[pavlom]

I have Lennox ML195UH series not igniting. Codes are alternating slow flashing.
According to the diagnostic code table the alternating slow flashing means: Watchguard - burners failed to ignite or lost flame sense 4 times in one heating cycle.
I am using this thread as my problem looks the same as stated above. That is why I followed the recommendations from this thread and cleaned the fire sensor with steel wool. But looks like the fire sensor cleaning didn't help as I am having the same issue with the furnace, it stops working once in a while and in order to make it working again I have to turn it off/on OR wait for one hour until it starts again by itself.
This problem first happened couple weeks ago. When I went down to check what was happening I noticed that my air filter was curved (bent) against the blower due to air flow as the filter was full of dust and air flow could not go through it easily. At that point I replaced the filter and turned off/on the furnace and it was working okay after that for one week. So at that point I thought that the problem was due to the restricted air flow through the air filter, and as long as I keep changing the air filter regularly it should be good.
Now I have a new clean air filter which doesn't restrict air flow very much but I still have the same issues with the furnace.
Maybe something was damaged when the air flow was restricted due to the full of dust air filter, and that is why I have problems since then.
Please help. Thank you so much.

 

[supers05]

Try cleaning again. Don't rush. The check the wire connector from the sensor to the board, as well as the grounding wire. Make sure that they are secure and corrosion free.

If that doesn't help, post a picture of the flame when it's on, if you can. does the flame get sucked into the tubes like a jet and blue or waffle a bit and have some yellow or orange to it?

PS. its better to start your own thread next time.

 

[beenthere]

Made this its own thread. Please don't hi jack other peoples threads, thank you.

 

[pavlom]

beenthere, thank you for making a new thread. I just thought that I can use somebody else's thread as long as I have the same issue.

supers05, thank you for the response. The fire sensor was cleaned perfectly. I also checked the wire connections and they look secure and corrosion free. The furnace is only 3 years old and it looks pretty shiny from inside.
I checked also the flame and the flame get sucked into the tubes like a jet and blue. Please see the attached pictures for more information.

 

[supers05]

Some images turn out so cool looking....

Anyways, what about the ground wire. Sometimes they don't add a dedicated wire to the burner. I've had to add a ground wire directly from the screw holding the burner to the housing, to a ground pin on the board. Another problem could be a loose ground or neutral wire going back to the panel.

You can measure the signal, but most people don't have a meter that sensitive. It requires uA DC.

All of the items in the picture look fine to me.

PS. Also make sure the burner face in front of the sensor is clean. The flame looks fine, but a bit of soot can cause some issues for the flame sensor.

Cheers!

 

[pavlom]

Supers05, your very first advice was the correct one - to clean the fire sensor again.
After a furnace rep cleaned the fire sensor one more time with some special kind of sandpaper the furnace has been working without any issues almost one week now. He also mentioned that steel wool is not good for this purpose.
So hopefully it will keep working for a long time.

Another issue that I have with my furnace Lennox ML195UH is a blower speed - it is very high. It would be much more comfortable for us if I could reduce the blower speed.
I just found in the manual that is very easy to do. Just to change out speed taps. There are four of them: black, brown, yellow, red; with black being the highest speed and red the lowest.
However, I heard that reducing the blower speed can overheat the furnace, as it will be adding the same amount of heat but to the smaller air flow, and therefore causing overheating.
Please advise.
If it is true and it will overheat, then I assume there should be some way how to reduce amount of heat added to the air flow.

 

[Bitters]

No way to reduce the amount heat unless you have two stage furnace. I would go with the yellow tap first. Don't use Red.

 

[beenthere]

Post the full model number of your furnace.

 

[pavlom]

ml195uh070xp36b

 

[pavlom]

I checked the furnace and currently the heat terminal is connected with the yellow tap (this is the factory setting), therefore I can go only one step down to the red tap, which will reduce the blower speed by approximately 200cfm (as per furnace manual).

I went through the entire manual, see the link below, and found that in order to reduce the blower speed I have to make sure only that the temperature rise between supply and return air is within the recommended range, which is from 40F to 70F in my case.
http://www.hvacpartsshop.com/manuals/ml195uh.pdf

I think I found the answer to my previous question, but please let me know if I am missing something. Otherwise I will go ahead and connect the red tap to the heat terminal, after I will check the temperature rise across the furnace to make sure it is not overheating.

 

[beenthere]

Check the temp rise now before slowing the blower.

 

[supers05]

Checking the temp rise first let's you know if your already close to the upper limit or not. If you go over the upper limit for more then just testing, the warranty will be void on the heat exchanger, and will probably lead to premature failure. The same goes if the temp rise is too low.


Steel wool can work, but you have to scrub pretty hard. Regular sand paper will tend to leave silica behind, attracting more carbon again. I like to use a very fine file. Everyone seems to have their preference. Some technicians will only replace the sensors.


Good luck

Cheers!

 

[yuri]

Steel wool works great as long as it is perfectly clean steel wool. Not some dish/pot scrubber steel wool which has soap or may be rusty.

#400 or higher emory paper works well too.

In larger commercial units techs usually replace the sensor and spark igniter assembly. In resi units cleaning is usually more economical.

 

[throrope]

Looks like others already provided same.

Supers05, your very first advice was the correct one - to clean the fire sensor again.
After a furnace rep cleaned the fire sensor one more time with some special kind of sandpaper the furnace has been working without any issues almost one week now. He also mentioned that steel wool is not good for this purpose.
So hopefully it will keep working for a long time.

Another issue that I have with my furnace Lennox ML195UH is a blower speed - it is very high. It would be much more comfortable for us if I could reduce the blower speed.
I just found in the manual that is very easy to do. Just to change out speed taps. There are four of them: black, brown, yellow, red; with black being the highest speed and red the lowest.
However, I heard that reducing the blower speed can overheat the furnace, as it will be adding the same amount of heat but to the smaller air flow, and therefore causing overheating.
Please advise.
If it is true and it will overheat, then I assume there should be some way how to reduce amount of heat added to the air flow.

Manufacturers often provide list or table for the minimum blower tap/speed based on rated capacity. The same box and fan is often used with different furnace sizes. My Trane heat pump manual has a table based on the electric heat strip kW. I did not find similar in my Lennox gas furnace manual. A search online or call to Lennox customer support may provide guidance. I suspect if you are on the highest speed tap, one down likely won't be an issue.