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Minisplit Head Blowing Unheated Air

JKeith52 | Posted in General Questions on

I have a Mitsubishi mini split that is continually blowing air that is not warm (I am in heat mode).  I understand there are times when the air is just circulated and not warmed.  However, both light indicators on the wall unit are lit.  This indicates that the unit is not reaching the desired temperature.  Therefore, I believe the unit should be blowing hot air until the desired temperature is reached.  Am I missing something or is there an issue with my mini split?  Thanks.

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  1. rondeaunotrondo | | #1

    It takes sometime for mine to blow hot air. Two lights indicates great than ?3-5 deg difference in temperature to set point. How long are you waiting to feel?

  2. walta100 | | #2

    I have to ask how long have you lived with this heat pump?

    The way mini splits achieve such high efficacies is by slowing the compressors speed so its output matches the buildings heat loss and lots of the time the air it is supplying is only a few degrees warmer than the return air 24-7. You will need real numbers from a thermometer not wild guesses based on your skin feels to tell if the unit is working.

    Note the unit will need to perform a defrost cycle for 3-5 minutes every 3-6 hours.

    Of course this would not be news to you if you have been operating this heat pump for a few years if the unit is still under warranty asking the installer to have a look seems like the way to go.


  3. JKeith52 | | #3

    Following are observations from 3 of my minisplits, including the one whose performance I am questioning.

    The "bad performing" minisplit -- air temperature inbound to minisplit is 70 degrees
    air temperature outbound is 78 degrees
    air temperature of corresponding room is 67.6 degrees

    Well performing minisplit number 1 --air temperature inbound to minisplit is 72 degrees
    air temperature outbound is 108 degrees
    air temperature of corresponding room is 65.5 degrees

    Well performing minisplit number 2 -- air temperature inbound to minisplit is 78 degrees
    air temperature outbound is 102 degrees
    air temperature of corresponding room is 66.7 degrees

    All temperature were measured directly at the inbound and outbound air vents of the minisplits. Due to the way rooms are interconnected the temperature in the room of "well performing minisplit number 1" is lower than expected because it would normally be gettint some help from the "bad performing minisplit." In any case, I would expect the outbound temperature of the "bad performing minisplit" to be much higher. Am I misunderstanding something? This is the third winter using the minisplits and my experience is better performance during the past 2 winters.

  4. bfw577 | | #4

    Are these heads connected to their own outside condensers or part of a multi split setup? My guess is a multi split.

    It sounds like bad performing mini split is not working at all based on a supply temp of 78. I usually get around 100-120 supply air out of my splits so sounds like 1 and 2 are working fine. If this is a multisplit then the coil on the bad performing split should be the same temperature as the other 2. Have you tried opening the cover on the bad one and seeing if the coil is hot and the same as the others?

  5. JKeith52 | | #5

    Actually, all 3 minisplits are connected to their own individual condensers. The coil on the bad minisplit is not hot. My HVAC contractor is coming tomorrow to check it out. Anyone else have a similar issue and, if so, what was the problem with the minisplit? Thanks.

    1. bfw577 | | #6

      My guess is the bad unit lost its refrigerant charge from a leak. Is the outside compressor and fan running on the bad unit when calling for heat?

  6. JKeith52 | | #7

    Yes, the outside compressor and fan are running. My suspicion is the same as yours. From anyone's experience, is it very common to get a refrigerant leak? Thanks.

  7. walta100 | | #8

    My guess is almost every heat pump/ AC system is replaced directly or indirectly because they leaked. People pay for a few feeble half hearted attempts and give up and replace the system. While the refrigerant is escaping thru the leak moisture is entering thru the same hole that moisture reacts with the oil in the system. The oil becomes acidic and will attack the insulation of the motors coils destroying the system indirectly.

    Finding and fixing leaks is frustrating and time consuming job. Finding a technician who is motivated to find the leak will not easy. A quality job will likely to take more than 2 hours and will not come cheap.

    A technician will need to connect a set of gages to the system and make a diagnosis. You likely have a leak but it is possible the compressor has failed the metering device is stopping the flow.


    1. Expert Member
      BILL WICHERS | | #9

      Moisture doesn't get in until enough refrigerant has leaked out to reduce the pressure in the lineset to near atmospheric. As long as the lineset is above the pressure of the atmosphere, it can only leak OUT. There are also dryers in the line (they look a little like a filter in the liquid line) that can take out small amounts of contaminents to protect the system.

