Central AC problem

[viviang]

I am averaging a tech every 2 months to work on my central air unit. (Heat Pump) Have had a new thermostat, new compressor, coils cleaned, freon replaced; all done this year. The unit iced over yesterday and technician returned today (after unit thawed) and stated that the "high side pressure" is running 325 psi and should be 275 psi. He further stated that this could be caused by low freon or dirty coils. Is this true? Coils were supposedly cleaned in July of this year (inside and outside coils). Compressor was replaced in February, so one would assume freon was full at that time. I need to figure out how to fix this thing instead of calling a tech every 2 months to come put freon in it. Help!! Tech today added freon and the high side pressure is still at 325 psi. Also, interestingly, noted that there is a switch (tech says high or low pressure switch to turn off unit if pressure was too high or too low) and the electrical leads were cut.

Would much appreciate any imput. I am trying to figure out who to call and how to handle this thing.

Vivian

[imeduc]

From the sound of this all, Id say first you need a good hvac tech to look at it for sure. I dont know in what order this was all done to the unit. But it dont sound right at all. Ice in the unit low freon. why didnt they find and fix the leak?????? High head, Can be they contaminated the system with noncondensables or with the new compressor and heatpump, did they put in the right driers in the high and low side can they be bad. ,TXV valve ok inside??? check valve ok there also???? WHO CUT THE HIGH AND LOW SWITCH OUT??????

You have to many ifs ifs here find a good tech

ED

My mistakes dont define me they inform me.

[LazyPup]

It is technically impossible to charge a unit unless both the evaporator coils (inside coil) and the condensing coil (outside coil) are clean.

Properly cleaning the coils should have been an integral part of the job when they changed the compressor.

If the refrigerant pressure is low, the operating temperature of the evaporator coil drops proportionally. Normally the pressures are adusted to maintain the evaporatore between 34 on 40 deg F to insure maximum cooling while still keeping the coil above the freezing temperature of condensate moisture on the coil. If the pressure drops, the temperature drops and ice forms.

Low refrigerant pressure will not cause high head pressure, in fact, the opposite is true, if refrigerant levels drop, the head pressure drops as well.

There are three common causes of high head pressure, 1. a dirty condenser coil, 2. a defective fan/motor combination or 3. a restriction in the liquid line.

One of the greatest contributors to a dirty condenser coil is cutting grass near the unit when the condenser is running. The grass clippings are sucked into the coils. It is best to make a point of shutting the AC off while cutting grass within about 25 feet of the unit. Trees, shrubs, weeds or other vegetation or any landscape materials such as fencing near the unit also restrict the airflow which results in high head pressure. For best operation an AC condensing unit should have a minimum of four feet assured clearance completely around the unit.

Check the amp draw on the fan motor. If the motor is working correctly and if the fan blade is properly positioned the motor should pull the rated amps +/- about 5%.

Note: to get a precise amp reading you have to compensate the manufactures rating for the actual voltage at the site. To compensate the reading, first compute the manufacturers rated wattage by multiplying the rated volts times the rated amps.

Example, assume the motor is rated at 2amps @ 230volts.
2 x 230 = 460 watts.

Now measure the actual voltage at the contactor primary. (It may range from 220 to 250volts.)

Divide the wattage by the actual voltage and it will yield the true amp draw for this location.

Example,,if your voltage measures 250 volts you would divide 460 watts by 250 volts. 460/250= 1.84 amps

If the amperage draw is low the most likely cause is a misspositioned fan blade, which reduces the amount of air it draws which further results in high head pressure.

If the amperage draw is high it indicates a problem with the motor bearings. You may just need to oil the bearings, or if excessivly high it could indicate defective motor bearings.

Pull the service disconnect then chech the motor shaft. If the fan turns hard when turning by hand the bearings are bad. Also check for wobble or excessive end play on the motor shaft, both of which indicate bad bearings.

Some condensing units have a metering orifice device in a final fitting where the liquid line attaches to the evaporator. If that orifice becomes obstructed it will result in a condition of high head pressure and low suction pressure, often resulting in ice on the evaporator coils.

If the compressor change out was a result of an electrical burnout, they should have installed both liquid and suction driers. An obstructed liquid drier will cause high head pressure, but is easily checkec by installing a tap valve on the discharge side of the drier and checking the line pressures.

Although the high and low pressure switches are not required, and not used on many machines, if the switches are present they most certainly should have been hooked up when installing the new compressor.

All of the afore mentioned points should have been checked during the compressor changeout.

Sounds like you may be dealing with a quick fix hit and run type contractor.