Second opinion on 410A “blend”
Due to mis-installation of my DIY mini-split, one of my connections failed (due to my own negligence) and the 1/4″ tubing blew out just a few seconds after I was basking in the warm air when I first turned on the system. Needless to say, I needed more refrigerant. The new HVAC tech I tried first tried to convince me to dump all the refrigerant and let him re-fill it with 10.1 pounds (and he charges for it in 1 lb increments, so that’s at the cost of 11). His explanation was (1) the units need the exact amount, and he can’t tell with precision how many pounds are missing, and (2) because 410A is a blend, he does not know which elements of the blend were lost when the tube blew out. Item (1) seems a bit suspect (and he eventually added enough to get the performance up, just by estimating), but (2) sounds totally bogus to me. My limited knowledge tells me that a “blend” means “blend,” such that if I have a refrigerant leak, I can just add more refrigerant, and don’t need to worry about whether one component of the blend went missing more than others. Sort of like if I spill half of my glass of blended Scotch whiskey, I can just refill my glass straight from the bottle, and need not pour out the remaining half first to ensure that the blend stays the same. Is there something about the 410A blend that is different than blended Scotch (for purposes of this analogy I mean — I don’t intend to sip the 410A).
Replies
I agree with the tech.
The boiling point of ethanol is about 170 degrees F. Otherwise it'd boil at room temperature.
Why Bother With School?
HVAC guys all learn one thing in school - then they all seem to toss that aside for shortcuts once they're working. Your problem is rooted in too many short cuts.
The original install should have been pumped down to a near-perfect vacuum, then checked to see that the vacuum held. This would have revealed the leak.
When the unit was charged, only the initial fill is based on the weight reccomendations. From there, the next step is to measure your 'super heat,' which will tell you exactly when to stop adding refrigerant.
Again, when the leak was discovered and repaired, the system should have been pumped down, and the refrigerant recovered. This material would have been returned to your system, and additional added, as needed, again referencing the super heat readings.
The new 'blends' are a bit more difficult to work with, simply because each part has different properties. Simply removing all the old and replacing it with new has some merit - depending on how much was lost. Forget textbook boiling points; the components have different 'partial pressures' and are radically affected by temperature differences. One part of the blend will work better at higher temperatures than the other; together they let the system cover the range of expected temperatures.