Question for the water heater service guys

40 gallon Bradford White conventional flue,8 years old.Heater has a good pilot flame and the burner fires OK.Problem is that it will not fire the burner until the hot water is pretty much exhausted from the tank,were there any problems with the dip tubes in any of these?It seems like it's not sensing the temp change at the bottom of the tank,or do I guess that its a problem in the control?The temp knob is labeled hot,a,b,c,d,very hot.If you fill it with it set at hot the burner comes on and shuts down in a reasonable amount of time,if you then move the control to anything hotter it wont fire until it once again runs out of hot water.

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