9% efficiency gain from thermal post-purge of cast iron boilers?

In my ongoing quest to improve the efficiency of our two Weil Mclain WGO-5 boilers, in November 2024 I installed a Resideo L6006 aquastat on each boiler, attached to the supply pipes coming out of the boilers. I wired them to continue supplying power to the circulators after boiler shutdown, until the supply pipe temperature drops to 90 F. Since each WGO-5 weighs over 700 pounds dry, there's quite a bit of residual heat stored in each boiler after shutdown, and my hope was to recapture some of that heat, which otherwise would go up the flue during the typical 2+ hours of idle time after a cycle. (These are cold start boilers that always start cold, run maybe 45 minutes, then shut down and idle until cold again).

I was hoping for maybe 5% efficiency improvement at best, and I was prepared to be disappointed by no measurable improvement. But after crunching the numbers for the first time tonight, after one partial heating season (2024-25) and one full heating season (2025-2026), it appears that the post-purge may be working better than I expected.

I've kept detailed consumption records for the past 7 seasons, and I have heating degree data from our local airport (Norwood MA, weather station KOWD). Taking the ratio of oil consumption per season total HDD for each year, for the two seasons prior to doing post-purge, the ratio was 0.236 gal/HDD for both previous seasons, a remarkably consistent result in consecutive years.

Then in the first partial season with post-purge (beginning late November 2024), that number dropped to 0.228 gal/HDD, a reduction of 3%.

Then last season, the first in which post-purge ran the entire season, that number dropped to 0.215 gal/HDD, a reduction of about 9% from the 0.236 gal/HDD baseline from two prior seasons.

Last season was also the coldest season in my 7-year database, with 6145 HDD's, which is almost 30% more than the two prior "baseline" seasons without post-purge. My understanding is that milder seasons typically result in lower BTU/HDD ratios, because solar heating does more of the "work" in milder seasons, resulting in lower BTU/HDD's. Conversely, colder seasons typically result in higher BTU/HDD ratios because the solar heating factor is relatively smaller. So in the absence of post-purge, I would expect last season's colder climate to have increased our gal/HDD number. But with post-purge running, it came in 9% lower than the two previous "baseline" seasons that had milder weather.

Of course other factors can be involved, but since I manage the building maintenance here I keep a close eye on whether people are leaving windows open, etc, during the winter, and we haven't made any envelope changes (in windows, insulation, etc) that would make any difference in the last several years. The only difference was the addition of post-purge.

I have fooled myself enough in the past to be cautious in how I interpret these numbers, but right now it looks like we got a 3% improvement in gal/HDD during the first partial season running post-purge, and a 9% improvement during the first full season running post purge. Since our annual oil bill runs about $5000, a 9% savings is $450. I think the two aquastats cost me $300 total, so if my math is correct, they've already paid for themselves.