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BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22633
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a Lennox Pulse G21 installed in 1996. Model # is

Customer Question

I have a Lennox Pulse G21 installed in 1996. Model # ***** ***** About 5 years ago the fan stopped cycling in AC mode. It worked as normal in heat but in AC it never shut off. No big deal as I'm in Minnesota and don't use AC much. This fall when I fired up the furnace now the fan runs all the time in heat too.
I started by completely disconnecting the thermostat. Flipped power back on to the furnace and the fan runs. Next up I replaced the fan timer part # ***** Flipped the power back on and the fan runs all the time. Double checked the thermostat by disconnecting and no change. On to the only other part in the circuit I replaced the ignition control module with the updated style and no change.
I'm at my whits end and have no idea what to try next. HELP!
Submitted: 1 year ago.
Category: HVAC
Expert:  HVAC Guru replied 1 year ago.

Hello and thanks for using this service, if you have been waiting do not worry or get upset I am just one of the techs on here and I try to get to as many customers as I can and I just picked up your question now. Please give me time to respond as typing, researching and adding links takes me time to do so please be patient and often I will ask you questions to narrow down the issue to get is diagnosed correctly. If another tech tries to post to you please only deal with me as this confuses the issue and further I receive no credit then for assisting....just kindly ask the other tech to leave and deal directly with me thanks!

Expert:  HVAC Guru replied 1 year ago.

So the fan runs without thermostat leads connected right?

Customer: replied 1 year ago.
I disconnected all the wires from the stat to the furnace at the furnace and when I turn the power back on to the furnace the fan runs. And just so you know I tried the same thing 5 years ago when it first started happening with the AC and it didn't change then either. So for 5 years the circuit worked fine with heat but ran all the time with AC and now it runs all the time.
Expert:  HVAC Guru replied 1 year ago.

If you take wire 6 off the relay does the motor stop?

Customer: replied 1 year ago.
I'll give it a whirl tonight. Can you give me the location of the relay and wire color I'm looking for? I assume it's on the same panel the fan timer is but want to be sure. I know there is a transformer and some other stuff in there but don't recall seeing a relay in there.
Expert:  HVAC Guru replied 1 year ago.

It should be numbered right on the relay for you to find.

Customer: replied 1 year ago.
How much experience do you have with Lennox Pulse units?
Expert:  HVAC Guru replied 1 year ago.

I have a lot of experience with them, but I will be offline for awhile, let me open this up to see if another expert can asisst

Expert:  BillyHvac replied 1 year ago.

Hello,

You have a relay that the green thermostat wire eventually lands on. This is listed as a K3 relay and is known as Part P-8-10283. See Here

The board is for time delay on and off for the heat cycle.

The K3 is for fan on and cooling function.

You would find that black blower motor wire attaches to this relay. Main power comes in one side of this relay and just sits there waiting. Once G terminal from thermostat sends 24v back on the green wire..it closes this relay.

In you case this relay could be stuck.

If you pull the black motor wire off and motor stops..then it is cooling K3 relay stuck.

If you pull black and it still runs, then it is a heat board/wire issue. Heat and cool use 2 different speeds.

Pull the black wire, confirm it stops and if so...put black back on...pull the green wire off that comes back from board. If it still runs replace K3.

Billy

Customer: replied 1 year ago.
You rock Billy! I'll get in there when I get home tonight and let you know what it does.
Expert:  BillyHvac replied 1 year ago.

Just let me know as go along troubleshooting.

Any questions just ask.

Billy

Customer: replied 1 year ago.
OK I pulled the black wire from the load side of the relay to the motor and the motor is stopped. But I was a tad confused on the last sentence in your instructions :Pull the black wire, confirm it stops and if so...put black back on...pull the green wire off that comes back from board. If it still runs replace K3.Do you mean the green thermostat wire? If so that is already tested when I disconnected the stat and no change. I added a photo so we are looking at the same thing.
Expert:  BillyHvac replied 1 year ago.

That is the culprit.

and you have tested correctly to find a bad relay. Well done!

Billy

Customer: replied 1 year ago.
Humm. Relay didn't even make my radar as I've heard them fail so they won't close but can't say I have ever heard them sticking closed. I'll get a relay coming and tell you how it comes out. Thank YOU Billy. 5 stars for sure!
Expert:  BillyHvac replied 1 year ago.

You are very welcome. Just return if any issues at all

Billy