heat pump compressor stalls sometimes
Ralph Bryan
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.
Any ideas, anyone? I need to fix real soon.
Thanks,
Ralph Bryan
Xenia, OH
LGJ57
Thus the compressor starts easily.
Something may be preventing the system from equalizing. Does the system use
CHECK VALVES?
Is there an external equalizer on the TXV?
Neon John
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.
John
--
John De Armond
johngd...@bellsouth.net
Neon John's Custom Neon
Cleveland, TN
"Bendin' Glass 'n Passin' Gas"
turtle4air
<Ralph...@worldnet.att.net> 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.
> Thanks,
> Ralph Bryan
> Xenia, OH
>
this is turtle.
try you a hard start kit on it and most of the time it will quiet that
false starting.
TURTLE
--
Posted via Talkway - http://www.talkway.com
Exchange ideas on practically anything (tm).
AllTemp
Your working to hard fella, the guys post has a 500% start kit already
installed...
The most recent HVAC tech installed a 500% hard-start capacitor,
> but it still happens.
Ralph Bryan
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
Neon John wrote:
> 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.
>
Neon John
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.
Jeff Upchurch
pot: How long is this unit off between cycles? If the thermostat is
restarting the unit too soon (short cycling) due to any number of reasons,
the system may not be equalizing quickly enough and trying to start under
load (single phase compressors don't do well at this). The installed hard
start will help avoid this, but may not eliminate it. Check for a anti-short
cycle timer in the low voltage controls and if it doesn't exist, add one set
for five minutes of delay (I'd do this at the same time I checked the
contactor for problems).
Just another $0.02 worth...
Jeff
Neon John wrote in message <37B7D0C2...@bellsouth.net>...