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Old 07-30-2012, 10:00 AM   #31
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So I want to replace the contactor in my outside unit


In cases like this, my short money is usually on the circuit boards (in the furnace, outdoor unit or tstat) followed closely by thermostat wiring to outdoor unit, followed by the transformer.

To test the tstat, just jump R to Y at the stat. If the unit comes on without chattering, you may have found your trouble. Put it back together, turn the system on, and next time it starts chattering you can then try jumping the R and Y leads AT THE AIR HANDLER (with the system on and running). if that stops the chattering, then chances are you've found the issue.

Hard to say with logic boards, since often they must be temporarily removed from the circuit and the items they control hardwired for testing. This can be tricky.

As to a transformer - you'd almost have to replace it with a known good one to eliminate that. Tformers are fairly cheap, of course, and generally fairly easy to get.

I'd also isolate the boiler temporarily (i.e. remove the stat wires from there).

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Old 07-31-2012, 09:35 AM   #32
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So I want to replace the contactor in my outside unit


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Originally Posted by scottmcd9999 View Post
In cases like this, my short money is usually on the circuit boards (in the furnace, outdoor unit or tstat) followed closely by thermostat wiring to outdoor unit, followed by the transformer.

To test the tstat, just jump R to Y at the stat. If the unit comes on without chattering, you may have found your trouble. Put it back together, turn the system on, and next time it starts chattering you can then try jumping the R and Y leads AT THE AIR HANDLER (with the system on and running). if that stops the chattering, then chances are you've found the issue.

Hard to say with logic boards, since often they must be temporarily removed from the circuit and the items they control hardwired for testing. This can be tricky.

As to a transformer - you'd almost have to replace it with a known good one to eliminate that. Tformers are fairly cheap, of course, and generally fairly easy to get.

I'd also isolate the boiler temporarily (i.e. remove the stat wires from there).
Would the circuit boards give *any* clues to failure via plain old eye-balling? I'm thinking I would have to rule out failure of other things and leave suspicion of the boards to last.

Replacing the transformer would be one way to go. And the relay that is near the boiler in the basement could be easily replaced.

My AC is doing something else besides chattering. It runs oh so briefly (30 sec to 2 minutes), then the compressor seems to bypass and either makes an awful noise and hot air stops coming out of the top of the condenser unit, or just shuts off altogether.

It is, I presume, overcharging.

I have seen some discussions elsewhere, and a hard start capacitor seemed to put things right in at least one similar case.

I have to go up to the attic, and see for myself whether there is a thermal expansion valve.
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Old 07-31-2012, 07:18 PM   #33
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So I want to replace the contactor in my outside unit


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It runs oh so briefly (30 sec to 2 minutes), then the compressor seems to bypass and either makes an awful noise and hot air stops coming out of the top of the condenser unit, or just shuts off altogether.
This is caused by a restriction in the lines somewhere (most often in the condenser, or in the liquid line or drier). This is not something a DIYer can fix.

A hard start kit won't resolve either of these issues.

Time to call out your contractor again.
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Old 07-31-2012, 10:25 PM   #34
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So I want to replace the contactor in my outside unit


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This is caused by a restriction in the lines somewhere (most often in the condenser, or in the liquid line or drier). This is not something a DIYer can fix.

A hard start kit won't resolve either of these issues.

Time to call out your contractor again.
My inclination is to call a new service co. at this point. My system worked great until it blew a cap; the banging and chattering was evident at first visit and the jerry-rigged single caps. It worked well for 24 hours after same banging situation (with intermittent voltage problem suggested).

Third visit it just bypasses every time an attempt is made to start the unit.

A line restriction was suggested.
Service tech took some refrigerant out (after making a big deal about service valve leaks -which btw secretly got my back up as, trust me on this one point, sealing the valve caps works FINE). He said pressures were high but wouldn't tell me the reading, and wouldn't write it down. He said it was high on the low side, too.

My unit may well have reached the end of it's mechanical life, but it is not getting replaced this year, so it's patch repair or nothing.

What do do about a line restriction? What's the best way to tell if it's in the liquid line or elsewhere?
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Old 08-01-2012, 05:59 AM   #35
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So I want to replace the contactor in my outside unit


I wouldn't call the single caps "jerry-rigged". Before dual caps, that's what was installed - a separate cap for both fan and compressor.

Sometimes capacitors can take out other parts when they fail, especially compressor caps. Most caps don't fail the day they're replaced; they get weaker over time, forcing the motor they're connected with to work harder and harder. This puts a strain on the motor, and can destroy an already weak motor. That's not the fault of the service company.

However, it does sound as if you've lost faith in the company, so it would probably be best if you contacted someone else.

If it is a restriction in the liquid line, then it's most likely in a drier or in the expansion device. However, it could be in the condenser itself. The only way to tell is to take pressure and temperature readings on the liquid line, and see where that change of state is happening.
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Old 08-02-2012, 10:44 AM   #36
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So I want to replace the contactor in my outside unit


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...(snip)

...If it is a restriction in the liquid line, then it's most likely in a drier or in the expansion device. However, it could be in the condenser itself. The only way to tell is to take pressure and temperature readings on the liquid line, and see where that change of state is happening.
What would cause high pressures on the suction line AND high head pressures?

This situation seemed to puzzle the HVAC tech exceedingly. I don't know what the pressure readings actually were - and they aren't documented on the service slip either.
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Old 08-02-2012, 07:37 PM   #37
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So I want to replace the contactor in my outside unit


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What would cause high pressures on the suction line AND high head pressures?

This situation seemed to puzzle the HVAC tech exceedingly. I don't know what the pressure readings actually were - and they aren't documented on the service slip either.
here are a few causes of high pressure in the suction side.....restricted air..{condenser}...air and non condensables... overcharge....excessive air..{evaporator}....overfeeding metering device....bad valves {low amp draw}
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Old 08-02-2012, 07:44 PM   #38
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So I want to replace the contactor in my outside unit


some causes of high head pressure,,,dirty condenser coils.....overcharged ..refrigerant flow restriction....extra hot indoor temperature...

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