7956-856 flame out problem

[jacsplace]

I have been troubleshooting my heater the best I can and cannot determine why I lose my flame. I have seen other similar posts that lead me to believe that the flame needs to be adjusted.

Here is my problem.

I can light the pilot just fine. The burner will light off and when hot enough the fan will kick on. After a couple of minutes I hear the dreaded click on the gas valve and then it's lights out. Burner and pilot. Fan stays on until ducting cooled down. I can relight pilot and redo the whole scenario over and over.

I've read your comment to make sure there is 24-28VAC at the GV. There are two posts on it. Should the power be at both terminals all the time? Also need a link to properly adjust the flame. I'm a mech by trade so shouldn't be a big deal to do it. Just would like to see some documentation. Seen that it should be a strong blue flame with a small bit of yellow at tip.

This all seems funny to have to do as the heater worked fine just the nights and weeks before tonight. And now all of a sudden the flame isn't heating the thermocouple?

Looking for your knowledge on this. Great site. Thank you in advance.


Scott

[Robert]

Hi Scott,

I'll help as far as I can, but adjusting the burner flame is a place I stay away from.

It's not that hard, but can become a problem if done wrong and is something I chose to avoid as a DIY repair.


At gas valve, you should read 24-28vac across the terminals at all times that there is a call for heat.


So, you need to see if the voltage stays there when you hear the clicks and everything shuts down.


If it does, the gas valve is possibly bad.


If it does not, then need to locate point of power interruption on low voltage circuit.


Check all limits, remove wires and ohm them to see if open.



Pilot light should cross top 1/2" to 3/4" of thermocouple tip.


Thermocouples are cheap and easy to replace if you feel it may be bad.


If you have natural gas, and 24-28vac stays at gas valve, see if you have 3.5"water column at gas valve outlet (manifold pressure).



Thanks for inquiring,
Robert

[jacsplace]

Mark, Thanks for the quick response.

Check all limits, remove wires and ohm them to see if open.

What limits can I check? I've taken the wires off the "circuit breaker" switch (red button) to bypass it in case it had a bad connection. The fan limit switch doesn't make sense and seems like it turns on and off normally anyway. The switch next to the fan limit switch I'm not sure of. I'm not sure what it does. 2 wires on one post and 1 wire on the other. Has 116vac on the terminals all the time. I'm not sure about this switch. All I can read on it is the "therm-o-disk". No other markings. It's in the hot section along side the fan limit switch.

I can ohm out any others but all wiring looks great visually.


Pilot light should cross top 1/2" to 3/4" of thermocouple tip.

This will be hard to see. It's tough to even light the pilot. The whole gas valve and thermocouple look good externally (like new even), even though I know it must have been there since we bought the place 10 years ago.

I'll start with the former before messing with the pilot unless the flame looks particularly weak.

Brings me to this question.

Can the thermocouple actually cool down during a heating cycle with all the flamage to think the pilot is not lit and shut off the GV? If that is the case then it sounds like a thermocouple problem. If not then I agree on the gas valve.

[Robert]

Hi Scott,

LOL, I'm not Mark . The red button switch is upper limit. Beside Fan

Switch is lower limit.


When it clicks and shuts down, IF you no longer have 24-28vac

at gas valve, immediately turn power OFF and remove wires

(remember where they go though) from limits.


They should always be closed and show continuity.



You can also jumper them and run a complete cycle. If it runs it without shutting down, you know it is one limit or the other, best to replace both at same time.



The thermocouple can stop working if not positioned right and below what I stated gets heated too much.


Thanks,
Robert

[jacsplace]

OK Mark Robert.....just kidding. Sorry bought that.


Just looked at the contraption. I have 24vac after the solenoid click. So we are down to GV or thermocouple. Agreed?

I pulled the TC and it was a little trashed down on the fire end. I cleaned it up and it ran for 4 minutes instead of 2. Being it is a $10 part as opposed to a $125 part I think I should start with that.

Being that there was 24vac still there it basically eliminated everything else. I'm not sure what the low limit switch is for though.

Whatcha think?

Scott, or maybe I'm Mark.....LOL

[Robert]

Hi Scott,

Lower limit is in lower furnace area, upper in upper, they both work as overheat limits.


Upper is mainly for when blower fails.


Lower for overheating period, but they both do basicly same thing.



Yep, try thermocouple first. Make sure flame is crossing top 1/2" and tighten thermocouple handtight plus 1/4 turn with small 4" wrench.



IF you have a call for heat and 24vac at GV, all should be working.


Try t-couple, but aside from a malfunction there, most likely GV.



Let me know if you need a price on anything.



Thanks,
Robert