Most efficient Differential setting

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Old 04-28-13, 09:20 AM
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Most efficient Differential setting

When the boiler will not be running to heat the house and there is very little need for domestic hot water, which is going to consume the least oil, setting the differential to 10 or setting it to 25?
 
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Old 04-28-13, 03:23 PM
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Hi Michael, I think it will be a 'wash'.

Set it at 10 and you will have shorter, but much more frequent firings of the boiler.

Set at 25 and the firings will be longer, but less frequent.

That said, the higher diff setting is preferred. You don't want 'short cycles'. A higher setting will give you slightly more hot water...

What is your LOW setting? (and your HIGH setting too, while we're at it...)
 
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Old 04-30-13, 03:24 PM
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if you have the ability, you can clock the run time.

Years ago i was investigating that very same thing.
I used and old electric plug in (120 Volt) clock in line with the motor. (I was on gas, so I used the boiler circ I think). Since your on oil, you should know your nozzle size, and the clock will give you your run time. You should be able to get some pretty accurate costs as long as the clock is good.
Try running for a day or 2, or week, and tally up how long the clock has run during the period with different differentials.

I ended up running my Indirect with a pretty large dif, I think about 30.
I have a mix valve on it so I was not concerned about over temps. I turned my boiler limit up quite high to allow the boiler to run the duration of the DHW call.

I know your talking about an oil boiler with a tankless coil in it, but its the same idea.
 
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Old 05-01-13, 05:30 AM
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Not taking the DHW into account (I am not using my coil), a larger differential will be more efficient (normally) if you are properly radiated or over radiated.
In my current case (changing this summer), I'm under radiated and a shorter differential is better (don't lose as much heat to the rads).
 
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Old 05-01-13, 05:02 PM
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FWIW: when there is little or no hot water draw (and the heat is off), some boilers will experience what is known as "heat soak", which can cause the boiler water to rise as much as 10 degrees (or more) after the burner stops firing. This might be something to keep in mind when you are setting the LL and DIFF during the warm season.
 
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Old 05-03-13, 05:14 AM
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May I ask a bumb newbie question here?

Are we talking about the differential of the DHW tank or the the boiler when there is a DHW call?

The reason I ask is I have had a new system installed in my older home and initally ran into DHW issues.

Would not a large differential allow the water to get too cool before the call for heat causing a catch up issue with the boiler?

Tom
 
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Old 05-03-13, 08:40 AM
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Would not a large differential allow the water to get too cool before the call for heat causing a catch up issue with the boiler?
It depends on whether the differential is additive or subtractive.

In this posters case, the aquastat on his boiler has BOTH...

The subtractive diff is FIXED at 10F , meaning that the boiler will always fire when the temp drops 10 below setpoint.

The ADJUSTABLE portion is ADDITIVE, meaning that a higher diff will allow the boiler to get hotter before shutting off.

I don't think this gent has returned to get his answers... a 'fly-by'.
 
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Old 05-03-13, 08:13 PM
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I see now.....Thanks NJ

Tom
 
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