Check blower wheel, and check blower motor
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I have a heat pump that has been freezing up in the evenings. The entire freon lines are covered with ice usually in the evenings around 8:30 or 9pm just after dark. I have cleaned the coil on the inside (in the attic) and on the outside unit, and replaced the filter. I also checked the coolant pressure 70 psi on the low side and 225 psi on the high side. After I notice the temp increase in the house and that the heat pump is frozen, I turn the unit off for 30 minutes and let it thaw and it works perfectly for until the next night.
The house is about 1500 sq ft on a slab, built 4 years ago. I live in Alabama, and when I have been having problems the outside temp as been in the high 80s to low 90s during the day. On cooler days (high in low to mid 80s) I have not been having the problem.
Does anyone have any ideas why tis may be happening?
Check blower wheel, and check blower motor
sounds as if the reversing valve is not closing all the way. its function is to reverse the flow of freon through condenser. one direction is for heat, the other is for cool. the unit cools by means of a pressure drop through an orifice.
We encountered a similar problem in our home in SC and the Technician found a leak in the freon line from the outside unit to the condenser in the attic. The leak caused the entire unit in the attic to freeze up as well as the outside unit. You may not have a leak, however depending on the age of your unit, you may need a recharge.
Originally Posted by geurd
This post have left me with far more questions than prospective solutions.
Properly high pressure liquid refrigerant from the condenser is passed through a capillary tube or expansion valve and is metered into the low pressure evaporator at a controlled rate. As the pressure drops the boiling temperature of the refrigerant drops proportional to the decrease in pressure. AT 70psig the actual boiling temperature of R22 refrigerant is 40degF. Understanding that there is moisture in the air it is an industry standard to set the low side pressure at 68 to 70psig to insure the coil will remain at 38 to 40degF, allowing maximum cooling while still keeping the coil above the freezing point of the water condensing on the coil. As heated air is passed through the evaporator coil the liquid refrigerant inside the coil absorbs heat by boiling into a low pressure gas which is then returned to the condensing unit via the suction line where it is then compressed to high pressure and passed to the condenser coil where the gas condenses back into liquid and repeats the cycle.
The presence of ice on the evaporator coil or suction line is indicative of insufficient air flow through the evaporator coil. As the air flow decreases the it also reduces the amount of heat energy entering the coil. The result is a portion of the liquid refrigerant is then carried over into the suction line where it continues to absorb heat energy through the tubing wall, causing atmospheric moisture to condense and freeze on the tubing. If left unchecked liquid refrigerant can be sucked into the compressor where it will damage the compressor valves and permanently damage the compressor.
The leading causes of insufficient air flow, in order are:
1.Dirty air filter
3.Furnishings or personal belongings blocking the return air grilles.
4.Defective fan or fan motor
5.low refrigerant charge which results in super cooling the evaporator.
The post states that the air filter was changed and the coils were cleaned. The question that comes to mind here is, how were the coils cleaned? Were they merely brushed off or were the coils professionally cleaned by means of an approved foaming coil clearner process?
Based upon other information in the post let us skip ahead to the refrigerant charge for a moment.
The post states that the operating pressure was measured at 70psig on the low side and 225degF on the high side. Obviously this measurement was taken after the ice was defrosted because 70psig would convert to a coil temperature of 40degF, well above the 32degF freezing point of water.
In order to compute the high side condensing temperature we take the outdoor ambient air temperature and add 30degF then compare the resultant temperature to a refrigerant enthalpy chart. For refrigerant R-22 a high side pressure of 225degF equates to a temperature of 109deF. Deducting 30degF we can then conclude that the test was taken when the outdoor air temp was 109-30= 79degF. which coincides with the posted average temps in that region. This alone would lead me to believe the refrigerant charge is fine, but for the record let us persue this a bit further.
The post states that the icing occurs on the hotter days but not on cooler days. If the icing condition were a result of refrigerant pressure just the opposite should occur.
We are still left with obstructed return air grilles or a defective fan or fan motor.
The post states that the icing generally occurs in the evening so it is remotely possible that someone is coming home from school or the office and setting a book bag or briefcase in front of the air grille, but that should be easy enough to check.
I would suspect the problem is more likely a dirt buildup on the squirrel cage wheel fan blades which has reduced the efficiency of the fan, a loose fan to motor shaft connection or the fan motor is not turning at the proper speed.
If one is fortunate enough to have access to a laser or optical tachometer you can actually measure the fan RPM, but due to the expense of those instruments it is highly unlikely that you would have one. A second option is to use a snap on ammeter to measure the operating current draw of the motor. If the fan blades were dirty or the fan loose the load on the motor would be reduced and result in a lower than normal amp draw.
The most common cause of reduced RPM is motor bearing friction. As a last resort you might try simply oiling the bearings and see if that alleviates the problem. If not you will need to consult a trained HVAC tech to perform further tests on your system.
FYI: In the post you stated that you had tested the refrigerant pressures. Federal law prohibits opening the refrigerant system for any purpose by anyone who is not an EPA certified refrigerant handler. On the one hand, The mere act of connecting a charging manifold to perform the pressure tests could result in fines up to $10,000 while on the other hand, unless you have been trained in HVAC there is very little information you could get from such a test because the pressures constantly vary by ambient conditions and require a thorough knowledge of HVAC to determine the results of such tests.
Oh where do I start? On cooler days the unit isn't running long enough to freeze up. The "add 30 degrees formula" is old school and doesn't apply to newer higher efficient units (above 10 SEER). You din't mention what the superheat or subcooling temps were. You need those temps along with if it is a cap tube or TXV system to determine proper refrigerant charge. A/C techs do not SET the low side at 70 degrees. The combination of outside air temp, indoor wet bulb temp and exspansion device determine low side psig. A motor won't slow down and keep running if the bearings are worn . The winding configuration in the motor determine the speed at which it rotates - if the bearings or anything else slows the motor down it will soon overheat and shut off on it's own internal temp protection. Since it isn't shutting off that way then that's not an issue. Some of LazyPups info was correct however so all those things concerning proper air flow need to be checked. If all those things are ok, then based on the info you gave, I would lean towards low refrigerant charge but it sounds like you need an experienced tech to come out.
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