Replies

1. Expert Member
   Dana Dorsett | Apr 24, 2019 01:51pm | #1

   The only way to answer any of it is to start with a room by room load calculation. It's just as likely that low-load rooms with oversized heads (all heads are oversized for the typical loads of average sized bedrooms) would be less comfortable than the one without, at least during the cooling season.

   Oversizing a multi-split compressor by more than 1.25x for the peak load can make both comfort and efficiency worse too. Oversizing is not your friend. Without a substantial duty cycle during not-so-hot but high humidity days it will oscillate between feeling sticky and clammy. With right-sized (or even undersized) equipment that doesn't happen.

   In a MA climate the night humidity/latent-load is the biggest issue, not heat gain from the outdoors. It's possible that a single right-sized or undersized ceiling cassette in the hallway or top of the stairwell would be a better solution for cooling the upstairs than a head in every room, if cooling for night time comfort is the primary consideration. The smallest 3 zone units are 2 tons, and it's highly unlikely that your cooling load is anywhere near that high.

   Use a freebie online tool such as LoadCalc.net to come up with reasonable estimates of the room by room cooling (and heating) loads and report back. I suspect you're looking at a total peak cooling load of less than 1.5 tons unless there is a lot of west facing window area on one or more of those rooms, maybe even less than 1 ton. It could be that the optimal solution is a 1.5 ton 2-zone system using a mini-duct cassette splitting the output for bedrooms with loads too low to warrant a separate head (which may be all of them.)

   [edited to add]

   A primer on the importance of right-sizing the equipment in the short intro-videos on these pages from Nate Adams. Though he is mostly discussing it in a heating context, it's also true in AC:

   http://www.natethehousewhisperer.com/home-comfort-101.html

   http://www.natethehousewhisperer.com/hvac-101.html

   http://www.natethehousewhisperer.com/hvac-102.html

   Whether central air or mini-split, take the time to get the load numbers right or you may live to regret it. Do NOT leave it up to the HVAC contractors to get the sizing right (the industry track record is atrocious on that). Either hire a certifed engineer or RESNET rater- someone who makes their living and reputation on the accuracy of their number rather than installing & servicing equipment, or do it yourself.

   Believe it or not, using a simplified version of a Manual-J tool (not all online tools are Manual-J) and aggressive assumptions on R-values and air tightness even a newbie homeowner is more likely to right-size it than the average AC installer. If using loadcalc do NOT upsize it from there "just to be sure". Even undersizing it by 15-20% would be better than upsizing by 15-20%, since it usually overshoots reality.

2. 
   joshdurston | Apr 24, 2019 02:44pm | #2

   It's frustrating that getting ducted cassettes installed (and quoted) is like pulling teeth.
   I think contractors love the simplicity of hanging a ductless wall mounted head, and are highly biased based on the ease of install to the point where they disregard comfort implications. In my early twenties I worked for a residential HVAC company installing AC and furnaces, and can say that unfortunately, in most cases the contractor isn't in the house long enough to have a valid opinion on operating costs, or comfort. I now work in the building controls industry and am hyper exposed to occupant comfort feedback and equipment performance implications.

   Dana's right... most bedrooms in a reasonably constructed house don't have enough load to warrant a dedicated head, and the best answer is often a properly sized ducted unit (1:1 indoor:outdoor).
   Having the ducted unit in the vicinity (against a wall, in a closet) with almost no return ducting, and a minimal amount of supply is will likely serve the best. Zoning it out room by room often isn't necessary for comfortable. But, proper sizing with long dehumidifying run times, and small temperature swings, with a modulating unit that purrs along and matches the load will be comfortable. Way better than the standard AC split unit with a oversized 4ton condenser and an air handler/furnace in the basement.

   In short stick to your guns and get a quote on properly sized ducted unit. Don't trust most HVAC people with sizing. You'll probably have to do the load calc or pay someone else, but either option will likely yield a more accurate number than the HVAC guy. And be prepared to have a bit of an argument about the size.

