Calculating Alde glycol quality with simple annual gravity drains and refills

I’ve read (or skimmed) most everything I could find on changing glycol in the T@B and see a mix of great DIY ideas, high costs from dealers and lot of hesitancy to dive in and try a DIY solution.  Throw on top of that the unknowns about the need to switch to the new Rhomar fluid and it can cause analysis paralysis or just plain denial of the need to do anything – at least it did for me.

After getting through the denial phase and considering the uncertainty of the need to do a complete flush and switch to a new fluid, I thought I could easily handle a gravity drain of the system through the drain plug, and a refill of the system.  I wondered, however, just how effective this would be.  To figure this out, I drained what fluid I could from the system via the rear drain -- managing to remove 6.5 quarts of glycol (2019 T@B 400).  Using a total capacity guestimate of 15 quarts, I calculated that each refill done in this manner would leave 57% of whatever fluid was in the system to begin with and add 43% new fluid each time.   Assuming an exchange of this type each year and starting with 2-year-old fluid, the average age of the fluid just prior to the yearly exchange starts at 2.0 years (prior to initial exchange) and slowly rises to and average age of 2.3 years prior to the 10th annual exchange.

I’m not certain that the average age of the fluid is a precise indicator of its quality, but  the calculations gave me the confidence that I could do a simple drain and refill each year without much risk of damage to the system.  All this, of course, pending any new info on why Alde is recommending a new type of fluid, and pending my need to tinker with pumps, shop vacs, low-point drains and all the other great ideas that are posted in this forum.

For me the partial exchange approach is at minimum a short-term solution that takes little skill or time, and the only cost is the fluid itself.  I’d love to hear any thoughts, especially on the assumption that the average age of the fluid is a reasonable indicator of quality.