Short cycling Weil-McLain GV90+4

wonderingY

We just completed installing a GV90+4 condensing boiler on a cast iron radiator system.  It is piped primary secondary.  Our problem upon start up was that the boiler was short cycling.  The boiler piping return was getting blazing hot after about 10 minutes of run time and then cycling the burner off.  It eventually heated the loops but this seemed very odd.  The return boiler piping was getting hot and the reurn cold.  Upon the burner shut down (apparently on its aquastat high limit) the temperatures would immediately flip flop back to "norma".  Boiler supply hot and return cooler.  And I'm there scratching my head for the first time in a long while.  It seems to me that there is something going on with the two internal circs, but I cant figure out what?? ANY INPUT?

JStar

Sounds like...

...the boiler pump is shutting down on boiler temperature, but the system pump is still running from the switching relay or other control from the thermostat. When the boiler pump is off, you pretty much have a "standard" system, with one pump. Primary/Secondary is going to cause a bit of natural short-cycling because you'll always be bypassing water right back to the boiler. So far, I haven't seen it cause a problem yet.

Ironman

More Info is Needed

Can you post some pics of the near boiler piping and perhaps a piping diagram and what size circulators you're using?

wonderingY

Loose return temp sensor

The plug to the return water temperature sensor was not corrected properly which caused the by-pass circulator to continuously run.  The boiler would cycle off on limit and then the system circ would work properly.  The end.

wonderingY

Loose return temp sensor

The plug to the return water temperature sensor was not corrected properly which caused the by-pass circulator to continuously run.  The boiler would cycle off on limit and then the system circ would work properly.  The end.