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Community and Q&A

Oversized Mini Split

Morris_Wanchuk | Posted in Mechanicals on

Hello, 

I purchased a home last year that has a few heat sources and I am trying to get the the best way to heat the house. House was built in 1980s, truss ranch with open living room/dinning, small hallway leading to two bedrooms. This main area of the house is about 1200sf, then off of one of the bedrooms (even with hallway) is a 400sf addition. Original double pain casement widows (pretty tight when locked down). Probably R45 in the attic (no cathedral ceilings), R3o in the floors. Location is central VT 

Home originally had electric baseboard, looks like 1994 they put in forced hot air oil, and in 2012 they put in two mini splits for cooling but they can heat too. One in the back addition which is 8kBTU and other in the living room which is 36kBTU! (Daikin RXS36LVJU) which was installed facing a wall. I ran the oil last year and used about 450 gal in an below average winter. Furnace worked fine but the oil tanks were in rough shape so I had them removed.  I am not sold on a new oil tank for a 26 year old furnace so I figured I would give the mini split a try this season. 

This fall I got the living room mini split relocated to blow down the hall as its old location did not move heat out of the living room. I have had it running for a few days and it is keeping the living room at 66 and the two non addition bed rooms at 63, and the back room around 60. (This is only using the 36k unit, not the smaller one.) However it is  duty cycling on and off once the room temp is reached. Temps have been in the 30s-40s the past few days. And side note, the remote is set to 62 but i have been using my thermostat/weather station thermometers to get the real temp.

I know the thing is way oversized but I was wondering what the best tips were for trying to have it work in the most efficient way 

I want to try and get as much heat into the addition bedroom as possible so I don’t have to turn on the small one if possible, this may be futile due to distance but with the oversized unit in the living room it would be nice to use it for its potential. 

1) Is there a recommended position for the louvers to move air down the hallway? Manual says for the louvers to point to the floor on heat but they also seem hyper concerned with people feeling the air so I was not sure if that was comfort or performance. I currently have is set at the smallest angle available for heat. 

2) I know a setback is not recommended with a mini split but with the unit duty cycling anyways, would I be better bringing the heat down at night and just having is blast the house with heat in the morning?

3) Should i just keep the living room warmer (wife/kids would be happy) to reduce the duty cycling and try and more more heat into the back room? I figure I might use the same amount of electricity anyways. 

4) Fan speed: I had it on auto but the fan speed comes on low which I think is adding to the duty cycling. I switched it to high today with again the hope it moves air further into the house. 

5) Both have a ECONO Mode, does this actually save $$ or is it just for electrical current?

Thanks in advance for the help. Attached is a basic floor plan 

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"1) Is there a recommended position for the louvers to move air down the hallway? Manual says for the louvers to point to the floor on heat but they also seem hyper concerned with people feeling the air so I was not sure if that was comfort or performance. I currently have is set at the smallest angle available for heat. "

    If you're trying to move heat down a hall you're looking for maximum throw, and aren't concerned about convection or localized wind chill near the head. Set it to blow as directly at the hall as possible, not down.

    >"2) I know a setback is not recommended with a mini split but with the unit duty cycling anyways, would I be better bringing the heat down at night and just having is blast the house with heat in the morning?"

    As long as the cycles are reasonably long (more than 5 minutes?), cycling on/off at it's MINIMUM compressor speed will usually be quite a bit more efficient than running at full speed to recover from a 5-10F setback.

    >"3) Should i just keep the living room warmer (wife/kids would be happy) to reduce the duty cycling and try and more more heat into the back room? I figure I might use the same amount of electricity anyways. "

    That's what I'd do. Running a higher duty cycle will increase the average efficiency of the mini-split when it's cycling on/off. Keeping the place at a much cooler than what keeps people happy is never a net win- comfort matters.

    >"4) Fan speed: I had it on auto but the fan speed comes on low which I think is adding to the duty cycling. I switched it to high today with again the hope it moves air further into the house. "

    Higher speed will definitely help with the room to room temperature differences, but will sometimes lead to wind-chill discomfort in the area nearest the ductless head, and is somewhat noisier. You may find it more comfortable overall at some intermediate speed.

