Would I Save Oil by Lowering the Boiler High-Limit?

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rberq
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Post by rberq » Thu. Apr. 14, 2011 9:25 pm

A little off-topic but it seems there are some knowledgeable heating people here, so I will ask.
My domestic hot water (130 degrees) comes from a cold-start oil boiler with an external storage tank. Since I burn coal, the oil boiler is used solely for dhw all but a few days per year. Would I save oil by lowering the boiler high-limit? I think it is now at 170 or so. If it does make sense to set it lower, then what temperature should I set it to?

 
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SMITTY
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Post by SMITTY » Fri. Apr. 15, 2011 9:04 am

You can lower the limit, but it will take longer for your hot water to recover during high demand. Not sure if there's any oil savings there, because the burner will run longer. I've tried a bunch of different combinations of high limit/low limit/differential settings without noticing any drastic effects on oil consumption.

The best thing to do, in my opinion, would be to install a hot water coil in your stove. Just don't buy one from Hilkoil (thermo-bilt) ....I think everyone knows that story by now. :x

But anyway ... if it's a cold-start, the coil will keep the boiler fairly warm ... and even up to temp when the stove is really crankin'. Mine have paid for themselves easily - even factoring in getting ripped off by Hilkoil.

 
rberq
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Post by rberq » Fri. Apr. 15, 2011 5:37 pm

SMITTY wrote:You can lower the limit, but it will take longer for your hot water to recover during high demand. Not sure if there's any oil savings there, because the burner will run longer.
When the burner shuts down due to the high-limit, but the external dhw storage tank is still calling for heat, will the circulator keep running?


 
franco b
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Post by franco b » Fri. Apr. 15, 2011 7:51 pm

Yes I think it will unless you also lower the setting on the external tank. If you lower the low limit on the boiler then it will run less except when the domestic tank calls for heat.

 
rberq
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Post by rberq » Fri. Apr. 15, 2011 9:19 pm

franco b wrote:Yes I think it will unless you also lower the setting on the external tank. If you lower the low limit on the boiler then it will run less except when the domestic tank calls for heat.
OK, thanks Smitty and franco, I set it significantly lower. At least I think I did. There is a control attached to the main pipe out of the boiler, with a transformer and thermostat connection and various electronic things and lots of wires, but nothing adjustable that I could see. There is another control box mounted atop the boiler itself, with a single adjustable dial that was set at 208, so I'm assuming that is the high limit. That's the dial I set lower. I could not find a low/high aquastat. I am guessing that, since it is a cold start boiler, the burner just runs flat out whenever any zone is calling for heat, and only shuts down the burner when all the zones are all satisfied or when the high limit is reached.

 
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Post by franco b » Sun. Apr. 17, 2011 9:57 am

By lowering that high limit you might save money if the oil burner is heating faster than the pump can transfer it to the domestic tank, causing it to hit the high limit. The only other way is to lower the temp. on the domestic tank which might be OK in the summer.

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