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Posted by Heatpro on December 03, 2005 at 10:11:14:
In Reply to: aqustat keeps tripping... posted by richie on December 02, 2005 at 23:21:25:
Did you mean that the high limit aquastat is stopping the burner at 200F?
You don't really have a problem, as heating the water to 200F or 180F is just too close a concern. Set the high limit to 210F and the Honeywell 3-function control will be beyond the 25F differential range. you probably have it set too close. Traditional tankles coil boiler settings had been 210F high limit, 180F low limit and 20F differential. Today's plumbers install a tempering valve on the domestic hot water line to reduce scalds and to give more hot water than from the coil alone. The settings are tweaked for satisfaction after that, but getting the settings too close can make strange boiler behavior.
Cast-iron stores heat, so more of the heat goes into the water when the burners shut off for a while to overheat the water in the boiler locally. You can probably install 100 high limits and have the same "problem" which is not a problem. With a 30-pound relief valve, the boiler can go to 275F before water turns to steam inside the boiler; the high limit stops getting even near that.
As you have a multi-zone system, when one zone calls for heat the burner runs heating the water. However; at that time there is only one zone open, which means that only part of the readiation to send heat out is available to discharge heat. So, if the boiler can put 90,000 btuh into the water, only the zone that is open, say, 30,000 btuh of radiation can emit heat. this overheats the water during the zone call for heat. Then, possibly, one or two zones call for heat - partially getting rid of the heat and overheating the water more. When the weather is cold, the water temperature is limited by the high limit due to this "multiple overlapping demand."
Ways to prevent this overheating of water as the natural result of zoning part of the radiation out of the circuit at times is to
+ install a Beckett Heat Manager control
+ two-stage thermostats on each zone
The easiest is probably the Manager.
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