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my coleman furnace 7966-856 flame looks to be half its usual size and wont heat up the house

Flame being smaller can be caused by a blockage in burner tube , gas valve, gas line, low gas pressure, gas valve going bad, ect.

What do you mean by wont heat up house, do you mean it wont reach temp srtting on t-stat,
wont ignite, low or no airflow from vents, some vents have airflow, can u be more specific?
It would help alot.

Look for a spider sac behind the orifice on the gas valve. This complaint is mentioned here many times and that is usually the problem.