Ductless minisplit question
AlanB4
| Posted in Energy Efficiency and Durability on
What models are available that will work at the lowest temperatures, it would be for a house in zone 4 (close to 5), about 4500HDD, at a design temp of -5F (-22C). The owner would likely keep the house temp at about 15C all winter, and the house probably has a heating load in the 20-30k range (at room temp so less at 15C).
Cost is of course a consideration, its an open concept two floor house with cathedral ceiling and unfinished basement,
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Alan,
The two brands of ductless minisplit usually chosen in cold climates are Mitsubishi and Fujitsu. A design temperature of -5 F is not particularly cold, so it will be easy to meet.
You wrote that the house "probably has a heating load in the 20-30 K range."
"Probably" isn't good enough. The first step is to perform a proper Manual J calculation.
In one of my articles, I described how a ductless minisplit is specified:
"The nominal heat output rating of a ductless minisplit is calculated at an outdoor temperature of 47°F. Since the unit’s heat output drops with the outdoor temperature, it’s important to check low-temperature performance before specifying a unit.
"For example, the Mitsubishi Mr. Slim Hyper-Heat unit (model PUZHA36NHA) has a nominal heat output rating of 38,000 Btuh. According to the manufacturer, at an outdoor temperature of -13°F, its heat output drops only 21%, to 30,000 Btuh.
"Another ductless minisplit, the Quaternity unit from Daikin (model FTXG15HVJU), has a heat output rating of 17,890 Btuh at 43°F. At -4°F, however, its heating output drops to 7,310 Btuh.
"As long as the heating system designer sizes the unit so that it will meet the building’s heating load at the design temperature, there’s no reason it won’t keep a home comfortable — even when the temperature drops well below zero."
For an overview of issues related to using ductless minisplit units for heating, see this article: Rules of Thumb for Ductless Minisplits.
Heat pumps can, in theory, operate significantly more efficiently if they are asked to keep a space at 15 C rather than 20 C, so your savings from only heating to a lower indoor temperature could greater than they would be if you were heating with combustion. Unfortunately, I don't know of any hard data on that effect.
Thanks for your replies, there was an energy audit done i believe 7 or 8 years ago so it must be kicking around somewhere and will have the proper heating load including the blower door result, i just don't have it available at the moment so of course it would be consulted for proper sizing.
The Fujitsu mini-duct type -_ _ RLFCD mini-splits are rated down to -22C, and can make heat distribution easier than the wall coil types, provided there is space to run the mini-ducts. The 15RLFCD has a nominal output of about 20,000 BTU/hr, and could handle several smaller rooms. For more open spaces the wall-coil types are more efficient, but that particular mini-duct series beats many of the better wall-coil types of only 4-5 years ago on raw efficiency.
Of the wall-coil types, the Mitsubishi MSZ/MUZ- FH_ _ NA series or the Fujitsu ASU/AOU-_ _ RLS3H series are both pretty good options. The smallest of the series -FH09NA can deliver about 11,000 BTU/hr @ -15C, the -9RLS3H delivers about 14.000 BTU/hr at -20C, but both will keep running down to -25C and beyond. The larger units have higher capacity, but also somewhat reduced efficiency (though modest oversizing improves average efficiency on these.) If the floor plan is really wide open and the individual heat loads of any doored off room is sufficiently low, you can probably do well with just one mini-split per floor.
If the unfinished basement is not yet insulated, it's worth doing that project, and it will affect the final heat load numbers.
If you have a heating fuel use history on the place it's possible to estimate the total heat load base on fuel use per heating degree day. That would require knowing your location with some precision, and the exact dates on the fuel fill-ups or meter readings over a winter period (for weather history precision) and the heating equipment used (for the efficiency factor, since that's the measuring instrument), as well as the thermostat settings. The error is no worse (and often better than) Manual-J calculation methods.