aquastat settings for two-boiler setup

RonV_2

My system has two boilers feeding one primary loop. I'm having a difficult time getting the boilers to run so that boiler #2 is the primary and boiler #1 is the backup for heavy loads. The problem is that boiler #1 sees the cooler return water first, while boiler #2 is downline and gets water that boiler#1 has already partly heated. So they are really setup in cascade. My goal was to have boiler #2 run most of the time, and boiler #1 come in only when under heavy load, but the best I've been able to do is to get them to both start and stop at about the same time. Aquastat setting for boiler #1 is 168degF with a 30 deg differential while #2 is setup for 183degF with a 13 deg differential.

I don't want to push #2 much higher, prefer to keep it around 180deg, but can't push #1 any lower, can I?

After watching these things run for hours, I don't believe that #1 is actually using a differential of 30deg because I never see its internal thermometer nor the loop thermometers ever get anywhere down near 140deg. Seems like it starts up at about 155 to 160.

Is there anything I can do to get closer to my goal?

By the way, its dead simple to get them to run the other way around with #1 as the primary and #2 backup, but #1 is giving me some trouble lately (airflow proving switch momentarily cutting out on lengthy runs) and I want to run it less.