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  1. #1
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    The most common complaint I get in my repair business is that the pilot light is lit but the main burner does turn on and stay on reliably when the electric toggle switch is turned on.

    Of course, there are lots of possible causes of that problem, the most common being a dirty pilot burner/orifice.

    But the second most common problem in my experience is a failing or defective toggle switch.

    One difficulty is that this is often an intermittent problem. When you get there to diagnose the problem, the fireplace switches on just fine, or the problem is so intermittent that you don't get a chance to observe the fireplace in a failed condition.

    So what to do?

    The most powerful check is to measure the milivolt loss across the circuit that energizes the main burner valve solenoid. These are the TH/TP and TH terminals on Robertshaw and many other milivolt gas valves.

    According to the specs packed with Robertshaw gas valves, voltage drops of up to 100 MV are acceptable, but in general I would want to see far lower voltage drops or know the reason why. Understanding the voltage drop you are getting across that circuit is one of the fundamental skills needed to effectively repair gas fireplaces of this type, in my opinion.

    In general, a MV drop of 15-20 is generally an indication that the circuit is working properly. Voltage drops of 30 MV or more require an explanation, especially when you have an intermittent failure of the burner to light and you can't observe the problem directly.

    Thye condition of wire nuts, wire connections to terminals, terminal screw tightness, wire gauge and length all need to be checked. I usually physically move the wires around to see if I can induce a failure of the burner due to some bad connection.

    Inspecting wall switch connections is the next things to check. Fairly often, installers use push in wire connections with small gauge wire, which are unreliable and prone to failure.

    24 volt thermostats are fairly common problems, since the heat anticipator often consumes a lot of the voltage. You may be able to turn the heat anticipator down to minimize this proble, although the thermostat probably doesn't track the temperature especially well when the heat anticipator isn't properly energized with 24 volts.

    If I don't find any other weaknesses, I'll usually replace a wall switch or toggle switch with a voltage drop of 30MV or more. I find this is an indication of a weak switch which at other times will have a much higher voltage drop and cause a failure of the burner to light.

    When the switch is causing a burner to fail to ignite, the voltage drop I find is often 100-250 millivolts.


    Usually this procedure solves the problem and avoids call backs. I've read through discussions we've had here on what switches people prefer to use. Personally, I don't use switch with silver contacts or whatever, I replace a bad switch with another regular wall switch for a $15 part charge, the part and labor guaranteed for a year.


    This procedure has worked well for me. Again, I'm only addressing switching problems in this post, not other causes of this kind of problem.



    Seattle Pioneer





  2. #2
    Join Date
    May 2005
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    Good way to test for a bad switch is to hook up your multimeter with gator clips or something and then go poke the switch all over with your finger. With a solid switch the MV wont fluctuate very much or not at all, if it jumps all over the place the switch is probably too worn out for the MV circuit.

    Also you can take the leads from the switch off the valve and hook the meter up to that set in impedance (OHMS), turn the switch on an off a few times and check the readins, the closer to 0 the better.

    One other thing to do is turn the firpelace on with the wall toggle and then gently tap or poke the wires and connections underneath, if the MV moves a round a lot or the FP turns off something is loose. A lot of times the fireplace will turn off when people walk hard on the floor, the vibration is just enough to jiggle the wires and cause it to go out and then come back on again in a second or two.

    When I get any fluctuating readings with any of these things I usually replace all the connectors underneath and make sure they are all tight.

    On a side note I just remebered this... we had a customer call a few weeks ago and said his stove worked fine until he jiggled the wire behind it. My question is... why wre you jiggling the wires then? That like saying my arm only hurts if I do this, as you bend it back behind your head in a way it was not meant to be bent.

  3. #3
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    Thread Starter
    Good additional comments/test ideas there, jtp 10181.

    I've never bothered measuring the resistance of switches directly myself. Even with a bad switch the reads are too close to zero to be useful in diagnosing a bad switch in my experience.

    So, what kinds of reads do you get with good switches vs bad switches?

    Personally, I've found the voltage drop across the switching circuit to be a useful and sensitive test as I described.


    But I'm willing to be convinced that an ohms test is useful. Can you describe in more detail the reads you typically get and how you use them to discriminate between good and bad switches? And do you consider this test to be better than the millivolt votage drop test I described?



    Seattle Pioneer

  4. #4
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    Usually the OHMS and MV Drop tests are about the same, the more ohms, the more MV drop you get. But by disconnecting the switch from the valve and everything else you eleminate all other variables and can test JUST the switch without any other factors possibly effecting it. Usually any reading not right close to 0 means the switch is junk but they will still turn the fireplace on even up to 0.050. I do the poke test and if the OHMS junps all over the place then thats probably the cause of ittermitant switch problems.

