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  1. #1

    Ductless Heat Pump for a few weeks - heating questions and concerns

    Hello,

    I recently had 2 Mitsubishi minisplits installed into my 5 year old 2 storey home and I'm hoping to get an answer to a few questions. First off, let me state that I have very little background in this area so my questions may seem pretty novice...and quite long winded as I have multiple questions. We live in Nova Scotia, Canada.

    Main floor - open concept - 900 square feet; 3 sections that form a circle of sorts...back two spots are main living space and kitchen and front space is sitting area (front door is located here and so are stairs leading to upstairs). Have GE18NA Mitsubishi minisplit on outside wall in back spaces between kitchen and main living space which is intended to heat the downstairs. We expected the front space to be slightly cooler since the hot air has to travel further.

    Upstairs - once you reach the top of the stairs to the right is an office space above a garage...we leave that door closed as it's quite cool and we don't use that room often. To the left are 3 bedrooms. A second mini split is installed in the master bedroom...FE12NA... Total upstairs is roughly another 900 square feet, but if you take the office space out of the formula it's closer to 600 or so.

    Onto my concerns:
    Both units continuously blow "cool" air 24/7...even though it feels cool to me I"m sure there is some heat in there and it's primary function is to circulate the air. Is that correct?

    However, when they really start to put out heat the vanes go down and the units push out hot air for about 2 minutes and then the vanes go back up and the regular fan starts blowing again. I'm not sure if this is how most minisplits work, but this what mine does. Unless it's morning and then the vanes stay down much longer as more heat is required for a longer period.


    Questions:


    1.The upstairs unit does this once every 10-15 minutes or so (blast of heat), but the downstairs unit does this every 5-6 minutes. Is this okay or is this abnormal in reference to the downstairs unit running twice as often? When I bought them I pictured them cutting in and out every 20 minutes or so and running for longer periods, but since I know nothing about them then maybe this is perfectly normal! I at least expected them to cut in and out equally so this is where my confusion lies since they seem to be doing different things. When I try and search online I can find problems with the units running constantly because it's too cold outside so the back up source needs to be turned on, but I can't seem to find an answer to my situation (which again makes me think that there's no problem). I guess I don't understand why one head might work twice as hard as the other...other than plain old thermodynamics and that I'm losing some of my heat upstairs and the internal thermostat on the machine downstairs isn't satisfied as long as the one in our master bedroom. The rooms do reach my set temps and maintain the constant temp so there is definitely heat being circulated, but it seems that the downstairs unit has to work "harder" to maintain it. Is there any risk to the outdoor units since it puts out the shot of heat every 5 minutes? These are variable speed compressors so is it okay that they do this? Or does something just seem plain wrong. Is there something I should change in terms of set up?


    2. What is the best way to utilize my minisplits? Should I turn the one down downstairs at night when I go to bed since we don't use the downstairs or do you leave it running at 20C (68F)? What about in the daytime (do I leave the upstairs one set at 20C when we wake up or turn it back)?

    3. Do I leave my back up heat source turned way back (unless necessary on really cold days) and just heat with the minisplit or do I set it to the same temperature as the minisplit?

    I'm sorry for the longwinded question everyone. If I've left pertinent info out please let me know and I certainly appreciate any advice/reassurance.

    Thanks so much!

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,775
    Variable speed minisplits should run almost constantly (outdoor unit) as it ramps up and down depending on demand. Your units may be way oversized for your house. What is your back up heat source? I would keep it a few degrees below the sets pint of the minisplits. The downstairs will have longer run times bc a lot of the heat from down travels up so the upstairs won't run as much. You probably have thermal bypasses from your garage to the office above causing that room to be cold bc cold air is traveling through gaps and holes in the thermal boundary between. The best solution to that problem is to spray foam the floor and outside wall above ceiling in garage to keep this air from getting into the house, the bypasses could also be letting dangerous carbon monoxide and other noxious gases into your home if the attached garage is not completely air sealed from the rest of the house. Check out one of these its cheap insurance for the safety and health of your family http://www.jpgorman.com/carbon-monoxide-detector.html I've seen some attached garages that actually have a supply and return air ducted above the ceiling of garage to keep the floors warm which doesn't work and sucks air out of garage into hvac system.

  3. #3
    Thanks for your response. Back up heat source is hot water baseboard (oil).The garage isn't my main concern in terms of the cool room above it, but thanks for the suggestions and I'll certainly look into them!

    I had various HVAC guys out for multiple quotes and most recommended the 18,000 unit downstairs (a couple said 12) and the FE12 upstairs. Nobody suggested anything smaller. They're argument was that because they were inverter driven that there wouldn't any concern of oversizing since I certainly wouldn't be way oversized and we leave bedroom doors open to try and heat the whole house. Again, keep in mind that I really don't know much about these...but it made sense to me since, for example, the ge18 is rated from 3500-25200BTU's at 47F. That's a pretty wide range.