    I haven't looked into wired or wireless wall remotes for Daikin, but most name-brand mini-splits have options for wall -remotes that can be programmed to behave like a thermostat. Most mini-splits by default sense the room temp by sensing the temperature at the incoming air at the head, but with wired or wireless wall remotes it can be programmed to track the temperature of a sensor in the remote, which often cures short-cycling when the head is in a constrained space. (Newer Midea mini-splits have a "follow me" function on the remote, with RF wireless communication between the remote & head that uses a temperature sensor in the hand held remote, so it will automatically try to bring the temp up or down in the room where the remote is located. I don't think Daikin has that function, but it would be great if it did.)

    >"5) Both have a ECONO Mode, does this actually save $$ or is it just for electrical current?"

    It depends on what that mode actually does on Daikin units. With several competitors' economy modes it does an automated set back function that steps the temp down (in heating mode) some time period after the temp was last tweaked. It may save a tiny amount of money in some seasons & some situations, but I'd be surprised if it does the right thing in your case. Read the manual to figure out how it actually behaves in ECONO mode.

  2. Morris_Wanchuk | | #2

    Thanks for the reply! I am not sure what the previous home Owner was thinking with the size of the unit. It is way oversized. It is currently running for about 2.5 minutes and off for 8. I am not sure the btu/hr of my house but in September it got down to 25 and the house lost about 7 degrees over night with no heat on. 36k seems double or triple what I need.

    Like I said above just trying to try and make this as efficient as it can be. I’ll probably still beat the oil but not as good as a smaller unit would have been if I had a new one installed.

    One thing I noticed is on cooling it never shuts off. Mind you we only really run it for a few hours around dinner on a very hot day because I’m VT it always cools down at night but I noticed the outdoor condenser unit fan will
    Slowly run on cooling mode. On heat mode it just turns on what looks like full speed then shut off. Is this a cooling vs heat difference?

    Thanks

  3. Expert Member
    Akos | | #3

    That is not a hyper-heat unit, definitely way oversized for cooling, but for a place like central VT, you'll be loosing at least 1/2 the rated capacity most of the winter.

    The unit should still have a better modulation range. It could be that it is recirculating the exhaust somehow and the unit thinks that the setpoint is met.

    Lot of the wall mounts work much better for heating if you use a wired thermostat with a remote sensor or if your remote has the option, select the "follow me" mode. Putting the remote sensor somewhere a bit out of the way of the airflow of the unit should get you much better runtimes.

    Some units also have a bunch of installer options you can configure through the remote, I would check the factory install manual to see if your has them. I could just be misconfigured.

    If you will continue to use it for heat for the winter, I would get a the pan heater kit for it. I'm in milder climate but it still builds up a fair bit of ice on the ground underneath from defrost. Without the pan heater the unit would be a block of ice.

    1. Steve Knapp CZ 3A Georgia | | #6

      My Daikin ducted unit has a "high-ceiling" mode that is accessible through the technician codes. The codes force the fan presets higher than the standard settings. I had to play around with my system to find the best combination of efficiency and comfort. Currently, I use the high-ceiling mode but keep the fan at its slowest speed. This delivers more runtime and greater comfort, but it took a bit of experimentation to get there.

  4. Morris_Wanchuk | | #4

    Just a note on the run times, it’s keeping the room the temp
    I want it, just the run times are really short. I’ll look into a wall mounted thermostat. Looks like my unit is from 2012 so I’ll se if they still make them.

  5. BFW577 | | #5

    According to the engineering data for your unit its minimum modulation is 10200 btus. So even running at its lowest speed it is probably way oversized when its in the 30s-40s outside.

    Also, your unit has really poor cold weather performance. At its 5 degree shutoff it has lost more than half its capacity. Its rated at only 16k btus at 5 degrees. Even at 14 degrees its only rated at 19k btu. It also has no basepan heater on the outside condenser. I would only run it above 25 degees in VT. At very cold temperatures its going to be using alot of electricity for very little heat. Its HSPF rating is only 8.3.

    Also, the engineering manual has a remote thermostat listed as an accessory. Part number
    BRC944B2-A08.

  6. Morris_Wanchuk | | #7

    Thanks for the replies. I figured it was less than ideal but I am going to install a wood stove next winter for the sub 30 degree temps. This year will be a bit of an experiment but the engineer in me wished it was more efficient/smaller/.

    When you say shut off temp, does that mean it will stop working all together below 5 degrees or will it just go way down in production?

    1. BFW577 | | #8

      It will stop operating at 5 degrees. This link has all the documents for your unit under datasheets and resources. The technical manual in pg 18 lists the heating capacity at different outdoor temperatures.

      https://m.johnstonesupply.com/storefront/product-view.ep?pID=B73-364

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