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  1. #1
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    fundamentals vs fundamental belief



    There is an inflexible belief here that freezing evaporator coils in the heat pumps is caused by low refrigerant. Always low refrigerant. We spend a lot on low refrigerant service calls.

    Consider the following statement.

    Low refrigerant levels: If your air conditioner has a refrigerant leak it will not have an adequate amount of refrigerant to absorb heat from your home’s air. Without something to remove the heat, the coils can freeze up.
    My background is mostly theoretical so I am open to the possibility that low refrigerant could cause evaporator freezing. But I need something more rational than the above statement to convince me. Is there something I am missing?

    The units are cheap 3-4 ton split systems with piston expansion devices. Most are R22, though the present problem child has R410.

    Thanks;
    Doug



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    Re: fundamentals vs fundamental belief

    Hi Doug
    There are many reasons evaporators freeze up, such as a low refrigerant charge ie the system has a leak !
    but having a broken evaporator fan motor or a choked air filter will all cause similar problems.
    With a low refrigerant charge the refrigerant quantity in the evaporator will be less than the quantity required as design. Lets say its a 3 ton unit, you have a 3 ton capacity compressor, evaporator,expansion valve and a slightly larger duty condenser. So you need sufficient refrigerant charge to circulate to give 3 ton of cooling effect. If the refrigerant charge is low, then the evaporator pressure will be low, due to the compressor trying to remove 3 tons worth of refrigerant, when say, only 2 tons is available. Therefore the compressor will cause the suction pressure to drop below 32deg F and cause the evaporator temperature to fall below freezing point, thus freezing the moisture in the air onto the evaporator coil.
    Also reduced air flow will cause the same problem caused by a broken evap fan/motor or a blocked air filter. The evaporator needs the correct air flow through it to boil the refrigerant and keep the suction pressure up, but with restricted air flow the refrigerant has less heat to absorb from the air and therefore the refrigerant doesnt boil at the correct condition again causing low suction condition and freezing of the coil.
    If the outside ambient temperature at the condenser is low causing low discharge temperatures/pressures, this can also cause a low suction and freezing evaps if the pressure drop across the expansion valve drops below its design conditions. From my experience AC systems only have the filters cleaned when the system stops working.
    Hope that helps

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    Re: fundamentals vs fundamental belief

    Deleted due to double post.
    Last edited by Doug30293; 11-09-2014 at 01:43 AM.

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    Re: fundamentals vs fundamental belief

    Thanks Glenn;

    I agree with you about air filters. Most of the problems I've encountered had to do with loss of air flow across the evap coil. Our plant is extremely dusty. I have to change some filters every week.

    It doesn't help that we have seven heat pumps with supply and return ducts crossed between separate office areas. Unless all interior doors are open we get pressure unbalance which pulls dust through the ceiling and into the return ducts. (There is no make-up air.)

    We also get iced coils on cooler days. But I thought this was usually due to excessively subcooled liquid entering the piston.

    The thing I am having trouble understanding is how insufficient refrigerant in the suction line causes lower evap temperatures. If there isn't enough refrigerant wouldn't it all just reach superheat farther back in the evap coil? What we are getting is ice all the way from the evap coil to the suction accumulator. The outdoor unit is about 80 feet away.

    I know we have low refrigerant today. The mechanics cleaned the coil with caustic detergent and a wire brush. But the ice is gone and now we have 60F at the exit of the coil. The HVAC people will be here tomorrow to quote another coil, the second in two years.

    This time I am hoping to observe the fun rather than participate. My interest in this is strictly a learning exercise.

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    Re: fundamentals vs fundamental belief

    When your system is lacking in refrigerant, you get very localized freezing and high outlet superheat, due to low evaporation temperatures. (lower pressure)
    OVERTIME, this localized freezing stops the air flow moving through that area, so little or no energy is absorbed (ice also acts an insulator) The ice then creeps through the whole coil, stopping the majority of the airflow. As a whole little energy is absorbed, and you get the liquid refrigerant moving back towards the compressor (ice on suction)
    The highlight comment, is rubbish. The amount of refrigerant does not change the amount of energy that "COULD" be absorbed from your area.

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    Re: fundamentals vs fundamental belief

    Now it makes sense. Low suction pressure causes the refrigerant to flash as soon as it leaves the expansion device. Once that spot covers with ice the flash occurs a little farther down the coil. The ice travels from piston to outlet.

    This differs from an airflow problem where I suppose the whole coil freezes more or less equally.

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by Doug30293 View Post
    Now it makes sense. Low suction pressure causes the refrigerant to flash as soon as it leaves the expansion device. Once that spot covers with ice the flash occurs a little farther down the coil. The ice travels from piston to outlet.

