Why did the voltage drop to 12 volts and hold there?

Joseph_4

<span>I was working on a  regular type gas steam system w/ Pa 404, flame rollout,float type low water cut off, spill switch and a  honeywell vr8300 gas valve. customer complaint that radiators are cold in the morning. get to the job, see that gas valve not opening on 24 volts (saw meter say 24 volts and not firing). ok changed gas valve. As I'm packing up my tools, see that boiler is erratically kicking on and off. Pull leads off new gas valve and see meter readings now at 12 volts  and holds there for 10 seconds. switches back to 24 then goes back to 12. and keeps happening.(sometimes fluctuated between 3 and 5 volts) so i change transformer. only cost $14 bucks. keeps happening. After lots of testing, I see that the flame rollout switch is weak/ has bad internal contacts.so it was cutting out. NOW for the QUESTION. if any of the safety switches fail shouldn't i get no volts not half volts?  

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Mike Kusiak_2

Not necessarily

A switch can fail so that the contacts have a high resistance  when they should be closed.  When the coil in the gas valve is activated and draws current, some of the 24V is lost in the resistance of the bad switch contacts, so you have less voltage available at the valve.



It is pretty common for switches to fail this way, typically when the contacts get burned or corroded.