Why a high limit on return?

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Old 12-14-12, 10:12 AM
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Why a high limit on return?

Another question about my NG boiler: What purpose does an open-on-rise high limit mounted to the return pipe serve? It's set to about 185f and only the thermostat wires go through it.
Under what condition would my return water temperature exceed the aquastat high limit? Is this just a redundant limit?

System is working fine--I'm just trying to understand.



 
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Old 12-15-12, 05:19 PM
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As to your question, my guess is that it's some sort of a high temperature safety control. Your description of how it's wired leads me to believe its purpose is to cut-off the t-stat's call for heat when the system water temps. climb too high.

The reason for a high limit safety temp. control is to shut down the boiler in case of a failure of the aquastat's high limit (hence it being called a "safety" control).
 
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Old 12-15-12, 05:40 PM
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I can't understand it either.
 
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Old 12-15-12, 06:10 PM
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If it is indeed a high temp. safety, then I'm not sure if it serves that purpose well. I'm thinking that any kind of failure of the aquastat's high limit (causing a potential "runaway" boiler) wouldn't be solved by simply cutting off the call for heat...
 
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Old 12-15-12, 09:01 PM
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Well I guess it makes about as much sense as the low water cutoff...in a basement location...meaning all the pipes would be empty before it would cut the power. Possibly someone thought these were prudent in a house that sits empty all winter.

The weird thing is I was told the heat got shut off and the water pipes drained each winter. Dont know why anyone would lie about that.

Also the aquastat settings are really jacked. The high limit is set to 200 and the low is 180. That would only make sense during winter in a low-efficiency house (log cabin). That's IF I understand past posts I have read.
 
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Old 12-15-12, 09:07 PM
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The low water cutoff is installed just above the water jacket in the boiler. It's there to protect the boiler only from low water.
 
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Old 12-16-12, 07:41 AM
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The boiler doesn't have it's own internal low water cutoff?
 
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