Arkansas HVACR NewsMagazine September 2025

HVACR NewsMagazine September 2025

Tech News

and heat and see if that will free the contaminants and catch them in the line drier. Otherwise, the piston should be removed and inspected before it is cleaned or replaced. Then, you would install a new line drier. 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 the proper bore size and signs of contamination around the outside or near the seal surfaces of the piston and the housing. In some cases, a heat pump piston may fail to unseat fully 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 no drop. For example, if you are running a system in cooling and 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, you can resolve this by switching back and forth from heat to cool a few times, but it will often require disassembly and inspection. Piston Bypassing (Overfeeding) Opposite Mode Piston Restriction

This condition is similar to what happens when an external TXV check valve fails.

In Closing

A piston is a simple little hunk of brass, and 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 could be the wrong size, or maybe something got into it and then 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 pistons in it, in case they 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. This article was furnished by Brian Orr and HVAC School For Techs by Techs. It is an excellent article on a subject we take for granted. Whether learning or reviewing, Bryan’s articles are valuable . We appreciate them sharing their excellent articles and encourage our readers to subscribe to their site at https://hvacrschool.com/start/ — Bryan

NewsMagazine Contributors

Bryan Orr https://hvacrschool.com/start/ Craig Migliacco, https://www.acservicetech.com Ruth King, ruthking@hvacchannel.tv Tom Turner, Air Evangelist Kirk Pierce, tpierce@summitutilities Plus Others