My Mother-in-law (MIL) calls to tell me she has no heat.
I go over to her place, and while checking things out my wife calls and tells me that the MIL has a "worry-free" contract with the gas co. I say "Good, call them - I'm coming home".

My wife meets the gas co. man today...he quickly finds a burst pipe on one of the baseboard heating units. He also says that the expansion tank needs to be replaced ~ both are NOT covered by the contract. I tell my wife.. "I'll fix the baseboard and we'll see what happens"

So I remove a 5' hydronic baseboard unit and replace and re-pressurize the system.

3 hours later, I head back to check that all is well.
Nope.
The baseboard are not getting hot. The boiler IS hot (180 +/- degrees), pressure seems good (15psi +/-).

Now I am left with a few questions:
1 - Is is the circ. pump that has decided to take an unapproved vaction?
I do have 120V across C1 + C2, so it seems to me that the relay and t-stat is working fine. I didn't have an amprobe to check for the .70A the motor plate states. Having power on C1/C2 also leads me to belive the control transformer is working "ok".

2 - Is it one - or both - of the 2 zone valves on the system?
The boiler supplies heat for 2 units in the 3-family apt. building the MIL owns. I could see one valve going, but not both at the same time.

3 - What role does the exp. tank play here? If it were broken, would that prevent the hot water from getting to the baseboards?

I am heavily leaning towards the circ. pump being bad here.

Any help folks?