boiler efficiency question

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  #1  
Old 12-04-07, 01:27 PM
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Location: wisconsin
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boiler efficiency question

setup: oil boiler; baseboard hot water; 4 zones.
on the honeywell controller there is a setting that programs the "differential" temp drop for the high temp cutoff. in other words, i can set the high temp to fire/stop the boiler at 180 +/- 10 or anthing else up to 180 +/- 25. example: temp falls to 170; boiler fires. temp rises to 190; boiler stops. that would be +/- 10. or, set high limit at 170 and differential at 20 to get: temp falls to 150; boiler fires. temp rises to 190; boiler stops.
is there a suggested differential and high limit setting that is optimal for efficiency? we live in wisconsin and my wife and i are happy if the house is a constant 65. are short cycles better or long cycles? is it better to circulate 180 water or 160 water. any thoughts appreciated.
 
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Old 12-04-07, 03:37 PM
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Your "heat emitters" (baseboard convectors) were designed to operate with a specific temperature of water circulating through them, probably 180 degrees, in the coldest weather. In milder weather (90+% of the time) they will function well with a bit lower temperature although performance will drop off pretty rapidly when the water temperature goes below 140 degrees.

Most (not all) high limit controls have what is called subtractive differential. This means that the temperature must drop the specified (differential) amount before the control will again close the circuit. A typical high limit set for 190 and a 20 degree differential would open (stop the burner) at about 190 and close (restart the burner) at about 170.

Some controls would have an additive differential and in this case the differential is added to the indicated set point.

There are several different applications of controls with either additive or subtractive differentials. I have never seen a control that the differential is +/- the set point.

Not knowing your particular system or the weather I cannot make any hard recommendations on what temperatures your system should operate. Generally, longer cycles are better than shorter cycles.
 
  #3  
Old 12-05-07, 09:01 AM
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furd
thanks for the info. i will try to get longer cycles as that does make sense. i can do this by setting the aquastat at 180 +/- 20 and see how that works.
the information about the aquastat is from the honeywell web site and regards the R8182D unit. their site claims that it is a +/- system and gives specific examples to show how it works. from there i gather that +/- 10 is the minimum differential and that +/- 25 is the maximum. with high limit at 180, the range of operation is from a minimum of +/- 10 to a maximum of +/- 25. i assume this means 170 to 190 or as far as 155 to 205.
GIVEN A 4 ZONE SYSTEM, DO YOU HAVE ANY THOUGHT AS TO WHY THE BOILER SOMETIMES SHORT CYCLES IE TURNS ON FOR A MINUTE OR LESS, EVEN IN COLD WEATHER??
 
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