   Sadly the most used method (if the calc is done at all) basically assumes the worst about everything, which will come up with a number that's 50-100% oversized, and then they will go up another size or two, "to be safe". Leaving you with a unit that is 250-400% oversized.

   Of course there are exceptions, and you may find a contractor who will carefully do an assertive load calc, and confidently spec the proper size equipment, and distribution ducting.

3. 
   redpurplegreen | Apr 24, 2019 04:27pm | #3

   This is all extraordinarily helpful! Are there people are specialize in calculating load? I looked at the calculator and I think I could benefit from some help on this. I think I'm definitely being oversold.

1. Expert Member
   Dana Dorsett | Apr 24, 2019 05:55pm | #4

   Many RESNET raters run Manual-J calculations as a service, as do many certified P.E.s

   Some HVAC contractors will too, but in my experience (and others) HVAC contractor Manual-Js almost always use conservative assumptions everywhere, as if your house was the leakiest, least insulated house in Massachusetts, with sub-code minimum windows. The instructions for using Manual-J explicitly requires that the assumptions for every thing from R-values to shading factors to air leakage be the most aggressive that would still be reasonable, taking into account every possible factor that would lower the load. I've seen HVAC contractor Manual-Js that came in 2x higher than those performed by an P.E.

   Rules of thumb based on floor area generally suck, and should never be used for anything other than sanity checking, but typical cooling load to floor area ratios for basically rectangular 2 story colonials in MA run about a ton per 1500' of conditioned space for insulated 2x4 construction with clear-glass double panes and at least R20 in the attic. With better shading factors it can be a ton per 2000' or more, with a lot of unshaded west facing window it could be a ton per 800', but those are rare. If punting a ton per 1000' would usually oversize by a bit, but not by a crazy amount.

   I suspect all four bedrooms combined add up to ~1000' or a bit less, add in the floor area for stairwells/halls/bathroom/closets etc maybe 1500'-1600' total for the second floor? A 1.5 tonner would almost always cover it with margin, and if it has a reasonable modulation ratio it wouldn't necessarily be overkill. With multi-splits in the 1.5-2.5 ton range don't assume more than a 3:1 turn-down ratio unless the engineering manuals or AHRI submittal sheets for the equipment indicate otherwise. Some of the "hyper heating" models have a bit more modulation range than that, but usually not more than 4:1. Some are only about 2:1, which would be an efficiency and comfort disaster if oversized by 2x. With single zone mini-splits the turn down ratios can be much higher. eg: Hyper heating vs. non-hyper Mitsubishi 2-ton multisplits:

   http://meus1.mylinkdrive.com/files/MXZ-3C24NA2_Submittal.pdf (~2:1 cooling turn down ratio)

   http://meus1.mylinkdrive.com/files/MXZ-3C24NAHZ2_ProductDataSheet.pdf (3.9:1 cooling turn down, hyper heating)

   Some single zone Mitsubishi that might be put to single-room use:

   http://meus1.mylinkdrive.com/files/MSZ-GL09NA-U1_MUZ-GL09NA-U8_Submittal.pdf (3.4:1 turn down, not hyper heating, a bit overkill for most bedrooms.)

   https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf (7:1 turn down, hyper heating, would still modulate some at typical top floor bedroom loads.)

   http://meus1.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NA_ProductDataSheet.pdf (5.3:1 turn down, hyper heating, would still modulate some.)

   If your 1% cooling load isn't more than 2x the minimum cooling output of the mini-split it's probably going to be more efficient and more comfortable to share a ducted mini-split with other rooms.

   So, sketch out the floor plan, run the cooling (and heating) load numbers and put the load numbers on the sketch to see what might make sense.

2. GBA Editor
   Martin Holladay | Apr 25, 2019 07:06am | #5

   Red,
   There's a GBA article addressing your question. Here is the link: "Who Can Perform My Load Calculations?"