Arkansas HVACR NewsMagazine November 2020

S tate, National, Chapter News Tech News

Any carbon, wax or other solid material that gets into the piston housing can cause one of three undesired conditions

For example, if you are running a system in cooling and you notice frost starting to form on the liquid line side of the outdoor, heat mode piston housing, you can be sure it is restricting in the opposite direction. Sometimes this can be resolved by switching back and forth from heat to cool a few times but often it will require disassembly and inspection. This condition is similar to what happens when an external TXV check valve fails. A piston is a simple little hunk of brass, it drives me nuts when a tech incompletes a call so that someone can “replace a failed piston”. A piston doesn’t just fail, if one does have an issue it’s either the wrong size or something got into it and got stuck in it or caused it to stop seating properly. Many of these issues lead back to improper vacuum, failing to flow nitrogen, getting copper shavings or sand in the system etc… Every good residential tech should have a little plastic container with various brands and sizes of piston in it in case you find one that is the wrong size or worn down from improper seating. I may be a little late to the game here since pistons are a dying breed but they are simple enough that a return trip for a “failed piston” seems like a huge waste. In Closing

Piston Restriction in the Desired Mode

If something gets into or covers the orifice bored into the piston, it can cause a restriction resulting in low evaporator pressure, low suction, high superheat and normal to high subcool. When a piston is restricted and the system is a heat pump with a liquid line filter/drier properly installed, we will often alternate the system into cool and heat and see if that will break free the contaminants and catch it in the line drier. Otherwise, the piston should be removed, inspected and cleaned or replaced and a new line drier installed. Keep in mind that some systems have a screen built into the piston housing inlet that can also block up. Look for this once the piston housing is disassembled. If the piston fails to seat properly it can overfeed the evaporator in the same way it would if the system had a larger bore size than it should. This will result in high suction pressure, low superheat and low subcooling. In these cases, the piston should be removed and inspected for proper bore size and signs of contamination around the outside or near the seal surfaces of the piston and the housing. Opposite Mode Piston Restriction In some cases, a heat pump piston may fail to fully unseat in the opposite mode. This will result in a pressure drop and an undesired restriction similar to a clogged liquid line filter drier. In this case there will be a clear temperature drop across that piston when there should be little to none. Piston Bypassing (Overfeeding)

— Bryan Bryan Orr, HVAC School ( Founder / Podcaster / Dad of 10 ) Bryan Orr is a lifelong learner, proud technician and advocate for the HVACR trade