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[Corngrinder]

Not air bound (I call it vapor lock).  I've taken everything apart.  Every line from A -> Z (except the 2 that I'm 100% sure are not blocked). 

I can get flow from the mixing valve to my DHW plate exchanger.  This has been 100% verfied.  My DHW plate HX is NOT blocked and it gets flow all the way through it.

Next is my water > air HX.  I opened both ends.  Blew through it, it was not blocked.  I blew water out, then I could physically feel air going through it.  When I hook the line back up between the DHW HX and the water > air HX, I don't get flow. 

Anyone have any ideas now?  I'm leaning towards a weak pump.  Does this make sense?  The distance between my two HXs is only about 3'; it will make it through one, but not the other (and I'm 100% sure it isn't blockage.

[Crow]

 Did you see something obviously wrong with the t-stat when you took it out? If you try to run the system with the stat out I think the water will take the path of least resistance which is not through the exchangers.

 If I looks ok you can experiment with it by placing it in a pot of water and start to heat it up. If you have a thermometer you can see what temp the stat opens. You should see a change in the 140* range.

 If that checks out you may have a pump issue. I know it's only a few years old but the impeller may be worn down ( furnace filled with hard water) or maybe even loose on the shaft??

[Corngrinder]

This is like a giant game of Clue...

I'm fairly confident the pump is not the issue.  I can open a port and water blasts out with as much force as I've ever seen.  The boiler was also filled with softened water and then treated with corrosion inhibitor.

However, I like what you said about "the path of least resistance."  It's leading me towards something...

With my T-stat broken (and now completely removed) the mixing valve is open.  Ergo, it has 3 "out ports" and 1 "in" port.  I'm thinking, perhaps, boiler water is going into the valve and then dead-heading itself again the top 2 "out" ports and then just looping down to the intended "out" port back to the OWB.

Here is a link to a Central Boiler brand mixing valve: http://store.myheatingoptions.com/thermostatic-valve-kit/

Look at the graphic of the valve cut in half, then imagine it without the regulating T-stat inside.  Do you guys think I might be on to something here?

Would something as simple as a check valve fix this, or do I need to create a bypass loop to force water in the correct direction?

[RSI]

Looks like you would need to block the internal bypass passage inside the housing.

[Corngrinder]

Update to all who might care:

The boiler is back in service!  I replaced the T-stat inside the mixing valve, and viola... magic.

[Roger2561]

Update to all who might care:

The boiler is back in service!  I replaced the T-stat inside the mixing valve, and viola... magic.

Congratulations!  I'm glad it's back in service and it wasn't an expensive fix.  Where did you get your new T-stat for the thermostatic mixing valve?  I've begun my 6th season my e-classic 1400 and I'd like to have a spare on hand for the "just in case scenario".  Roger

[mlappin]

Update to all who might care:

The boiler is back in service!  I replaced the T-stat inside the mixing valve, and viola... magic.

Congratulations!  I'm glad it's back in service and it wasn't an expensive fix.  Where did you get your new T-stat for the thermostatic mixing valve?  I've begun my 6th season my e-classic 1400 and I'd like to have a spare on hand for the "just in case scenario".  Roger

From the pictures I’ve seen of em, almost looks like a standard automotive thermostat with a rubber ring on it, kinda like Cummins uses in the 5.9’s.

[slimjim]

Sometimes things get overthunked, it is usually something mechanical and personally the mixing valve would have been my first guess, I would keep a spare on hand or eliminate it all together!

[Corngrinder]

I bought a new mixing valve from my local dealer (~$50) - opened it up and took the T-stat out and put it in mine.  I don't think you can use an automotive type due to the dual temperature settings inside the mixing valve.  Although, automotive type T-stat gaskets work in a pinch.

We're planning to build a new house on the property in a few years.  I'll definitely be creating bypass loops for mine in future... I'm actually very excited about being able to re-plumb my entire set-up now that I've fiddled with it a bit.  It's going to have so many valves and loops and bypasses, I won't even know what to do with myself.