1. Have them install valves so you can isolate and drain that loop. 2.Also have you tried letting it run a couple days maybe the clog will break loose by itself after it gets good and hot. 3.Is this loop pump fed or does it rely on gravity? If it has a low point and relys on gravity all the crap is going to settle in that one spot.

Thanks for the feedback. I had also contacted Taco-Hvac, the manufacturer of the unit.

They explained that I did not really have a circulation pump, but a cartridge circulator. They recommend the Taco Series 110 as the solution.

I will probably go that route.

If it were me I would provide a means of isolating that loop and a method of turning the pump off upon start up.Let the boiler run a day of so. After you blow the boiler down a few times to get all the mud out of it and water comes clear,then start up that loop. Your idea is good too.
Regards.

The boiler water is clean. I drain it regularly, so there is no build up there.

Because of the cold snap today in NY, the boiler has been on all day.

I did try leaving the zoned heat on today, but that did not work.

This has been going on for three replacements one year, one year and two years apart.

Seems I can buy the pump myself and have it installed by my plumber for $400 to $450. If it gives me a long term solution, it is worth it.

Sounds good.