    This differs from an airflow problem where I suppose the whole coil freezes more or less equally.
    Sort of right, but sort of wrong.
    Flashing always occurs after the expansion device (on standard systems), it is the temperature of the flashing that is important (vaporization), when short of refrigerant the flashing stops shortly after the expansion device, only when there is no load at that point ( the liquid does not have enough energy to boil), does it move further through the coil.

    When you have low airflow, or low liquid line temps (need not be subcooled liquid), the temp of the refrigerant is low more evenly over the coil, but when ice covers the whole coil regardless, then the load drops of totally. A new low temp equilibrium is achieve.

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by mad fridgie View Post
    Sort of right, but sort of wrong.
    Flashing always occurs after the expansion device (on standard systems), it is the temperature of the flashing that is important (vaporization), when short of refrigerant the flashing stops shortly after the expansion device, only when there is no load at that point ( the liquid does not have enough energy to boil), does it move further through the coil.
    It makes more sense when I think in terms of suction pressure. Low suction pressure means low evaporation temperature. If I understand you correctly though, icing from low refrigerant happens slowly, whereas icing from low airflow happens more quickly because the entire coil temperature drops at once.

    So is icing from low refrigerant a common problem you encounter in the field?

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    Re: fundamentals vs fundamental belief

    .

    I agree with Mad Fridgie, different things cause the AC evaporator
    to freeze over but the term freeze over is a broad statement and
    not all ice is the same.

    When an evap ices up due to poor air flow the ice formed tends to
    be frozen water ice and very hard, very solid. The ice that is formed
    from a low refrigerant level is more frost or snow and as the snowy ice
    insulates the evap it prevents the heat exchange so the evaporation
    process slowly moves out of the evap and can go into the suction pipe.

    That is why frost forms on the suction valve (on the outside unit) when
    the system is short of refrigerant.

    Also as it has been explained earlier if the system looses refrigerant the
    evaporating pressure tends to drop. With a room temperature at +21 degsC
    the air off the evap coil is going to be about 15 ish degsC so the evaporating
    temp will be about 5 ish degsC.

    5 degsC is above freezing so ice is very slow to form, when the evap pressure drops
    the evap temp drops and instead of evaporating at a temp above freezing it evaps
    at a temp below freezing.

    Regards

    Rob

    .
    Last edited by Rob White; 11-09-2014 at 01:31 PM.
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

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    Re: fundamentals vs fundamental belief

    but now on a heat pump the evap Is the condesnor, so what mod are we talking here, as if on heat mode then freezing is expected and there is defrost circuitry built into it for that reason
    Now in Redvers Sask.

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by Rob White View Post
    .

    I agree with Mad Fridgie, different things cause the AC evaporator
    to freeze over but the term freeze over is a broad statement and
    not all ice is the same.

    When an evap ices up due to poor air flow the ice formed tends to
    be frozen water ice and very hard, very solid. The ice that is formed
    from a low refrigerant level is more frost or snow and as the snowy ice
    insulates the evap it prevents the heat exchange so the evaporation
    process slowly moves out of the evap and can go into the suction pipe.
    This is good information. In every case where we had air flow problems the ice was dense, shiny, and somewhat translucent. It also developed quickly. Change the filters, thaw the coil, open the interior office doors, and all is fine.

    I have a window shaker at home that develops snowy frost, beginning at the cap tube and slowly crawling across the evaporator. It typically takes about six hours to frost over. So this is low refrigerant.

    Well now I know how to tell the difference between low refrigerant and restricted air flow.

    Thanks everyone!
    Doug

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by Doug30293 View Post
    I know we have low refrigerant today. The mechanics cleaned the coil with caustic detergent and a wire brush. But the ice is gone and now we have 60F at the exit of the coil. The HVAC people will be here tomorrow to quote another coil, the second in two years.
    This is a follow-up and correction to my above quoted remarks. The leak turned out to be a failed solder joint where the liquid line enters the air handler, not another failed coil. My guess is that with four of us climbing around the unit and stepping on the line we broke the joint. The contractor repaired the connection, recharged the unit, and it is now cooling properly.

    I checked the suction line at the condensing unit yesterday and got 42F with an IR gun. Seems a little low after nearly 100 feet of suction line. It will be interesting to see if we get solid ice again after the evap coil gets dirty.

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by Doug30293 View Post
    This is a follow-up and correction to my above quoted remarks. The leak turned out to be a failed solder joint where the liquid line enters the air handler, not another failed coil. My guess is that with four of us climbing around the unit and stepping on the line we broke the joint. The contractor repaired the connection, recharged the unit, and it is now cooling properly.

