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Originally Posted by Mr Bill

I just personally did this on Ruud furnace about two weeks ago, I got .210 volts, there is a point there "pun intended".

This is an opportunity to remind the guys here about grounding and bonding.

The main panel cabinet is the only place in the system where the neutral is connected to the equipment grounding (bonding) conductors. The two should never have another connection in an appliance, or a sub panel. That is no longer an approved practice. You still see it in old dryers and ranges, with the copper strip used to bond the unit cabinet to the neutral. Now, a four wire scheme is used, with a separate neutral and equipment grounding conductor.

If you find voltage between the unit cabinet and the neutral, you need to repair the grounding circuit going back to the main.

Originally Posted by timebuilder

I would expect that value to be close to zero, and not as high a two volts.

I just personally did this on Ruud furnace about two weeks ago, I got .210 volts, there is a point there "pun intended".

Be sure to check the air intake and vent pipes for partial blockage. Also, the reason not to use sand paper is that small particles of sand will get embedded in the sensor and then the flame will cause them to turn to glass and insulate the sensor. I use a 3M scouring pad like you use to clean dishes. This works as well as steel wool but you can actually keep it in your pocket.

Originally Posted by timebuilder

What was the reason that was given for that check?

A properly grounded (bonded) appliance should have the cabinet and the neutral at the same potential, and since the transformer secondary is bonded to the cabinet (24 volt common) you should show no voltage from neutral (the grounded line conductor) to the cabinet. I would expect that value to be close to zero, and not as high a two volts.

His reason was for board and flame sensor issues. I agree that the closer to zero the better. I was unsure if 2v is acceptable or not. I haven't had a chance to try it out yet. With all the ground and flame sensor talk in this thread it seems like a good place to get some input.

Originally Posted by Hvac216

We had a goodman tech at our shop today for a training class and he says you can check between 24v com and 120v n. you should get less than 2v with the furnace running. Is there any validity to this?

What was the reason that was given for that check?

A properly grounded (bonded) appliance should have the cabinet and the neutral at the same potential, and since the transformer secondary is bonded to the cabinet (24 volt common) you should show no voltage from neutral (the grounded line conductor) to the cabinet. I would expect that value to be close to zero, and not as high a two volts.

Originally Posted by Ac_man

Well I went back to this call earlier today and double checked all my ground connections. i removed one that was connected to the frame of the transformer and placed it with the rest of the ground on the furnace cabinet and it tested fine. I cycled the furnace 3x and everything was fine after that, man I'm happy I'm involved with hvac talk. I will def learn a lot from seasoned techs. Thanks guys

Please, tell me you took a picture before you removed that wire, so I can see what you did....

Originally Posted by Ac_man

Well I went back to this call earlier today and double checked all my ground connections. i removed one that was connected to the frame of the transformer and placed it with the rest of the ground on the furnace cabinet and it tested fine. I cycled the furnace 3x and everything was fine after that, man I'm happy I'm involved with hvac talk. I will def learn a lot from seasoned techs. Thanks guys

What kind of readings did you get before u determined it bad? I asume u didnt do any testing after it worked, i dont either but maybe in the future on this subject ill check volts to flame sensor befor and after and uA after i clean it and get it to work. If nothing else for refrence but judging from (what little) experience i have and from what i can tell based on this thread it depends on the monster your taming no set logic is sufficient for all flame sensors. Can u call a tech support for facs? Im not a pro tech ?

Well I went back to this call earlier today and double checked all my ground connections. i removed one that was connected to the frame of the transformer and placed it with the rest of the ground on the furnace cabinet and it tested fine. I cycled the furnace 3x and everything was fine after that, man I'm happy I'm involved with hvac talk. I will def learn a lot from seasoned techs. Thanks guys

Originally Posted by Hvac216

We had a goodman tech at our shop today for a training class and he says you can check between 24v com and 120v n. you should get less than 2v with the furnace running. Is there any validity to this?

I was taught between 30 and 50v from from flame rod to common or ground. This is with or without flame and this works like a charm on are trane 90+% ers at school but other ones ive tested dont work out the same mine at me house has a smaller flame sensor coming from an ignition module ive read 5v on it and cant get a uA at all off of it but it works
Im thourghly confused but it seems im not the only one. Are you talking about checking for good ground connection?

We had a goodman tech at our shop today for a training class and he says you can check between 24v com and 120v n. you should get less than 2v with the furnace running. Is there any validity to this?

tech rep or salesman...?

Originally Posted by wdshea

I wouldnt hold it against him, he doesn't NEED to know, and is confused.

Truth is, i didnt even know until earlier today...

But he does need to know. He is the ICP tech rep trainer for the northeast. If he doesn't now how can he trouble shoot flame rectification without iunderstanding how it works?

Originally Posted by ebierley

I wish more people understood how flame rectification worked. I just came from a manufacturer's training class where the rep said the flame sensor works because it is made of two dissimilar metals that produce a micro amp current when heated. I almost fell out of my chair.

I wouldnt hold it against him, he doesn't NEED to know, and is confused.

Truth is, i didnt even know until earlier today...

I wish more people understood how flame rectification worked. I just came from a manufacturer's training class where the rep said the flame sensor works because it is made of two dissimilar metals that produce a micro amp current when heated. I almost fell out of my chair.

If you read the article i posted, it would be more aptly named a "rectified current sensor".

would it not then be called a heat sensor instead of a flame sensor?

dogboy

http://www.wardburner.com/flamerectification.html

Great explanation on flame sensor...,hell I think alot of techs still think it "heat" they sense!

Originally Posted by timebuilder

You MUST have a good connection between the burners and the ground connection on the board or module.

Amen Brother Ben, shot at the fox and killed the hen.

The process of ground checking in this situation is to eliminate any high resistance points between the board/module and the burners. The burners are the negative side of the rectification circuit, and the flame sense rod is the positive side. The board or module supplies an AC voltage to the sensor rod, referenced to the burner ground, and then it looks for the DC microamp current, as a result of the flame rectification.

You MUST have a good connection between the burners and the ground connection on the board or module. In rooftop units, you can apply a jumper from the burner array to the module, and if the problem goes away, you know you have a ground connection issue.