  5. #5
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    Typically a wall switch will begin to create high resistance over a short period of time due to the contacts inside becomming corroded. What I have found that seems to work is that if I find high millivolt loss thru the switch itself, I will flip the switch on/off several times, 20or30 times very rapidly, and this usally will drop the loss back down to an acceptible reading...under 20mv. While we're discussing switches here,I was wondering about automotive type toggle/rocker switches. They are rated for 12V so would they not be considered a "low voltage switch" ??? I recall going to a no heat fireplace call a couple years ago and the lady had a gas insert and the main burner would not come on. After doing all my voltage checks I found that the rocker switch on the side of the unit was faulty so I ran down to the local automotive supply house and bought her a new rocker switch. The switch fit perfectly into the unit and I had 0 voltage loss thru it. Sorry for the long post, just thinking out loud again.

  6. #6
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    Thread Starter
    Hello blk----

    Very good idea on using automobile switches, I've never thought of that. I'll have to check out what may be available. Of course, you'd have to check the rating and see what voltage they are designed for ----some may be no different than the typical 120 volt switch.

    But I'll check it out. Thanks for the idea.


    Seattle Pioneer

  7. #7
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    No problem SP, I believe they are rated for 12V and not much more, but I could be wrong. The only problem I can see using them is that they only come in black and most homeowners may not be too fussy about a black colored toggle switch sticking out of a white/cream colored wall plate, although it would make identifying which switch operates the burner and which one operates the fan easier! Let us know if you find something that is usable.

  8. #8
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    Switch resistance: Mechanical vs Digital thermostats

    Thanks for the tips. Regarding "turning the heat anticipator down": on the Braeburn 505 mechanical magnetic reed switch thermostat I am looking at, rotating the heat anticipator to the LONGER cycle reduces the overall resistance across the thermostat's W R terminals (the shortest cycle adds an additional ohm or two resistance) and so would reduce the voltage drop over the switch. (on this thermostat, the switched current flows through the coiled bi-metallic thermometer coil; thus, when the bi-metallic coil cools, tripping the switch, current flows through the bi-metallic coil which warms it slightly and the anticipator adds to that resistance reducing current).

    I am interested in this question as it applies to 750 mV thermopile wall furnaces. I am leaning these days toward magnetic reed switch thermostats with no batteries. I have often seen failures with simple battery operated non-programmable thermostats, plus the hassle of changing batteries.

    I recently looked into the supposedly simple Honeywell Round non-programmable manual thermostat", which, at first glance, requires no batteries. But actually the Honeywell Round has a 10-year (?) lithium battery squirreled away on a control board because its internal switching would be power transistors of some sort. Unlike the classic Honeywell Round units which had the extremely reliable and now illegal liquid mercury switch. This really bugs me since I put a Honeywell Round set to 45 degrees in an outside shop to keep pipes from freezing. But now I don't trust it since who knows when that lithium battery will die.

  9. #9
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    Quote Originally Posted by ryeisenman View Post
    Thanks for the tips. Regarding "turning the heat anticipator down": on the Braeburn 505 mechanical magnetic reed switch thermostat I am looking at, rotating the heat anticipator to the LONGER cycle reduces the overall resistance across the thermostat's W R terminals (the shortest cycle adds an additional ohm or two resistance) and so would reduce the voltage drop over the switch. (on this thermostat, the switched current flows through the coiled bi-metallic thermometer coil; thus, when the bi-metallic coil cools, tripping the switch, current flows through the bi-metallic coil which warms it slightly and the anticipator adds to that resistance reducing current).

    I am interested in this question as it applies to 750 mV thermopile wall furnaces. I am leaning these days toward magnetic reed switch thermostats with no batteries. I have often seen failures with simple battery operated non-programmable thermostats, plus the hassle of changing batteries.

    I recently looked into the supposedly simple Honeywell Round non-programmable manual thermostat", which, at first glance, requires no batteries. But actually the Honeywell Round has a 10-year (?) lithium battery squirreled away on a control board because its internal switching would be power transistors of some sort. Unlike the classic Honeywell Round units which had the extremely reliable and now illegal liquid mercury switch. This really bugs me since I put a Honeywell Round set to 45 degrees in an outside shop to keep pipes from freezing. But now I don't trust it since who knows when that lithium battery will die.
    By the way, I note that the Braeburn install instructions indicate that one should turn the heat anticipator all the way in the LONGER CYCLE position for millivolt systems. So that makes sense since the resistance shown on my multimeter Ohms scale is lowest in that position.

    And I think my explanation for WHY heat anticipator set to reduced resistance results in longer cycle may be faulty, so ignore that in previous post.

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