Compressor Sluggin-Best Place For P-Trap?

[LibertyTree]

Hi Everyone, we just installed a system with a fairly long (75' or so) 1 1/8 lineset. The evaporator is around 15-20' higher than the compressor. I started the system up and dialed the charge in. After the system was turned off for about 45 min I made a call for cooling and went out to the heat pump. The compressor was trying to slug liquid. I'm guessing the oil migrated over the 45 min. I'm fairly certain the fix for this is to install a P-trap. Where is the best location to put the trap???

[timetable15]

at the bottom of the riser

[hvacrmedic]

If oil migrates to the compressor, then that's always a good thing because that's where it's needed. Oil traps are designed to help oil return to the compressor, not to prevent it from getting there. It's more likely that you're talking about flooded starts, which are due to migration of liquid refrigerant into the compressor during the off cycle. With the evaporator above the compressor an inverted trap is required at the evaporator outlet. It involves running the suction line up higher than the top of the evaporator coil before running it out and down to the compressor. The inverted trap prevents gravity flow of liquid refrigerant out of the evaporator. You don't need an oil trap with your configration, since it would be counter-productive to oil return.

[joemach]

Just thinking out loud, but would a liquid line solenoid help? Like just after the condenser?

[timetable15]

good catch medic

[Thorazine]

crankcase heater working?

[HVAC_HUSKER]

i believe every 25 ft. vertical a p-trap must be added i may be wrong but for some reason in my head i remember learning this. please feel free to correct me... you can always contact the manufacturer and ask there recommendations they should help you with that.

[mark beiser]

With that long of a line set, you need 3 things to prevent liquid migration.
1) An inverted trap at the evaporator, like Hvacrmedic said.
2) Either a hard shutoff TXV at the indoor coil, or a LL solenoid.
3) A crankcase heater.

Any time the evaporator is higher than the condenser, even a little, the suction line should exit the coil and go up above the level of the evaporator before going back down to the condenser.

With the evaporator above the condenser, no p-trap is ever required, or useful. All it would be is a useless pressure drop.
Even in applications where the condenser is significantly higher than the evaporator, p-traps are going out of style.

[JKopp]

OP, you did not state the metering device type piston, TXV?

[LibertyTree]

Thanks for the replies fellas. Isnt the suction side supposed to be pulling in vapor not liquid? I have always thought that liquid in the suction line was hard on the compressor, which is why you cant dump a bunch of liquid refrigerant in on the suction side of the system while charging. The lineset does go up about 3" before going horizontally to the ODU location but the compressor is still getting inundated. Could it be because of the long length of the large 1 1/8 lineset holding a few pounds of refrigerant? Maybe there's enough liquid in their to give the compressor a hard start?

[LibertyTree]

The unit has a factory crankcase heater and I have an inverted trap already. How does the liquid line solenoid work? Closes up the LL when the unit is not running I would assume. How would this prevent liquid migrating through the suction line? Thanks

[saintmichael36]

The liquid line solenoid would pump down the unit and trap all of the liquid refrigerant before it hits the low side of the system.


The Day You Think You Know Everything Is The Day You Stop Learning.

[SouthTex]

You said this was a heat pump outdoor, would not use a solenoid with a heat pump with out some specical consideration for the heating cycle. Is there a suction acculator installed between the reversing valve and the compressor? May be an option

[joemach]

You said this was a heat pump outdoor, would not use a solenoid with a heat pump with out some specical consideration for the heating cycle. Is there a suction acculator installed between the reversing valve and the compressor? May be an option

I have systems I maintain that are heat pumps with long line sets with a LL solenoid that have run for years with no problem.

I think Mark B. nailed it on what is needed.

[mark beiser]

The liquid line solenoid would pump down the unit and trap all of the liquid refrigerant before it hits the low side of the system.

No, it would be energized and de-energized by calls to Y to the outdoor unit, so it will close when the system cycles off, and open when the system cycles on.
It simply stops the flow of refrigerant when the compressor stops running, no pump down.

You said this was a heat pump outdoor, would not use a solenoid with a heat pump with out some specical consideration for the heating cycle.

There is no consideration needed, other than making sure the LL solenoid is a bi-flow type.

In a long line set application, with the majority of the liquid line above the condenser, the LL solenoid should be installed within a couple of feet of the outdoor unit.
In addition to reducing refrigerant migration to the low side, it prevents the refrigerant in the liquid line from draining back to, and filling the condenser coil.
When the condenser is filled with liquid there can be some really nasty spikes in discharge pressure, and extreme vibration and compressor noise when a compressor starts up, especially with scroll compressors.

[joemach]

Now that makes TOTAL sense. That is exactly what I was thinking, for some reason just could not get it formulated into words.

Thanks Mark, at least I know I was going in the right direction.

[bunny]

Bi directional solenoid valve would do the trick.

And no need for a p-trap if the evaporator is HIGHER than the condensing unit.

And compressor was "trying to slug liquid"....what on earth does that mean???

[joemach]

Bi directional solenoid valve would do the trick.

What is a bi directional solenoid valve?

Never heard of such a thing?

[LibertyTree]

Cool thanks guys (especially Mark Beiser) for the information and explanations. I will throw a LL solenoid in tomorrow. One last quick question, I am going to pump the system down and I'm wondering how many pounds of refrigerant you can jam into a 5 ton compressor. The reason I ask is because I added several pounds to the system to get the charge dialed in, the factory charge is about 15 if my memory serves me. Are these things built to hold 18-20 pounds of refrigerant in a pump down? Also, I'm not 100% sure how much is in the system, I think there was a loss of charge somewhere while it sat in the warehouse for years. I did not hear a refrigerant rush when I cracked the valves and it was way undercharged after adding refrigerant for the lineset length.

[mark beiser]

Since it is a heat pump, make sure you get a bi-flow LL solenoid.

With your lineset length, you won't be able to pump the entire charge down, as there is more refrigerant in the system than can be stored in the condenser.
Use the system to push some out into a recovery cylinder, then pump the system down.

Also, if the system does not have a TXV at the indoor coil, I'd highly recommend installing one, preferably a non bleed type.

A non bleed TXV will help prevent migration to the low side, as well as increase cyclical efficiency in the cooling mode.
LL solenoid at the outdoor unit prevents liquid migration/drain back to the outdoor coil.
Crankcase heater prevents migration to the compressor.
Inverted trap at the indoor coil prevents liquid refrigerant, that condenses in the indoor coil, from running out all at once and slugging the compressor when it starts.

These are the things that will help a system with long lines live a long and happy life.