      It doesn't have to be a super big job to find a leak, unless the lines are inaccessible -- but remember that it's usually FITTINGS that leak, not the lines themselves. The first step is to check if the system is low, which can be done with pressure checks pretty quickly (a few minutes). If the system is low, have the tech "wand" the system. The wands are like electronic noses that sniff for leaks, and they're pretty sensitive. Any significant leak should be picked up by the wands and be able to be localized enough to know what needs to be fixed. Finding a leak with the wands takes well under an hour even on a good-sized system. How long it actually takes to fix whatever is leaking depends on what is leaking.

      If the wands don't find anything, but a leak is suspected, then you probably have a slow leak. What I like to do in this case is to put dye in when the charge is topped up on the system. The dye is UV active, so it glows under ultraviolet light, and it circulates around the system with the refrigerant. The dye is great for finding really small leaks, because the dye will come out at the leaky spot and build up over time. Have the techs come back a few days after putting in the dye to check the system with a UV light. They should be able to find even very small leaks this way. I think in my near 25 years of working with these systems, I've only had a leak one time that we couldn't figure out.

      If you're using a blended refrigerant, which, as far as I know, is only used as a retrofit on older systems using R22, then you have a problem since the different components that make up the blend leak out at different rates. With these systems, you have to evacuate the system and completely recharge to repair a leak. Most newer residential systems are probably using R410A which doesn't have this problem. You only need to evacuate a system if it's REALLY low, which I call "vented", or if a compressure burned out and contaminated the system (in which case you need to flush the system, replace the dryer, evacuate, and then recharge to really do it "right").

      Refrigerant leaks aren't uncommon, and the more fittings you have, the more likely you'll have a leak. I've also seen joints fail at stress points (sharp bends usually, or retention clamps, especially in areas where there is a lot of vibration). I'd check all the mechanical fittings first though as those are the weak spots for leaks.


  8. Expert Member
    Akos | | #10

    I've yet to find an HVAC tech that would try and locate leaks. Best case scenario is one that will refill and add dye so you can see the leak, most would just want to recharge and go.

    Unless you find the leak, you'll be back to exact same spot next year, so you need to find the leak.

    Since almost no residential trade will do this for you, the best bet is pick up an AC sniffer and check the fittings yourself. You want to check the flare connections on the outdoor unit as well as the ones to the indoor unit. Sometimes the indoor connections are behind the indoor wall mount which means having to swing it out a bit or could be just outside the wall. Make sure not to kink any lines if you need to move things for access.

    Do not buy the AC sniffer from the typical online marketplaces. I've tested a couple of overseas brands and none of them work. You want to pick one up from an HVAC supply house, they are not cheap, but it will cost you less than refilling the unit every couple of years.

    Sometimes if the leak is small you have to tent the area around the fitting with a bit of plastic to keep the refrigerant from being blown away. Also best to do this on a no wind day. Most of the time the leak will be at the flare fittings, rarely it could be corrosion on the indoor unit coil, ever rarer somewhere in the outdoor unit.

    Once you find the leak, you can try to tighten the fitting yourself. Once the leak is fixed call an hvac tech to reclaim than refill the setup. Make sure they don't just top up the unit, there is no accurate way to fill a modulating unit based on pressure like older AC units, they need to reclaim and refill by weight.

    If you can't find the leak with a wand, you might be looking at having to replace the whole unit.

  9. JKeith52 | | #11

    The HVAC tech came and verified that there is a leak. He recharged the unit to get back up and running. He also scheduled an appointment to come back in 2 weeks to replace a joint where he believes the leak is occurring and to doublecheck for any other potential leakage. He needed an additional appointment where he said he could schedule 4 hours, as during my visit he was in troubleshoot mode for multiple people with no heat (as it was an 18 degree day).

  10. _jt | | #12

    I suffered from excessive ice formation last year. Fast forward a year and my air temperatures were off. I checked the pressure and did some basic maintenance and found the leak - There was a tiny leak through the shut off valves. Nylog over the cap and re-charge seems to have done the trick. Not sure if leaking shut off valves is that common - but seems likely that it is given the nature of the valves.

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