Heat pump with low suction pressure

[Ford3517]

if the suction line is stopped up,(unlikely,but ya never know) the head pressure is still going to be high because it can't get thru the suction line ,backing up and resulting in high head pressure. When you pump a unit down the compressor and every thing is assumed to be working fine the reason you dont see the head pressure going up when you pump down a unit is because the accsess port is AFTER the shut off valve. It is physics,when the pump runs it is pushing liquid and pulling vapor and any blockage in between said push and pull ports is going to adversly effect pressure. A restriction is usually going to cause the symptoms orriginally described in this thread . Cause if that pump is still pumping and it cant get thru to the pump return , the pressure is going to rise BEHIND the point of blockage and the pressure AFTER the blockage is going to be low. Remember the lines make a loop to an from the compressor being that that is where the magic happens. The whole pump down thing is bad example IMO. I base this on what I said about the accsess port being being AFTER the valve

It doesn't matter that the service port is after the valve. I have pumped systems down with a solenoid valve leaving the service port open the gauge. I didn't believe my teacher when he said a restriction wouldn't cause a high head pressure so I did it and the head pressure did not go up.

A suction line restriction won't cause high head pressure. The condenser will just condense the additional refrigerant. Then as the suction drops the head will also drop.

I agree

On systems without a receiver (or too small of one) a liquid line restriction could cause high head pressure if the line set is long enough after the restriction to cause the condenser to flood.

I also agree, and I am not intending to be rude but I did put a PS disclaimer to note that what I posted was not always the case. Please don't think I am stepping on any toes because I know I don't know everything and I learn a lot just lurking around here.

[PalmettoMaintPro]

I am sorry but I still stand by the physics of it , If a line some thing goes thru is stopped up no matter where it is in the system is going to cause some type of adverse/unwanted operating condition and pressure changes . I am one of those have to see to believe it guys and the pumpdown test just does not make sense to me. A hot gas valve for instance if I am not mistaken traps a small amount of hot gas out of the compressor before it goes into the condenser and stores it behind the H/G valve and then the gas is used during the defrost cycle of ice machines and other types of refrigeration applications. My point is that we are creating a little electrically controlled mini restriction and using it to our advantaged ,but it is controlled and planned and therefore does not create any adverse or unwanted operating conditions. Which IMO an uncontrolled line restriction is going to cause some type of pressure change somewhere in the system. I realize that refrigerant is a strange beast sometimes especailly in large quantities or in elaborate piping schemes where it changes state many times . But physical space is physical space and only so much any thing can fit in a predetermined space . Also I wanna say that I do NOT know it all ,I just think simple things should be kept simple. I am always willing to learn new things and feel like knowledge is the best tool any tech can have. Sorry if I sound"that way" I just feel like right is right and when there is an equation that does not have variables ,there should be none added for the sake of conversation.

[chuckcrj]

I am sorry but I still stand by the physics of it , If a line some thing goes thru is stopped up no matter where it is in the system is going to cause some type of adverse/unwanted operating condition and pressure changes . I am one of those have to see to believe it guys and the pumpdown test just does not make sense to me. A hot gas valve for instance if I am not mistaken traps a small amount of hot gas out of the compressor before it goes into the condenser and stores it behind the H/G valve and then the gas is used during the defrost cycle of ice machines and other types of refrigeration applications. My point is that we are creating a little electrically controlled mini restriction and using it to our advantaged ,but it is controlled and planned and therefore does not create any adverse or unwanted operating conditions. Which IMO an uncontrolled line restriction is going to cause some type of pressure change somewhere in the system. I realize that refrigerant is a strange beast sometimes especailly in large quantities or in elaborate piping schemes where it changes state many times . But physical space is physical space and only so much any thing can fit in a predetermined space . Also I wanna say that I do NOT know it all ,I just think simple things should be kept simple. I am always willing to learn new things and feel like knowledge is the best tool any tech can have. Sorry if I sound"that way" I just feel like right is right and when there is an equation that does not have variables ,there should be none added for the sake of conversation.

Please don't take offense, but you need to study refrigeration theory some more.

Do you ever work on refer systems with receivers? The head pressure actually falls when that solenoid valve on the liquid line closes.

Do you work in the hvac field?

[john.0522]

if the suction line is stopped up,(unlikely,but ya never know) the head pressure is still going to be high because it can't get thru the suction line

The head pressure is low because it can’t get thru sum thing has to get thru for the compressor to pump it’s not; the mass flow rate of refrigerant is decreased through the system.

[luskys a/c]

I am working on a ruud heat pump MN# upne-048jaz with an electric air handler inside mn#uhla-hm48 with R-22 coil mn#rcsa-hm4824au. The home owner called with a complaint of freezing up on hot days with 85 degrees and up. Unit is 4 years old and this is the first hot summer we have had since it's install (not an install of mine). With 80 degree ambient outdoor it has 180# head, 50# suction with 50 degree line temp. We sucked it down and purged with nitrogen and vacuum and weighed in a new charge. Checked the pressure drop across filter drier and all seems to be well there. Replaced txv and no change with that either. Have cycled unit into heat mode in hopes that if there was oil in distributor tubes when in heat mode i do have a slight pressure difference from each port. Any ideas?

You need to take basic steps before you go taking the refrigerant out and changing the TEV. Subcooling would have been the first route to take . 4 years old and no problems till this summer. You know that the ductwork is probably fine. Was the filter ok? Check the a-coil. I am assuming your standing pressure test passed before you continued on with your vacuum? Should have changed your filter drier since you opened the system up. If you tested your TEV then you would already know it wasn't it. Let us know what your subcooling is.

[bob_scheel]

You need to take basic steps before you go taking the refrigerant out and changing the TEV. Subcooling would have been the first route to take . 4 years old and no problems till this summer. You know that the ductwork is probably fine. Was the filter ok? Check the a-coil. I am assuming your standing pressure test passed before you continued on with your vacuum? Should have changed your filter drier since you opened the system up. If you tested your TEV then you would already know it wasn't it. Let us know what your subcooling is.

Line temp 50 and pressure 50# is about 25 degree superheat. If subcooling is good then the problem is TXV or its adjustment. Assuming the measurements were made at the evap coil it doesn't look like an air flow problem. If low subcooling then its a low charge problem.

[PalmettoMaintPro]

The head pressure is low because it can’t get thru sum thing has to get thru for the compressor to pump it’s not; the mass flow rate of refrigerant is decreased through the system.

I know this sounds dumb but I thought about that same thing after my little rant up there. Makes sense though

[Tommy1010]

It doesnt matter what the dry bulb is indoor or outdoor for the coil to freeze. As soon as you said 180 head and 50 suction, my first "guess" was low charge. 50 sat temp is 25-26 = below freezing.

Any moisture, humidity or condensate is gonna freeze on that coil. And unless the tstat satisfies and the fan stays in ON, its gonna keep freezing and spreading.

You head is also too low for an 80° day. I would like to see in the 230 - 260 range. But anything under 200psig for R22 head pressure, either theres an issue, or the darn thing shouldnt even be running.

But no ideas count with limited info giving. SH/SC is a must for this type of question.

Good Luck