faces mostly east-west so the western and middle rooms get especially warm in the summer months.

I'm looking for some input from those experienced with sizing these units.

Thanks in advance,

Ed





Comments

  • yellowdog
    yellowdog Member Posts: 157
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    heat pumps are not sized by square footage. step 1 is to do an accurate heat loss/gain to see what the design day loads are. after that the results need to be run thru diamond system builder to get the actual outputs at design temperatures. more than likely the 6k units are the correct ones, but i would never sell or install any heat pumps without running the info thru diamond system builder first.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
    edited May 2023
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    You’re approaching a significant problem here: ductless minisplits should not be installed like this unless you have no other options, especially when it’s multiple indoor units connected to one outdoor unit. Mitsubishi themselves says not to do it. Use ductwork or one outdoor unit per bedroom.  
  • EdSheridan
    EdSheridan Member Posts: 16
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    YellowDog, I am aware that Heat pumps are not simply based on sq. ft. (that's why i'm here with this question) I did do a DSB and a basic Manual J, (I say 'basic' because I have not done a blower door evaluation on the building or the rooms but my total heat load for the building is estimated at 76k BTU for Heating and 54k BTU for cooling, I am planning a 78k capacity system. I am new to the DSB software but i'm getting 8k BTU C/ 8.5k at design day with the 9k BTU Units, and 5.5k BTUh Cooling / 6.1k BTUh Heating with the 6k Units. I realize this is only half of the data I need because I need the heat load for the individual rooms. and the adjacent attic seems like a wildcard.

    I guess the reasonable question to follow this would be, what do I stand to loose if I go with the 9k BTU units, Is there a potential downside to the 3k difference? I know that before the 6k was on the market, guys put the 9k units even in small rooms. anyone have experience with things to look out for - associated with this?

    Hot_Water_Fan, Can you explain more about why you're saying this? Can you point me to where Mitsubishi say this?
  • yellowdog
    yellowdog Member Posts: 157
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    there is nothing wrong with a properly sized multi head system. i prefer them over doing what a lot of other contractors do and put multiple condensers on every side of your house with too short of linesets. once i figure what needs to go in each room, my goal then becomes trying to keep the outside appearance of the customers house as clean as i can. by putting larger heads in than required, you are shooting yourself in the foot by being grossly oversized on your cooling side and also your heating side. it also affects your connected capacity, so you would have to install larger more expensive condensers than necessary. before the 6k ceilings came out, you had to use the 9k's. they were always oversized, but was all that was available at the time. if you think that your attic space above your rooms gets hotter than the ambient temperature outside, you just need to factor that into your heat gain calc with your delta t for those rooms.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
    edited May 2023
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     I have not done a blower door evaluation on the building or the rooms but my total heat load for the building is estimated at 76k BTU for Heating and 54k BTU for cooling, I am planning a 78k capacity system. 
    That’s very high. Have you lived in the house for a winter already? How much fuel and what kind did you use? Manual Js are usually junk, and as you see, the blower door test is a significant variable to leave out.

    Can you explain more about why you're saying this? Can you point me to where Mitsubishi say this?
    Sure can: https://www.greenbuildingadvisor.com/article/multi-zone-heat-pump-issue

    There is a technical bulletin, I just can’t find it at the moment too. The issue is that multi-splits can’t turndown much AND are put into situations where they really need to turndown. Like a bedroom that’s 150 sqft. To illustrate this: pretend the system was only the 3 bedrooms. 

    This is the smallest 3 head multisplit that Mitsubishi makes: https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006%5CM_SUBMITTAL_MXZ-3C24NAHZ3_en.pdf

    it has a minimum heating (at 47F) and cooling (at 95F!) outputs of about 12,000 Btu. That’s just too big for a space that’s a total of 450sqft probably even on design day. The rest of the time, dramatically oversized. It’ll cycle on and off, poorly dehumidifying and being noisy the entire time. Treat multi-splits as the last resort, they’re about the level of window units, just tens of thousands of dollars more. Now, combining adjacent rooms into a small ducted system would fit the space!
    GGross
  • EdSheridan
    EdSheridan Member Posts: 16
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    Hot_water_Fan, Thank you for the additional information. Basically, my objective is to avoid oversizing the system. And I agree based on my limited knowledge, that the Manual J seems high. As YellowDog was mentioning, I am trying to steer away from having to hang a bunch of condenser units all over the building.

    I don't think the ducted option will work as the attic space height in the rooms which need the units is inadequate, and access is limited. It could be done but it would involve tearing out a ceiling and then getting access to it for service would be a pain.

    The Green Building Advisor segment you linked too was informative, Although it seems to be dealing with client demands which lead to oversizing a system, which is easy to do with passive buildings, or buildings with air-tight high performance building envelopes -- which is far from what i'm dealing with. This house was built in 1907 it's a balloon-framed wood structure with way more infiltration than id like to admit.

    The key point of the Mitsubishi Bulletin you posted is "If an indoor unit nominal capacity is more than 50% higher than the maximum heating or cooling load in a space, it will be oversized and humidity, overheating and higher than expected system energy usage can occur. This space should not have its own indoor unit."


