old hot water boiler-aquastat

Hello, I have a house with a 1956 Bryan A-14 copper tube hot water boiler. I noticed that my gas usage was skyrocketing with 100's of cubic feet of gas going out each week. I changed the old thermostat and saw that the thermostat correctly turns the boiler on and off. I then watched the hot water gauge controlled by the aquastat. It was originally set to 100 and I changed it to 140 and watched it burn.. It got up to temp at 140 and then correctly shutdown and occassionally refired. But then after sometime it stopped firing and the temp in the water dropped down to 60. Then when the house temp dropped it would refire and bring the water temp up again and repeat.
I noticed that the aquastat is a L4006A and is obviously set as a high limit cutoff. My book says that there is an automatic differential of 5 built in, non-adjustable. Shouldn't the boiler kick back on when the water temp drops below 5 degrees of the set point? It was doing the same thingy of me bob when the temp was set to 100, except it was constantly cycling on and off and I could figure out why?
Thanks, Ed
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It will only kick back on the 5 degree drop if the house thermostat is calling heat and the boiler has not met that yet. 140f is still a low boiler temp, Tonight here will be 0f , 180f is a common temp, at 100f your circulator will be always pumping and the boiler calling for heat in cold weather. It sounds if its working right. But for me 140 is to low.
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