Hot Water Boiler Flame Sensor

Old 11-20-07, 09:41 AM
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Hot Water Boiler Flame Sensor

Last year I was having a problem with maintaining operation of the gas firing. I had noticed that if I touched the flame sensor where it connects into the gas valve that I could get it to work for a while more (days / weeks). As time progressed, through trial and error, I had to tap this area more and more and harder and harder to get it working. Looking at the schematic, I noticed that it was a single pole / double throw switch operated by the expansion of mercury within the cappillary tube. Naturally it gave up the ghost at the very worst time and I verified it with a meter and a propane torch that it would no longer work. I read somehwere that these devices have a life span and eventually leak and become defective requiring replacement.

My new one from last year had worked fine through the heating season but this year stopped after just a month of operation. I found that although it didn't work when just the pilot light was touching it, it did function when there was more heat applied.

I temporarily installed a double pole light switch in it's place to verify that the failure was solely caused by the flame sensor and it bore it out.

Could my mercury bulb be just not placed properly with respect to the pilot flame. The replacement wasn't exactly the same as I had- there was a different mounting method and I didn't notice exactly how the origianl was installed before removing it.

Any thoughts????
Old 11-20-07, 02:47 PM
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Make sure the flame really is up against, even engulfing the thermocouple. Unless it is, you'll never know anything. Also make sure the nut that connects it to the gas valve is not loose at all. In fact, I'd try backing it off, and then retightening just to make more certain of good contact.

If you have a mercury flame sensor [according to your 5th and 6th lines of your post, it sounds like it], read this: (Pretty enlightening)

Last edited by ecman51; 11-20-07 at 02:59 PM. Reason: added found info

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