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Discussion Starter · #1 ·
Hi.
My Lennox furnace (or at least the fan) is powering on every hour at exactly the same time, runs for exactly 5 minutes in an idling cycle, and powers down again without any thermostat inputs. A red light is constantly flashing on the control board.
The thermostat is in the off position the entire time and there are no thermostat inputs!

I have replaced the air filter, the flame sensor was recently replaced, I cleaned the furnace igniter, vacuumed around the blower and inducer fan. I have also replaced the Ecobbee Power Extender Kit connected to the furnace control board but the furnace is still turning on and off repeatedly.

What is driving this behavior?

The furnace runs regularly when I am changing the temperature, and turns on fully for heating (or cooling when I turn on the A/C)
 

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When you say without thermostat inputs - did you disconnect all the t-stat wires at the control board?

What does the board display when this happens?

Those wire saver devices can leak current which can wreck havock with some boards. The ecobee itself can do the same, doesn't use electro-mechanical relays.

Best to run a new wire without enough conductors, if not

Really, that furnace should be on a lennox communicating t-stat so that the heat output can directly track heat loss.

The next best is a 2-stage stat (W2 connected on both ends) - the board calculates firing rate based on length of previous cycles and second stage call causes it to increase the firing rate.

4/5 wire setup - RWYGC, i believe it doesn't really modulate and just runs as a timed 3-stage. could be wrong.
 

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Discussion Starter · #4 ·
When you say without thermostat inputs - did you disconnect all the t-stat wires at the control board?

What does the board display when this happens?

Those wire saver devices can leak current which can wreck havock with some boards. The ecobee itself can do the same, doesn't use electro-mechanical relays.

Best to run a new wire without enough conductors, if not

Really, that furnace should be on a lennox communicating t-stat so that the heat output can directly track heat loss.

The next best is a 2-stage stat (W2 connected on both ends) - the board calculates firing rate based on length of previous cycles and second stage call causes it to increase the firing rate.

4/5 wire setup - RWYGC, i believe it doesn't really modulate and just runs as a timed 3-stage. could be wrong.
This is how my wire setup looks like:

From the control board...
Cable management Electrical wiring Electronics Electronic component Technology


... to the Power Extender Kit...
Product Wire Electronics Technology Electronic device


...to the wire connector screws...
Orange Wire Technology Electronic device Electrical wiring


...to the thermostat
Technology Electronic device Electrical supply Switch Power plugs and sockets


With this setup, the board display is constantly flashing a red light:
Electronics Computer hardware Electronic engineering Electronic component Technology


Do I understand you correctly that you want me to completely disconnect all wires from the first picture from the control board to see if the problem persists?
 

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Yes, disconnect the wires and restore power.

Is the problem new?

Your furnace is crippled the way it's wired, but for now focus on eliminating the issue.

I believe this furnace has a separate display, not just flashing led which indicates operating mode, any error conditions.
 

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Discussion Starter · #6 ·
Yes, disconnect the wires and restore power.

Is the problem new?

Your furnace is crippled the way it's wired, but for now focus on eliminating the issue.

I believe this furnace has a separate display, not just flashing led which indicates operating mode, any error conditions.
It is a fairly new problem. I replaced the old thermostat over a year ago and this issue started a few weeks ago.

I disconnected the wires and restored power. The display still keeps blinking which stands for "Idle Mode", according to the diagnostic code table. (it is the only display and when I turn on heating/cooling the same display will show different letter/number combinations to indicate the corresponding status. Again, the system works just fine when operating the furnace or A/C)

Electronics Electronic engineering Computer hardware Electrical network Technology
 

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Discussion Starter · #8 ·
Thank you so much for your help so far. I will wait to see if the fan cycling turns on again while the wires are disconnected. Will report back tomorrow.

In the meanwhile, could you please elaborate how a "wire saver devices can leak current which can wreck havock with some boards. The ecobee itself can do the same, doesn't use electro-mechanical relays"
Could the ecobee or PEK cause permanent damage to the circuit board and is there a way to prove that?
 

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The switching is done with triacs instead of relays and they can leak a tiny amount of current.

Relays physically break the circuit.

Triacs are electronic gates.

I don't think there would be permanent damage from ecobee products - perhaps some of the crappier ones.

The slp98 is lennox's most advanced furnace and the electronics may be very sensitive.

The pek, the ecobee, or even a minor short between thermostat wires could cause issues.

Ecobee is supposed to be a good product, much more likely to have issues using a nest.

Still, might as well rule out the easy stuff.
 

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Check the programming of the thermostat/tstat to see if it has a Ventialtion mode/cycle.

Some of them do and will run the fan for ventilation on a timer.

Ideally you want to use a 2 stage tstat or Lennox communicating tstat with that furnace so it can use the modulating feature.

Now it is using a timer and works like a 3 stage furnace, 40/70/100 % firing rate. However a lot of them are run that way and people like them that way.

That power extender/cheater device can cause those circuit boards to get damaged and I refuse to use them. And yeah that board is VERY sensitive.
 

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Discussion Starter · #11 ·
Check the programming of the thermostat/tstat to see if it has a Ventialtion mode/cycle.

Some of them do and will run the fan for ventilation on a timer.
I am such an idiot, there was indeed a mode activated in the tstat to turn on the fan every hour for 5 min :bangin:
I must have accidentally activated that when I was playing with the comfort settings a while ago.
 

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Aha.

Glad you found it.:biggrin2:

I am a Lennox tech and yeah it is built into the higher end thermostats plus some HRV heat recovery ventilators.
 
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