Mitsubishi Mini split -should I worry more about enough heat or short cycling?

Discussion in 'HVAC Heating & Cooling' started by rgs, Dec 6, 2013.

  1. rgs

    rgs New Member

    Messages:
    18
    Location:
    Nova Scotia
    Wondering between a Mitsubishi Mr Slim MSZ-FE12NA or MSZ-FE18NA which way to go.

    I have a 170 (approx) year old 1 1/2 story house, post and beam construction built on a stone foundation with a cellar under about 1/3 of the first floor. No south windows. Unheated enclosed porch covers south side and unheated sun porch covers 2/3 of the west side.

    Location about 4 miles inland from Port Hood, Nova Scotia with a mean January temperature of about -10C.

    1st floor is open U shaped 625 sq ft with 7 ft ceilings. Walls are 8 to 9 inches thick. They had no installation when I moved here. I've since had insulation blown in along with 6 inches or so in the attic but there is no insulation under the floor - impossible to put any there. Metal roof. There are 7 windows on the first floor - 4 are double pane in wood frames and 3 are thermopane in vinyl frames. One of the 7 is a short window over the sink and the other six are 3 x 4 1/2 feet in size. Two windows face north and three east. The other two face west but one has the sunporch in between it and the outdoors so there's some protection from the wind.

    Two outside doors - one to an unheated porch and the other to an unheated sunporch. One door to enclosed stairs to the 2nd floor. I've stopped as many drafts as possible in a house this old but there are still some, particularly around the floor. We have to bank the house every winter.

    We've heated for the last 8 years with a Japanese Toyotomi Oil Miser stove burning about 150 gallons of oil a year at a cost of about $1000. We also have a wood burning stove that we often light in the afternoon and we burn about two cords of wood a year to supplement the oil stove. We don't heat the second floor (bedrooms and bathroom); enough heat gets up there to keep it tolerable and good for sleeping.

    The Oil Miser (90% efficiency) has 3 outputs 22,000, 15,000, and 12,000 and steps down automatically as the room warms. It's done a remarkable job with this old house but it's 8 years old now and starting to show some problems. We plan to replace it with a Mitsubishi Mr Slim MSZ-E12NA or MSZ-FE18NA. The installer says a 1 ton will do the job but I'm leaning towards the 1 1/2 ton unit which should have an easier time when the colder temperatures arrive. Just a bit worried about its being too big and short cycling when the weather gets milder. Will also need a 40 foot line set which may affect things.

    I understand that a Manual J is the way to go but no one knows how to do them around here. Does it sound as if a 1 1/2 ton would be a better bet?
  2. Dana

    Dana In the trades

    Messages:
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    Location:
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    The maximum output of the FE18 at -10C is about 22,000BTU/hr, and drops to about 15,000BTU/hr @ -25C, so it's not that different from the output of your Oil Miser. The FE12 delivers about 13,600BTU/hr @ -10C running full-out.

    According to weatherspark.com datasets -10C is about the midwinter mean daily LOW temp, not the binned hourly mean for Port Hood. The binned hourly mean is closer to -5C, but your 99% outside design temp is probably about -19C (slightly warmer than New Glasgow) At -20C the output of the FE18 is about 18KBTU/hr, and the FE12 is good for maybe 10K.

    All but the smallest wood stoves can average outputs over 15KBTU/hr in an overnight burn, so the woodstove may have been important for handling your peak loads on the coldest days. If the plan is to always use the wood stove during colder weather the FE12 would probably handle your average load (at -5C) just fine on it's own. But if you hope to stay warm with just the mini-split during a -20C night, the FE18 would be a better bet, and it just might cover it.

    To get a handle on the heat load calculations, if it's a plaster wall with plank sheathing and wood clapboard or shingles, with 6-7" of depth in the wall losses at -20C in a +20C room (a 40C delta-T) are about 3.5 BTU/hr per square foot of exterior wall area, maybe a little less if the beams are widely spaced.

    Clear double pane windows (no low-E) lose between 35-40 BTU/hr per square foot of total framed window area (sash and framing included) A typical double-hung window is about 10-11 square feet, but measure them. (Yes, an order of magnitude lossier than the walls, per square foot.)

    With 6" of blown cellulose or rock wool you're again looking at about 3.5 BTU/hr per square foot of attic floor. If the upstairs is doored off and basically unheated, dropping to near 0 when it's -20C out, it's probably more like 2 BTU/foot for the combined upstairs wall area + attic floor area- it depends on just how cold it really runs up there at the temperature extremes.

    For the floor losses add up the total square feet of the above grade portion of the foundation, and figure on maybe 50 BTU per square foot. It depends a lot on just how cold the basement gets, but it's likely to be a significant fraction of the total heat load.

    This is an extremely crude estimate- the true loads could be either higher or lower, but it won't be 2x higher, nor will it be half. I'm sure the wood stove could handle the entire load by itself if you were keeping it stoked overnight, unless it's the worlds smallest parlor stove.
  3. rgs

    rgs New Member

    Messages:
    18
    Location:
    Nova Scotia
    Thanks, Dana. I did some measuring based on your figures and came up with a heat lose of about 21,500 btus and hour at -5F which the FE18 should be able to handle more easily than the FE12. Our wood stove is average size but can easily heat the whole house by itself when we use it and open it up. We usually light it around noon and let it burn out in the evening. Only when there's a power outtage do we burn it over night as the Oil Miser does fine for that period. We'll have the wood to make up the difference on the coldest days of the year. I like wood heat but I'm getting old (76) and don't know how much longer I'll be cutting, splitting and hauling in several cords of wood a year. Hopefully, the FE18 will be able to handle things in all but the worst weather.
  4. Dana

    Dana In the trades

    Messages:
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    Location:
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    There is nothing wrong with supplementing the bedroom heat on the absolute coldest nights when the mini-split can't keep up with a 1500 watt oil-filled electric radiator type heater, as long as it's plugged in to a 20A service with house wiring in good shape. A 1500W space heater can deliver about 5000 BTU/hr. The oil-filled radiator type heaters are less of a fire hazard than anything with glowing red elements or blowers. (And they're quieter than the ones with the blowers too.) Electric blankets to get you through the wee hours of the coldest nights in comfort work too.

    But it does sound like the -FE18 (and not the -FE12) is the right choice here.


    FWIW: My mother will be 79 in a couple of months, and heats her place with an FE18. She has a higher outside design temp, but is heating more space. Her Manual-J came in at a bit under 20KBTU/hr, but on the coldest nights she turns on a space heater in the doored-off bedroom (rather than sleeping in the living room where the mini-split head is located.) I can't recall her ever firing up the wood stove that came with the place.
    Last edited: Dec 9, 2013
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