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The burners ignite, main circulation blower kicks on...and both should remain on until thermostat turns off burner,,and 90 seconds later the blower stops.
If the burner cuts off befor the thermostat set point is reached, then there is a defect somewhere
Is the there any set pattern? Do flames go out exactly when blower comes on?...or does the flame go out and blower continues and then flame comes back on?
Since it involves both burner and blower I am led to believe circuit board. But with this particular set of symptoms it would take sitting there with a volt meter to see what drops out when, and why,
Have you check thermostat batteries?
Very inefficient...but runs.
The unit may be worth a service call to get straight. But I can tell you it is not operating as designed.
You are very welcome.
I am available for 30 days of follow up if you have questions on this situation.