The boiler loop sensor is on the boiler loop, between the radiant return and the boiler inlet. It's the only sensor that was in the proper location! (out of three).
You are correct about the pump ramping up and down, this was corrected by a plumber a few years ago by installing a manual valve in the radiant return line. If we put the valve at about 48%, the injection pump runs around 70%, and the boiler runs constantly until the supply loop is brought up to temp.
Put the valve at 100%, the boiler still runs constantly, but the injection pump spins up and down and up and down - never really get a consistant temperature out of it.
Putting it below 40% is a bad idea, the boiler starts to short cycle again (It hits its 180 F high limit and shuts off over and over).
What's the WM Ultra? I *think* the WM CGb Gold we have in the other end of the building is a condensing boiler, I like that thing. Never any issues, no rust, no complaining... Not very powerful though, have to supplement that one with the Trane's furnace when it gets below about 10. Just low BTU output, no mechanical problems (150,000 BTU).
It sounds like your boiler is under fired. Has anyone tried to "tune" the boiler with a combustion analyzer? I'd be willing to bet you have a very low stack temperature. All that short cycling is bad, but I suspect you more issues than that.
Rust and corrosion are caused by two things and neither is low water temperature or short cycling. Water heaters have run with low temperatures for years and they only rust for the same reasons below.
1. Improper venting
Using a match to check venting does not verify you are venting flue gasses. If fact it verifies you are venting room air instead of flue gasses.
I think we may have an answer - at some point several years ago, one of the building maintenance staff thought the room was too cold and blocked the fresh air intake.
This forced the boiler to suck in combustion air from the main building - which is always extremely humid due to snow melting on to the radiant floor, its a vehicle maintenance garage.
So there you go - hot boiler, humid air - isn't that the equation for rust?
I removed the piece of cardboard the first time i saw it because I also saw the automatic damper in the duct...
He also corrected me - our second boiler is a CG-S not a CG-B