Split system location
build2580
| Posted in Mechanicals on
If I am going to install a mini-split in a very small, well sealed, well-insulated new home that will need airconditioning rarely (maybe never) but heat more often, does it make sense to install the interior unit at floor level instead of ceiling level? Is there generally an option offered by mfgr of low or high mounted units?
Thanks!
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Replies
A Lange,
Some HVAC contractors and energy experts recommend that the indoor unit of a ductless minisplit unit used for heating be installed close to the floor. For more information, see this article: Ductless Minisplits May Not Be As Efficient As We Thought.
If you decide that you like the idea of mounting the indoor unit near the floor, you have two options:
1. You can simply mount the unit that is usually installed near the ceiling where your prefer it -- near the floor (top image below).
2. You can order and install an indoor unit designed for floor mounting. (The lower image below shows one option from Mitsubishi.)
.
Thank you. Fantastic suggested article! Now the crux is to figure out the correct size as I know the HVAC folks can not be trusted. I was told today I need a 12kW system- Tiny house, hyper insulated, well sealed, average low in coldest month is 37F and record low ever is only 21F.
12kw (= 40,944 BTU/hour ) is the heat load of my not-small not super well insulated (well below current code minimums) 1920s vintage home when its -18C / 0F outside.
The smallest single-head mini-splits out there are rated for 9000BTU/hour cooling, and can usually deliver something like 10 -12000 BTU/hr (= 3 to 3.5kw) @ +21F in heating mode.
Run a heat load calculation on the house to see where it ends up.
When comparing models of "inverter drive" modulating minisplits, look at their minimum output at some tested-rated temperature as well as it's maximum capacity. To be most efficient it needs to be able to modulate rather than cycling on/off in milder weather when the heat loads are low. If oversized to the point where it rarely operates are part load, and only cycles on/off it's efficiency drops significantly. In the US +47F is a standard test temperature used in the efficiency ratings tests (HSPF or Heating Season Performance Factor), and the test data submittal documents usually indicate both a minimum and maximum output range at that temperature. eg:
http://www.centralmaineheatpumps.com/sites/default/files/ec_pro/centralmaineheatpumps/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf
http://www.fujitsugeneral.com/PDF_06/Submittals/9RLS3Submittal.pdf
Note, both of those units have over 10,000 BTU/hr of heating capacity in your coldest temperature ranges, but the minimum modulation of one is nearly twice that of the other. In a very low load home with a maximum heat load WELL under 10,000 BTU/hr it's better to go with the unit that can modulate lower during the milder weather, since it will be able to spend more time in a highly efficient non-cycling mode.
Maybe the HVAC contractor meant 12kBtu/hr...not 12kW. Still, based on what's known about the house here...there's a good chance you'll only need a 9kBtu/hr unit.
In fact, at those modest temperatures if the loads are such that a mini-split would rarely modulate, it may be better just use a much less expensive 6000-9000 BTU/hr point terminal heat pump (PTHP) rather than a split system.
A PTHP doesn't modulate, but they have reasonable performance down to the mid to low 20sF, and cost half the money of a decent mini-split. Only if the heating head needs to be mounted somewhere other than an exterior wall would a mini-split make sense for heat load of say 3000-4000 BTU/hr @ 21F.
But we don't really know what the load is yet.
Awesome information THANKS. I know I have seen some free software for calculating heat load (I may even have it saved somewhere). I will try to find that and see if I can figure it out, as obviously the
"experts" aren't even starting from that point.
For running a heat load calculation, use only the 99th percential temperature bin and 68F/20C for an interior design temp. A list of design temps for many locations in US & Canada compiled by the ACCA can be found here:
https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
...with a second opinion by ASHRAE using different data sets here:
https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf
For tiny houses a simple I=B=R spreadsheet is probably going to be just as quick & accurate for sizing purposes. A primer on how to do that lives here:
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-2
If you have unusual non-standard construction methods that may not be covered in online U-factor searches, with the details we could cook up some quick & dirty U-factor estimates for you.
In considering where to mount your mini-split, keep in mind the need to clean the filters frequently. I have very high ceilings and access to the units, mounted about 9-10 feet from the floor, requires a ladder. No big deal, but worth some consideration.
Using Marc Rosenbaum's Residential Heat Loss Calculator, I came up with a heat loss of 6000BTU/hr. I did have to do a lot of metric to imperial conversions and made an educated guess at the infiltration rate (no blower door test yet) and calculated on a design temp between Tuscon and Prescott (from what I know of those places and looking at their temp data it seems about right). Is this a reasonable figure for a super small (850ft2), super insulated, well sealed, strawbale house with triple glazed European windows, or am I completely out of the ballpark and need to try again before deciding on the appropriate mini-split?
Also, in comparing units, can anyone explain how a unit with a lower COP could get more stars than one with a higher COP?
Dana, thanks for the suggestion, but the interior unit does need to be on an interior wall so will need to stick with the split system.
Thanks all for your input.
A Lange,
Q. "Is this a reasonable figure for a super small (850ft2), super insulated, well sealed, strawbale house with triple-glazed European windows?"
A. I would say yes, although there is no way to know for sure without verifying each of your assumptions and calculations.
Thanks! So long as that doesn't look totally out of whack to an expert, I feel pretty confident of the data entered, so i am going to run with it. That translates to 1.8 kW and the smallest system operates at 1.1-2.5-3.5 (min-rated-max) with a COP just over 4 so unless there is any good reason you can suggest to go bigger I will stick with that.