Circulator on In Floor Radiant Stops Circulating

MRC

On a service call last Friday. American Standard Boiler. 1350 sq. ft home built in 1957. Has a copper tube in floor radiant system. On arrival the circulator (Bell & Gossett wet rotor) not running. Voltage to motor was 120 volts.
Condemned the circulator and installed a Grunfos UP15 (wet rotor) circulator. Started system back up and was cleaning up when after about 10 minutes the circulator stopped circulating. A check of amperage indicated that the motor had not shut off. The motor is rated to run at .6 amps. When I first restarted system that was the amperage. Now it was only .2 amps. A few minutes later water temp reached the high limit setting (180) and shut off the burners. I shut the system off. This boiler has its circulator in the return just ahead of a plunger type purge valve. I found this valve was not in the normal position but not in the purge position either. Somewhere in between but probably more in a purge position. I could not get the plunger to move out of this position at first but after lubricating the stem with wd40 I was able to get it to retract to a normal open position. I thought this surely would solve my problem. On restart the circulator started and the water temp dropped quickly indicating that water was again moving thru the boiler. It ran for approx ten minutes this way. The boiler water gauge showed a consistent 140 degree temp. Then the water temp in the boiler started rise quickly, the amperage draw dropped to .2 amps and the burners cycled off on high limit. The only way it seemed to get the water flowing again was to wait a few minutes then apply voltage to motor. Sometimes it would circulate other times not.
The only other work on this system that I am aware of is in February of 2013 I replaced a water logged expansion tank and replaced an automatic fill valve.
Could it be the circulator is becoming air bound? The boiler drain valve does not open so is difficult to power purge the system. Could there be a restriction in the loop?

Stumped...need some advice.

Mike

Ironman

Most likely air bound. It could be that there's a leak in the floor piping. I would recommend that you isolate it and see if it holds pressure.

GW

Stupid question time, are you losing your neutral somehow?

hot_rod

maybe pull the motor from the old circa, see if something is causing the rotor to stick.

If in fact the rotor is spinning freely, sound like an air lock or frozen lines perhaps?