    Everything I read, including the technical bulletin you posted, seems to indicate that oversizing system or the G.B.A. interview with Dana, is having way too big of an outdoor unit, so you're cycling and loosing efficiency to heat or cool a single room. It seems like the way to avoid this is to stay within that 30% scale down range, from your actual heating or cooling loads, and if you don't, this is where you can go wrong.

    I'm not so quick to Junk the Manual J. I'll admit that if you don't know what you're doing, or how to change the appropriate inputs the Manual J can swing 15 or 20k BTUs based only a few changed parameters. But if you can't depend on a Manual J, what else is there? How do you properly size the units. I'm not sure looking at the natural gas usage of the building over the last year would be of any use because our boiler was 70% oversized, because it too was insanely oversized.

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
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    Well what was the gas usage? If the boiler is old and inefficient, that can be your ceiling. I’d be shocked if 450 sqft needed 18,000 Btu or 18,000 x .7 either. 

    Remember, multi-splits are usually installed in old, leaky homes as most newer homes have ductwork. So all of these painful customer experiences are in houses just like yours. 
  • EdSheridan
    EdSheridan Member Posts: 16
    edited May 2023
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    We did an oil to gas conversion several years ago and the job was done by contractors who did not make any effort to size the new steam boiler. The boiler is a new Burnham IN8 which is massively oversized. its rated for 168k BTU, The above grade structure is about 2,500 sq ft. even just running the numbers from the old existing steam radiators translates 68k BTU, from 296 EDR but this is way more than what is actually needed now, (two radiators were removed from the original schemata). The building has been retrofit with cavity insulation, house wrap, double pane windows etc.

    We have natural gas appliances which would have to be factored out of the NG usage to answer your question and give you a realistic calculation. I'll try to do that, but I think the heating load for the building is closer to 50k BTU.

    Getting back to the Mini-split question - I believe what your saying and I want to heed good advice but I can help but question the numbers from the Manual J estimate, I don't know if this is just subjective or whether this is really the scenario, but the cooling demands for these bedrooms according the Manual J runs from 3k to 4k. per room. Frankly, I find this hard to believe because they are situated beneath a poorly insulated, unvented attic and at the top of the main stairwell which functions giant heat chimney), and during the cooling season they are scorching hot, there is often with no A/C a 10-20 degree temperature differential. I really am inclined to think that the Manual J software i'm using is not able to account for these factors, or i'm unable to figure out how to realistically represent them. Additionally the enclosing attic space which surrounds each room is generally 15-20 degrees hotter than the ambient air outside.

    For the purpose of argument, assuming the Manual J is correct, then I am in fact oversizing for these small spaces. Mitsubishi says of the nominal size is greater than 50% of the total load is oversized, the downside to this being that the system would cycle somewhat and be less efficient, If I still wanted to pursue this route not have a great alternative - i'd be better off splitting up a larger unit to use a 24k/ 3-Zone Unit which can turn down to ~8400k (using the 35% benchmark).

    The biggest obstacle I see is determining how to get an accurate BTU estimate based on these other factors, and knowing what is right or wrong. Any Ideas?
  • pecmsg
    pecmsg Member Posts: 4,895
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    Get a 6000 BTU window shaker plug it in and see if it can do the job. That will confirm or deny the manual J
    Larry Weingarten
  • EdSheridan
    EdSheridan Member Posts: 16
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    Thats what I cooled these rooms with last few seasons,
  • pecmsg
    pecmsg Member Posts: 4,895
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    Thats what I cooled these rooms with last few seasons,

    and did they cycle on and off on a hot day?
  • EdSheridan
    EdSheridan Member Posts: 16
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    No they ran constantly
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
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    Oh I understand - retrofits will always be compromises and it’s about picking the best one. 

    Which 24kbtu unit turns down to 8400? 

    For the purpose of argument, assuming the Manual J is correct, then I am in fact oversizing for these small spaces. Mitsubishi says of the nominal size is greater than 50% of the total load is oversized, the downside to this being that the system would cycle somewhat and be less efficient, If I still wanted to pursue this route not have a great alternative - i'd be better off splitting up a larger unit to use a 24k/ 3-Zone Unit which can turn down to ~8400k (using the 35% benchmark).

  • pecmsg
    pecmsg Member Posts: 4,895
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    If 2 6000 BTU/h window shakers did the job than a 12,000 Ducted mini split would be fine.

    Assuming there a little attic space.
  • EdSheridan
    EdSheridan Member Posts: 16
    edited June 2023
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    Okay,

    So the decision I made here is to go with a 3 zone 24k outdoor unit with 3 6K BTU ceiling cassettes. I think this was the right move. The install went smoothly - for the most part - Here's the catch, two of the three units are working great, but the third is acting "funny" I will start a new post about to try to figure out what help I need. I think this problems is completely unrelated to the oversizing question, but who knows. I'm trying to determine why this would only impact a single zone.

    Ed.