Does anyone on AH repair HVAC?

[highfigh]

I have a Reznor unit heater in my garage (I call it 'Trent') and it started to interrupt the flame last year, but it happened near the end of Winter, so I didn't really need to do anything about it until this Fall because I now have more to do there and I really want to be heated.

Model- FT45
Symptom: induction motor runs normally and the flame starts (spark ignitor), but flame stops after a few seconds, as if the flame sensor is bad.

Cleaned the sensor, no change.

Measured voltage at the sensor- ~62VAC at the beginning of the flame cycle, drops to ~31VAC once the flame sensor reacts.
Measured current through the sensor- saw >6.2uA, which is within the acceptable window.
I don't have a megger, so I can't measure resistance with a high AC voltage but when I measured DC resistance between the rod and the mounting plate, I didn't see any continuity, even on the high resistance scale.
I have seen no trouble codes, so far.

I decided to replace the fame sensor because it's a cheap part and it just arrived, but I won't replace it immediately because it's snowing and I'll need to move my boat out of the way.

If anyone has ideas for what to check in addition to what I have done, please let me know.

 

[TLS Guy]

I have a Reznor unit heater in my garage (I call it 'Trent') and it started to interrupt the flame last year, but it happened near the end of Winter, so I didn't really need to do anything about it until this Fall because I now have more to do there and I really want to be heated.

Model- FT45
Symptom: induction motor runs normally and the flame starts (spark ignitor), but flame stops after a few seconds, as if the flame sensor is bad.

Cleaned the sensor, no change.

Measured voltage at the sensor- ~62VAC at the beginning of the flame cycle, drops to ~31VAC once the flame sensor reacts.
Measured current through the sensor- saw >6.2uA, which is within the acceptable window.
I don't have a megger, so I can't measure resistance with a high AC voltage but when I measured DC resistance between the rod and the mounting plate, I didn't see any continuity, even on the high resistance scale.
I have seen no trouble codes, so far.

I decided to replace the fame sensor because it's a cheap part and it just arrived, but I won't replace it immediately because it's snowing and I'll need to move my boat out of the way.

If anyone has ideas for what to check in addition to what I have done, please let me know.

Sounds like the flame detector thermocouple has gone bad, or it could be the electronics associated with it. I would put money on the thermocouple.
I would caution you though, that you need a license to work on this type of equipment. If you do a DIY job and there is an incident as a result, you will be in very serious trouble.

 

[highfigh]

Sounds like the flame detector thermocouple has gone bad, or it could be the electronics associated with it. I would put money on the thermocouple.
I would caution you though, that you need a license to work on this type of equipment. If you do a DIY job and there is an incident as a result, you will be in very serious trouble.

I seriously doubt the flame sensor replacement could cause a problem- it has one wire and is held in by one small bolt.

The part arrived yesterday, I'm hoping the weather is good today because I need to move my boat out a bit to access the area of the heater where the control panel is located.

 

[TLS Guy]

I seriously doubt the flame sensor replacement could cause a problem- it has one wire and is held in by one small bolt.

The part arrived yesterday, I'm hoping the weather is good today because I need to move my boat out a bit to access the area of the heater where the control panel is located.

No, I doubt it would. The way this works is that the thermocouple generates enough current to keep a small sensitive relay open that keeps the gas flowing. If the relay does not open the valve in the required time, then the flame goes out. So when your condition occurs, it is usually the thermocouple or the relay. I suspect some units may now have electronic relays rather than the electromechanical type.

The only safety issue you could get into, is having a situation where the gas keeps flowing even if the flame goes out because the system does not shut off the flow of gas. That could certainly cause a gas explosion.

 

[mtrycrafts]

... I would put money on the thermocouple.
....

I replaced a few of these in my water heaters over the years.
Same issue

 

[highfigh]

No, I doubt it would. The way this works is that the thermocouple generates enough current to keep a small sensitive relay open that keeps the gas flowing. If the relay does not open the valve in the required time, then the flame goes out. So when your condition occurs, it is usually the thermocouple or the relay. I suspect some units may now have electronic relays rather than the electromechanical type.

The only safety issue you could get into, is having a situation where the gas keeps flowing even if the flame goes out because the system does not shut off the flow of gas. That could certainly cause a gas explosion.

If I disconnect anything in the gas stream, there's no way I would think of trying to activate the heater without making sure it's not leaking and that's easy- disconnect the igniter. The Flame lasts for a few seconds before shutting off- it looks the same as when it would operate properly and doesn't seem to be caused by low pressure and as I wrote, I did check the current on the sensor, which was in the 'normal' window. It was higher than what I saw in diagnostic videos from HVAC pros, so I ordered a new sensor.

Thanks

 

[isolar8001]

Thank You, Bot.

 

[cpp]

As always, proceed with caution, safety first