Help with lennox pulse Fan control

mike585

Lennox Pulse Gas G21Q. Fan timing not working. Switched stat to control. This disables on and off delay timing, so I get cold air at cycle start and lose the delay on shutdown. Obviously I want these features back. I replaced the fan timer (51K46). No remedy. I replaced this control 6 years ago and had favorable result. There is a field modification that the Lennox service man made about 10 years ago in the control circuit.



 With stat fan control switched to the fan timer the burner fires, but trips on high limit when the fan fails to start. For now I am back on stat control, so as to have heat. As a side note, I sometimes hear a loud buzzing noise from the furnace at the end of the cycle, about 3 seconds long



I'm not a pro, but I have done most repairs myself and I have a decent idea how the controls work.



Help would be appreciated.

Tim McElwain

It sounds like the control board

is faulty. What modification did they make previously and what is the make and number of the control board.



I am going to see if I can find my manuals on that model of Lennox Pulse.

mike585

Lennox control

The attached .pdf has the mod scribbled by the tech after he finished. I should have made clearer notes on the diagram.  I never got a clear explanation from him as to the purpose of the change.  I found the mod on the fan control circuit  board the first time I replaced it 6 yrs ago.



By the control board I assume you mean the main control, and not the fan control that I just replaced. The main control is Johnson Part Number 34K8301 Mod Num. G891TCA-8102. It  too was replaced a long time ago. I can't see how the fan control is influenced by the main board. It looks to me like relay K3 enables relay K43 which times the fan.

Tim McElwain

It looks to me like he

bypassed the K43-1 contact and the limit circuit, hard to tell from the scribble. It looks like there may be a short. I would suggest getting whoever did that back and have him fix the problem. You might not want to fool with it yourself as you may damage something.





When "R" on the t'stat makes to "G" then  Relay K-3 is energized on a call for heat and brings in K3-1 and K3-2. K3-2 closes and powers K-43 which should bring in K43-1 which looks like it has been bypassed. 





I would get them back as I said.

Tim McElwain

I found my Lennox G21Q

manual. Do you have a copy? If so try going through the troubleshooting charts on pages 20 through 23.

mike585

K3

I tested the K36 timer relay and contacts tonight and they function as they should.  Tested the K-3 and it seems messed up, NC contacts are open. Also found out that K-3 makes the noise on shutdown. Going to buy a new one tomorrow.  

mike585

K-3

Relay replaced. Works as it should.