Bryant propane heater can't possibly be wired reversed (red LED blinks constantly)

[Danny D.]

Stormin Mormon wrote, on Wed, 31 Dec 2014 08:18:12 -0500:

Two out of three is pretty good, compared to 1 out of 3 before.

[Danny D.]

trader_4 wrote, on Wed, 31 Dec 2014 05:29:47 -0800:

I am clueless.

Maybe cracked ceramic that only opened up under red hot heating of the metal rod?

Even then, what's the failure mode? It would just be easier for the electrons to get to the chassis (which is what they're trying to do through the flame).

I really do not understand the failure mode, especially as this thing is clean as can be.

I wish I understood HOW it failed.

[Danny D.]

VinnyB wrote, on Wed, 31 Dec 2014 07:22:32 -0600:

We don't do FWD. But a Toyota will work fine! :)

[Danny D.]

Pat wrote, on Wed, 31 Dec 2014 07:21:06 -0500:

We thought the same thing too... at first. :)

[Danny D.]

dpb wrote, on Wed, 31 Dec 2014 09:00:35 -0600:

I find it interesting that the electrons flow, in effect, backward, in the opposite direction of the flame, from the stainless steel rod, along the flame, back to the burner metal to ground.

[Danny D.]

dpb wrote, on Wed, 31 Dec 2014 09:00:35 -0600:

PHYSICAL CONFIGURATION

1. The flame rod consists of a center electrode and an outer metallic sheath.
2. The electrode is isolated from the outer sheath by an insulating material.
3. The end of the flame rod, in the flame path, has a special high temperature hermetic seal with a threaded adaptor which is connected to the center electrode.
4. A high temperature sensing rod is attached to the threaded adaptor.

I'm confused. Is the "outer sheath" the ceramic? Or, is the outer sheath the stainless steel rod exterior?

[dpb]

As it says, the center electrode is isolated from the outer metallic sheath by the ceramic. IOW, it's a center rod in the center of a ceramic insulator all surrounded by the outer case that you see. It's not a "rod" at all but a composite assembly that looks solid from the outside.

[trader_4]

IDK what flame sensor you have there, but from the dimensions it looks like it's a foot or more long, 1/2" in diameter and certainly not for a typical home furnace. The one Danny has pics of does look like a small rod, mounted on an insulator, connected to a wire. The descriptions I've read about rod to flame conductivity also don't say anything special about the reqts for the rod. They just say that a metal rod inserted into a flame will conduct a current when voltage is applied. That would seem to coincide with what he has, ie a metal rod on an insulator mount.

[dpb]

On 12/31/2014 7:52 PM, trader_4 wrote: ...

Those are commercial for larger boilers, etc., but the physics is the same. There must be an isolated section for the current flow; otherwise it would also be grounded. The _precise_ configuration is immaterial...

[trader_4]

I agree. The isolated section is the metal rod with a wire attached. It's housed in a ceramic insulator that holds it and keeps it isolated from the furnace metal. The flame completes the circuit. The controller board sends it a small current, at about 90V?. If the flame is there, the controller detects that current is flowing, because the flame can conduct a small current, it actually rectifies it. I had no idea how this worked before, but that's what I learned from googling. I haven't seen anything about any special requirements beyond that. And the pics Danny posted of his flame sensors sure look like just a metal rod housed in a ceramic insulator. So, from everything I've seen so far about how it works, the physics, etc it sounds like just a metal rod.

So, if that's right, then the question remains, what failed in Danny's detector? There have been so many posts, I'm unsure of is what exactly he measured, what the readings were, etc. I think he measured that there was infinite resistance between rod and furnace metal, ie no shorting of the insulator. I think he verified conductivity from end of wire to tip of rod, but not sure what exactly he measured there. So, I don't understand what could be wrong with the old one......

[Stormin Mormon]

I'd eat Gretel first, Hansel can still help fix the overhead deck you had blow down.

- . Christopher A. Young Learn about Jesus

.

