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Question DetailsAsked on 3/18/2015

Cost to replace the coil and condenser on a AC

The unit inside the house keeps freezing up after about two months after we have it service and they say they can't find a leak

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2 Answers


Did they say to replace the system?

Very unusual that they can't find the leak.

When they add refrigerant , how long before it's low and icing up again?

Where do the copper refrigerant lines run , from the indoor unit to the outdoor unit? Attic, under concrete slab, between walls????

To provide answers , click Answer below


Answered 4 years ago by BayAreaAC


To respond back with more details like BayAreaAC suggested, use the Answer This Question yellow button right below your question. In particular, please clarify whether you have an A/C or a Heat Pump, and exactly what you mean about "freezing up" - do you mean frosting/ice on the evaporator coil or on one or more refrigerant lines, or do you just mean it stops cooling but compressor is running - so you mean possibly a vapor lock or stuck valve or lost refrigerant situation rather than actually "freezing". Also, have they checked the gas pressure to determine if it has the correct amount of gas in it ? Also, when they serviced it, did they change from R-22 (Freon) to another gas type ?

I am assuming you are talking a conventional A/C or heat pump with indoor condenser coils installed in the forced air heating system ductwork, or possibly several distributed evaporators with their own fans, all charged from an outside unit. The way these units work is they compress a gas to a liquid, cool it at the outdoor condenser, then pipe it to the evaporator (the Coil or Radiator to most people but is technically a Heat Exchanger like the one in a forced air furnace) where it is expanded or "boiled" into the coil. This expansion causes the gas emperature to drop dramatically in temperature, cooling the coil unit which then cools the air blowing over it. The amount of expansion in the coil is regulated to match the amount of heat transfer that can occur from the coil to the air passing over it in the normal air conditioning range - so excess or too low a gas flow rate or less than design airflow over it can cause overcooling at the surface with resultant icing up.

Several reasons a coil can ice up - assuming you are talking icing up on its surface:

1) A/C is running in weather that is too cold, so the unit freezes up with moisture from the surrounding air - but that is almost always at the condenser outside, because most people willnot run an A/C in a cold house.

2) unit has been recharged with the wrong gas so it is operating at the wrong temperature differential (very rare)

3) incoming air over the coil is so cold or airflow is so low that it drops well below freezing at the coil surface - usually an indication of a fan not running or running at low when it should be at high, or an airflow blockage from a mostly closed duct damper or a plugged or too restrictive a filter so the airflow over the unit is too low for its rated heat transfer capacity, causing it to run well below freezing and frosting up from the moisture in the air passing over it

4) excessive dirt and lint on evaporator reducing airflow and trapping moisture which then freezes on it

5) a leak at the coil would also chill it more than usual, but enough of a leak to cause it to freeze up for any period of time would quickly exhaust your gas supply. Any such leak should be readily found using a leak detector meter or injected dye in the refrigerant - or usually just by seeing where there is an accumulation of frost on the outside of the piping or coils.

6) low refrigerant level, so the incoming liquid is cooler than it should be and outgoing pressure is lower than it should be, so the unit cools more than it should, causing surface frost buildup. An eroded out or missing coil inlet restrictor (if so equipped) can also do this by allowing more gas flow than it is designed for, causing overcooling of the coil. This latter factor is not common in residential units because coils and compressor/condenser units are matched in capacity but can occur in evaporators that are designed for several different sized units so they have a flow restrictor installed to match the coil to the compressor/condensor unit. More common in larger commercial installations.

7) excessive temperature refrigerant due to dirty condensor or malfunctioning condensor fan, resulting in the outbound "liquid" line from the outdoor unit to the coil having gas in it or mixed gas and liquid, which significantly reduces the amount of coil cooling - a simple temperature differential check between inbound and outbound temperatures would resolve if this is happening

If you mean the unit is actually freezing up INSIDE the coil or tubing so refirgernt flow is stopped, then that can only happen if you have water in the system, meaning someone got very sloppy in recharging it, because there is no way the compressor/evaporator can get the gas down to the temperature it frosts up or freezes as a solid internal to the unit - about 200 some degrees below zero. However, some types of design can get a vapor lock in the gas control valve if they are running too cold due to low pressure, or too hot due to inadequate gas flow from a line blockage issue or low refrigerant. Some units have an in-line debris screen just before the compressor to prevent compressor damage from any bits of metal in the system from working on tubing or incomplete coil cleaning in manufacturing and if this screen becomes clogged with debris or refrigerant oil sludge it can act to restrict refrigerant flow, causing problems as well.

Assuming your unit worked OK for the two months after servicing, and assuming your fan is running OK and the coil and air filter are clean, then all I can think of is the unit is passing too much gas through it - which would indicate the gas regulator (flow control valve) may be letting to high a flow through the unit AND the compressor is capable of turning out more cooling than the evaporator coil is designed for - indicating a mismatched unit. Has anything been changed on the system ?

I would say if that vendor can't figure out the problem, time to either go to another one or, if you like that vendor and want to use him in the future, ask them to have the manufacturer rep come look at it - they commonly will come on a referral from an HVAC contractor (particularly if one who distributes their A/C line) for free though you will probably still have to pay your HVAC guy to be there at the same time to talk to him - but cheaper than a new unit. Since you say it "keeps freezing up" that seems to indicate it works for awhile then quits, so sounds like a gas charge or gas flow control issue rather than a failed coil or compressor which might lead you to consider a new unit instead - though of course if looking at an expensive repair AND an older (7-10 years +) unit or running R-22 (Freon) rather than on e of the newer replacement gases, that would weigh in favor of a new unit rather than sinking a lot of money into an older one.

Answered 4 years ago by LCD

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