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my outside lennox air conditioning unit turns on but then shuts

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off after 15-20 seconds-does this...
my outside lennox air conditioning unit turns on but then shuts off after 15-20 seconds-does this mean the compressor coil is leaking somewhere and no freon left inside? this is a ten yr old 3/5 ? ton unit with new capacitator and new coil-everything seems fine except it wont stay on-a repair man told me that the compressor coil was leaking and that it was not repairable-i am not sure that it cannot be patched for a few months-this company is very anxious to sell me a new unit-do you think it is worth my trying to identify leak area,patch it with liquid weld and put in more freon?
Submitted: 7 years ago.Category: HVAC
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Answered in 5 minutes by:
6/15/2010
HVAC Technician: Tom, HVAC Technician replied 7 years ago
Tom
Tom, HVAC Technician
Category: HVAC
Satisfied Customers: 770
Experience: 16+yrs HVAC/R service. Residential Commerical Industrial
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Tom :

Hi, sorry you're having trouble. If your unit is equipped with a low pressure switch, that could be the case.

Customer:

where is the low pressure switch and does this mean the freon has leaked out?

Tom :

Ok, the low pressure switch would be located inside the condensing unit. It will be on the larger of the two refrigerant lines.

Tom :

We can find out...first need to know if it has a low pressure switch.

Tom :

The switch is usually about a couple of inches high and about the diameter of a quarter,, it would have two wires coming out of the top of it.

Customer:

let me go outside and look-give me a minute-

Tom :

You'll have to pull the side panel off of your a/c unit...trace the larger of the two lines and check for a switch.. standing by..Tom

Customer:

side panel off-be right back

Customer:

does this switch have a button on it and is located over by compressor coil entrance-i followed larger of two lines and this is only thing i found that resembles a switch but it is a red push button with two wires below it-

Tom :

That is the high pressure reset...lets say the fan motor dies and unit overheats...the switch would trip and you would have to manually reset it...

Tom :

The auto reset low pressure would be on the larger line going back to the compressor and would have no button..

Tom :

You stated that your unit has a new coil...which coil are you referring to/

Customer:

not sure just know that they did major work on it two years ago and pretty sure they said they replaced the coil-assuming it was not the compressor coils-let me go and see if i see the other switch

Tom :

Ok..

Customer:

dont see anything else with wires on any refrigerant lines-there is a valve on the larger refrigerant line- the make of this unit is HS25-651-2P if that is any help

Tom :

Ok..it does't sound like there is a low pressure switch...so...that shouldn't be the issue..Here is a test you can try if you are willing.

Tom :

Locate the compressor contactor..

Full Size Image

Tom :

Find a good insulated tool or piece of wood..to hold the contacts closed ...by pressing on the black bar in the center of the contacts...

Tom :

This would 'manually' hold the relay in just for a minute or so to see if the refrigerant line gets cold.

Tom :

If you are not comfortable doing this...please don't.

Customer:

i took the outside breaker out -obviously i need to put this back in

Tom :

You'll want to set the thermostat to cooling so the indoor fan kicks in and put the plug back in outdoors.

Tom :

After 30 seconds to a minute you should feel the larger of the two refrigerant lines turning cold if the charge is up on the unit.

Tom :

In the meantime I will see if I can find some info on your unit and see if it has a low pressure cut out switch.

Customer:

roger-give me a sec to go and turn on etc=still waiting for it to kick on-had thermostat too high -used a shim and tripped it but it has stayed on-will keep pressing after i hear outside blower go on-be back in a few-

Tom :

Ok, no problem....from what I'm seeing, I'm pretty sure there is a low pressure cut out somewhere on your unit.

Customer:

of course i will up the ante on my price for your time on this question-never know how this "just ask" stuff is going to go-be back

Customer:

pouring rain here-need to wait till it stops

Tom :

Ok, if I happen to go off line I will check back later...but I'll share my thoughts with you at this time.

Tom :

First off, if we have a refrigerant leak...it's going to take some detective work to find without an electronic sniffer. Sometimes you get luck and can find it with soapy water.

Tom :

Check the connections where the refrig. lines connect indoors and out. If they have flare fittings with large nuts to lock the joints, they are prone to leaking.

Tom :

Look for traces of oil anywhere. If you are leaking oil, you're leaking refrigerant.

Tom :

If it is in the coil itself, they are a lot harder to find and pinpoint. Even then, due to the pressures involved, any type of epoxy will not work. It would have to be soldered. I've repaired quite a few over the years. It is tedious, and time consuming. You have to tear fins out to get at the actual pinhole spot on the copper tube to fix it.

Tom :

Most contractors will not touch a coil in this sort of need for warranty reasons. Also, it can take alot of time and that just racks up the bill. In the end, usually no one is happy.

Tom :

I'm not trying to discourage you, just being honest. If we determine it is low on gas...a guage set would be necessary to recharge and figure out just how low it was. Look for the obvious signs of oil first. If you find some, hit the joints with some soapy water. That would be much easier to repair than an actual coil leak.

Tom :

Do you want to get back with me later when it stops raining?

Tom :

The other way to find the low pressure switch...

Tom :

is to trace the wires from the high pressure switch...one will go to the contactor coil or power...the other one should connect to the low pressure switch as the two switches should be wired in series.

Customer:

i kick started unit but now the blower fan outside will not go on-one of the coolant lines is hot-is that normal? what do you suggest i try next? is it worth trying a freon recharge? the system seems to operate fine-goes on automatically but just kicks off after 10 -15 secs and now the fan which did go on before doesnt go on when system kicks on-

Tom :

The refrigerant line will get hot without the fan going. It may not be running if it is low on refrigrant. I think your unit may have a 2 speed fan motor that is probably operated off of refrigerant pressure.

Tom :

I'm sure it's low on refrigerant, but I'm a little weak on the Lennox units. I think I should opt out of this question and see if one of our other techs can either confirm my suspicion or give you a more solid answer. Tom

Tom :

If we had a gauge on it, it would be a simple matter to tell. Tom

Customer:

i am sure it is low on refrigerant and i am sure the leak is also in the coil-there is one weld epoxy that works as good as soldering gun-i would give it a try-i think i have isolated the area with leak and have already removed fins etc-i dont mind trying this semi-solder route and then adding freon if there is a chance this will hold for a few weeks/months-i am really not in the properfinancial state to buy a new unit this week as i just had a tree thru a large window last week during a storm and my $2500 deductible left me with total repair costs out of pocket- i need at least a month to replenish my emergency fund.

Tom :

I understand and can sympathise. I guess I didn't know you already had the fins out and located the leak. The only problem is the pressure..You're generally looking at 250 lbs or so refrigerant peressure.


 

Tom :

If you have a method of removing the refrigerant, then I would go ahead and try to solder it. I'm not recommending this, but I've seen guys use a piece of rubber and a hose clamp around the pipe to get a unit by until a permanent repair can be made. Tom

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