Posted by Harold Kestenholz - Hydronic Network on September 27, 2001 at 15:50:24:
In Reply to: Temperature limit control posted by Jason Peterson on September 27, 2001 at 15:29:16:
Actually, the reason a hot water boiler cycles quickly on the high limit is that there is insufficient water flow through the boiler to take the heat away from the boiler to the radiation. This occurs more often in mild weather with a multizone system as not all the zones are open while the boiler puts its rated input into the boiler at every cycle.
Two-stage thermostats eliminate this problem in multi-zone systems. Proper circulator pumping rates with attention to the proper boiler sizing will eliminate it in single-zone systems. Use a thermometer to check if the water leaving the boiler is coming back from the radiation within 20 F degrees difference. If the temperature difference is more than 20F, the circulator is not moving enough water through the radiation and the boiler overheats.
You can add fiberglass to cover any missing from the jacket.