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From: John De Armond
Subject: Re: heat pump compressor stalls sometimes
Message-ID: <>
Date: Sun, 08 Aug 1999 03:11:41 EDT
Newsgroups: sci.engr.heat-vent-ac

Ralph Bryan wrote:
> I have a 3 ton water-source (aka "geothermal") heat pump with a Tecumseh
> reciprocating compressor.  Sometimes, the compressor attempts to start
> but seems to be stalled. It just makes a loud humming noise until the
> thermal protector shuts it off.  The circuit breaker (20A) does not
> blow.  The most recent HVAC tech installed a 500% hard-start capacitor,
> but it still happens.  The R-22 charge has been removed and measured
> (weighed) properly back in.  Water temps and flows are OK.  Starting
> current and voltage are OK when it doesn't stall.  Freon suction and
> discharge pressures measure OK. System has a TXV. The problem happens
> approx. once per day now, but wasn't caught when tech was here.  This
> can happen also in winter heat season.  One fix which often works is to
> switch from cool to heat for approx. 10 seconds, and then back to cool.

The problem obviously is that the system isn't equalizing
sufficiently for the compressor to be able to start.  TXV-equipped
systems won't equalize unless it is designed specifically as an
equalizing valve - usually by including a small groove in the seat. 
You manually equalized the system when you flipped it over to heat
which switched the crossover valve.

The big question is why this problem is just now cropping up on an
existing unit.  Has it been working OK and just started acting up or
is this a new system?  If it is an existing system, has it been
worked on lately.  If it has not been worked on, the most likely
cause is the pending failure of the compressor, perhaps via a
shorted winding.  This equals less starting torque.  Does it make an
excessive noise?  Other electrical possibilities include an
intermittent starting relay if it has one or an intermittent
connection somewhere in the starting cap circuit.  When it fails to
start, is it pulling locked-rotor amps as listed on the compressor
nameplate?  If not, it is likely to be in the starting circuit.

I've seen heat pumps that tripped the crossover valve in order to
equalize the system when the thermostat opens.  Rare but possible. 
If yours is designed to work like that, there may be something wrong
in the control logic.

Has the TXV been replaced?  If so, the service tech may not have
replaced it with a bleed-type TXV (if the system required one.) 
There are so many possible causes that one would have to spend some
time diagnosing the system to tell for sure.  Meanwhile you CAN get
by with your flip-to-heat trick.


From: John De Armond
Subject: Re: heat pump compressor stalls sometimes
Message-ID: <>
Date: Mon, 16 Aug 1999 04:50:15 EDT
Newsgroups: sci.engr.heat-vent-ac

Ralph Bryan wrote:
> Thanks for your info on the possible TXV problem.  The tech that was
> here thought about that too, but also thought that would cause a run
> problem. When the compressor was stalled the other night, I hurried down
> to the crawl space and hooked up refrig. gauges to see if pressures were
> not equalized.  (I had already switched unit from "cool" to "off".) Both
> pressures were about the same, around 130 psi (R-22).  I also measured
> that the reversing valve is not energized in either the "off" position
> (of course!) or in the "cool" position.  (I guess that means it is
> energized in the "heat" position.) Therefore I concluded, that I had not
> equalized the pressures by switching from "cool" to "off".
>     Do you have any ideas on how to isolate whether or not a motor winding
> is bad?  If the power is shut off while the unit is stalled, could I
> measure resistance in either the run or start leg and determine
> anything?  Would those be very low resistances?
> Thanks again,
> Ralph Bryan

I'll run a few other things past you that may be the problem.  But
you're really getting to the stage where you're going to have to be
there (either in person or via recording instruments) when the
problem happens in order to diagnose it.  These intermittent
problems are the ones that make you pull your hair out.

Some things to consider:

*	Contactor could not be making up.  I've run into this a lot
lately.  Contactors are getting cheaper and cheaper.  It'll look
like the contacts have made up but there'll be either no or low
conductivity on one pole.  You can check that with a voltmeter or
ampclamp if you're there when it fails to start.

*	If it has a start relay (usually a potential relay on larger
units), that relay may not be picking up reliably.  Most of them
have to be level to work right.  

*	If it has one, the start cap may be bad or marginal. Need to check
the microfarads on a cap meter.

*	The run cap could be marginal.  Need to check the microfarads.

*	Either of the caps could be the wrong size.  I don't know any
generally available reference for compressor model number vs cap
size so you're probably going to have to ask the service company to
check this.  This is a possibility if a tech changed out a cap in
the past and just used whatever he had on his truck that was close.

*	Compressor could be tight and/or have some other intermittent
problem.  You'll have to record the starting amperage when it fails
to know.  I have a recording ammeter for such occasions.  Given that
you don't have one, Repeatedly starting the unit (with enough time
between starts to allow the start winding to cool) while monitoring
an amp clamp is the only other way.

*	Compressor could have an intermittent electrical problem.  A
megger might find a grounding winding.  An ohmmeter won't work
unless the ground is on the verge of blowing because it doesn't
supply enough compliance voltage.  A megger supplies up to 500 volts
which drives leakage current through faults that are voltage
dependent.  Other kinds of faults can be diagnosed with a "ring
scope" (a device that hits the winding with a short, high voltage
pulse and displays the ringing waveform on a scope) but you probably
don't have one of those at your disposal either.

Since you're not working by the hour, you can spend some time doing
detailed troubleshooting.  I'd check the contactor, the start relay
if it has one and the caps.  The contactor and start relay are cheap
so it might be easier to just replace them than try to catch an
intermittent.  If you get that far and don't see an obvious problem,
you're going to have to babysit the thing and catch it not starting
with test equipment attached.  But if you do get that far, you're
probably looking at a defective compressor, assuming the pressures
look equalized when it won't start.


From: John De Armond
Subject: Re: Great tips..thanks
Message-ID: <>
Date: Fri, 09 Jun 2000 04:43:24 EDT
Newsgroups: rec.outdoors.rv-travel

Mike Niemela wrote:
> "Gas Passin" John is right on target, again, with his description of
> checking out the compressor on an AC unit.
> Many of these units also have a control package consisting of assorted
> relays and a transformer for the low voltage control circuit.  Under the
> lid covering these devices you will, in most cases, find a schematic
> showing which wire goes where and what that relay, device does.
> Use caution as 120 VAC can kill.
> As a last resort, if the compressor checks out ok with the resistance
> and voltage tests John outlined and the unit will still not run, an HVAC
> tech can "rock" the compressor by attempting to alternately start it
> backwards and forwards. This will often break loose a stuck compressor.
> No guarantees on how long it will last however.

Ok, now let's tell 'em how to do it.  Get a KickStarter (hard start
kit) from the HVAC supply store and install according to 
instructions. This provides extra torque.  Then reverse the "start"
and "run" leads on the compressor (those two wires other than the
common one) and attempt to start it.  Only for a few seconds.  This
tries to start the compressor backwards which will hopefully
dislodge whatever is sticking it but the smaller starting winding is
now the run winding.  It can't take more than a few seconds of this
sort of duty.  I usually try alternating starting reversed and
straight before I give up and yank the compressor.


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