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Multisplit – does load/sizing effect defrosting frequency?

Dave Jones | Posted in General Questions on

Over this past winter our Daikin 36k multisplit unit (4 heads – ground floor) did good until the outdoor ambient temp got down 37F or below.  Then it would start defrost cycling.  Cooling season last summer was fine.  Worked like a champ all summer.  We’re now getting ready to install another multisplit system on our second floor level.

My question is, if we change from a 36k outdoor unit to a 48k outdoor unit (pretty simple swap – pump down + swap + charge + test), would that decrease the likelihood of it defrost cycling during heating season?  If so, I will move the 3 ton unit to the top floor and power the ground floor with a bigger 4 ton unit.

I guess, generally speaking, I’m ignorant on how compressor capacity effects heating performance when it comes to heat pumps.  Do they perform better when the demand:capacity ratio is kept low?

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Replies

  1. Walter Ahlgrim | | #1

    In my opinion defrost mostly a function the outdoor temp and humidity.

    Yes a larger unit has larger coils but the BTU per square foot will be almost the same so the defrost about as often.

    I think it would be a big mistake to replace a perfectly good unit with a larger unit. Units with a single head are able to slow down more than multi head units and cycle on and off less and cost less to operate.

    "Do they perform better when the demand:capacity ratio is kept low?"
    No they cycle on and off more and cost more to operate and remove less water from the air when cooling.

    Walta

    1. Dave Jones | | #6

      I'm looking at the capacity table for our configuration (9+12+12+12) and it shows that would result in heating capacity at each head being: 6.91, 9.69, 9.69, 9.69 respectively. So, in that respect, this unit was maxed out.

      That also corresponds to what I was seeing in the load being reported by the compressor itself in November. It showed the compressor demand bumping 100% pretty constantly (chart attached). This chart data is coming from the unit, not an external energy monitor.

      1. Dave Jones | | #7

        Here is our ground floor layout. Units are mounted about 7 feet above the floor.

  2. Jay Thomas | | #2

    There is something else wrong here. At close to +/- 32 on my Midea mini splits I might get one defrost cycle a day. Even in the teens and on humid days I still only get a couple defrost cycles per day.

    Before I had adequate ground clearance I did get hourly defrost cycles. That may not be your issue, but I do suspect something is "not quite right" in your setup.

    How many square feet do you have on your first floor? 36 k is probably over sized to begin with. As others have pointed out in the past - the first thing you need to do is get an energy meter and keep an eye on your energy load. It will point a finger as to what's going wrong. (For instance if your COP is low)

    1. Dave Jones | | #4

      What do you consider adequate ground clearance? I looked at the installation manual and it doesn't specify a minimum height from the ground. It only talks about leveling and wall/ceiling clearances. Ours is sitting on about a 4" plastic pad laid directly on the ground.

      1. Jay Thomas | | #8

        I think that's your problem. I had the same frosting issues on mine before I raised it a few inches - you need that space for the condensate to drain. Otherwise that's what will freeze.

        A "hat" does help a well when you have freezing rain, but feet are essential. If you followed Walter's advice of keeping it above the snow line you would also be fine.

        36 k BTU should be more than enough for the roughly 1000 ft2 you show in your diagram. I would not oversize this to 48k BTU. Solve the defrost issue and you'll have plenty of capacity.

        https://www.amazon.com/Senville-Mounting-Condenser-Anti-Vibration-Absorbing/dp/B085PZFH3C/ref=sr_1_1_sspa?dchild=1&keywords=mini+split+feet&qid=1590108723&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzQURLTlJEOE1QMFpCJmVuY3J5cHRlZElkPUEwNjM5NTc4MlM4RzBVVldYOFNTVSZlbmNyeXB0ZWRBZElkPUEwMzI2MzUwMlVXNlpVSUMxTVZVQiZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

        1. Dave Jones | | #9

          Thanks for that link. Just ordered.

          I’m guessing the idea here is that the condensate can freeze/frost even if the ambient temp is above 32F since there is air flow across the coil causing the temp near the coil to be lower than ambient?

          1. Jay Thomas | | #10

            The issue is the air around the mini split is sub zero when the mini split is at 37. Before I had feet it would literally build an ice wall - then the defroster runs to keep that under control. Depending on your climate you may want to have a real stand and a hat to avoid freezing rain from freezing in front of the split as well.

            I put a piece of plywood on when we were in freezing rain season - it did help but the feet made a bigger difference.

          2. Walter Ahlgrim | | #11

            What happens is the heat pump is removing heat from the air out doors so the coil is colder than the outdoor air.

            The frost builds up when the coil is colder than the outdoor frost point temp the air is always below freezing when you have a frost point. Enough frost on the coils will restrict the air flow limit heat transfer and put the unit out of order.

            https://en.wikipedia.org/wiki/Dew_point
            “Frost point
            The frost point is similar to the dew point in that it is the temperature to which a given parcel of humid air must be cooled, at constant atmospheric pressure, for water vapor to be deposited on a surface as ice crystals without undergoing the liquid phase (compare with sublimation). The frost point for a given parcel of air is always higher than the dew point, as the stronger bonding between water molecules on the surface of ice requires higher temperature to break.”

            I have had icing problems with my conventional heat pump with the fan on top freezing rain falls on the stopped fan blades and bridges to the shroud locking the fan in place putting the heat pump out of order. Most minis avoid this problem by not blowing air out the top. Freezing rain is a fairly rare event. It takes a lot of energy to change liquid rain to ice generally the amount of liquid rain falling will melt any ice that may freeze on a coil.

            The other ice problem is what happens to the frost that is melted when you defrost? Generally the water runs off the now warm coil and drips into a pan with a drain to get the water out of the unit. If you have weeks or months without getting the pan above freezing the pan can fill with ice and stop the fan, block the air flow or throw off the coil temp sensor. Heated pans are made for locations like Maine best avoided if possible as it wastes away 200 watts or so.

            My conventional heat pump has logged 1435 defrost cycles in 5925 hours or a defrost cycle every 4.1 hours of operation so not an uncommon event with my weather.

            Walta

  3. BFW577 | | #3

    My Midea unit does not defrost much as well.

    I second getting an energy meter. You can visually see exactly what your unit is doing. In my opinion you can run your mini split so much more efficient
    I am using a 120$ Efergy Engage.

    Here is an example. It was chilly here this morning so I cranked up my Midea Premier floor console. It ran from its max power of about 2.4kw all the way to its lowest modulation of 220 watts. The spikes are to boost oil pressure at its lowest modulation.

  4. Walter Ahlgrim | | #5

    Most install instructions require the unit to be mounted above the level of any snow one would normally expect.

    Walta

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