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Posted by HeatPro on March 27, 2005 at 11:02:12:
In Reply to: Temp. overshoot posted by Rick on March 27, 2005 at 10:45:55:
The gauges and the hottest water are often in different sections of the boiler while the boiler is running. Different flows can be in different sections of a boiler while the burner is on; but the water mixes when the circulator stops. This is often because the water flow is restricted to the rate of the tubing it is attached to. If the piping is changed to a loop-subloop configuration so the water flow in the boiler is at maximum no matter what the flow rate required for the radiation, the variation would most likely cease.
The charm of cast-iron is that it 'holds heat' and it has a somewhat delayed transfer of heat that keeps the fire side hotter than the water side when the burner is on and the water flowing. This helps assure that the fireside remains above condensing temperature, as water on the fireside of exposed hot cast-iron will rust it severely. Stop the water flow and the higher fireside temperature is then transferred to the water side. Another possibility is to add a delay to the circulator control so it continues running for a minute or so after the fire dies so the fireside heat is absorbed and distributed through the system rather than heating the still water inside the boiler.