From: John De Armond Subject: Re: heat pump compressor stalls sometimes Date: Sun, 08 Aug 1999 03:11:41 EDT > I have a 3 ton water-source (aka "geothermal") heat pump with a TecumsehSometimes, 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 notThe 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. > current and voltage are OK when it doesn't stall. > discharge pressures measure OK. System has a TXV. > approx. once per day now, but wasn't caught when tech was here. > 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. 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 anwhat size ac unit do i need for a 1200 sq ft houseHas it been working OK and just started acting up orair conditioning fan shuts off is this a new system? car ac repair in san joseIf it is an existing system, has it beenIf it has not been worked on, the most likely cause is the pending failure of the compressor, perhaps via aThis equals less starting torque.

Does it make anOther 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 compressorIf 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. 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 Date: Mon, 16 Aug 1999 04:50:15 EDT > 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 runWhen the compressor was stalled the other night, I hurried down > to the crawl space and hooked up refrig. gauges to see if pressures were(I had already switched unit from "cool" to "off".) > pressures were about the same, around 130 psi (R-22). > that the reversing valve is not energized in either the "off" positionor 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 windingIf the power is shut off while the unit is stalled, could I > measure resistance in either the run or start leg and determineWould those be very low resistances? I'll run a few other things past you that may be the problem. 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. 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 lotContactors are getting cheaper and cheaper. 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. have to be level to work right. * If it has one, the start cap may be bad or marginal. 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 toThis 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 intermittentYou'll have to record the starting amperage when it failsI have a recording ammeter for such occasions. 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. 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 voltageOther 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 doingI'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 anIf 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. Subject: Re: Great tips.. Date: Fri, 09 Jun 2000 04:43:24 EDT > "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. > 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 toThis 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. 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 thisI usually try alternating starting reversed and straight before I give up and yank the compressor.