laars jvs minitherm pilot but no burner

TSV_Skibum

I have a Laars Minitherm JVS.  I have a pilot but no burner when the t-stats call for heat.  It appears, the cut off circuit (flame roll-out , blocked vent , hi-limit) has continuity but I am not able to measure voltage anywhere.  In addition I jumpered the entire protective circuit and still no burner and no function of the automatic flue damper.  It appears the pumps are working and t-stat side is also functional.  I  

Charlie from wmass

time to call a pro

jumping out the safety equipment is a bad idea even for a test. With the proper tools a decent pro will beable to get to the bottom of this for you and get the proper parts.

TSV_Skibum

noted, thank you

noted, thank you

Plumdog_2

Check the fuse

Inside a  little black plastic holder (to the left of the transformer and relay) there is a fuse. If it blew, replace it with an identical one. The vent damper must operate before the firing cycle. You can manually open the damper (gently), and either place a switch in manual position or relocate a jumper wire inside the damper motor cover to provide temporary heat. Put the safety string back exactly as it was! That type of boiler can and WILL clog up and spill products of combustion (Carbon Monoxide). They have a recommended cleaning schedule.

TSV_Skibum

checked fuse

the fuse was good, I defeated the safety string while I was there with no intention of running for more than a few seconds to isolate the problem to the safety string. After the test all  safeties were returned to the  as designed configuration  



Subsequent to this posting, I found the problem to be the hi-limit switch. The factory recommends its setting be 190 degrees, I found it to be set somewhat lower, about 140.  I reset the dial to the factory setting and the unit worked.  What bothers me is why the as found value was 140 in 1st place.  I am watching the operation closely, everything seems to be nominal.