Heat pump problem

[cinergi]

Hi,

I'm a homeowner with a heat pump problem. My service tech has been here twice, but he's not sure what it is. He checked the charge (it's fine). I'd really appreciate your input to help us diagnose the problem, since he doesn't want to just start replacing parts.

Background: All-Carrier system (1997 vintage) with 2.5-ton heat pump, air handler and electric heat strips. The heat pump ran fine all winter. On Saturday I switched it to cooling for the first time this year. I noticed the following problem:

- Heat pump starts normally and cools fine for the first 20 minutes or so (my tech checked the output temperature and said it was fine).

- After running for about 20 minutes, a hissing noise can be heard from inside the heat pump (like refrigerant flowing in the lines, only louder). This noise gets progressively louder over a 10-minute period, until eventually it is quite loud and can be heard from 10 feet away above the fan and compressor noise. Along with this hissing noise, I noticed that the air blowing from the top of the heat pump, which is normally warm, gets progressively cooler until it is the same as the ambient temperature.

- Eventually (after a total running time of about 30 minutes), the compressor and hissing noise both stop. The fan continues to run (since the thermostat is still calling for cooling).

- At some point after that (say, a few hours - I'm not sure), the compressor restarts and the same cycle repeats again.

That's it... To me, the hissing noise suggests some sort of gradual pressure build-up inside the heat pump, eventually causing the compressor safety mechanism to kick in. Trouble is, we're not sure what is causing the build-up. My tech said it might be the piston in the reversing valve gradually shifting due to a leak or crack in it, but he's not sure.

Any ideas? We'd really appreciate your help on diagnosing this one.

Thanks,
cinergi

[iraqveteran]

yeah, you need a new technician

Your compressor head pressure is through the roof. The compressor is going into overload. hence the hissing.

Any good technician can diagnose and repair the problem. you need a tech that will spend the time on the system.....

Not just take temps and gone 20 minutes later.

[cinergi]

Well, in his defense, it was Sunday afternoon, so I suppose it was good that he came at all. He's supposed to come back tomorrow for a more thorough diagnosis - just thought I'd get a head start in case this is a common / easy problem. He's been very helpful with my other issues in the past.

He did put his gauges on; the pressures were good when the unit was running OK, but he didn't stay long enough for the problem to occur.

-cinergi

[iraqveteran]

There shouldn't be a "in his defense"

Yes it is Sunday. I am on call. My phone rang at 530am. I got in the truck, drove 2 hours and spent 4 hours on the call.

I cleaned and fixed an ice machine. They have an A/C down. Low on refrigerant. I performed a leak serch, found the leak, also found a bad expansion valve that needs to be replaced. I checked the compressor windings, checked the electrical and did a quick fix on the leak to prevent further contamination.

I then called all the local suppliers after hours numbers and already have a new expansion valve on order. Gonna be here Tuesday.


The fact that it is Sunday shouldn't mean anything to a technician that takes pride in his work. You called him, your probably paying good money for after hours service, and now he has to return to finish the diagnosis???? That seems a little off to me.

[beenthere]

So if you are taken to the hospital on a Sunday, and the ER blotches your care. Its ok because its Sunday.

[cinergi]

I agree with both of you that the problem must be diagnosed and fixed properly, and hopefully it will be. I simply meant that I called the tech at short notice, and by the time he got here (he was on another job at the time and had to finish that one first), it was already getting dark. He did not charge me for his visit, and said he would return to finish today. Although he said it might be the reversing valve, he didn't want to just replace it without confirming, which a less principled tech out to make a quick buck may have been inclined to do.

Of course, you are both correct that the problem has to be diagnosed and fixed correctly. If he does, great; otherwise, I'll have someone else look at it. Does anyone know a good tech in the Montreal, Canada area?

As for my original question, is there a common problem that could lead to the symptoms I described?

Thanks,
cinergi

[beenthere]

Several thigs could cause it.
Unfortunately. We are not allowed to give instructions or guidance of that type by forum rules.

I won't say its 100% not the RV. But its very doubtfull that it is.

If it happens every 30 minutes of run time. See if you can get a hold of him today yet before he gets there. And have him call you 15 minutes before he will get there. Then you can start your A/C and have it running so that he can hook his gauges and probes, and only have 10 or 15 minutes till it does it.

Always easier if they do it while your there to see it.

[model m-man]

Finished up a reversing valve replacement Sat night at 11:30. I understand where your tech is coming from. I would have done the same thing if I had got there at dark and suspected a bad RV.

Emergencies are one thing, but you ain't gonna die from doing without AC for one night. That's a major job and if it's done in a couple of hours, the tech is superman or it's going to fail. JMO

[cinergi]

The tech has diagnosed my problem. I'm not sure what it's called, but it's the valve on the thinner of the two refrigerant lines inside the house at the evap coil. Apparently this valve is sticking intermittently, resulting in a pressure build-up and eventually compressor shutdown.

The part (what's it called?) is on order.

-cinergi

[beenthere]

TXV.

[james farrish]

Replaced 6-wire Trane heat pump with 5-wire Amana. Brown wire not connected. Auxiliary light on at Trane t-stat. Should I change t-stat to 5-wire?

[cinergi]

The TXV was replaced, lines nitrogen purged, system recharged, and a new filter drier installed. System runs great now.

-cinergi

[iraqveteran]

glad to hear.

Did he clean the coils while he was at it???

[cinergi]

He cleaned the condenser coil. The evap was clean (I'm careful about my filter maintenance!)

-cinergi

[snewman24]

OK, guys. Here's what I don't understand: it sounds like the system was building up head pressure and eventually the internal relief valve was venting back to suction side. This would continue until the internal overload in the compressor would finally kick out. After cooling off awhile, it would reset. My question is this: how would a bad TXV cause this to happen? Why wouldn't it just pump down, and cut out on low suction pressure? Or just continue to run into a vacuum? Why the excess head pressure?

[snewman24]

Or... is the TXV somehow sticking wide-open and causing the high head??

[beenthere]

Condenser may not be able to hold the entire operating charge.

[iraqveteran]

TXV could be hunting severly. Causing a constant change in pressures and temps.

[flange]

many resi units do not have low presssure switches.

[snewman24]

many resi units do not have low presssure switches.

So where is all the heat coming from to elevate the Head pressure? If the low side is into vacuum it's not moving any heat from the evap. coil. All the refrigerant is stacked in the condenser and liquid line, being cooled down to ambient. Is it possible that the high compression ratios and the heat of compression are responsible for all the excessive head pressure? I've never witnessed a system running long term into a vacuum with no LPCO to know the effects this has.