Gas and Oil Home Heating Furnaces - Lennox Value Series cycling on/off

Good Morning,

I was searching the web for some information on my Lennox Value Series furnace and came upon this forum and found a wealth of information so I figured this is the place to post my problem...

It is a G40DF (x) Series (at least it looks most like this one from the online manuals I've been able to find) and here is what I've been experiencing. The thermostat will kick on the purge, then the ignitor will test and it will fire. The ignitor will turn off and it will continue to burn for less than 1 minute. The purge will come on again, and repeat the cycle. At the end of the 2nd go-round the furnace will burn for well over a minute and then turn out before satisfying the thermostat.

May times the indoor blower will not come on until after the flame has gone out. It is frustrating to say the least. Eventually it does satisfy the thermostat but it can take quite a while.

Typically I will hear it kicking on and off and go in and trip the power to restart the process so that it will continue to burn off/on instead of staying off.

When it stays off I most often see LED 1 with slow flash and LED 2 on. The yesterday day I saw LED 1 off and LED 2 with a slow flash.

To the most common condition. I see that this is due to the "primary or secondary limit switch" being open. What are these switches, what causes them to open, how can I replace them or remedy the issue causing them to open?

As I understand it the primary switch will open when it gets too hot. To keep it from getting too hot a large volume of air needs to flow across the heat exchange area. So perhapps the switch is bad, or the blower isn't coming on soon enough (though it does come on and continues to run after the primary limit kills the flame) or there isn't enough volume going across the heat exchange.

To the last point I've been thinking of a repair I made last summer in the crawl space. The builders or previous home owners left a 2"x18" gap between the vent and floor joist. From the crawl space I could see the AC coil and at that time the cold AC was blowing into the crawl space. I couldn't figure out why they would leave this gap in the ventilation system so I plugged it with some angled sheet metal. The AC system worked much better but now I'm thinking that gap was to help the furnace cool and stay between the limiters?

Drivin me nuts, any help would be appreciated.

We are WELL into the winter now. You said you sealed off that rectangle cut-out last summer. So I have to ask if the furnace has been working fine up until recently, or if the problem has been there ever since you have ran the furnace this year, since you did that cover-over job.

What 'vent' is that, that you speak of, that is next to the rectangle cut-out in that crawl space? Is it a 'vent' at all?, or is it a duct?

As it got colder this week I called an HVAC tech in to take a look. Turns out the flame sensor had a crack in the white part and the previous owner had put filters in the furnace and in the ceiling return air duct. We didn't know about the ones in the furance so they hadn't been changed in at least 18 months and from the looks of things they may have never been changed at all. They were nasty.

The HVAC tech said we didn't need the redundant filters so we took them out and the system works fine now.

Lennox won't stay burning
________________________________________
It is a G40DF (x) Series and here is what I've been experiencing. The thermostat will kick on the purge, then the ignitor will test and it will fire. The ignitor will turn off and it will continue to burn for less than 1 minute. The purge will come on again, and repeat the cycle. At the end of the 2nd go-round the furnace will burn for well over a minute and then turn out before satisfying the thermostat.

Many times the indoor blower will not come on until after the flame has gone out. It is frustrating to say the least. Eventually it does satisfy the thermostat but it can take quite a while.

Typically I will hear it kicking on and off and go in and trip the power to restart the process so that it will continue to burn off/on instead of staying off.

(1) When it stays off I most often see LED 1 with slow flash and LED 2 on. Then (2) yesterday day I saw LED 1 with slow flash and LED 2 off.


I would first make sure return-air is not being starved. This could happen if too many of the return registers are closed, or if the filter is making it difficult for the air to flow (excessively dirty, or, perhaps you have in place one that is way too efficient – and hence restrictive).

Condition (1) is indeed an open limit condition. This is happening b/c the burners are firing and heating-up the plenum without the house blower being allowed to run.
Why is the flame present for such short periods of time? Likely the flame-rod is not doing a good job in sensing the presence of the flame and informing the module. Its tip may be sooted-up (clean with emery cloth), its bracket loose so that the tip is no longer in the flame (re-align & tighten), the electrical wire connecting it to the module is loose and/or the module connection to ground is loose or lost (tighten).

Condition (2) tells the tale of a direct connection to ground from the flame-rod. Maybe the tip of the rod really did become loose and tilted so much so that is now touching a metal (re-align)…or the ceramic shield has cracked and hence the rod needs replacement (replace with new one). Depending on the M/N of the module of your unit, a DC electrical signal lo lesser than 0.18 micro-amperes is expected thru the flame-rod circuit…it does not take much to mess with such a minute current.

Hope this helps.

Ooops!
I should have read the threads first.
Obviously the problem was already taken care of. :o