[dpb]

On 01/01/2015 8:41 AM, trader_4 wrote: ...

I'd guess the measurements he took aren't precise enough (or weren't actually measuring what thought he was is also a possibility of course).

Being as it's a minute current, _any_ breakdown in the insulation and/or hermetic seal could lead to having enough leakage current to essentially short out the actual current but still measure a high (but not infinite) impedance.

Hypothesis; we don't have enough info to be able to diagnose remotely exactly. The key is it doesn't work; a new one does. That's good enough for me...

In a "former life" was responsible for incore self-powered neutron detectors for a particular commercial power reactor vendor. These are essentially a current source also in the 100-1000 nano-amp range and are a swaged leadwire on a Rh emitter surrounded by a ceramic in a metallic sheath. (Sound familiar?)

Their OD is only about 1/16", the lead wire and ceramic are thus tiny. Turned out that even though one could measure a very high bulk resistance between the emitter and sheath that miniscule locations along the length of the detector could have very localized thin spots and become a shunt path for leakage currents. Also turned out that moisture could infiltrate the end seal and gradually diffuse down the length of the detector over time. This also reduced the overall resistance and showed up as a systematically lower output signal but of great variability as again the existence of the surface roughness and local thickness made exponential changes in local conductivity.

The detectors had been tested for years at the corporate research reactor without ever discovering the issues--turned out it required the higher power density of the real thing for the issue to become significant. While they're not identical, given the similarity in construction and function I can imagine many such similar scenarios with these devices. Mostly I imagine it is a moisture infiltration problem with these as a first guess.

In the end, the reactor detector problems turned out to be a secondary blessing; I got a MS thesis out of it while it kept me gainfully employed to pay for the course work while working around the issues for operating reactors w/o requiring warranty replacements and solving the manufacturing problems for new product. :)

[Danny D.]

Oren wrote, on Wed, 31 Dec 2014 15:46:19 -0800:

I just noticed this, so sorry for not responding sooner. I'm still amazed that the heat sensor was bad, simply because there was almost nothing to it.

I guess if, under heat, a crack allows electrons to "jump" to ground, both ways, then the control board wouldn't measure any DC current (because it would be AC). I'm guessing though.

[Danny D.]

trader_4 wrote, on Wed, 31 Dec 2014 17:52:16 -0800:

The one I have is only a couple of inches long, and maybe something like 1/8 of an inch in diameter of the rod itself.

[Danny D.]

trader_4 wrote, on Thu, 01 Jan 2015 06:41:52 -0800:

All this is true.

I'm "guessing" that a crack developed in the ceramic that allowed electrons to flow both ways to the metal chassis that the assembly was bolted to.

But that's just a guess.

[Danny D.]

UPDATE:

The heater has been working (although it has been almost 70 degrees lately) just fine with the new flame sensor.

Apparently, this is what happened:

1. Heater wasn't working for so long that it "forgot" the prior error code, so it was just blinking incessantly.

1. Eventually, the heater got around to blinking a code for a bad flame sensor circuit.

2. The observation was the flames would light and then shut off within seconds, four times, and then lockout would occur until the power to the furnace was turned off and back on.

1. All measurements to determine whether the flame sensor was broken had failed, mainly because 5uA is hard to measure and because the meter I had borrowed must have a blown fuse.

2. However, replacing the flame sensor worked.

How the flame sensor failed is a mystery, as it clearly was not tarnished (it was sanded with steel wool, emery cloth, and a file, over subsequent attempts). The ceramic "looked" intact, but may have developed a fault such as a crack.

The assumption is that the flame sensor failed, although the only proof of that is that the new flame sensor worked as soon as it was installed.

Thanks for all your help and advice. Who knew that the flames form part of an electrical circuit!

[makolber]

If you really want to verify a bad flame sensor, wait till spring, and put the bad one back in, and see if the problem comes back.

But wait till spring Mark

[Danny D.]

makolber wrote, on Fri, 09 Jan 2015 18:16:08 -0800:

:)