30% efficiency gain on Burnham 206 gas boiler

A quick update on a previous thread about my sister's Burnham 206 atmospheric gas boiler. She bought the house last summer in the Boston suburbs and got slammed with a $6k+ gas bill for 3300 sq ft last winter. I visited periodically through the winter and noticed the boiler room was always hot and the boiler water temp was always 170-180. And it looked to me like the boiler was short-cycling. I suspected the boiler was running inefficienctly, but we didn't know how bad it was until this summer when the gas company sent her a letter saying they had mistakenly billed her for the wrong meter, and instead of a $3k gas bill, her actual bill was over $6k. Fortunately because it was their error, they didn't make her pay the extra $3k.

When I heard how much gas she had used, I ran some house heat loss numbers and felt certain the heat loss should not be bad enough to fully account for the high gas usage. I then did a survey of the heating system and found numerous problems: too many zones, thermostats with differentials set too low, no automatic vent damper, and boiler short-cycling, typically bouncing off the aquastat high limit of 180 F.

With help from the good HH'ers here, I did the following:

1. Changed thermostat settings to maximize differential/swing.
2. Disconnected some thermostats and jumpered several contiguous zones together on the Taco 6-zone controller so those circs ran off one thermostat.
3. Installed a new auto vent damper to replace the one that had originally come with the boiler but then was removed at some point.
4. Installed a new Tekmar 256 reset controller at the suggestion of @Steamhead . I have it set for a minimum water temp of 140 and a max of 160 at outdoor design temp. Those are "first cut" settings that can be adjusted later if needed.

Now in the first month of the heating season here in New England, I have some preliminary results.

Last year's heating season total average usage was 0.40 therms/HDD65 (after subtracting for DHW).

Last year's October billing period usage was also 0.40 therms/HDD65. So that number appears to have been stable across a range of temperatures throughout the winter.

This year's October billing period usage (for the first 3 weeks in October) was 0.27 therms/HDD.

That is a 30%+ reduction in therms/HDD65 (after correcting all usage for DHW).

With the Tekmar 256 running ODR, the boiler stops firing noticeably sooner while the circulators continue running. I'm further encouraged by the fact that I didn't install the Tekmar until 1 week into this 3-week test period. (The actual bill from the gas company will come next week, but I read the meter myself today and did the math for the 3 weeks ending today). So future billing periods in which the Tekmar is running the entire time should be slightly better.

I will continue to monitor future billing periods but am optimistic/hopeful that the 30%+ reduction in therms/HDD65 will continue through the winter.

Thanks to everyone who made suggestions, especially @Steamhead for the Tekmar 256, which is working great.