Boiler Limit Switch - Replace with bigger Differential

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  #1  
Old 01-09-19, 04:53 PM
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Boiler Limit Switch - Replace with bigger Differential

Peoples thoughts on replacing a gas boilers 15 degree limit switch with a new one that can go from 5 -30 degrees.

Does it make sense to open up the differential to allow the system to extract more of the BTU heat while the gas is not running but the circulator is??

OR

While I get no benefit because the boiler then has to just run back up the extra differential degrees?

So at 15 degrees limit the boiler will run from 165 to 180 or I could also go 175 to 190 degrees. So what ever gas is used to get back up the 15 degrees.

Or at 25 degrees limit the boiler will run from 155 to 180 or I could also go 165 to 190 degrees. So what ever gas is used to get back up the 25 degrees.
 
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Old 01-09-19, 05:37 PM
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Don't know the model of the limit switch you have but I'm fairly certain the limit portion is adjustable. If you want to go from 180-190 or whatever temp you want to aquire is fine and your 15deg. diff. will work accordingly, if that's what you are asking.

Differential settings are there to keep a span between high limit shutdown and to when it refires to help keep the boiler from short cycling.

Keep in mind that the lower the boiler temp the less heat the emitters will put out.
 
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Old 01-09-19, 09:16 PM
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Spott thanks for the reply.

The Limit is I think 140-210. With 15 Degree diff.

My question is if you install a different limit switch with a 20, or 25, or even 30 diff provided you don't set it so the lower end drop below 145 degrees are you getting more out of the boiler? Versus the 15 degrees then boiler turns back on.

So hi limit set at 190 with 15 degree diff, Limit is hit gas goes off circulator runs until limit hits 175 degrees gas turns back on till it hits 190 again

If hi limit set at 190 with 30 degree diff, Limit is hit gas goes off circulator runs until limit hits 160 degrees gas turns back on till it hits 190 again


Question - does this result is savings???
 
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Old 01-10-19, 01:35 PM
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That's a question with a lot of variables to it.

It depends on the age of the boiler. If it's a high water content boiler, how efficient it is, what you have you have for emitters and of course your comfort level.

That being said if you have a fairly current boiler of around 100,000 or so BTU's you probably have about 13 gals. of water in the boiler which hardly takes any time to heat up so in that respect in my opinion there are no appreciable amount of fuel savings to be had.

Where increasing your differential would help is with your equipment, like your burner. Stopping and start continuously will shorten the life of the equipment. Other than that I don't think you would see any real savings in your life time.

Just my thoughts. Hope this helps a little.
 
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Old 01-12-19, 09:10 AM
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The boiler looks brand new, but it's 8 years old. Its input is 130,000 with an output of 108,000. I don't see this boiler really short cycling. Just thought I could use more BTU's left in the pipes if I go from 15 degree diff to 20 or 25 diff
 
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Old 01-13-19, 07:54 AM
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Öcirculator runs until limit hits 175 degreesÖ
Maybe I donít understand but it sounds like you believe the circulator shuts off when the water temp drops below the [High Limit Ė Differential]. I donít think thatís the case. The circulator runs as long as the thermostat is calling for heat. I believe the burner operation is independent of the circulator operation.

The water will continue to circulate if the tístat wants heat, regardless of how much, or when or if, the burner has heated the water.
 
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Old 01-13-19, 10:41 AM
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The gas burner flame shuts off when it hits high limit, the circulator keeps running, The burners kick back on when the water temp goes down to 165 degree.

My question is could I benefit a little more by installing a differential that keeps the burners from kicking on til it reaches 160 or 155 degrees. Then the burners will kick on. Essentially I'm using a little more of the btus in the pipe.
 
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Old 01-13-19, 11:49 AM
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Iím no expert and I think the dynamics of a boiler system are pretty complicated (at least to me-lol). But I donít see what good opening up the differential would do. It seems to me that the temperature of the water you will circulate on average will be halfway between the HIGH LIMIT(HL) and the HIGH LIMIT(HL) minus the DIFFERENTIAL (DF).

So for example if HL = 180 and DF = 15, then the average water temp you are circulating would be 173, i.e., [ (180 + (180-15)]/2.

But if HL = 180 and DF = 30, then the average water temp you are circulating would be 165,
i.e.,[ (180 + (180-30)]/2.

So it seems to me if HL=180/Diff=30 works for the house, then why not just set HL= 170 and DF= 10, which still gives you 165 average water temp. In other words, isnít opening up the Diff just dropping the average temp of the water that is being circulated(and also changing the cycle length)?

Well I guess actually the change in water temperature over time in the [HL-DF] temperature interval would probably not be linear. So probably the average water temperature is not a simple as I made it out above. But maybe itís not a bad approximation.
 
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Old 01-14-19, 06:47 AM
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2014gt "Question - does this result is savings???
As I posted recently: “many users have a poor understanding of heating systems and controls ”

If 2014gt learned heating system basics he would understand how boiler delta-T impacts burner run time, stack temperature, BTU output, efficiency and savings. Until he learns , this thread is like a dog chasing its tail.
 

Last edited by doughess; 01-14-19 at 08:00 AM.
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