Charley B or David A

Or anyone else who might know the answer… :slight_smile:

I had a system yesterday (hack job overall) where the lineset was installed through the attic. The suction line (large diameter) was kept as short as possible, while the liquid line (small diameter) had about 15 extra feet coiled up in the attic before it connected to the roof-top mounted condensing unit.


What (if any) effect will there be on a system with unequal lengths of liquid-line and suction line?

Good question. Would like to know the answer as well…

Pretty good question for your 10,000th post! I’d also like to know the answer

There is no effect on the length of the two lines being different per se.

The primary concern with excessive length of liquid line is that it requires an increased amount of refrigerant to properly charge the system. It is imperative that we keep the refrigeration charge as small as possible, whenever possible. I’m not going to go into all of the potential issues unless we have an issue to discuss. Just take my word for it, the less refrigerant the better.

The most likely issues you may encounter are:

The installation design with the condensing unit on top of the roof (with an excessive liquid line refrigerant charge) can be problematic because of the elevation between the indoor and outdoor components. Excessive refrigerant and excessive elevation changes cause head pressure issues. Depending on the type of metering device in the evaporator, this refrigerant could flood the low side coil during the off cycle and potentially overload the compressor at start up. Most metering devices have an equalization feature that allows the valve to “leak by” and equalize the high and low pressure components to the same pressure. Once the pressures are equalized, the increased head pressure of the liquid refrigerant line (due to elevation) and its increased volumetric capacity may cause liquid to fill the evaporator coil on the low side of the metering device.

The location of large amounts of liquid refrigerant in an attic can also be problematic depending on the temperature of the ambient attic air. We want the liquid line to cool off as much is possible before it makes it to the evaporator metering device. When it sits around in the attic it picks up heat higher than the outdoor air temperature which is at the condenser saturation temperature/pressure. This can cause the liquid refrigerant to flash into a gas (before it’s time at the metering device) which causes surging and potential valve erosion in the metering device. This also reduces capacity as the liquid refrigerant that passes through the metering device must flash off and cool the rest of the liquid refrigerant to the evaporator saturation temperature. This uses up a larger amount of refrigerant and reduces the evaporator capacity. “Change of state” between liquid and vapor refrigerant is where the system picks up its heat. We want this to happen inside the evaporator, not before or after it.

If we identify problematic conditions such as this, the fix will be to reduce the length of the piping (obviously), but in occasions where the lines are short as possible and it still happens, a solenoid valve is utilized at the lower elevation that closes off the liquid refrigerant from the evaporator when the system shuts down (not a pump down application, just a “shut off” application), or (but less likely) a thermal expansion valve without a pressure equalization port can be utilized.

Hope this helps…

Ha! Hadn’t noticed that :smiley:

Yes, it does. Thank you.

The entire installation was pretty poor-quality workmanship, this was just one indication that it was installed by someone other than a “professional.”

I knew that the lineset should be as short as practical, if for no other reason than the extra refrigerant that would be necessary. The basic physics behind hydraulics told me that the length difference shouldn’t make much difference, but I just wasn’t sure.

Thanks Dave, I learned something new today!

I agree with the most part of what David is posting when ever the condensing unit is above the evaporator poses its own set of problems not only with refrigerant but also on the oil return to the compressor which would require a P-trap in the suction line.

MFG do not have a problem with what some might consider excessive distance between condensing unit and evaporator in order for this to be accomplished MFG will list the distance in feet and the size of the liquid line and the suction line diameter increase for the line set. When ever the line set is increased in size additional refrigerant must be added to compensate for the distance. MFG do not state how many oz/lbs of refrigerant has to be added in the initial start up to compensate for distance the HVAC tech makes this determination.

As David Stated liquid migration back to the evaporator during shut down can be a problem over time. My thinking concerning the extra coils of liquid lines in the attic was the installers frugual attempt to prevent liquid back to the evaporator during shut down. A simple pump down install would have prevented this