    Like I said in my original post, both units constantly produce air (they never shut off), but the air feels cool...though I'm sure there is some heat mixed in there...it just feels cool to the skin. Am I right in saying that the compressor must still be running to produce this air? During these times the fans on the outdoor units isn't moving and I can't hear anything coming from them, but I don't know if this means the compressor isn't running.

    When I get the short bursts of heat the outdoor units fans start to spin and I can hear the compressor ramp up. Would the compressor be so quiet during the other times that I wouldn't be able to hear it? Or is the compressor only running when I see the fan spinning?

    Thanks again!

  4. #4
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,775
    If the outdoor unit isn't running no heat is being produced, just the circulating fan. My guess is either it is not a variable drive compressor and either is on 100% or off or there is a problem with the unit where it is only running at max capacity or it is way oversized and it only has to run a few minutes and satisfies the temp. I believe those units come on full speed for the first few minutes then ramp down depending on the conditions of indoor temp. It must be satisfying the temp in the first couple minutes and shutting of which would mean its way oversized. You could try moving the temp sensor further away from the unit so it doesn't satisfy so quickly but then you will probably over heat the room. The baseboard may be causing it to satisfy too quickly try running the back up heat stats down to 60F or so and see if you get longer rub times.

  5. #5
    Yeah something is off then...they are definitely supposed to be variable speed as it's Mr Slim and the website makes it pretty clear. I've been using them without my back up heat even on...today it's around 0C. The entire downstairs reaches 20 degrees celcius with just the minisplit, but they seem to be short cycling for whatever reason to achieve and maintain this temperature...they never seem to ramp down to put out less heat....it's on for a few minutes and then off for a few minutes, but based on the specs I can't see why this is happening as I don't think I'm drastically oversized by any stretch of the imagination. Any possibility of some sort of installation error that might come to mind? Also, the sensor that monitors the return air is on the unit itself if I understand correctly so I don't really think it's possible to move the temp sensor...I dread having to call the installer.

  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,775
    It is possible to move the sensor but its not DIY and probably not reccomended my the manufacture. You may have a problem with the circuit board not controlling the unit properly. I wouldn't think it's an installation problem but it could be. Look on Mitsubishi site and find another certified contractor and let them give it a look if you're not confident in the installing contractor.

  7. #7
    Thanks for your time! If there's any other thoughts please let me know. I'll get in touch with Mitsubishi in the meantime.

  8. #8
    Join Date
    Oct 2008
    Posts
    37
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 11-25-2012 at 09:44 PM.

  9. #9
    Obviously I have no idea, but I will certainly bring it up as maybe it's something the installer has to adjust. As an aside:

    The downstairs 18GE unit is really out in the open so I can't fathom that the heat is hitting a wall or something causing it to inadvertently trip the internal temp sens. The specs for the GE18 at 8C in terms of BTU's range from 3500-25200. The FE12 which certainly wouldn't be oversized downstairs is 3000-21000. It's 0C here today so I don't feel oversizing is an issue based on those specs if I interpret what I've read about the machines correctly.

    I picture the outdoor unit ramping up to reach my setpoint and then slowing back once it's achieved to maintain it. Certainly 3500BTU's shouldn't cause it to turn off if that's all it had to put out at 0C. I thought that was the whole point of variable speed compressors...to avoid short cycling.

  10. #10
    Join Date
    Oct 2008
    Posts
    37
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 11-25-2012 at 09:44 PM.

  11. #11
    I've read all the literature that came with the units and this isn't really addressed in any of them...it's likely something beyond the scope of me. The registration was done by the installer and I believe Mitsubishi only talks directly to the installers so that's might next call is to the installer to see if they can come out and diagnose the issue. Other than the typical things like oversizing and incorrect placement of the heads I can't really come up with an obvious explanation...and I like I said I don't think either of those things are issues in this case, but I'm not the expert.

  12. #12
    Join Date
    Oct 2008
    Posts
    37
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 11-25-2012 at 09:44 PM.

  13. #13
    The only thing I can add to what you've said is that if I set the temp downstairs back to 16C before going to bed and then turn it back up to 20C in the morning the blower will put out hot air for 20 to 30 minutes at a time...then once it gets to around 20C it starts this whole short cycling bit. The temp difference it the room seems to be great enough in the morning that it will initially blow continuosly for awhile. Not sure if that helps clarify/eliminate anything or not.

    I will definitely report back with my findings.

    Thanks!

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