Upstairs Heat won't turn off!

Hello:

I have an old boiler which heats the hot water tank and 3 zones. The upstairs heat (Z3) won't stop pumping out heat even when all 3 zone thermostats are turned "off" ... Possible check valve issue?

History --- Last week I lost heat in the middle zone (Z2). I called a local guy who was available and $700 later he drained the system and replaced my old red circulator (Z2) with a new green Taco 0015 circulator. All seemed fine, but then a few days ago I noticed the upstairs heat (Z3) was not turning off and in fact was continuously heating up even when all thermostats were in the "off" posiiton.

I began trouble shooting with the Z3 thermostat doing all the standard checks... I even disconnected the Z3 thermostat wires from both the thermostat and the boiler but the Z3 radiators STILL blasted out heat... The only way I can now turn off the Z3 radiators is by by flipping the entire boiler system off using the red master switch --- which obviously turns off the entire house heat and hot water as well.

A close friend, who is a master plumber, thinks the Taco 0015 is too powerful and the original plumber should have used a 007... he feels the new 0015 (Z2) is pushing water back through the check valves of the other two zones...

But this doesn't make sense to me because - as mentioned - Z3 gets hot even when all the zones are "off." Therefore the new 0015 isn't even on when the Z3 is continuing to pump out heat...

For example, after a night with the boiler completely shut down the next morning I tuned just Z3 on for a few minutes until the heat first started coming up, then shut it down immediately... never turn on Z1 or Z2... However, Z3 kept getting hot for hours while Z1 and Z2 stayed cool... And all this while the Z3 circulator was clearly not running... I eventually turned the red switch off to cool everything down again...

I paid for a big company to come check it out and they feel it is a flow-check valve problem for Z3... They want to drain the system and replace the flow check... which stinks because I just paid the first guy to drain the system last week!

So... Wondering if anybody has come by this issue before and what they think about the possible issues... Could the 0015 be too powerful? It's just a flow check valve issue and fixing that should solve the problem? I don't need this expense, but especially if it is likely not to actually solve the problem.

I also can't help but feel like the first guy messed around with the flow check somehow causing this subsequent issue, but that is hard to prove.

THANK YOU for your thoughts.
NICK

 

I believe the check valves I have are spring loaded.