View Full Version : Discharge valve or TXV
06-17-2005, 09:20 PM
outdoor temp 71
indoor temp 71
liq press 150
suct press 5
liq temp 68
suct temp 67
Indoor coil is not freezing up. 5 Ton H/P
Weighed in R-22
06-17-2005, 11:58 PM
My money's on the TXV. I had similar symptoms last week on a Lennox heat pump, except the OAT was a lot higher than 71 degrees. More like 95. Still, 5 psig suction and 175 head. No subcooling. Changed out TXV and it's cruising, now.
Indoor coil may not be freezing up but is there frost right at the TXV?
06-18-2005, 12:08 AM
shophound what cause it to go wrong was it the sensing bulb?
06-18-2005, 12:54 AM
Theres no frost @ the TXV.I was thinking the Cond. Sat.Temp. should of been about 100*
Could the discharge valve be stuck closed?
Found out TXV was replaced already.
Thanks for replying, still triing to fig this out.
06-18-2005, 06:52 AM
The power head on the TXV failed. Once that happens a TXV will not meter, it will slam shut. The only opening force in a TXV is the charge in the thermostatic bulb acting on the power head. If either one of those go bad, the TXV becomes a restriction by not allowing hardly any flow through it. The valve is seated.
When you mention discharge valve, are you talking about the compressor discharge valve or the condensing unit's service valve on the liquid line (if this is a split system) leaving the condenser?
For your heat pump I assume we're discussing operation in cooling mode. Perchance was there ever a liquid line drier installed in the compressor discharge line vs. the liquid line? Or, was the liquid line drier in the right place (liquid line) but it was not a bi-flow drier? Or, worst of all, a liquid line drier that was NOT bi-flow was installed on the compressor discharge line?
Here's my thoughts...once upon a time there was a liquid line drier that was not a bi-flow drier that lived on a compressor discharge line in a heat pump. One day the little drier couldn't take it anymore and shed its dessicant beads throughout the system. TXV and checks in the condensing unit fouled and plugged. In heat mode the liquid line frosted as it went into the building. Only solution was for condenser coil to be replaced and outdoor TXV's and checks.
In your case you mentioned a liquid line temp lower than ambient in what I would assume is cooling mode operation of the heat pump. I'm also assuming your device to measure liquid line temp and the device to measure air temp agree. If perchance there is a liquid line drier installed on the compressor discharge line, or even a liquid line drier installed on the liquid line that's not a bi-flow drier, there may be trash in the outdoor check that bypasses the outdoor TXV (if its a piston, the piston could be fouled as well). This trash would cause a restriction from that point to the indoor TXV, thereby starving the TXV of almost ALL liquid.
Only thing that would trash that theory is where you measure liquid line pressure. If it's at the condenser service valve and there's a liquid line drier downstream, I'm probably on target. That drier may be plugged beyond belief. If the restriction is upstream of the service valve, seems most everything downstream of it would be closer to suction pressure you gave.
All unless you measure "true discharge" off the compressor vs, the liquid line pressure as liquid leaves the condenser in cooling mode. Some heat pumps have the ability to measure "true suction" and "true discharge" by providing schraders on the common suction leading into the compressor from the reversing valve and the discharge line off the compressor back into the reversing valve.
I'm kind of thinking out loud...hopefully there's something here that can help you.
[Edited by shophound on 06-18-2005 at 06:55 AM]
06-18-2005, 07:59 AM
15 degrees of subcooling and 96 degrees of superheat. That has to be obvious. An almost closed dishcharge valve will cause head pressure to rise high.
Something is stopping that flow between where you measured subcooling. What is subcooling right at LL just before TXV?
Why nobody uses these measurments is absolutely beyond me.
06-18-2005, 10:49 AM
they dont know how to use them. i would check drier, then remove sensing bulb and see if suction pressure rises. if it does then you may have a restriction in the coil if it doesnt replace txv.
06-18-2005, 12:40 PM
Shophound thanks for all the info. I'll be going back to run test later today.
Let me tell you something GREAT.(LOL) This is a GMC Cond.
unit with a Trane Evap. unit. Tell me how this passed inspection.
Anyway I'll post again tonight if I get back out.
06-18-2005, 03:35 PM
I’m assuming you presupposed this system to be a split system based on scrmndmn’s posted “Could the discharge valve be stuck closed? comment, but what posted information did you use to determine this was a heatpump system, or were you just guessing?
Also, with the obviously large restriction problem, shouldn’t scmndmn just physically check between the hot gas discharge of the compressor and the point on the liquid line that he recorded his surface temperature for the anticipated temperature differential that will reveal and confirm where the restriction is located?
And for those of you that are still suggesting that the TXV is the possible location of this restriction, I would point you to scrmndmn’s first posted readings that point out that the restriction is “upstream” of the TXV based on the liquid line temperature reading being three degrees lower than the outside ambient.
Just my option.
John J. Dalton
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