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Flame rod voltage drops after a while - temperature unchanged

Flame rod voltage drops after a while - temperature unchanged

Flame rod voltage drops after a while - temperature unchanged

(OP)
Hi All,
I have got a curious situation where on a start-up the burner goes through the normal sequence ( i.e pilot flame followed by the staged flame ramp up) with no issues.
After about 15 min. the controller reads the ‘full flame’ or ‘full voltage’ (5V) from the flame rod . The furnace reaches the operating temperature in about 30min and from then on everything is stable for a while.
Anywhere between 15 and 24 hours into operation the voltage on the controller ( from the flame rod) drops down to 2-3 V while the furnace temperature remains unchanged (at the operation set point).
By now I have got the flame rod, the controller and the earth ( ground) wires replaced -to no avail. The only item in the loop that wasn’t changed is the flame rod power cable.
As mentioned the furnace temperature remains unchanged (fact) after the voltage drop and the flame as seen through the sight glass appears unchanged too – but that’s subjective.
Any ideas what the problem could be?

Kind regards

RE: Flame rod voltage drops after a while - temperature unchanged

Head scratcher.. But. Actually, the flame rod power cable is a likely suspect to what you're seeing. As the cable temp goes up its resistance is going to go up. That's going to cause a voltage drop that will increase with temperature.

I can see the cable slowly being heated in the furnace space(area) over many hours resulting in what you're seeing.
It will be greatly emphasized at any connections that get warmed up too.

A potential troubleshooting effort would be to cool the cable over it's length, also focusing on any connections.

Not seeing the cable and its layout and situation I can't suggest how you might cool it. Not sure if crushed ice in a baggy would work or cold spray or ice water or a vacuum sucking cool air somewhere blowing on the cable or a cold gun.

Another angle you've probably thought of: If the stack is large and tall and could slowly increase draft you might have the furnace staying the same temp, the fire staying the with the same look but subtle draft increase pulling more air in past the flame rod causing it to run slowly cooler.

The increased draft could also be from ambient conditions like colder external ambient causing greater draft. It could be you're seeing the flame voltage tapering off due to the time of day rather than the hours running but not noticing that correlation.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Flame rod voltage drops after a while - temperature unchanged

(OP)
Many thanks Sir!
Great answer and great help!

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