    I checked the suction line at the condensing unit yesterday and got 42F with an IR gun. Seems a little low after nearly 100 feet of suction line. It will be interesting to see if we get solid ice again after the evap coil gets dirty.
    42˚ F is low. Typical cooling equipment is designed to operate at saturated evaporator temps in the 40˚-50˚F range, with most (I see) operating nearer 40F. Superheat (suction line temp minus evaporator temp) with fixed metering (pistons) varies with indoor and outdoor conditions, but with moderate indoor and outdoor temps, you will generally see a minimum of 10˚F (most equipment includes service literature "charts" for determining target superheats).

    So, properly charged systems with design airflow (400 CFM/ton) will generate suction line temps of 50˚F minimum. At 42˚F, I'd guess the evaporator coil is restricted (dirty), and the system possibly overcharged.

    Piston systems can be overcharged to raise the suction pressure (evaporator temp), as the excess charge raises the head pressure which in turn, increases the rate of liquid metering, thus increasing suction pressure/evaporator temp. But that tactic also reduces superheat, and low to non-existent superheat values are potentially detrimental to compressor service life.

    I've seen my share of "dusty plant" equipment. Without some very "efficient" return air filtering systems (and airtight return ducting), indoor coils will restrict prematurely, creating low airflow problems, namely low superheat and frosting coils, with or without refrigerant leaks.
    Last edited by Saturatedpsi; 17-09-2014 at 01:20 PM.

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by Saturatedpsi View Post
    So, properly charged systems with design airflow (400 CFM/ton) will generate suction line temps of 50˚F minimum. At 42˚F, I'd guess the evaporator coil is restricted (dirty), and the system possibly overcharged.

    Piston systems can be overcharged to raise the suction pressure (evaporator temp), as the excess charge raises the head pressure which in turn, increases the rate of liquid metering, thus increasing suction pressure/evaporator temp. But that tactic also reduces superheat, and low to non-existent superheat values are potentially detrimental to compressor service life.
    The data plate shows 14F as the target superheat. I'm going with overcharged.

    It is possible the contractor intentionally overcharged the unit to raise the suction pressure above freezing to compensate for restricted airflow and eventually dirty coils (the evap was just cleaned). We haven't had a frozen coil since the repair.

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    Re: fundamentals vs fundamental belief

    You can not use IR gun for measuring superheat.
    You can not use rules of thumb to determine actual system performance.
    Stated performance is at a pre determined set of conditions, if the conditions are different, then the performance will be different.

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    Re: fundamentals vs fundamental belief

    Quote Originally Posted by mad fridgie View Post
    You can not use IR gun for measuring superheat.
    You can not use rules of thumb to determine actual system performance.
    The OP asked for an expanded explanation for evaporator coil icing, relative to "low refrigerant", with little information to do any analysis. If the coils are icing, and he's being told it's due to lost charge, we can only agree the diagnosis could be correct, since low charge generates low saturated temps that can dip far enough below 32˚F to freeze the condensed H2O.

    Had he provided IDWB/OD temps, pressures, line temps and equipment SEER, we could offer a more specific analysis. 40˚- 50˚ evaporator temps are design values and 40˚ vapor lines are atypical occurrences. Though the IR measurement likely has an error factor, it still provides a glaring symptom of overcharge or low airflow with fixed metered systems, which was the only point I was attempting to make.

    I suspect the only level of "performance" Doug's hoping for, is a system that will blow some cold air without freezing up.

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    Re: fundamentals vs fundamental belief

    Mad Fridgie, I agree that we do not have adequate data to completely assess the system. Attaching my personal gauges and T-couples right now is not politically feasible. I am attempting to learn more about the possible causes of freezing, doing so without encroaching on another person's territory.

    My earlier mention of 14F superheat listed on the plate was an error of assumption. Looking at it more carefully yesterday I saw that it was the target subcooling for a TXV, which this unit does not have.

    I also checked the suction temp at the exit from the evap coil and got 52F with the IR gun. Yes, I know the IR is not very accurate but it does indicate two things. First, we have reasonable evap exit temperature. Second, we are getting about 12F temperature drop in the suction line.

    Thus it appears we have no superheat in the evap coil. The refrigerant is boiling off in the suction line. The 12F temperature drop indicates a pressure drop, possibly because the line is too small for its length (about 90 feet).

    The filter was plugged to the point that it collapsed yesterday. Still no ice.

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    Re: fundamentals vs fundamental belief

    With clean filters, see what the run state is after an houres opperation, it is all a fine balance.
    Now in Redvers Sask.

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    Re: fundamentals vs fundamental belief

    In my opinion, you can read SH with an IR gun: spot on the first tubes of the evaporator and then spot on the leaving one. You can see this very good with an IR camera.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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